****************************************************************************** AUSTRALASIAN SOCIETY FOR GENERAL RELATIVITY AND GRAVITATION Electronic Newsletter -- #25, Summer 2021-22 ****************************************************************************** The ASGRG has a home web page at http://www.asgrg.org Items for this newsletter should be emailed to the editor: asgrg@hotmail.com The deadline for the next issue is 30 November, 2022. ****************************************************************************** CONTENTS: * REPORT ON 10TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG10): Victoria University of Wellington, New Zealand, 10-13 December, 2019 * MINUTES OF THE 11^TH BIENNIAL GENERAL MEETING OF THE ASGRG held at Victoria University of Wellington, New Zealand, Tuesday 10 December, 2019 * 2020 PRIME MINISTERS PRIZE FOR SCIENCE awarded to ASGRG members Susan Scott, David Blair, David McClelland and Peter Veitch * 11TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG11): University of Tasmania, Hobart, 2-4 February, 2022 * 12TH BIENNIAL GENERAL MEETING OF THE ASGRG, 3 February 2022 * MEMBERSHIP DETAILS ONLINE at http://www.asgrg.org/membership/index.php * FORTHCOMING MEETINGS * MEMBERS' ABSTRACTS at gr-qc, December 2019 - November 2021 * ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc, December 2019 - November 2021 ****************************************************************************** REPORT ON 10TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG10) Victoria University of Wellington, New Zealand, 10-13 December, 2019 ACGRG10 was the tenth in the series of biennial conferences run by the ASGRG. The venue was the Kelburn campus of Victoria University of Wellington. Keynote talks were given by Jörg Hennig (solitons and black holes), Volker Schlue (expanding black hole spacetimes), Maria Eugenia Gabach Clement (quasilocal quantities in GR), Karl Wette (continuous GWs from rapidly rotating neutron stars), Krzysztof Bolejko (non-Riemannian signatures in cosmological data), and Robert Ward (precision measurement of space-time). In addition, there were another 18 contributed talks on a range of mostly theoretical topics, covering inflation, cosmic expansion, wormholes, Hawking radiation, Bianchi models, the Sagnac effect, modified gravity, neutron stars and black hole perturbation theory. The winner of the Kerr Prize for the best student talk at ACGRG10 was Jessica Santiago of Victoria University of Wellington, who spoke on Thermodynamic equilibrium in General Relativity. ACGRG10 also included an Education and Outreach session for science teachers on 13 December, which featured workshop-style presentations by Shon Boubil, Matt Visser and Jackie Bondell. ****************************************************************************** MINUTES OF THE 11TH BIENNIAL GENERAL MEETING OF THE ASGRG held at Victoria University of Wellington, New Zealand, Tuesday 10 December, 2019 The meeting opened at 4.15 p.m. Present: Jörg Frauendiener (President), Malcolm Anderson (Secretary), Susan Scott, David Wiltshire, Karl Wette, Krzysztof Bolejko, Colin Maclaurin, Woei Chet Lim, Matt Visser, Nikodem Poplawski Apologies: Todd Oliynyk (Treasurer), Leo Brewin 1. The minutes of the 10th Biennial General Meeting, held at the Gravity Discovery Centre, Gingin, Western Australia on Monday 27 November, 2017, were presented to the meeting. Woei Chet Lim moved that the minutes be accepted, and David Wiltshire seconded. The motion was approved, with the following corrections: David Wilshire should be David Wiltshire Gervais Bay should be Jervis Bay Barry Barrish (twice) should be Barry Barish Matters Arising: Susan Scott mentioned that the ASGRG was not represented by a plenary speaker at the last AIP Congress. Ray Weiss was invited as a plenary speaker but pulled out at the last minute, so Susan Scott and Paul Lasky instead gave a combined plenary talk. Previous plenary speakers in GR at AIP Congresses have included Bruce Allen, Steve Carlip, Gary Horowitz and Barry Barish. Jörg promised to submit names of potential plenary speakers for the next AIP Congress (in Adelaide in December 2020) early next year. 2. President's Report: Jörg Frauendiener informed the meeting that (i) The OzGrav Centre [the ARC Centre of Excellence for gravitational wave discovery, hosted by Swinburne University] is up and running. (ii) David Wiltshire, Richard Easther and Jenny Adams have submitted a proposal for a New Zealand Centre of Excellence in Astrophysics, which if funded will be a network for planetary and space science with 16 principal investigators, costing $5.5 million a year for 8 years. The proposed name for the project is the Kerr-Tinsley Centre [Beatrice Tinsley was the first female professor of astrophysics at Yale University]. (iii) He [Jörg] delivered reports to the AIP Council Meetings in 2018 and 2019, as ASGRG is a cognate society of the Australian Institute of Physics. ASGRG members are eligible for a fee reduction if they wish to join the ISGRG. The general advice of the AIP was that the ASGRG should remove the option of lifetime membership, which was restricting the income of the society. 3. Treasurer's Report: The Treasurer, Todd Oliynyk, was unable to attend the BGM and sent his apologies. He reported by email that the Society's current account contained $10,702 and there was another $5,305.75 in Paypal. The net balance of funds was therefore $16,009.75. 4. Auditor's Report: No auditors report was available at the BGM. 5. Appointment of Auditor for the next session: Malcolm Anderson would ask John Schutz if he was willing to remain the Auditor of the Society's accounts. 6. Election of officers: The following people were elected officers of the ASGRG Committee for the 2019-2021 session unopposed: President: Jörg Frauendiener (nominated: David Wiltshire, seconded: Susan Scott) Treasurer: Karl Wette (nominated: Susan Scott, seconded: David Wiltshire) Secretary: Malcolm Anderson (nominated: David Wiltshire, seconded: Jörg Frauendiener) Officer: Susan Scott (nominated: David Wiltshire, seconded: Jörg Frauendiener) Officer: David Wiltshire (nominated: Susan Scott, seconded: Jörg Frauendiener) Krzysztof Bolejko was also co-opted as a Committee member. Karl Wette was appointed as a signatory to all the Societys bank accounts. 7. Date and venue for ACGRG11: The date of the next Conference was set for December 2021, but the venue remained undecided. The next Amaldi meeting would be held in July 2021 at Swinburne University in Melbourne. One possible venue for ACGRG11 was Hobart. Krzysztof Bolejko would investigate this. ACGRG12 might be hosted by the ANU in December 2023. 8. Other business There will be a Summer School in Mathematical Relativity in Nelson, New Zealand in January 2020, organised by the New Zealand Mathematics Institute. 30 students will attend, including 6 from Australia. It was agreed that the advertising for ACRGR10 was sent out too late, and an effort should be made to advertise ACGRG11 much earlier. It was agreed to keep the fees for membership of the ASGRG unchanged, namely $40 a year for full membership, $20 a year for student or unwaged membership, and $250 for lifetime membership. The ASGRG website is hosted by Monash University, and there is always a need for someone who can spare the time to update it regularly. Karl Wette agreed to look at the website and see if he could take it over. Susan Scott stated that the ASGRG needs more members from OzGrav, and should actively try to recruit some. The meeting closed at 5.35 pm. ****************************************************************************** 2020 PRIME MINISTERS PRIZE FOR SCIENCE On 28 October 2020 the $250,000 Australian Prime Ministers Prize for Science was awarded jointly to David Blair (UWA), David McClelland (ANU), Susan Scott (ANU) and Peter Veitch (Adelaide) for their work on gravitational wave astronomy, particularly their contributions to the first direct detection by LIGO of gravitational waves in 2015. All four are members of ASGRG. For more information about the Prize and the scientific contributions of the recipients, see pmc.gov.au/news-centre/government/2020-prime-ministers-prizes-science ****************************************************************************** 11TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG11) University of Tasmania, Hobart, 2-4 February, 2022 ACGRG11 is the eleventh in a series of biennial conferences run by the ASGRG with the aim of bringing together researchers from around the world to discuss all aspects of General Relativity, Cosmology and Relativistic Astrophysics including theory and experiment. The programme will include both experimental and theoretical plenary sessions, with invited speakers Vaishali Adya (ANU), Dan Brown (Adelaide), Hayley Macpherson (Cambridge), Meg Millhouse (Melbourne), Roger Penrose (Oxford), Ryan Shannon (Swinburne), Rory Smith (Monash), Chris Stevens (Canterbury) and Magdalena Zych (UQ). ACGRG11 will be hosted by the University of Tasmania from 2 to 4 February, 2022, and is open to anyone with an interest in general relativity. Attendance is possible both in person and via zoom. In addition, Roger Penrose will give a Public Talk (remotely) from 8 to 9 pm on Wednesday 2 February. Local Organising Committee: Krzysztof Bolejko, Earl Lester, Ben Whale, Karelle Siellez Scientific Organising Committee: Joerg Frauendiener (Otago), Carl Blair (UWA), David Wiltshire (Canterbury), Susan Scott (ANU), Cullan Howlett (UQ), Jade Powell (Swinburne), Malcolm Anderson (Brunei), Eric Thrane (Monash), Karl Wette (ANU) For further details, visit the conference website at asgrg2021.org/home ****************************************************************************** 12TH BIENNIAL GENERAL MEETING OF THE ASGRG The 2022 Biennial General Meeting of the ASGRG will be held in conjunction with ACGRG11, at 5 pm on Thursday 3 February 2022. All ASGRG Executive Committee positions will be filled by election at the BGM. The outgoing Executive Committee members are: President Jörg Frauendiener Treasurer - Karl Wette Secretary - Malcolm Anderson Officer - Susan Scott Officer - David Wiltshire Co-Opted committee member: Krzysztof Bolejko ****************************************************************************** MEMBERSHIP DETAILS ONLINE: ASGRG members are invited to renew their subscriptions by visiting the Membership web page at: http://www.asgrg.org/membership/index.php Membership is open to anyone interested in General Relativity. Post-graduate students and early career researchers are particularly encouraged to apply. The annual subscription is A$40 (A$20 for students and retirees). Life membership is available for a one-off payment of A$250. Members of the Australian Institute of Physics (AIP) are entitled to a 10% discount on all memberships. ****************************************************************************** FORTHCOMING MEETINGS January 19-24, 2022: Nordic Winter School on Particle Physics and Cosmology Thon Hotel Skeikampen Svingvoll, Norway http://indico.nbi.ku.dk/event/1324 February 14-18, 2022: SIGRAV International School 2022 on Cosmology: from Theory to Observation Italian Society of General Relativity and Gravitation (online) http://agenda.infn.it/event/28785 February 14-18, 2022: Gravity The Next Generation Yukawa Institute for Theoretical Physics, Kyoto University Kyoto, Japan http://www2.yukawa.kyoto-u.ac.jp/~gc2022/YKIS/index.php February 21-23, 2022: 12^th Central European Relativity Seminar Budapest, Hungary http://univie.ac.at/cers/cers12/undex.html March 14-25, 2022: Theoretical Aspects of Astroparticle Physics, Cosmology and Gravitation Galileo Galilei Institute for Theoretical Physics Arcetri, Florence, Italy http://agenda.infn.it/event/28760 April 25-28, 2022: Gravitational Wave and Multimessenger Astronomy Physikzentrum Bad Honnef Hannover, Germany http://we-heraeus-stiftung.de/veranstaltungen/gravitational-wave-and-multimessenger-astronomy/main/ May 1-7, 2022: Hot Topics in Modern Cosmology Institut dEtudes Scientifiques de Cargese Cargese, France http://cpt.univ-mrs.fr/~cosmo/SW_2022/index.php May 19-21, 2022: ICASU Inaugural Conference Illinois Center for Advanced Studies of the Universe University of Illinois Urbana-Champaign http://icasu.illinois.edu/inaugural-conference May 30-June 3, 2022: Informational Architecture of Spacetime Workshop Okinawa Institute of Science and Technology Okinawa, Japan http://groups.oist.jp/iasw June 27-July 1, 2022: Metric-Affine Frameworks for Gravity 2022 Institute of Physics, University of Tartu Tartu, Estonia http://geomgrav.fi.ut.ee/conf/maffgrav2022/ July 3-8, 2022: 23^rd International Conference on General Relativity and Gravitation (GR23) Institute of Theoretical Physics, Chinese Academy of Sciences Beijing, China http://gr23beijing.com July 21-23, 2022: Global Structure in Semi-Classical Gravity Munich Centre for Mathematical Philosophy, LMU Munich, Germany http://mcmp.philosophie.uni-muenchen.de/events/workshops/container/scg2022/index.html July 25-29, 2022: Frontiers in Numerical Relativity TPI, FSU Jena, Germany http://indico.nbi.ku.dk/event/1324 ****************************************************************************** MEMBERS' ABSTRACTS at gr-qc, December 2019 - November 2021 We list here all new abstracts that we are aware of that have been submitted by our members to gr-qc, or which are cross-linked at gr-qc. (We have not searched for abstracts on other Cornell University archives which are not cross-linked to gr-qc.) If you do not send your papers to gr-qc but would like to have them noted in the newsletters, please send them to the Editor. Note that the 208 papers listed here and in the LIGO section represent 1.69% of the 12273 papers posted or cross-linked to gr-qc between December 2019 and November 2021. ---------------------------------------------------------------- arXiv:2012.15479 gr-qc astro-ph.SR *Algorithms for Generating All Static Spherically Symmetric (An)isotropic Fluid Solutions of Einstein's Equations * *Authors*: M M Akbar, R Solanki Abstract: We study the Einstein equations of the static spherically symmetric anisotropic fluid system in curvature coordinates to find algorithms that generate all solutions and all solutions that are regular at the center. All possible combinations of input functions from the set of four functions that characterize the anisotropic system are considered and all equivalent conditions for central regularity are determined (for both isotropic and anisotropic systems). We provide the first regularity analysis of the known algorithm that uses the potential function and anisotropy as inputs. For three other choices of input function pairs (any two of the potential function, density, or radial pressure), a remarkably straightforward algorithm follows, which is very efficient in generating regular anisotropic solutions. This is because the equivalency of the three pairs in this algorithm arises precisely from the same algebraic relation that made the different equivalent sets of regularity conditions possible. In addition, the choice of functions makes this algorithm very suitable for finding particular solutions that admit other desirable physical properties; we construct three examples. This algorithm does not admit an isotropic limit although all isotropic solutions are produced as part of the anisotropic system. The remaining two choices of input function pairs (anisotropy with the radial pressure or density) lead to the old barriers one encounters in the isotropic system: Riccati and Abel equations. However, with any solution generated by the new and existing algorithms, one can now construct the general solution of the corresponding Riccati equation to obtain a one-parameter family of geometries for each input solution. We discuss the regularity of the resulting solutions. ---------------------------------------------------------------- arXiv:2108.13489 gr-qc *Generating Spherically Symmetric Static Anisotropic Fluid Solutions of Einstein's Equations from Hydrostatic Equilibrium * *Authors*: M M Akbar, R Solanki Abstract: For static fluid spheres, the condition of hydrostatic equilibrium is given by the generalized Tolman--Oppenheimer--Volkoff (TOV) equation, a Riccati equation in the radial pressure. For a perfect fluid source, it is known that finding a new solution from an existing solution requires solving a Bernoulli equation, if the density profile is kept the same. In this paper, we consider maps between static (an)isotropic fluid spheres with the same (arbitrary) density profile and present solution-generating techniques to find new solutions from existing ones. The maps, in general, require solving an associated Riccati equation, which, unlike the Bernoulli equation, cannot be solved by quadrature. In any case, it can be shown that the output solution is not, in general, regular for a given regular input solution. However, if pressure anisotropy is kept the same, the new solution is both regular and can be found by solving a Bernoulli equation. We give a few examples where the generalized TOV equation, under algebraic constraints, can be converted into a Bernoulli equation and thus, solved exactly. We discuss the physical significance of these Bernoulli equations. Since the density profile remains the same in our approach, the spatial line element is identical for all solutions, which facilitates direct comparison between various equilibrium configurations using fluid variables as functions of the radial coordinate. Finally, combining with the previous study on generation algorithms, we show how this study leads us to a new three-parameter family of exact solutions that satisfy all desirable physical conditions. ---------------------------------------------------------------- arXiv:2108.05169 quant-ph gr-qc *Relativistic Bohmian trajectories of photons via weak measurements * *Authors*: Joshua Foo, Estelle Asmodelle, Austin P. Lund, Timothy C. Ralph Abstract: Bohmian mechanics is a nonlocal hidden-variable interpretation of quantum theory which predicts that particles follow deterministic trajectories in spacetime. Historically, the study of Bohmian trajectories has been restricted to nonrelativistic regimes due to the widely held belief that the theory is incompatible with special relativity. Here we derive expressions for the relativistic velocity and spacetime trajectories of photons in a Michelson-Sagnac-type interferometer. The trajectories satisfy quantum-mechanical continuity and the relativistic velocity addition rule. Our new velocity equation is operationally defined in terms of weak measurements of momentum and energy. We finally propose a modified Alcubierre metric which could give rise to these trajectories within the paradigm of general relativity. ---------------------------------------------------------------- arXiv:2003.00477 gr-qc astro-ph.HE hep-th *Shadow and Quasinormal Modes of a Rotating Loop Quantum Black Hole * *Authors*: Cheng Liu, Tao Zhu, Qiang Wu, Kimet Jusufi, Mubasher Jamil, Mustapha Azreg-Aïnou, Anzhong Wang Abstract: In this paper, we construct an effective rotating loop quantum black hole (LQBH) solution, starting from the spherical symmetric LQBH by applying the Newman-Janis algorithm modified by Azreg-Aïnou's non-complexification procedure, and study the effects of loop quantum gravity { (LQG) on its shadow}. Given the rotating {LQBH}, we discuss its horizon, ergosurface, and regularity {as} r0. Depending on the values of the specific angular momentum a and the polymeric function P arising from {LQG}, we {find} that the rotating solution we obtained can represent a regular black hole, a regular extreme black hole, or a regular spacetime {without horizon (a non-black-hole solution)}. We also {study} the effects of {LQG} and rotation, and {show} that, in addition to the specific angular momentum, the polymeric function {also} causes deformations in the size and shape of the black hole shadow. Interestingly, for a given value of a and inclination angle _0 , the apparent size of the shadow monotonically decreases, and the shadow gets more distorted with increasing P. We also {consider the effects of P on the deviations from the circularity of the shadow, and find} that the deviation from circularity increases with increasing P for fixed values of a and 0. Additionally, we explore the observational implications of P in comparison with the latest Event Horizon Telescope (EHT) observation of the supermassive black hole, M87. The connection between the shadow radius and quasinormal modes in the eikonal limit as well as {the} deflection of massive particles are also considered. Less Journal reference: Phys. Rev. D 101, 084001 (2020); Phys.Rev.D 103 (2021) 8, 089902 (erratum) ---------------------------------------------------------------- arXiv:2004.02602 gr-qc astro-ph.HE hep-th *Orbital mechanics and quasiperiodic oscillation resonances of black holes in Einstein-Æther theory * *Authors*: Mustapha Azreg-Aïnou, Zihang Chen, Bojun Deng, Mubasher Jamil, Tao Zhu, Qiang Wu, Yen-Kheng Lim Abstract: In this paper, we study the motion of test particles around two exact charged black-hole solutions in Einstein-Æther theory. Specifically, we first consider the quasi-periodic oscillations (QPOs) and their resonances generated by the particle moving in the Einstein-Æther black hole and then turn to study the periodic orbits of the massive particles. For QPOs, we drop the usually adopted assumptions _U =_ , _L =_r , and _U /_L =3/2 with _U (_L ) and _r (_ ) being the upper (lower) frequency of QPOs and radial (vertical) epicyclic frequency of the orbiting particles, respectively. Instead, we put-forward a new working ansatz for which the Keplerian radius is much closer to that of the innermost stable circular orbit and explore in detail the effects of the æther field on the frequencies of QPOs. We then realize good curves for the frequencies of QPOs, which fit to data of three microquasars very well by ignoring any effects of rotation and magnetic fields. The innermost stable circular orbits (isco) of timelike particles are also analyzed and we find the isco radius increases with increasing c_13 for the first type black hole while decreases with increasing c_14 for the second one. We also obtain several periodic orbits and find that they share similar taxonomy schemes as the periodic equatorial orbits in the Schwarzschild/Kerr metrics, in addition to exact solutions for certain choices of the Einstein-Æther parameters. The equations for null geodesics are also briefly considered, where we study circular photon orbits and bending angles for gravitational lensing. Journal reference: Phys. Rev. D 102, 044028 (2020) ---------------------------------------------------------------- arXiv:2005.00184 gr-qc *X-ray reflection spectroscopy with Kaluza-Klein black holes * *Authors*: Jiachen Zhu, Askar B. Abdikamalov, Dimitry Ayzenberg, Mustapha Azreg-Ainou, Cosimo Bambi, Mubasher Jamil, Sourabh Nampalliwar, Ashutosh Tripathi, Menglei Zhou Abstract: Kaluza-Klein theory is a popular alternative theory of gravity, with both non-rotating and rotating black hole solutions known. This allows for the possibility that the theory could be observationally tested. We present a model which calculates the reflection spectrum of a black hole accretion disk system, where the black hole is described by a rotating solution of the Kaluza-Klein theory. We also use this model to analyze X-ray data from the stella-mass black hole in GRS 1915+105 and provide constraints on the free parameters of the Kaluza-Klein black holes. Journal reference: Eur. Phys. J. C (2020) 80:622 ---------------------------------------------------------------- arXiv:2007.11023 gr-qc astro-ph.CO math-ph *Dynamical and static solutions to R=0-scalar-tensor theory * *Authors*: Mustapha Azreg-Aïnou Abstract: We consider the most cosmologically interesting and relevant case of scalar-tensor theory (STT) and derive new normal and phantom, dynamical and static, solutions. We determine the Bianchi I Kasner exponents and show that the dynamical solutions are heteroclinic orbits connecting two singularities. Approaching the singularities, a purely transverse expansion (no radial expansion or collapse) may occur. Journal reference: EPL, vol. 130 (2020) 60003 ---------------------------------------------------------------- arXiv:2008.08450 gr-qc *Quasinormal modes, quasiperiodic oscillations and shadow of rotating regular black holes in non-minimally coupled Einstein-Yang-Mills theory * *Authors*: Kimet Jusufi, Mustapha Azreg-Aïnou, Mubasher Jamil, Shao-Wen Wei, Qiang Wu, Anzhong Wang Abstract: In this paper we obtain an effective metric describing a regular and rotating magnetic black hole (BH) solution with a Yang-Mills electromagnetic source in Einstein-Yang-Mills (EYM) theory using the Newman--Janis algorithm via the non-complexification radial coordinate procedure. We then study the BH shadow and the quasinormal modes (QNMs) for massless scalar and electromagnetic fields and the quasiperiodic oscillations (QPOs). To this end, we also study the embedding diagram for the rotating EYM BH. The energy conditions, shadow curvature radius, topology and the dynamical evolution of scalar and electromagnetic perturbations using the time domain integration method are investigated. We show that the shadow radius decreases by increasing the magnetic charge, while the real part of QNMs of scalar and electromagnetic fields increases by increasing the magnetic charge. This result is consistent with the inverse relation between the shadow radius and the real part of QNMs. In addition, we have studied observational constraints on the EYM parameter via frequency analysis of QPOs and the EHT data of shadow cast by the M87 central black hole. We also find that the decaying rate of the EYM BH is slower than that of the neutral and ends up with a tail. We argue that the rotating EYM black hole can be distinguished from the Kerr-Newman black hole with a magnetic charge based on the difference between the angular diameters of their shadows. Journal reference: Phys. Rev. D 103, 024013 (2021). ---------------------------------------------------------------- arXiv:2008.09115 gr-qc *Constraining the Generalized Uncertainty Principle Through Black Hole Shadow and Quasiperiodic Oscillations * *Authors*: Kimet Jusufi, Mustapha Azreg-Aïnou, Mubasher Jamil, Tao Zhu Abstract: In this paper we study the effect of the Generalized Uncertainty Principle (GUP) on the shadow of GUP-modified Kerr black hole and the correspondence between the shadow radius and the real part of the quasinormal modes (QNMs). We find that the shadow curvature radius of the GUP-modfied Kerr black hole is bigger compared to the Kerr vacuum solution and increases linearly monotonically with the increase of the GUP parameter. We then investigate the characteristic points of intrinsic curvature of the shadow from a topological point of view to calculate the the angular size for these curvature radii of the shadow. To this end, we have used the EHT data for the M87* black hole to constrain the upper limits of the GUP parameter red and our result is <10^95 . Finally, we have explored the connection between the shadow radius and the scalar/electromagnetic/gravitational QNMs. The GUP-modified Kerr black hole is also used to provide perfect curve fitting of the particle oscillation upper and lower frequencies to the observed frequencies for three microquasars and to restrict the values of the correction parameter in the metric of the modified black hole to very reasonable bound <10^77 . ---------------------------------------------------------------- arXiv:2010.09698 gr-qc math-ph *On "Rotating charged AdS solutions in quadratic f(T) gravity": New rotating solutions * *Authors*: Mustapha Azreg-Aïnou Abstract: We show that there are two or more procedures to generalize the known four-dimensional transformation, aiming to generate cylindrically rotating charged exact solutions, to higher dimensional spacetimes . In the one procedure, presented in Eur. Phys. J. C (2019) \textbf{79}:668, one uses a non-trivial, non-diagonal, Minkowskian metric ¯_ij to derive complicated rotating solutions. In the other procedure, discussed in this work, one selects a diagonal Minkowskian metric _ij to derive much simpler and appealing rotating solutions. We also show that if (g_ ,_ij ) is a rotating solution then (g¯_ ,¯_ij ) is a rotating solution too with similar geometrical properties, provided ¯_ij and _ij are related by a symmetric matrix R: ¯_ij =_ik R_kj . Journal reference: Eur. Phys. J. C 80, 998 (2020) ---------------------------------------------------------------- arXiv:2011.02276 gr-qc astro-ph.HE hep-th *Shadow, quasinormal modes and quasiperiodic oscillations of rotating Kaluza-Klein black holes * *Authors*: M. Ghasemi-Nodehi, Mustapha Azreg-Aïnou, Kimet Jusufi, Mubasher Jamil Abstract: In this paper we study the shadow of rotating Kaluza-Klein (KK) black holes and the connection between the shadow radius and the real part of quasi-normal modes (QNMs) in the eiokonal limit. In addition we have explored the quasi-periodic oscillations (QPOs) in the rotating KK black hole. Journal reference: Phys. Rev. D 102, 104032 (2020) ---------------------------------------------------------------- arXiv:2012.03431 gr-qc astro-ph.CO *Rotating cosmological cylindrical wormholes in GR and TEGR sourced by anisotropic fluids * *Authors*: Mustapha Azreg-Aïnou Abstract: Given an anisotropic fluid source, we determine in closed forms, upon solving the field equations of general relativity (GR) and teleparallel gravity (TEGR) coupled to a cosmological constant, cylindrically symmetric four-dimensional cosmological rotating wormholes, satisfying all local energy conditions, and cosmological rotating solutions with two axes of symmetry at finite proper distance. These solutions have the property that their angular velocity is proportional to the cosmological constant. Journal reference: Physics of the Dark Universe 32 (2021) 100802 ---------------------------------------------------------------- arXiv:2106.08070 gr-qc astro-ph.HE *Constraining Wormhole Geometries using the Orbit of S2 Star and the Event Horizon Telescope * *Authors*: Kimet Jusufi, Saurabh K., Mustapha Azreg-Aïnou, Mubasher Jamil, Qiang Wu, Cosimo Bambi Abstract: In this paper we study the possibility of having a wormhole (WH) as a candidate for the Sgr A^ ^central object and test this idea by constraining their geometry using the motion of S2 star and the reconstructed shadow images. In particular, we consider three WH models, including WHs in Einstein theory, brane-world gravity, and Einstein-Dirac-Maxwell theory. To this end, we have constrained the WH throat using the motion of S2 star and shown that the flare out condition is satisfied. We also consider the accretion of infalling gas model and study the accretion rate and the intensity of the electromagnetic radiation as well as the shadow images. ---------------------------------------------------------------- arXiv:2109.08150 gr-qc *Axion-plasmon or magnetized plasma effect on an observable shadow and gravitational lensing of a Schwarzschild black hole * *Authors*: Farruh Atamurotov, Kimet Jusufi, Mubasher Jamil, Ahmadjon Abdujabbarov, Mustapha Azreg-Aïnou Abstract: In this paper, we study the influence of the axion-plasmon, as proposed in (Phys. Rev. Lett. 120, 181803 (2018)) on the optical properties of the Schwarzschild black hole. Our aim is to provide a test to detect the effects of a fixed axion background using black holes. To accomplish our goal, we explore the effect of the axion-plasmon coupling on the motion of photons around the Schwarzschild black hole and check the possibility of observing those effects upon the black hole shadow, the gravitational deflection angle, Einstein rings and shadow images obtained by radially infalling gas on a black hole within a plasma medium. We find that these quantities are indeed affected by the axion-plasmon coupling parameters which consequently generalize some of the well-known results in the literature. It is shown that the size of the black hole shadow decreases with increasing axion-plasmon if observed from sufficiently large distance. Journal reference: Physical Review D 104, 064053 (2021) ---------------------------------------------------------------- arXiv:2110.07258 gr-qc astro-ph.CO hep-th *Constraints on Barrow entropy from M87* and S2 star observations * Authors: Kimet Jusufi, Mustapha Azreg-Aïnou, Mubasher Jamil, Emmanuel N. Saridakis Abstract: We use data from M87* central black hole shadow, as well as from the S2 star observations, in order to extract constraints on Barrow entropy. The latter is a modified entropy arising from quantum-gravitational effects on the black hole horizon, quantified by the new parameter . Such a change in entropy leads to a change in temperature, as well as to the properties of the black hole and its shadow. We investigate the photon sphere and the shadow of a black hole with Barrow entropy, and assuming a simple model for infalling and radiating gas we estimate the corresponding intensity. Furthermore, we use the radius in order to extract the real part of the quasinormal modes, and for completeness we investigate the spherical accretion of matter onto the black hole, focusing on isothermal and polytropic test fluids. We extract the allowed parameter region, and by applying a Monte-Carlo-Markov Chains analysis we find that 0.0036^+0.0792 _0.0145 . Hence, our results place the upper bound 0.0828 at 1, a constraint that is less strong than the Big Bang Nucleosynthesis one, but significantly stronger than the late-time cosmological constraints. ---------------------------------------------------------------- arXiv:1912.05305 gr-qc *LRS Bianchi I model with perfect fluid equation of state * *Authors*: Vijay Singh, Aroonkumar Beesham Abstract: The general solution of the field equations in LRS Bianchi-I space-time with perfect fluid equation-of-state (EoS) is presented. The models filled with dust, vacuum energy, Zel'dovich matter and disordered radiation are studied in detail. A unified and systematic treatment of the solutions is presented, and some new solutions are found. The dust, stiff matter and disordered radiation models describe only a decelerated universe, whereas the vacuum energy model exhibits a transition from a decelerated to an accelerated phase. Journal reference: International Journal of Modern Physics D, Vol. 28, Page 1950056 (2019) ---------------------------------------------------------------- arXiv:1912.05850 gr-qc *LRS Bianchi I model with constant deceleration parameter * *Authors*: Vijay Singh, Aroonkumar Beesham Abstract: An LRS Bianchi I model is considered with constant deceleration parameter, q=1, where 0 is a constant. The physical and kinematical behaviour of the models for =0 and 0 is studied in detail. The model with =0 describes late time acceleration, but eternal inflation demands a violation of the NEC and WEC. The acceleration is caused by phantom matter which approaches a cosmological constant at late times. The solutions with a scalar field also show that the model is compatible with a phantom field only. A comparison with the observational outcomes indicates that the universe has entered into the present accelerating phase in recent past somewhere between 0.2z0.5. The model obeys the "cosmic no hair conjecture". The models with 0<<1 describe late time acceleration driven by quintessence dark energy. A violation of the NEC and WEC is required to accommodate the early inflationary epoch caused by phantom matter. The models with 1<<3 describe decelerating phases which are usually occur in the presence of dust or radiation. These models are also found anisotropic at early times and attain isotropy at late times. The model for =3 represents a stiff matter era which also has shear at early stages and becomes shear free at late times, but it evolves with an insignificant ceaseless anisotropy. The models with >3 violate the DEC and the corresponding scalar field models have negative potential which is physically unrealistic. Journal reference: General Relativity and Gravitation (2019) ---------------------------------------------------------------- arXiv:2002.08654 gr-qc *Plane symmetric model with constant deceleration parameter * *Authors*: Vijay Singh, Aroonkumar Beesham Abstract: A plane symmetric Bianchi I model is considered with constant deceleration parameter, q=1, where 0. The model with =0 violates the NEC throughout the evolution, and hence provides a physically unrealistic scenario. The model with 0 obeys the NEC and WEC at late times, which shows that the models in this case can render a physical realistic cosmological scenario, though for a restricted period of time. It is also shown that the physical and kinematical behaviour of both models remain similar to an LRS Bianchi I model. ---------------------------------------------------------------- arXiv:2003.04602 gr-qc *LRS Bianchi I model with constant expansion rate in f(R,T) gravity * *Authors*: Vijay Singh, Aroonkumar Beesham Abstract: An LRS Bianchi-I space-time model is studied with constant Hubble parameter in f(R,T)=R+2T gravity. Although a single (primary) matter source is considered, an additional matter appears due to the coupling between matter and f(R,T) gravity. The constraints are obtained for a realistic cosmological scenario, i.e., one obeying the null and weak energy conditions. The solutions are also extended to the case of a scalar field (normal or phantom) model, and it is found that the model is consistent with a phantom scalar field only. The coupled matter also acts as phantom matter. The study shows that if one expects an accelerating universe from an anisotropic model, then the solutions become physically relevant only at late times when the universe enters into an accelerated phase. Placing some observational bounds on the present equation of state of dark energy, _0 , the behavior of (z) is depicted, which shows that the phantom field has started dominating very recently, somewhere between 0.2z0.5. Journal reference: Astrophysics and Space Science, Vol 365 Page 125 (2020) ---------------------------------------------------------------- arXiv:2003.08665 gr-qc *Plane symmetric model in f(R,T) gravity * *Authors*: Vijay Singh, Aroonkumar Beesham Abstract: A plane symmetric Bianchi-I model is explored in f(R,T) gravity, where R is the Ricci scalar and T is the trace of energy-momentum tensor. The solutions are obtained with the consideration of a specific Hubble parameter which yields a constant deceleration parameter. The various evolutionary phases are identified under the constraints obtained for physically viable cosmological scenarios. Although a single (primary) matter source is taken, due to the coupling between matter and f(R,T) gravity, an additional matter source appears, which mimics a perfect fluid or exotic matter. The solutions are also extended to the case of a scalar field model. The kinematical behavior of the model remains independent of f(R,T) gravity. The physical behavior of the effective matter also remains the same as in general relativity. It is found that f(R,T) gravity can be a good alternative to the hypothetical candidates of dark energy to describe the present accelerating expansion of the universe. Journal reference: Eur. Phys. J. Plus 135, 319 (2020) ---------------------------------------------------------------- arXiv:2008.00169 astro-ph.CO gr-qc *Emergent Universe Scenario in Modified Gauss-Bonnet Gravity * *Authors*: B. C. Paul, S. D. Maharaj, A. Beesham Abstract: We present modified Gauss-Bonnet gravity without matter in four dimensions which accommodates flat emergent universe (EU) obtained in Einstein's general theory of gravity with a non-linear equation of state. The EU model is interesting which is free from big-bang singularity with other observed features of the universe. It is assumed that the present universe emerged out from a static Einstein universe phase exists in the infinite past. To obtain a flat EU model we reconstructed mimetic modified f(G)-gravity (G representing Gauss-Bonnet terms) without matter. The functional form of f(G)-gravity is determined which accommodates the early inflation and late accelerating phases without matter. ---------------------------------------------------------------- arXiv:2106.01757 gr-qc *LRS Bianchi I model with bulk viscosity in f(R,T) gravity * *Authors*: S. Jokweni, Vijay Singh, Aroonkumar Beesham Abstract: Locally-rotationally-symmetric Bianchi type-I viscous and non -viscous cosmological models are explored in general relativity (GR) and in f(R,T) gravity. Solutions are obtained by assuming that the expansion scalar is proportional to the shear scalar which yields a constant value for the deceleration parameter (q=2). Constraints are obtained by requiring the physical viability of the solutions. A comparison is made between the viscous and non-viscous models, and between the models in GR and in f(R,T) gravity. The metric potentials remain the same in GR and in f(R,T) gravity. Consequently, the geometrical behavior of the f(R,T) gravity models remains the same as the models in GR. It is found that f(R,T) gravity or bulk viscosity does not affect the behavior of effective matter which acts as a stiff fluid in all models. The individual fluids have very rich behavior. In one of the viscous models, the matter either follows a semi-realistic EoS or exhibits a transition from stiff matter to phantom, depending on the values of the parameter. In another model, the matter describes radiation, dust, quintessence, phantom, and the cosmological constant for different values of the parameter. In general, f(R,T) gravity diminishes the effect of bulk viscosity. Journal reference: Gravitation and Cosmology, Vol 27, page 169 (2021) ---------------------------------------------------------------- arXiv:1912.06305 astro-ph.IM astro-ph.HE gr-qc *Ground-Based Gravitational-Wave Astronomy in Australia: 2019 White Paper * *Authors*: Matthew Bailes, David McClelland, Eric Thrane, David Blair, Jeffrey Cooke, David Coward, Robin Evans, Yeshe Fenner, Duncan Galloway, Jarrod Hurley, Li Ju, Paul Lasky, Ilya Mandel, Kirk McKenzie, Andrew Melatos, David Ottaway, Susan Scott, Bram Slagmolen, Peter Veitch, Linqing Wen, Chunnong Zhao Abstract: The past four years have seen a scientific revolution through the birth of a new field: gravitational-wave astronomy. The first detection of gravitational waves---recognised by the 2017 Nobel Prize in Physics---provided unprecedented tests of general relativity while unveiling a previously unknown class of massive black holes, thirty times more massive than the Sun. The subsequent detection of gravitational waves from a merging binary neutron star confirmed the hypothesised connection between binary neutron stars and short gamma-ray bursts while providing an independent measurement of the expansion of the Universe. The discovery enabled precision measurement of the speed of gravity while shedding light on the origin of heavy elements. At the time of writing, the Laser Interferometer Gravitational-wave Observatory (LIGO) and its European partner, Virgo, have published the detection of eleven gravitational-wave events. New, not-yet-published detections are announced on a nearly weekly basis. This fast-growing catalogue of gravitational-wave transients is expected to yield insights into a number of topics, from the equation of state of matter at supra-nuclear densities to the fate of massive stars. The science potential of 3G observatories is enormous, enabling measurements of gravitational waves from the edge of the Universe and precise determination of the neutron star equation of state. Australia is well-positioned to help develop the required technology. The Mid-term Review for the Decadal plan for Australian astronomy 2016-2025 should consider investment in a scoping study for an Australian Gravitational-Wave Pathfinder that develops and validates core technologies required for the global 3G detector network. ---------------------------------------------------------------- arXiv:2001.11173 astro-ph.IM gr-qc physics.ins-det *A Cryogenic Silicon Interferometer for Gravitational-wave Detection * *Authors*: Rana X Adhikari, Odylio Aguiar, Koji Arai, Bryan Barr, Riccardo Bassiri, Garilynn Billingsley, Ross Birney, David Blair, Joseph Briggs, Aidan F Brooks, Daniel D Brown, Huy-Tuong Cao, Marcio Constancio, Sam Cooper, Thomas Corbitt, Dennis Coyne, Edward Daw, Johannes Eichholz, Martin Fejer, Andreas Freise, Valery Frolov, Slawomir Gras, Anna Green, Hartmut Grote, Eric K Gustafson , et al. (86 additional authors not shown) Abstract: The detection of gravitational waves from compact binary mergers by LIGO has opened the era of gravitational wave astronomy, revealing a previously hidden side of the cosmos. To maximize the reach of the existing LIGO observatory facilities, we have designed a new instrument that will have 5 times the range of Advanced LIGO, or greater than 100 times the event rate. Observations with this new instrument will make possible dramatic steps toward understanding the physics of the nearby universe, as well as observing the universe out to cosmological distances by the detection of binary black hole coalescences. This article presents the instrument design and a quantitative analysis of the anticipated noise floor. ---------------------------------------------------------------- arXiv:2002.02637 gr-qc astro-ph.IM *Ground Based Gravitational Wave Astronomy in the Asian Region * *Authors*: Vaishali Adya, Matthew Bailes, Carl Blair, David Blair, Johannes Eichholz, Joris van Heijningen, Eric Howell, Li Ju, Paul Lasky, Andrew Melatos, David Ottaway, Chunnong Zhao Abstract: The current gravitational wave detectors have identified a surprising population of heavy stellar mass black holes, and an even larger population of coalescing neutron stars. The first observations have led to many dramatic discoveries and the confirmation of general relativity in very strong gravitational fields. The future of gravitational wave astronomy looks bright, especially if additional detectors with greater sensitivity, broader bandwidth, and better global coverage can be implemented. The first discoveries add impetus to gravitational wave detectors designed to detect in the nHz, mHz and kHz frequency bands. This paper reviews the century-long struggle that led to the recent discoveries, and reports on designs and possibilities for future detectors. The benefits of future detectors in the Asian region are discussed, including analysis of the benefits of a detector located in Australia. ---------------------------------------------------------------- arXiv:2007.03128 astro-ph.HE astro-ph.IM gr-qc *Neutron Star Extreme Matter Observatory: A kilohertz-band gravitational-wave detector in the global network * *Authors*: K. Ackley, V. B. Adya, P. Agrawal, P. Altin, G. Ashton, M. Bailes, E. Baltinas, A. Barbuio, D. Beniwal, C. Blair, D. Blair, G. N. Bolingbroke, V. Bossilkov, S. Shachar Boublil, D. D. Brown, B. J. Burridge, J. Calderon Bustillo, J. Cameron, H. Tuong Cao, J. B. Carlin, S. Chang, P. Charlton, C. Chatterjee, D. Chattopadhyay, X. Chen , et al. (139 additional authors not shown) Abstract: Gravitational waves from coalescing neutron stars encode information about nuclear matter at extreme densities, inaccessible by laboratory experiments. The late inspiral is influenced by the presence of tides, which depend on the neutron star equation of state. Neutron star mergers are expected to often produce rapidly-rotating remnant neutron stars that emit gravitational waves. These will provide clues to the extremely hot post-merger environment. This signature of nuclear matter in gravitational waves contains most information in the 2-4 kHz frequency band, which is outside of the most sensitive band of current detectors. We present the design concept and science case for a neutron star extreme matter observatory (NEMO): a gravitational-wave interferometer optimized to study nuclear physics with merging neutron stars. The concept uses high circulating laser power, quantum squeezing and a detector topology specifically designed to achieve the high-frequency sensitivity necessary to probe nuclear matter using gravitational waves. Above one kHz, the proposed strain sensitivity is comparable to full third-generation detectors at a fraction of the cost. Such sensitivity changes expected event rates for detection of post-merger remnants from approximately one per few decades with two A+ detectors to a few per year, and potentially allows for the first gravitational-wave observations of supernovae, isolated neutron stars, and other exotica. Journal reference: PASA (2020) 37, e047 ---------------------------------------------------------------- arXiv:2003.06528 astro-ph.CO gr-qc *Cosmological signatures of torsion and how to distinguish torsion from the dark sector * *Authors*: Krzysztof Bolejko, Matteo Cinus, Boudewijn F. Roukema Abstract: Torsion is a non-Riemannian geometrical extension of general relativity that allows including the spin of matter and the twisting of spacetime. Cosmological models with torsion have been considered in the literature to solve problems of either the very early (high redshift z) or the present-day Universe. This paper focuses on distinguishable observational signatures of torsion that could not be otherwise explained with a scalar field in pseudo-Riemannian geometry. We show that when torsion is present, the cosmic duality relation between the angular diameter distance, D_A , and the luminosity distance, D_L , is broken. We show how the deviation described by the parameter =D_L /[D_A (1+z)^2 ]1 is linked to torsion and how different forms of torsion lead to special-case parametrisations of , including _0 z, _0 z/(1+z), and _0 ln(1+z). We also show that the effects of torsion could be visible in low-redshift data, inducing biases in supernovae-based H0 measurements. We also show that torsion can impact the Clarkson-Bassett-Lu (CBL) function C(z)=1+H^2 (DDD2)+HHDD, where D is the transverse comoving distance. If D is inferred from the luminosity distance, then, in general non-zero torsion models, C(z)0. For pseudo-Riemannian geometry, the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric has C(z)0; thus, measurement of the CBL function could provide another diagnostic of torsion. Journal reference: Physics Review D 101 (2020) 104046 ---------------------------------------------------------------- arXiv:2105.12933 astro-ph.CO gr-qc *Exploring the Redshift-Space Peculiar Velocity Field and its Power Spectrum * *Authors*: Lawrence Dam, Krzysztof Bolejko, Geraint F. Lewis Abstract: Redshift-space distortions (RSD) generically affect any spatially-dependent observable that is mapped using redshift information. The effect on the observed clustering of galaxies is the primary example of this. This paper is devoted to another example: the effect of RSD on the apparent peculiar motions of tracers as inferred from their positions in redshift space (i.e. the observed distance). Our theoretical study is motivated by practical considerations, mainly, the direct estimation of the velocity power spectrum, which is preferably carried out using the tracer's redshift-space position (so as to avoid uncertainties in distance measurements). We formulate the redshift-space velocity field and show that RSD enters as a higher-order effect. Physically, this effect may be interpreted as a dissipative correction to the usual perfect-fluid description of dark matter. We show that the effect on the power spectrum is a damping on relatively large, quasilinear scales (k>0.01hMpc^1 ), as was observed, though unexplained, in N-body simulations elsewhere. This paper presents two power spectrum models for the peculiar velocity field in redshift space, both of which can be considered velocity analogues of existing clustering models. In particular, we show that the "Finger-of-God" effect, while also present in the velocity field, cannot be entirely blamed for the observed damping in simulations. Our work provides some of the missing modelling ingredients required for a density--velocity multi-tracer analysis, which has been proposed for upcoming redshift surveys. Journal reference: JCAP09(2021)018 ---------------------------------------------------------------- arXiv:1912.08839 gr-qc Using Cadabra for tensor computations in General Relativity *Authors*: Leo Brewin Abstract: Cadabra is an open access program ideally suited to complex tensor commutations in General Relativity. Tensor expressions are written in LaTeX while an enhanced version of Python is used to control the computations. This tutorial assumes no prior knowledge of Cadabra. It consists of a series of examples covering a range of topics from basic syntax such as declarations, functions, program control, component computations, input and output through to complete computations including a derivation of two of the BSSN equations from the ADM equations. Numerous exercises are included along with complete solutions. All of the source code for the examples, exercises and solutions are available on GitHub. ---------------------------------------------------------------- arXiv:2009.11516 gr-qc *Non-aligned Einstein-Maxwell Robinson-Trautman fields of Petrov type D * *Authors*: Norbert Van den Bergh, John Carminati Abstract: We discuss Petrov type D Einstein-Maxwell fields in which both double null eigenvectors of the Weyl tensor are non-aligned with the eigenvectors of a non-null electromagnetic field and are assumed to be geodesic, shear-free, diverging and non-twisting. We obtain the general solution of the Einstein-Maxwell equations under the extra condition that the complex null vectors of the Weyl canonical tetrad are hypersurface orthogonal. The corresponding space-times are all conformally related to a Killing-Yano space and are described by a 5-parameter family of metrics, admitting two commuting Killing vectors and having the C-metric as a possible vacuum limit. ---------------------------------------------------------------- arXiv:2111.06303 gr-qc *Comment on "Shear-free barotropic perfect fluids cannot rotate and expand simultaneously" by R. Goswami and G.F.R. Ellis * *Authors*: Norbert Van den Bergh, John Carminati Abstract: We point out an error in a recent paper by Goswami and Ellis. As a consequence the question of whether shear-free barotropic perfect fluids (with p+0) can or cannot rotate and expand simultaneously, is still wide open. ---------------------------------------------------------------- arXiv:1912.08807 gr-qc astro-ph.HE *Joint search for isolated sources and an unresolved confusion background in pulsar timing array data * *Authors*: Bence Bécsy, Neil J. Cornish Abstract: Supermassive black hole binaries are the most promising source of gravitational-waves in the frequency band accessible to pulsar timing arrays. Most of these binaries will be too distant to detect individually, but together they will form an approximately stochastic background that can be detected by measuring the correlation pattern induced between pairs of pulsars. A small number of nearby and especially massive systems may stand out from this background and be detected individually. Analyses have previously been developed to search for stochastic signals and isolated signals separately. Here we present BayesHopper, an algorithm capable of jointly searching for both signal components simultaneously using trans-dimensional Bayesian inference. Our implementation uses the Reversible Jump Markov Chain Monte Carlo method for sampling the relevant parameter space with changing dimensionality. We have tested BayesHopper on various simulated datasets. We find that it gives results consistent with fixed-dimensional methods when tested on data with a stochastic background or data with a single binary. For the full problem of analyzing a dataset with both a background and multiple black hole binaries, we find two kinds of interactions between the binary and background components. First, the background effectively increases the noise level, thus making individual binary signals less significant. Second, weak binary signals can be absorbed by the background model due to the natural parsimony of Bayesian inference. Because of its flexible model structure, we anticipate that BayesHopper will outperform existing approaches when applied to realistic data sets produced from population synthesis models. Journal reference: Bence Becsy and Neil J. Cornish 2020 Class. Quantum Grav. 37 135011 ---------------------------------------------------------------- arXiv:2001.00595 astro-ph.HE astro-ph.IM gr-qc *Modeling the uncertainties of solar-system ephemerides for robust gravitational-wave searches with pulsar timing arrays * *Authors*: M. Vallisneri, S. R. Taylor, J. Simon, W. M. Folkner, R. S. Park, C. Cutler, J. A. Ellis, T. J. W. Lazio, S. J. Vigeland, K. Aggarwal, Z. Arzoumanian, P. T. Baker, A. Brazier, P. R. Brook, S. Burke-Spolaor, S. Chatterjee, J. M. Cordes, N. J. Cornish, F. Crawford, H. T. Cromartie, K. Crowter, M. DeCesar, P. B. Demorest, T. Dolch, R. D. Ferdman , et al. (39 additional authors not shown) Abstract: The regularity of pulsar emissions becomes apparent once we reference the pulses' times of arrivals to the inertial rest frame of the solar system. It follows that errors in the determination of Earth's position with respect to the solar-system barycenter can appear as a time-correlated bias in pulsar-timing residual time series, affecting the searches for low-frequency gravitational waves performed with pulsar timing arrays. Indeed, recent array datasets yield different gravitational-wave background upper limits and detection statistics when analyzed with different solar-system ephemerides. Crucially, the ephemerides do not generally provide usable error representations. In this article we describe the motivation, construction, and application of a physical model of solar-system ephemeris uncertainties, which focuses on the degrees of freedom (Jupiter's orbital elements) most relevant to gravitational-wave searches with pulsar timing arrays. This model, BayesEphem, was used to derive ephemeris-robust results in NANOGrav's 11-yr stochastic-background search, and it provides a foundation for future searches by NANOGrav and other consortia. The analysis and simulations reported here suggest that ephemeris modeling reduces the gravitational-wave sensitivity of the 11-yr dataset; and that this degeneracy will vanish with improved ephemerides and with the longer pulsar timing datasets that will become available in the near future ---------------------------------------------------------------- arXiv:2003.09456 gr-qc *Reconstructing gravitational wave signals from binary black hole mergers with minimal assumptions * *Authors*: Sudarshan Ghonge, Katerina Chatziioannou, James A. Clark, Tyson Littenberg, Margaret Millhouse, Laura Cadonati, Neil Cornish Abstract: We present a systematic comparison of the binary black hole (BBH) signal waveform reconstructed by two independent and complementary approaches used in LIGO and Virgo source inference: a template-based analysis, and a morphology-independent analysis. We apply the two approaches to real events and to two sets of simulated observations made by adding simulated BBH signals to LIGO and Virgo detector noise. The first set is representative of the 10 BBH events in the first Gravitational Wave Transient Catalog (GWTC-1). The second set is constructed from a population of BBH systems with total mass and signal strength in the ranges that ground based detectors are typically sensitive. We find that the reconstruction quality of the GWTC-1 events is consistent with the results of both sets of simulated signals. We also demonstrate a simulated case where the presence of a mismodelled effect in the observed signal, namely higher order modes, can be identified through the morphology-independent analysis. This study is relevant for currently progressing and future observational runs by LIGO and Virgo. Journal reference: Phys. Rev. D 102, 064056 (2020) ---------------------------------------------------------------- arXiv:2004.08464 gr-qc astro-ph.HE astro-ph.IM *Global Analysis of the Gravitational Wave Signal from Galactic Binaries * *Authors*: Tyson Littenberg, Neil Cornish, Kristen Lackeos, Travis Robson Abstract: Galactic ultra compact binaries are expected to be the dominant source of gravitational waves in the milli-Hertz frequency band. Of the tens of millions of galactic binaries with periods shorter than an hour, it is estimated that a few tens of thousand will be resolved by the future Laser Interferometer Space Antenna (LISA). The unresolved remainder will be the main source of ``noise'' between 1-3 milli-Hertz. Typical galactic binaries are millions of years from merger, and consequently their signals will persist for the the duration of the LISA mission. Extracting tens of thousands of overlapping galactic signals and characterizing the unresolved component is a central challenge in LISA data analysis, and a key contribution to arriving at a global solution that simultaneously fits for all signals in the band. Here we present an end-to-end analysis pipeline for galactic binaries that uses trans-dimensional Bayesian inference to develop a time-evolving catalog of sources as data arrive from the LISA constellation. Journal reference: Phys. Rev. D 101, 123021 (2020) ---------------------------------------------------------------- arXiv:2005.03610 gr-qc astro-ph.HE *Black Hole Hunting with LISA * *Authors*: Neil J. Cornish, Kevin Shuman Abstract: The Laser Interferometer Space Antenna (LISA) will be able to detect massive black hole mergers throughout the visible Universe. These observations will provide unique information about black hole formation and growth, and the role black holes play in galaxy evolution. Here we develop several key building blocks for detecting and characterizing black hole binary mergers with LISA, including fast heterodyned likelihood evaluations, and efficient stochastic search techniques. Journal reference: Phys. Rev. D 101, 124008 (2020) ---------------------------------------------------------------- arXiv:2009.00043 gr-qc astro-ph.HE *Time-Frequency Analysis of Gravitational Wave Data * *Authors*: Neil J. Cornish Abstract: Data from gravitational wave detectors are recorded as time series that include contributions from myriad noise sources in addition to any gravitational wave signals. When regularly sampled data are available, such as for ground based and future space based interferometers, analyses are typically performed in the frequency domain, where stationary (time invariant) noise processes can be modeled very efficiently. In reality, detector noise is not stationary due to a combination of short duration noise transients and longer duration drifts in the power spectrum. This non-stationarity produces correlations across samples at different frequencies, obviating the main advantage of a frequency domain analysis. Here an alternative time-frequency approach to gravitational wave data analysis is proposed that uses discrete, orthogonal wavelet wavepackets. The time domain data is mapped onto a uniform grid of time-frequency pixels. For locally stationary noise - that is, noise with an adiabatically varying spectrum - the time-frequency pixels are uncorrelated, which greatly simplifies the calculation of quantities such as the likelihood. Moreover, the gravitational wave signals from binary systems can be compactly represented as a collection of lines in time-frequency space, resulting in a computational cost for computing waveforms and likelihoods that scales as the square root of the number of time samples, as opposed to the linear scaling for time or frequency based analyses. Key to this approach is having fast methods for computing binary signals directly in the wavelet domain. Multiple fast transform methods are developed in detail. ---------------------------------------------------------------- arXiv:2009.04496 astro-ph.HE astro-ph.GA gr-qc *The NANOGrav 12.5-year Data Set: Search For An Isotropic Stochastic Gravitational-Wave Background * *Authors*: Zaven Arzoumanian, Paul T. Baker, Harsha Blumer, Bence Becsy, Adam Brazier, Paul R. Brook, Sarah Burke-Spolaor, Shami Chatterjee, Siyuan Chen, James M. Cordes, Neil J. Cornish, Fronefield Crawford, H. Thankful Cromartie, Megan E. DeCesar, Paul B. Demorest, Timothy Dolch, Justin A. Ellis, Elizabeth C. Ferrara, William Fiore, Emmanuel Fonseca, Nathan Garver-Daniels, Peter A. Gentile, Deborah C. Good, Jeffrey S. Hazboun, A. Miguel Holgado , et al. (36 additional authors not shown) Abstract: We search for an isotropic stochastic gravitational-wave background (GWB) in the 12.5-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. Our analysis finds strong evidence of a stochastic process, modeled as a power-law, with common amplitude and spectral slope across pulsars. The Bayesian posterior of the amplitude for an f2/3 power-law spectrum, expressed as the characteristic GW strain, has median 1.92×1015 and 5%--95% quantiles of 1.37--2.67×1015 at a reference frequency of fyr=1 yr1. The Bayes factor in favor of the common-spectrum process versus independent red-noise processes in each pulsar exceeds 10,000. However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with general relativity. We find that the process has neither monopolar nor dipolar correlations, which may arise from, for example, reference clock or solar system ephemeris systematics, respectively. The amplitude posterior has significant support above previously reported upper limits; we explain this in terms of the Bayesian priors assumed for intrinsic pulsar red noise. We examine potential implications for the supermassive black hole binary population under the hypothesis that the signal is indeed astrophysical in nature. Journal reference: The Astrophysical Journal Letters, Volume 905, Number 2 (2020) ---------------------------------------------------------------- arXiv:2010.11950 astro-ph.HE astro-ph.GA gr-qc *Astrophysics Milestones For Pulsar Timing Array Gravitational Wave Detection * *Authors*: Nihan S. Pol, Stephen R. Taylor, Luke Zoltan Kelley, Sarah J. Vigeland, Joseph Simon, Siyuan Chen, Zaven Arzoumanian, Paul T. Baker, Bence Bécsy, Adam Brazier, Paul R. Brook, Sarah Burke-Spolaor, Shami Chatterjee, James M. Cordes, Neil J. Cornish, Fronefield Crawford, H. Thankful Cromartie, Megan E. DeCesar, Paul B. Demorest, Timothy Dolch, Elizabeth C. Ferrara, William Fiore, Emmanuel Fonseca, Nathan Garver-Daniels, Deborah C. Good , et al. (27 additional authors not shown) Abstract: The NANOGrav Collaboration reported strong Bayesian evidence for a common-spectrum stochastic process in its 12.5-yr pulsar timing array dataset, with median characteristic strain amplitude at periods of a year of Ayr=1.92+0.750.55×1015. However, evidence for the quadrupolar Hellings \& Downs interpulsar correlations, which are characteristic of gravitational wave signals, was not yet significant. We emulate and extend the NANOGrav dataset, injecting a wide range of stochastic gravitational wave background (GWB) signals that encompass a variety of amplitudes and spectral shapes, and quantify three key milestones: (I) Given the amplitude measured in the 12.5 yr analysis and assuming this signal is a GWB, we expect to accumulate robust evidence of an interpulsar-correlated GWB signal with 15--17 yrs of data, i.e., an additional 2--5 yrs from the 12.5 yr dataset; (II) At the initial detection, we expect a fractional uncertainty of 40% on the power-law strain spectrum slope, which is sufficient to distinguish a GWB of supermassive black-hole binary origin from some models predicting more exotic origins;(III) Similarly, the measured GWB amplitude will have an uncertainty of 44% upon initial detection, allowing us to arbitrate between some population models of supermassive black-hole binaries. In addition, power-law models are distinguishable from those having low-frequency spectral turnovers once 20~yrs of data are reached. Even though our study is based on the NANOGrav data, we also derive relations that allow for a generalization to other pulsar-timing array datasets. Most notably, by combining the data of individual arrays into the International Pulsar Timing Array, all of these milestones can be reached significantly earlier. ---------------------------------------------------------------- arXiv:2011.01942 gr-qc astro-ph.HE *Bayesian search for gravitational wave bursts in pulsar timing array data * *Authors*: Bence Bécsy, Neil J. Cornish Abstract: The nanohertz frequency band explored by pulsar timing arrays provides a unique discovery space for gravitational wave signals. In addition to signals from anticipated sources, such as those from supermassive black hole binaries, some previously unimagined sources may emit transient gravitational waves (a.k.a. bursts) with unknown morphology. Unmodeled transients are not currently searched for in this frequency band, and they require different techniques from those currently employed. Possible sources of such gravitational wave bursts in the nanohertz regime are parabolic encounters of supermassive black holes, cosmic string cusps and kinks, or other, as-yet-unknown phenomena. In this paper we present BayesHopperBurst, a Bayesian search algorithm capable of identifying generic gravitational wave bursts by modeling both coherent and incoherent transients as a sum of Morlet-Gabor wavelets. A trans-dimensional Reversible Jump Markov Chain Monte Carlo sampler is used to select the number of wavelets best describing the data. We test BayesHopperBurst on various simulated datasets including different combinations of signals and noise transients. Its capability to run on real data is demonstrated by analyzing data of the pulsar B1855+09 from the NANOGrav 9-year dataset. Based on a simulated dataset resembling the NANOGrav 12.5-year data release, we predict that at our most sensitive time-frequency location we will be able to probe gravitational wave bursts with a root-sum-squared amplitude higher than 5×10^11 Hz^1/2 , which corresponds to 40Mc^2 emitted in GWs at a fiducial distance of 100 Mpc. Journal reference: Bence Becsy and Neil J. Cornish 2021 Class. Quantum Grav. 38 095012 ---------------------------------------------------------------- arXiv:2011.05055 gr-qc *Spectral separation of the stochastic gravitational-wave background for LISA: observing both cosmological and astrophysical backgrounds * *Authors*: Guillaume Boileau, Nelson Christensen, Renate Meyer, Neil J. Cornish Abstract: With the goal of attempting to observe a stochastic gravitational wave background (SGWB) with LISA, the spectral separability of the cosmological and astrophysical backgrounds is important to estimate. We attempt to determine the level with which a cosmological background can be observed given the predicted astrophysical background level. We predict detectable limits for the future LISA measurement of the SGWB. Adaptive Markov chain Monte-Carlo methods are used to produce estimates with the simulated data from the LISA Data challenge (LDC). We also calculate the Cramer-Rao lower bound on the variance of the SGWB parameter uncertainties based on the inverse Fisher Information using the Whittle Likelihood. The estimation of the parameters is done with the 3 LISA channels A, E, and T. We simultaneously estimate the noise using a LISA noise model. Assuming the expected astrophysical background, a cosmological background energy density of around GW,Cosmo1×10^12 to 1×10^13 can be detected by LISA. Journal reference: Phys. Rev. D 103, 103529 (2021) ---------------------------------------------------------------- arXiv:2011.09494 gr-qc astro-ph.HE *The BayesWave analysis pipeline in the era of gravitational wave observations * *Authors*: Neil J. Cornish, Tyson B. Littenberg, Bence Bécsy, Katerina Chatziioannou, James A. Clark, Sudarshan Ghonge, Margaret Millhouse Abstract: We describe updates and improvements to the BayesWave gravitational wave transient analysis pipeline, and provide examples of how the algorithm is used to analyze data from ground-based gravitational wave detectors. BayesWave models gravitational wave signals in a morphology-independent manner through a sum of frame functions, such as Morlet-Gabor wavelets or chirplets. BayesWave models the instrument noise using a combination of a parametrized Gaussian noise component and non-stationary and non-Gaussian noise transients. Both the signal model and noise model employ trans-dimensional sampling, with the complexity of the model adapting to the requirements of the data. The flexibility of the algorithm makes it suitable for a variety of analyses, including reconstructing generic unmodeled signals; cross checks against modeled analyses for compact binaries; as well as separating coherent signals from incoherent instrumental noise transients (glitches). The BayesWave model has been extended to account for gravitational wave signals with generic polarization content and the simultaneous presence of signals and glitches in the data. We describe updates in the BayesWave prior distributions, sampling proposals, and burn-in stage that provide significantly improved sampling efficiency. We present standard review checks indicating the robustness and convergence of the BayesWave trans-dimensional sampler. Journal reference: Phys. Rev. D 103, 044006 (2021) ---------------------------------------------------------------- arXiv:2101.01188 gr-qc astro-ph.HE *Rapid and Robust Parameter Inference for Binary Mergers * *Authors*: Neil J. Cornish Abstract: The detection rate for compact binary mergers has grown as the sensitivity of the global network of ground based gravitational wave detectors has improved, now reaching the stage where robust automation of the analyses is essential. Automated low-latency algorithms have been developed that send out alerts when candidate signals are detected. The alerts include sky maps to facilitate electromagnetic follow up observations, along with probabilities that the system might contain a neutron star, and hence be more likely to generate an electromagnetic counterpart. Data quality issues, such as loud noise transients (glitches), can adversely affect the low-latency algorithms, causing false alarms and throwing off parameter estimation. Here a new analysis method is presented that is robust against glitches, and capable of producing fully Bayesian parameter inference, including sky maps and mass estimates, in a matter of minutes. Key elements of the method are wavelet-based de-noising, penalized maximization of the likelihood during the initial search, rapid sky localization using pre-computed inner products, and heterodyned likelihoods for full Bayesian inference. Journal reference: Phys. Rev. D 103, 104057 (2021) ---------------------------------------------------------------- arXiv:2101.01200 gr-qc astro-ph.HE *Modeling compact binary signals and instrumental glitches in gravitational wave data * *Authors*: Katerina Chatziioannou, Neil Cornish, Marcella Wijngaarden, Tyson B. Littenberg Abstract: Transient non-gaussian noise in gravitational wave detectors, commonly referred to as glitches, pose challenges for inference of the astrophysical properties of detected signals when the two are coincident in time. Current analyses aim towards modeling and subtracting the glitches from the data using a flexible, morphology-independent model in terms of sine-gaussian wavelets before the signal source properties are inferred using templates for the compact binary signal. We present a new analysis of gravitational wave data that contain both a signal and glitches by simultaneously modeling the compact binary signal in terms of templates and the instrumental glitches using sine-gaussian wavelets. The model for the glitches is generic and can thus be applied to a wide range of glitch morphologies without any special tuning. The simultaneous modeling of the astrophysical signal with templates allows us to efficiently separate the signal from the glitches, as we demonstrate using simulated signals injected around real O2 glitches in the two LIGO detectors. We show that our new proposed analysis can separate overlapping glitches and signals, estimate the compact binary parameters, and provide ready-to-use glitch-subtracted data for downstream inference analyses. Journal reference: Phys. Rev. D 103, 044013 (2021) ---------------------------------------------------------------- arXiv:2103.14598 astro-ph.IM astro-ph.GA gr-qc *Characterization of the stochastic signal originating from compact binaries populations as measured by LISA * *Authors*: Nikolaos Karnesis, Stanislav Babak, Mauro Pieroni, Neil Cornish, Tyson Littenberg Abstract: The Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the early 2030s, is a gravitational wave observatory in space designed to detect sources emitting in the milli-Hertz band. In contrast to the present ground based detectors, the LISA data are expected to be a signaldominated, with strong and weak gravitational wave signals overlapping in time and in frequency. Astrophysical population models predict a sufficient number of signals in the LISA band to blend together and form an irresolvable foreground noise. In this work, we present a generic method for characterizing the foreground signals originating from a given astrophysical population of coalescing compact binaries. Assuming idealized detector conditions and perfect data analysis technique capable of identifying and removing the bright sources, we apply an iterative procedure which allows us to predict the different levels of foreground noise. Journal reference: Phys. Rev. D 104, 043019 (2021) ---------------------------------------------------------------- arXiv:2104.04596 gr-qc *New binary pulsar constraints on Einstein-æther theory after GW170817 * *Authors*: Toral Gupta, Mario Herrero-Valea, Diego Blas, Enrico Barausse, Neil Cornish, Kent Yagi, Nicolás Yunes Abstract: The timing of millisecond pulsars has long been used as an exquisitely precise tool for testing the building blocks of general relativity, including the strong equivalence principle and Lorentz symmetry. Observations of binary systems involving at least one millisecond pulsar have been used to place bounds on the parameters of Einstein-æther theory, a gravitational theory that violates Lorentz symmetry at low energies via a preferred and dynamical time threading of the spacetime manifold. However, these studies did not cover the region of parameter space that is still viable after the recent bounds on the speed of gravitational waves from GW170817/GRB170817A. The restricted coverage was due to limitations in the methods used to compute the pulsar sensitivities, which parameterize violations of the strong-equivalence principle in these systems. We extend here the calculation of pulsar sensitivities to the parameter space of Einstein-æther theory that remains viable after GW170817/GRB170817A. We show that observations of the damping of the period of quasi-circular binary pulsars and of the triple system PSR J0337+1715 further constrain the viable parameter space by about an order of magnitude over previous constraints. Journal reference: Class. Quantum Grav. 38 195003 (2021) ---------------------------------------------------------------- arXiv:2105.02943 gr-qc astro-ph.HE *Massive Black Hole Binaries and Where to Find Them with Dual Detector Networks * *Authors*: Kevin J. Shuman, Neil J. Cornish Abstract: A single space-based gravitational wave detector will push the boundaries of astronomy and fundamental physics. Having a network of two or more detectors would significantly improve source localization. Here we consider how dual networks of space-based detectors would improve parameter estimation of massive black hole binaries. We consider two scenarios: a network comprised of the Laser Interferometer Space Antenna (LISA) and an additional LISA-like heliocentric detector (e.g. Taiji); and a network comprised of LISA with an an additional geocentric detector (e.g. TianQin). We use Markov chain Monte Carlo techniques and Fisher matrix estimates to explore the impact of a two detector network on sky localization and distance determination. The impact on other source parameters is also studied. With the addition of a Taiji or TianQin, we find orders of magnitude improvements in sky localization for the more massive MBHBs, while also seeing improvements for lower mass systems, and for other source parameters. ---------------------------------------------------------------- arXiv:2105.04283 gr-qc *Spectral separation of the stochastic gravitational-wave background for LISA in the context of a modulated Galactic foreground * *Authors*: Guillaume Boileau, Astrid Lamberts, Nelson Christensen, Neil J. Cornish, Renate Meyer Abstract: Within its observational band the Laser Interferometer Space Antenna, LISA, will simultaneously observe orbital modulated waveforms from Galactic white dwarf binaries, a binary black hole produced gravitational-wave background, and potentially a cosmologically created stochastic gravitational-wave background (SGWB). The overwhelming majority of stars end their lives as white dwarfs, making them very numerous in the Milky Way. We simulate Galactic white dwarf binary gravitational-wave emission based on distributions from various mock catalogs and determine a complex waveform from the Galactic foreground with 3.5×107 binaries. We describe the effects from the Galactic binary distribution population across mass, position within the Galaxy, core type, and orbital frequency distribution. We generate the modulated Galactic white dwarf signal detected by \textit{LISA} due to its orbital motion, and present a data analysis strategy to address it. The Fisher Information and Markov Chain Monte Carlo methods give an estimation of the \textit{LISA} noise and the parameters for the different signal classes. We estimate the detectable limits for the future LISA observation of the SGWB in the spectral domain with the 3 \textit{LISA} channels A, E, and T. We simultaneously estimate the Galactic foreground, the astrophysical and cosmological backgrounds. Assuming the expected astrophysical background and a Galactic foreground, a cosmological background energy density of around GW,Cosmo8×1013 could be detected by LISA. LISA will either detect a cosmologically produced SGWB, or set a limit that will have important consequences. ---------------------------------------------------------------- arXiv:2105.06793 gr-qc *Spectral separation of the stochastic gravitational-wave background for LISA: galactic, cosmological and astrophysical backgrounds * *Authors*: Guillaume Boileau, Astrid Lamberts, Nelson Christensen, Neil J. Cornish, Renate Meyer Abstract: In its observation band, the Laser Interferometer Space Antenna (LISA) will simultaneously observe stochastic gravitational-wave background (SGWB) signals of different origins; orbitally modulated waveforms from galactic white dwarf binaries, a binary black hole produced background, and possibly a cosmologically produced SGWB. We simulate the emission of gravitational waves from galactic white dwarf binaries based on the Lamberts \cite{Lamberts} distributions and determine a complex waveform from the galactic foreground. We generate the modulated galactic signal detected by LISA due to its orbital motion, and present a data analysis strategy to address it. The Fisher Information and Markov Chain Monte Carlo methods give an estimate of the LISA noise and parameters for the different signal sources. We simultaneously estimate the galactic foreground, the astrophysical and cosmological backgrounds, and estimate detection limits for the future LISA observation of the SGWB in the spectral domain with the 3 LISA channels A, E and T. In the context of the expected astrophysical background and a galactic foreground, a cosmological background energy density of about GW,Cosmo8×10^13 could be detected by LISA with our spectral separation strategy. ---------------------------------------------------------------- arXiv:2107.02292 gr-qc astro-ph.CO *A comparison of maximum likelihood mapping methods for gravitational-wave backgrounds * *Authors*: Arianna I. Renzini, Joseph D. Romano, Carlo R. Contaldi, Neil J. Cornish Abstract: Detection of a stochastic background of gravitational waves is likely to occur in the next few years. Beyond searches for the isotropic component of SGWBs, there have been various mapping methods proposed to target anisotropic backgrounds. Some of these methods have been applied to data taken by the Laser Interferometer Gravitational-wave Observatories (LIGO) and Virgo. Specifically, these directional searches have focused on mapping the intensity of the signal on the sky via maximum likelihood solutions. We compare this intensity mapping approach to a previously proposed, but never employed, amplitude-phase mapping method to understand whether this latter approach may be employed in future searches. We build up our understanding of the differences between these two approaches by analysing simple toy models of time-stream data, and run mock-data mapping tests for the two methods. We find that the amplitude-phase method is only applicable to the case of a background which is phase-coherent on large scales or, at the very least, has an intrinsic coherence scale that is larger than that of the detector. Otherwise, the amplitude-phase mapping method leads to a loss of overall information, with respect to both phase and amplitude. Since we do not expect these phase-coherent properties to hold for any of the gravitational-wave background signals we hope to detect in the near future, we conclude that intensity mapping is the preferred method for such backgrounds. ---------------------------------------------------------------- arXiv:2109.02728 gr-qc *Heterodyned Likelihood for Rapid Gravitational Wave Parameter Inference * *Authors*: Neil J. Cornish Abstract: Inferring the source properties of a gravitational wave signal has traditionally been very computationally intensive and time consuming. In recent years, several techniques have been developed that can significantly reduce the computational cost while delivering rapid and accurate parameter inference. One of the most powerful of these techniques is the heterodyned likelihood, which uses a reference waveform to base-band the likelihood calculation. Here an efficient implementation of the heterodyned likelihood is presented that can be used for a wide range of signal types and for both ground based and space based interferometers. The computational savings relative to direct calculation of the likelihood vary between two and four orders of magnitude depending on the system. The savings are greatest for low mass systems such as neutron star binaries. The heterodyning procedure can incorporate marginalization over calibration uncertainties and the noise power spectrum. ---------------------------------------------------------------- arXiv:2109.14706 gr-qc astro-ph.GA astro-ph.HE *The NANOGrav 12.5-year data set: Search for Non-Einsteinian Polarization Modes in theGravitational-Wave Background * *Authors*: Zaven Arzoumanian, Paul T. Baker, Harsha Blumer, Bence Becsy, Adam Brazier, Paul R. Brook, Sarah Burke-Spolaor, Maria Charisi, Shami Chatterjee, Siyuan Chen, James M. Cordes, Neil J. Cornish, Fronefield Crawford, H. Thankful Cromartie, Megan E. DeCesar, Dallas M. DeGan, Paul B. Demorest, Timothy Dolch, Brendan Drachler, Justin A. Ellis, Elizabeth C. Ferrara, William Fiore, Emmanuel Fonseca, Nathan Garver-Daniels, Peter A. Gentile , et al. (46 additional authors not shown) Abstract: We search NANOGrav's 12.5-year data set for evidence of a gravitational wave background (GWB) with all the spatial correlations allowed by general metric theories of gravity. We find no substantial evidence in favor of the existence of such correlations in our data. We find that scalar-transverse (ST) correlations yield signal-to-noise ratios and Bayes factors that are higher than quadrupolar (tensor transverse, TT) correlations. Specifically, we find ST correlations with a signal-to-noise ratio of 2.8 that are preferred over TT correlations (Hellings and Downs correlations) with Bayesian odds of about 20:1. However, the significance of ST correlations is reduced dramatically when we include modeling of the Solar System ephemeris systematics and/or remove pulsar J0030+0451 entirely from consideration. Even taking the nominal signal-to-noise ratios at face value, analyses of simulated data sets show that such values are not extremely unlikely to be observed in cases where only the usual TT modes are present in the GWB. In the absence of a detection of any polarization mode of gravity, we place upper limits on their amplitudes for a spectral index of =5 and a reference frequency of f_yr =1yr^1 . Among the upper limits for eight general families of metric theories of gravity, we find the values of A^95% _TT =(9.7±0.4)×10^16 and A^95% _ST =(1.4±0.03)×10^15 for the family of metric spacetime theories that contain both TT and ST modes. Journal reference: The Astrophysical Journal Letters, vol. 923, no. 2, p. L22, Dec. 2021 ---------------------------------------------------------------- arXiv:2110.06238 gr-qc astro-ph.HE *Low Latency Detection of Massive Black Hole Binaries * *Authors*: Neil J. Cornish Abstract: The next decade is expected to see the launch of one or more space based gravitational wave detectors: the European lead Laser Interferometer Space Antenna (LISA); and one or more Chinese mission concepts, Taiji and TianQin. One of the primary scientific targets for these missions are the mergers of black holes with masses between 10^3 Mand 10^8 M. These systems may produce detectable electromagnetic signatures in additional to gravitational waves due to the presence of gas in mini-disks around each black hole, and a circumbinary disk surrounding the system. The electromagnetic emission may occur before, during and after the merger. In order to have the best chance of capturing all phases of the emission it is imperative that the gravitational wave signals can be detected in low latency, and used to produce reliable estimates for the sky location and distance to help guide the search for counterparts. The low latency detection also provides a starting point for the ``global fit'' of the myriad signals that are simultaneously present in the data. Here a low latency analysis pipeline is presented that is capable of analyzing months of data in just a few hours using a laptop from the last decade. The problem of performing a global fit is avoided by whitening out the bright foreground produced by nearby galactic binaries. The performance of the pipeline is illustrated using simulated data from the LISA Data Challenge. ---------------------------------------------------------------- arXiv:2001.09793 gr-qc *Prospects for Fundamental Physics with LISA * Authors: Enrico Barausse, Emanuele Berti, Thomas Hertog, Scott A. Hughes, Philippe Jetzer, Paolo Pani, Thomas P. Sotiriou, Nicola Tamanini, Helvi Witek, Kent Yagi, Nicolas Yunes, T. Abdelsalhin, A. Achucarro, K. V. Aelst, N. Afshordi, S. Akcay, L. Annulli, K. G. Arun, I. Ayuso, V. Baibhav, T. Baker, H. Bantilan, T. Barreiro, C. Barrera-Hinojosa, N. Bartolo , et al. (296 additional authors not shown) Abstract: In this paper, which is of programmatic rather than quantitative nature, we aim to further delineate and sharpen the future potential of the LISA mission in the area of fundamental physics. Given the very broad range of topics that might be relevant to LISA, we present here a sample of what we view as particularly promising directions, based in part on the current research interests of the LISA scientific community in the area of fundamental physics. We organize these directions through a "science-first" approach that allows us to classify how LISA data can inform theoretical physics in a variety of areas. For each of these theoretical physics classes, we identify the sources that are currently expected to provide the principal contribution to our knowledge, and the areas that need further development. The classification presented here should not be thought of as cast in stone, but rather as a fluid framework that is amenable to change with the flow of new insights in theoretical physics. Journal reference: Gen.Rel.Grav. 52 (2020) 8, 81 ---------------------------------------------------------------- arXiv:2110.03428 gr-qc astro-ph.CO *Dynamical Friction From Ultralight Dark Matter * *Authors*: Yourong Wang, Richard Easther Abstract: We simulate the gravitational dynamics of a massive object interacting with Ultralight / Fuzzy Dark Matter (ULDM/FDM), non-relativistic quantum matter described by the Schrodinger-Poisson equation. We first consider a point mass moving in a uniform background, and then a supermassive black hole (SMBH) moving within a ULDM soliton. After replicating simple dynamical friction scenarios to verify our numerical strategies, we demonstrate that the wake induced by a moving mass in a uniform medium may undergo gravitational collapse that dramatically increases the drag force, albeit in a scenario unlikely to be encountered astrophysically. We broadly confirm simple estimates of dynamical friction timescales for a black hole at the center of a halo but see that a large moving point mass excites coherent "breathing modes" in a ULDM soliton. These can lead to "stone skipping" trajectories for point masses which do not sink uniformly toward the center of the soliton, as well as stochastic motion near the center itself. These effects will add complexity to SMBH-ULDM interactions and to SMBH mergers in a ULDM universe. ---------------------------------------------------------------- arXiv:2002.06759 gr-qc *Asymptotically flat vacuum initial data sets from a modified parabolic-hyperbolic formulation of the Einstein vacuum constraint equations * *Authors*: Florian Beyer, Jörg Frauendiener, Joshua Ritchie Abstract: In this paper we continue earlier investigations of evolutionary formulations of the Einstein vacuum constraint equations originally introduced by Rácz. Motivated by the strong evidence from these works that the resulting vacuum initial data sets are generically not asymptotically flat we analyse the asymptotics of the solutions of a modified formulation by a combination of analytical and numerical techniques. We conclude that the vacuum initial data sets generated with this new formulation are generically asymptotically flat. Journal reference: Phys. Rev. D 101, 084013 (2020) ---------------------------------------------------------------- arXiv:2005.11936 gr-qc math-ph *Explorations of the infinite regions of space-time * *Authors*: Florian Beyer, Jörg Frauendiener, Jörg Hennig Abstract: An important concept in Physics is the notion of an isolated system. It is used in many different areas to describe the properties of a physical system which has been isolated from its environment. The interaction with the `outside' is then usually reduced to a scattering process, in which incoming radiation interacts with the system, which in turn emits outgoing radiation. In Einstein's theory of gravitation isolated systems are modeled as asymptotically flat space-times. They provide the appropriate paradigm for the study of gravitational waves and their interaction with a material system or even only with themselves. In view of the emerging era of gravitational wave astronomy, in which gravitational wave signals from many different astrophysical sources are detected and interpreted, it is necessary to have a foundation for the theoretical and numerical treatments of these signals. Furthermore, from a purely mathematical point of view, it is important to have a full understanding of the implications of imposing the condition of asymptotic flatness onto solutions of the Einstein equations. While it is known that there exists a large class of asymptotically flat solutions of Einstein's equations, it is not known what the necessary and sufficient conditions at infinity are that have to be imposed on initial data so that they evolve into regular asymptotically flat space-times. The crux lies in the region near space-like infinity i0 where incoming and outgoing radiation `meet'. In this paper we review the current knowledge and some of the analytical and numerical work that has gone into the attempt to understand the structure of asymptotically flat space-times near space-like and null-infinity. Journal reference: Int. J. Mod. Phys. D, 2030007, (2020) ---------------------------------------------------------------- arXiv:2104.13646 gr-qc math-ph *A new look at the Bondi-Sachs energy-momentum * *Authors*: Jörg Frauendiener, Chris Stevens Abstract: How does one compute the Bondi mass on an arbitrary cut of null infinity $\scri$ when it is not presented in a Bondi system? What then is the correct definition of the mass aspect? How does one normalise an asymptotic translation computed on a cut which is not equipped with the unit-sphere metric? These are questions which need to be answered if one wants to calculate the Bondi-Sachs energy-momentum for a space-time which has been determined numerically. Under such conditions there is not much control over the presentation of $\scri$ so that most of the available formulations of the Bondi energy-momentum simply do not apply. The purpose of this article is to provide the necessary background for a manifestly conformally invariant and gauge independent formulation of the Bondi energy-momentum. To this end we introduce a conformally invariant version of the GHP formalism to rephrase all the well-known formulae. This leads us to natural definitions for the space of asymptotic translations with its Lorentzian metric, for the Bondi news and the mass-aspect. A major role in these developments is played by the "co-curvature", a naturally appearing quantity closely related to the Gauß curvature on a cut of~$\scri$. Journal reference: Class. Quantum Grav. 39 025007 (2022) ---------------------------------------------------------------- arXiv:2105.01906 gr-qc *Can gravitational waves halt the expansion of the universe? * *Authors*: Jörg Frauendiener, Jonathan Hakata, Chris Stevens Abstract: We numerically investigate the propagation of plane gravitational waves in the form of an initial boundary value problem with de Sitter initial data. The full non-linear Einstein equations with positive cosmological constant are written in the Friedrich-Nagy gauge which yields a wellposed system. The propagation of a single wave and the collision of two with colinear polarization are studied and contrasted with their Minkowskian analogues. Unlike with =0, critical behaviours are found with >0 and are based on the relationship between the wave profile and . We find that choosing boundary data close to one of these bifurcations results in a "false" steady state which violates the constraints. Simulations containing (approximate) impulsive wave profiles are run and general features are discussed. Analytic results of Woodard and Tsamis, which describe how gravitational waves could affect an expansion rate at an initial instance of time, are explored and generalized to the entire space-time. Finally we put forward boundary conditions that, at least locally, slow down the expansion considerably for a time. ---------------------------------------------------------------- arXiv:2105.09515 gr-qc *The non-linear perturbation of a black hole by gravitational waves. I. The Bondi-Sachs mass loss * *Authors*: Jörg Frauendiener, Chris Stevens Abstract: The excitation of a black hole by infalling matter or radiation has been studied for a long time, mostly in linear perturbation theory. In this paper we study numerically the response of a Schwarzschild black hole to an incoming gravitational wave pulse. We present a numerically well-posed initial boundary value problem for the generalized conformal field equations in which a wave profile for the ingoing wave is specified at an outer time-like boundary which then hits an initially static and spherically symmetric black hole. The non-linear interaction of the black hole with the gravitational wave leads to scattered radiation moving back out. The clean separation between initial state and incoming radiation makes this setup ideal to study scattering problems. The use of the conformal field equations allows us to trace the response of the black hole to null infinity where we can read off the scattered gravitational waves and compute the Bondi-Sachs mass and the gravitational flux through I. In this way we check the Bondi-Sachs mass loss formula directly on null infinity. We also comment on comparisons with quasinormal modes. ---------------------------------------------------------------- arXiv:2001.08951 astro-ph.HE gr-qc *A universal formula for the relativistic correction to the mutual friction coupling time-scale in neutron stars * *Authors*: Lorenzo Gavassino, Marco Antonelli, Pierre Pizzochero, Brynmor Haskell Abstract: Vortex-mediated mutual friction governs the coupling between the superfluid and normal components in neutron star interiors. By, for example, comparing precise timing observations of pulsar glitches with theoretical predictions it is possible to constrain the physics in the interior of the star, but to do so an accurate model of the mutual friction coupling in general relativity is needed. We derive such a model directly from Carter's multifluid formalism, and study the vortex structure and coupling time-scale between the components in a relativistic star. We calculate how general relativity modifies the shape and the density of the quantized vortices and show that, in the quasi-Schwarzschild coordinates, they can be approximated as straight lines for realistic neutron star configurations. Finally, we present a simple universal formula (given as a function of the stellar compactness alone) for the relativistic correction to the glitch rise-time, which is valid under the assumption that the superfluid reservoir is in a thin shell in the crust or in the outer core. This universal relation can be easily employed to correct, a posteriori, any Newtonian estimate for the coupling time-scale, without any additional computational expense. Journal reference: Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 3, May 2020, Pages 3562-3580 ---------------------------------------------------------------- arXiv:2002.10357 astro-ph.HE astro-ph.SR gr-qc *Magnetic field configurations in neutron stars from MHD simulations * *Authors*: Ankan Sur, Brynmor Haskell, Emily Kuhn Abstract: We have studied numerically the evolution of magnetic fields in barotropic neutron stars, by performing nonlinear magnetohydrodynamical simulations with the code PLUTO. For both initially predominantly poloidal and toroidal fields, with varying strengths, we find that the field settles down to a mixed poloidal-toroidal configuration, where the toroidal component contributes between 10% and 20% of the total magnetic energy. This is, however, not a strict equilibrium, as the instability leads to the development of turbulence, which in turn gives rise to an inverse helicity cascade, which determines the final 'twisted torus' setup. The final field configuration is thus dictated by the non-linear saturation of the instability and is not stationary. The average energy of the poloidal and toroidal components, however, is approximately stable in our simulations, and a complex multipolar structure emerges at the surface, while the magnetic field is dipolar at the exterior boundary, outside the star. Journal reference: Monthly Notices of the Royal Astronomical Society, Volume 495, Issue 1, June 2020 ---------------------------------------------------------------- arXiv:2003.04609 gr-qc astro-ph.HE *Bulk viscosity in relativistic fluids: from thermodynamics to hydrodynamics * *Authors*: Lorenzo Gavassino, Marco Antonelli, Brynmor Haskell Abstract: The approach of extended irreversible thermodynamics consists of promoting the dissipative fluxes to non-equilibrium thermodynamic variables. In a relativistic context, this naturally leads to the formulation of the theory of Israel and Stewart (1979), which is, to date, one of the most successful theories for relativistic dissipation. Although the generality of the principle makes it applicable to any dissipative fluid, a connection of the Israel-Stewart theory with microphysics has been established, through kinetic theory, only for the case of ideal quantum gases. By performing a convenient change of variables, we provide, for the case of bulk viscosity, an equivalent reformulation of the equations at the basis of extended irreversible thermodynamics. This approach maps any thermodynamic process which contributes to the bulk viscosity into a set of chemical reactions, whose reaction coordinates are abstract parameters describing the displacement from local thermodynamic equilibrium of the fluid element. We apply our new formalism to the case of the relativistic fluids, showing that the Israel-Stewart model for bulk viscosity is just the second-order expansion of a minimal model belonging to a larger class of non-perturbative theories for bulk viscosity which include the nuclear-reaction-mediated bulk viscosity of neutron star matter as a particular case. Furthermore, we show with concrete examples that our formalism provides new ways of computing the bulk viscosity directly and defines a simple prescription for constructing the Israel-Stewart model for a generic bulk-viscous fluid. Journal reference: Class. Quantum Grav. 38 075001 (2021) ---------------------------------------------------------------- arXiv:2006.09843 gr-qc astro-ph.HE hep-th *When the entropy has no maximum: A new perspective on the instability of the first-order theories of dissipation * *Authors*: Lorenzo Gavassino, Marco Antonelli, Brynmor Haskell Abstract: The first-order relativistic fluid theories of dissipation proposed by Eckart and Landau-Lifshitz have been proved to be unstable. They admit solutions which start in proximity of equilibrium and depart exponentially from it. We show that this behaviour is due to the fact that the total entropy of these fluids, restricted to the dynamically accessible states, has no upper bound. As a result, these systems have the tendency to constantly change according to the second law of thermodynamics and the unstable modes represent the directions of growth of the entropy in state space. We, then, verify that the conditions of stability of Israel and Stewart's theory are exactly the requirements for the entropy to have an absolute maximum. Hence, we explain how the instability of the first-order theories is a direct consequence of the truncation of the entropy current at the first order, which turns the maximum into a saddle point of the total entropy. Finally, we show that recently proposed first-order stable theories, constructed using more general frames, do not solve the instability problem by providing a maximum for the entropy, but, rather, are made stable by allowing for small violations of the second law. Journal reference: Phys. Rev. D 102, 043018 (2020) ---------------------------------------------------------------- arXiv:2007.09481 gr-qc astro-ph.HE *Multifluid Modelling of Relativistic Radiation Hydrodynamics * *Authors*: Lorenzo Gavassino, Marco Antonelli, Brynmor Haskell Abstract: The formulation of a universal theory for bulk viscosity and heat conduction represents a theoretical challenge for our understanding of relativistic fluid dynamics. Recently, it has been shown that the multifluid variational approach championed by Carter and collaborators has the potential to be a general and natural framework to derive (hyperbolic) hydrodynamic equations for relativistic dissipative systems. Furthermore, it also allows to keep direct contact with non-equilibrium thermodynamics, providing a clear microscopic interpretation of the elements of the theory. To provide an example of its universal applicability, in this paper we derive the fundamental equations of the radiation hydrodynamics directly in the context of Carter's multifluid theory. This operation unveils a novel set of thermodynamic constraints that must be respected by any microscopic model. Then, we prove that the radiation hydrodynamics becomes a multifluid model for bulk viscosity or heat conduction in some appropriate physical limits. ---------------------------------------------------------------- arXiv:2009.00030 astro-ph.HE astro-ph.SR gr-qc *Return of the Big Glitcher: NICER timing and glitches of PSR J0537-6910 * *Authors*: Wynn C. G. Ho, Cristobal M. Espinoza, Zaven Arzoumanian, Teruaki Enoto, Tsubasa Tamba, Danai Antonopoulou, Michal Bejger, Sebastien Guillot, Brynmor Haskell, Paul S. Ray Abstract: PSR J0537-6910, also known as the Big Glitcher, is the most prolific glitching pulsar known, and its spin-induced pulsations are only detectable in X-ray. We present results from analysis of 2.7 years of NICER timing observations, from 2017 August to 2020 April. We obtain a rotation phase-connected timing model for the entire timespan, which overlaps with the third observing run of LIGO/Virgo, thus enabling the most sensitive gravitational wave searches of this potentially strong gravitational wave-emitting pulsar. We find that the short-term braking index between glitches decreases towards a value of 7 or lower at longer times since the preceding glitch. By combining NICER and RXTE data, we measure a long-term braking index n=-1.25+/-0.01. Our analysis reveals 8 new glitches, the first detected since 2011, near the end of RXTE, with a total NICER and RXTE glitch activity of 8.88x10^-7 yr^-1. The new glitches follow the seemingly unique time-to-next-glitch---glitch-size correlation established previously using RXTE data, with a slope of 5 d microHz^-1. For one glitch around which NICER observes two days on either side, we search for but do not see clear evidence of spectral nor pulse profile changes that may be associated with the glitch. Journal reference: Mon. Not. R. Astron. Soc. 498, 4605-4614 (2020) ---------------------------------------------------------------- arXiv:2010.15574 astro-ph.HE astro-ph.SR gr-qc doi 10.1093/mnras/stab307 Gravitational waves from mountains in newly born millisecond magnetars Authors: Ankan Sur, Brynmor Haskell Abstract: In this paper we study the spin-evolution and gravitational-wave luminosity of a newly born millisecond magnetar, formed either after the collapse of a massive star or after the merger of two neutron stars. In both cases we consider the effect of fallback accretion, and consider the evolution of the system due to the different torques acting on the star, namely the spin up torque due to accretion and spin-down torques due to magnetic dipole radiation, neutrino emission, and gravitational wave emission linked to the formation of a `mountain' on the accretion poles. Initially the spin period is mostly affected by the dipole radiation, but at later times accretion spin the star up rapidly. We find that a magnetar formed after the collapse of a massive star can accrete up to 1 M_{\odot} , and survive on the order of 50 s before collapsing to a black hole. The gravitational wave strain, for an object located at 1 Mpc, is h_c \sim 10^{-23} at kHz frequencies, making this a potential target for next generation ground based detectors. A magnetar formed after a binary neutron star merger, on the other hand, accretes at the most 0.2 M_{\odot}, and emits gravitational waves with a lower maximum strain of the order of h_c \sim 10^{-24} , but also survives for much longer times, and may possibly be associated with the X-ray plateau observed in the light curve of a number of short gamma-ray burst. Journal reference: Monthly Notices of the Royal Astronomical Society, Volume 502, Issue 4, April 2021, Pages 4680-4688 ---------------------------------------------------------------- arXiv:2011.10557 astro-ph.HE gr-qc *Axisymmetric models for neutron star merger remnants with realistic thermal and rotational profiles * *Authors*: Giovanni Camelio, Tim Dietrich, Stephan Rosswog, Brynmor Haskell Abstract: Merging neutron stars are expected to produce hot, metastable remnants in rapid differential rotation, which subsequently cool and evolve into rigidly rotating neutron stars or collapse to black holes. Studying this metastable phase and its further evolution is essential for the prediction and interpretation of the electromagnetic, neutrino, and gravitational signals from such a merger. In this work, we model binary neutron star merger remnants and propose new rotation and thermal laws that describe post-merger remnants. Our framework is capable to reproduce quasi-equilibrium configurations for generic equations of state, rotation and temperature profiles, including nonbarotropic ones. We demonstrate that our results are in agreement with numerical relativity simulations concerning bulk remnant properties like the mass, angular momentum, and the formation of a massive accretion disk. Because of the low computational cost for our axisymmetric code compared to full 3+1-dimensional simulations, we can perform an extensive exploration of the binary neutron star remnant parameter space studying several hundred thousand configurations for different equation of states. Journal reference: Phys. Rev. D 103, 063014 (2021) ---------------------------------------------------------------- arXiv:2012.10288 astro-ph.HE cond-mat.quant-gas gr-qc physics.flu-dyn *Superfluid dynamics in neutron star crusts: the Iordanskii force and chemical gauge covariance * *Authors*: Lorenzo Gavassino, Marco Antonelli, Brynmor Haskell Abstract: We present a geometrical derivation of the relativistic dynamics of the superfluid inner crust of a neutron star. The resulting model is analogous to the Hall-Vinen-Bekarevich-Khalatnikov hydrodynamics for a single-component superfluid at finite temperature, but particular attention should be paid to the fact that some fraction of the neutrons are locked to the motion of the protons in nuclei. This gives rise to an ambiguity in the definition of the two currents (the normal and the superfluid one) on which the model is built, a problem that manifests itself as a chemical gauge freedom of the theory. To ensure chemical gauge covariance of the hydrodynamic model, the phenomenological equation of motion for a quantized vortex should contain an extra transverse force, that is the relativistic version of the Iordanskii force discussed in the context of superfluid Helium. Hence, we extend the mutual friction model of Langlois et al. (1998) to account for the possible presence of this Iordanskii-like force. Furthermore, we propose that a better understanding of the (still not completely settled) controversy around the presence of the Iordanskii force in superfluid Helium, as well as in neutron stars, may be achieved by considering that the different incompatible results present in the literature pertain to two, opposite, dynamical regimes of the fluid system. Journal reference: Universe 2021, 7(2), 28 ---------------------------------------------------------------- arXiv:2104.03137 gr-qc *Gravitational waves from isolated neutron stars * *Authors*: Brynmor Haskell, Kai Schwenzer Abstract: Neutron star interiors are a fantastic laboratory for high density physics in extreme environments. Probing this system with standard electromagnetic observations is, however, a challenging endeavour, as the radiation tends to be scattered by the outer layers and the interstellar medium. Gravitational waves, on the other hand, while challenging to detect, interact weakly with matter and are likely to carry a clean imprint of the high density interior of the star. In particular long lived, i.e. `continuous' signals from isolated neutron stars can carry a signature of deformations, possibly in crystalline exotic layers of the core, or allow to study modes of oscillation, thus performing gravitational wave asteroseismology of neutron star interiors. In this article we will review current theoretical models for continuous gravitational wave emission, and observational constraints, both electromagnetic and gravitational. Finally we will discuss future observational possibilities. ---------------------------------------------------------------- arXiv:2105.14621 gr-qc astro-ph.HE hep-ph physics.flu-dyn *Thermodynamic stability implies causality * *Authors*: Lorenzo Gavassino, Marco Antonelli, Brynmor Haskell Abstract: The stability conditions of a relativistic hydrodynamic theory can be derived directly from the requirement that the entropy should be maximised in equilibrium. Here we use a simple geometrical argument to prove that, if the hydrodynamic theory is stable according to this entropic criterion, then localised perturbations to the equilibrium state cannot propagate outside their future light-cone. In other words, within relativistic hydrodynamics, acausal theories must be thermodynamically unstable, at least close to equilibrium. We show that the physical origin of this deep connection between stability and causality lies in the relationship between entropy and information. Our result may be interpreted as an ``equilibrium conservation theorem'', which generalizes the Hawking-Ellis vacuum conservation theorem to finite temperature and chemical potential. ---------------------------------------------------------------- arXiv:2108.11858 astro-ph.HE astro-ph.SR gr-qc *Long-term GRHMD simulation of magnetic field in isolated neutron stars * *Authors*: Ankan Sur, William Cook, David Radice, Brynmor Haskell, Sebastiano Bernuzzi Abstract: Strong magnetic fields play an important role in powering the emission of neutron stars. Nevertheless a full understanding of the interior configuration of the field remains elusive. In this work, we present General Relativistic MagnetoHydroDynamics simulations of the magnetic field evolution in neutron stars lasting 500 ms (5 Alfven crossing times) and up to resolutions of 0.231 km using Athena++. We explore two different initial conditions, one with purely poloidal magnetic field and the other with a dominant toroidal component, and study the poloidal and toroidal field energies, the growth times of the various instability-driven oscillation modes and turbulence. We find that the purely poloidal setup generates a toroidal field which later decays exponentially reaching 1% of the total magnetic energy, showing no evidence of reaching equilibrium. The initially stronger toroidal field setup, on the other hand, loses up to 20% of toroidal energy and maintains this state till the end of our simulation. We also explore the hypothesis, drawn from previous MHD simulations, that turbulence plays an important role in the quasi equilibrium state. An analysis of the spectra in our higher resolution setups reveal, however, that in most cases we are not observing turbulence at small scales, but rather a noisy velocity field inside the star. We also observe that the majority of the magnetic energy gets dissipated as heat increasing the internal energy of the star, while a small fraction gets radiated away as electromagnetic radiation. ---------------------------------------------------------------- arXiv:2110.05546 gr-qc astro-ph.HE nucl-th physics.flu-dyn *Extending Israel and Stewart hydrodynamics to relativistic superfluids via Carter's multifluid approach * *Authors*: Lorenzo Gavassino, Marco Antonelli, Brynmor Haskell Abstract: We construct a relativistic model for bulk viscosity and heat conduction in a superfluid. Building on the principles of Unified Extended Irreversible Thermodynamics, the model is derived from Carter's multifluid approach for a theory with three currents, where the quasi-particle current is an independent hydrodynamic degree of freedom. For small deviations from local thermodynamic equilibrium, the model reduces to an extension of the Israel-Stewart theory to superfluid systems. It can, therefore, be made hyperbolic, causal and stable if the microscopic input is accurate. The non-dissipative limit of the model is the relativistic two-fluid model of Carter, Khalatnikov and Gusakov. The Newtonian limit of the model is an Extended-Irreversible-Thermodynamic extension of Landau's two-fluid model. The model predicts the existence of four bulk viscosity coefficients and accounts for their microscopic origin, providing their exact formulas in terms of the quasi-particle creation rate. Furthermore, when fast oscillations of small amplitude around the equilibrium are considered, the relaxation-time term in the telegraph-type equations for the bulk viscosities accounts directly for their expected dependence on the frequency. ---------------------------------------------------------------- arXiv:2009.03992 gr-qc math-ph *Axis potentials for stationary n-black-hole configurations * *Authors*: Jörg Hennig Abstract: We extend earlier discussions of the balance problem for two black holes and study stationary spacetimes containing an arbitrary number of n aligned rotating and (possibly) charged black holes. For these hypothetical equilibrium configurations, we obtain the most general form of the boundary data on the symmetry axis in terms of a finite number of parameters. Hence future investigations of n-black-hole configurations can be restricted to studying properties of these finite families of solutions. Journal reference: Class. Quantum Grav. 37, 19LT01 (2020) ---------------------------------------------------------------- arXiv:2012.02240 gr-qc math-ph *Fully pseudospectral solution of the conformally invariant wave equation on a Kerr background * *Authors*: Jörg Hennig, Rodrigo Panosso Macedo Abstract: We study axisymmetric solution to the conformally invariant wave equation on a Kerr background by means of numerical and analytical methods. Our main focus is on the behaviour of the solutions near spacelike infinity, which is appropriately represented as a cylinder. Earlier studies of the wave equation on a Schwarzschild background have revealed important details about the regularity of the corresponding solutions. It was found that, on the cylinder, the solutions generically develop logarithmic singularities at infinitely many orders. Moreover, these singularities also `spread' to future null infinity. However, by imposing certain regularity conditions on the initial data, the lowest-order singularities can be removed. Here we are interested in a generalisation of these results to a rotating black hole background and study the influence of the rotation rate on the properties of the solutions. To this aim, we first construct a conformal compactification of the Kerr solution which yields a suitable representation of the cylinder at spatial infinity. Besides analytical investigations on the cylinder, we numerically solve the wave equation with a fully pseudospectral method, which allows us to obtain highly accurate numerical solutions. This is crucial for a detailed analysis of the regularity of the solutions. In the Schwarzschild case, the numerical problem could effectively be reduced to solving (1+1)-dimensional equations. Here we present a code that can perform the full 2+1 evolution as required for axisymmetric waves on a Kerr background. Journal reference: Class. Quantum Grav. 38, 135006 (2021) ---------------------------------------------------------------- arXiv:2001.06102 astro-ph.HE gr-qc nucl-th *Gravitational waves or deconfined quarks: what causes the premature collapse of neutron stars born in short gamma-ray bursts? * *Authors*: Nikhil Sarin, Paul D. Lasky, Gregory Ashton Abstract: We infer the collapse times of long-lived neutron stars into black holes using the X-ray afterglows of 18 short gamma-ray bursts. We then apply hierarchical inference to infer properties of the neutron star equation of state and dominant spin-down mechanism. We measure the maximum non-rotating neutron star mass M_TOV =2.31^+0.36 _0.21 Mand constrain the fraction of remnants spinning down predominantly through gravitational-wave emission to =0.69^+0.21 _0.39 with 68% uncertainties. In principle, this method can determine the difference between hadronic and quark equation of states. In practice, however, the data is not yet informative with indications that these neutron stars do not have hadronic equation of states at the 1 level. These inferences all depend on the underlying progenitor mass distribution for short gamma-ray bursts produced by binary neutron star mergers. The recently announced gravitational-wave detection of GW190425 suggests this underlying distribution is different from the locally-measured population of double neutron stars. We show that M_TOV and constraints depend on the fraction of binary mergers that form through a distribution consistent with the locally-measured population and a distribution that can explain GW190425. The more binaries that form from the latter distribution, the larger M_TOV needs to be to satisfy the X-ray observations. Our measurements above are marginalised over this unknown fraction. If instead, we assume GW190425 is not a binary neutron star merger, i.e the underlying mass distribution of double neutron stars is the same as observed locally, we measure M_TOV =2.26^+0.31 _0.17 M. Journal reference: Phys. Rev. D 101, 063021 (2020) ---------------------------------------------------------------- arXiv:2003.09780 astro-ph.IM astro-ph.HE gr-qc *The Parkes Pulsar Timing Array Project: Second data release * *Authors*: M. Kerr, D. J. Reardon, G. Hobbs, R. M. Shannon, R. N. Manchester, S. Dai, C. J. Russell, S. -B. Zhang, W. van Straten, S. Osowski, A. Parthasarathy, R. Spiewak, M. Bailes, N. D. R. Bhat, A. D. Cameron, W. A. Coles, J. Dempsey, X. Deng, B. Goncharov, J. F Kaczmarek, M. J. Keith, P. D. Lasky, M. E. Lower, B. Preisig, J. M. Sarkissian , et al. (5 additional authors not shown) Abstract: We describe 14 years of public data from the Parkes Pulsar Timing Array (PPTA), an ongoing project that is producing precise measurements of pulse times of arrival from 26 millisecond pulsars using the 64-m Parkes radio telescope with a cadence of approximately three weeks in three observing bands. A comprehensive description of the pulsar observing systems employed at the telescope since 2004 is provided, including the calibration methodology and an analysis of the stability of system components. We attempt to provide full accounting of the reduction from the raw measured Stokes parameters to pulse times of arrival to aid third parties in reproducing our results. This conversion is encapsulated in a processing pipeline designed to track provenance. Our data products include pulse times of arrival for each of the pulsars along with an initial set of pulsar parameters and noise models. The calibrated pulse profiles and timing template profiles are also available. These data represent almost 21,000 hrs of recorded data spanning over 14 years. After accounting for processes that induce time-correlated noise, 22 of the pulsars have weighted root-mean-square timing residuals of < 1 s in at least one radio band. The data should allow end users to quickly undertake their own gravitational-wave analyses (for example) without having to understand the intricacies of pulsar polarisation calibration or attain a mastery of radio-frequency interference mitigation as is required when analysing raw data files. ---------------------------------------------------------------- arXiv:2006.04396 astro-ph.HE gr-qc *Detection and parameter estimation of binary neutron star merger remnants * *Authors*: Paul J. Easter, Sudarshan Ghonge, Paul D. Lasky, Andrew R. Casey, James A. Clark, Francisco Hernandez Vivanco, Katerina Chatziioannou Abstract: Detection and parameter estimation of binary neutron star merger remnants can shed light on the physics of hot matter at supranuclear densities. Here we develop a fast, simple model that can generate gravitational waveforms, and show it can be used for both detection and parameter estimation of post-merger remnants. The model consists of three exponentially-damped sinusoids with a linear frequency-drift term. The median fitting factors between the model waveforms and numerical-relativity simulations exceed 0.90. We detect remnants at a post-merger signal-to-noise ratio of 7 using a Bayes-factor detection statistic with a threshold of 3000. We can constrain the primary post-merger frequency to ±^1.4 _1.2 % at post-merger signal-to-noise ratios of 15 with an increase in precision to ±^0.3 _0.2 % for post-merger signal-to-noise ratios of 50. The tidal coupling constant can be constrained to ±^9 _12 % at post-merger signal-to-noise ratios of 15, and ±5% at post-merger signal-to-noise ratios of 50 using a hierarchical inference model. Journal reference: Phys. Rev. D 102, 043011 (2020). ---------------------------------------------------------------- arXiv:2006.11525 gr-qc astro-ph.CO *Mapping the Universe Expansion: Enabling percent-level measurements of the Hubble Constant with a single binary neutron-star merger detection * *Authors*: Juan Calderón Bustillo, Samson H. W. Leong, Tim Dietrich, Paul D. Lasky Abstract: The joint observation of the gravitational-wave and electromagnetic signal from the binary neutron-star merger GW170817 allowed for a new independent measurement of the Hubble constant H_0 , albeit with an uncertainty of about 15% at 1. Observations of similar sources with a network of future detectors will allow for more precise measurements of H_0 . These, however, are currently largely limited by the intrinsic degeneracy between the luminosity distance and the inclination of the source in the gravitational-wave signal. We show that the higher-order modes in gravitational waves can be used to break this degeneracy in astrophysical parameter estimation in both the inspiral and post-merger phases of a neutron star merger. We show that for systems at distances similar to GW170817, this method enables percent-level measurements of H_0 with a single detection. This would permit the study of time variations and spatial anisotropies of H_0 with unprecedented precision. We investigate how different network configurations affect measurements of H_0 , and discuss the implications in terms of science drivers for the proposed 2.5- and third-generation gravitational-wave detectors. Finally, we show that the precision of H_0 measured with these future observatories will be solely limited by redshift measurements of electromagnetic counterparts. Journal reference: The Astrophysical Journal Letters, Volume 912, Number 1 (2021) ---------------------------------------------------------------- arXiv:2006.11957 stro-ph.CO gr-qc *Searching for anisotropy in the distribution of binary black hole mergers * *Authors*: Ethan Payne, Sharan Banagiri, Paul Lasky, Eric Thrane Abstract: The standard model of cosmology is underpinned by the assumption of the statistical isotropy of the Universe. Observations of the cosmic microwave background, galaxy distributions, and supernovae, among other media, support the assumption of isotropy at scales 100 Mpc. The recent detections of gravitational waves from merging stellar-mass binary black holes provide a new probe of anisotropy; complementary and independent of all other probes of the matter distribution in the Universe. We present an analysis using a spherical harmonic model to determine the level of anisotropy in the first LIGO/Virgo transient catalog. We find that the ten binary black hole mergers within the first transient catalog are consistent with an isotropic distribution. We carry out a study of simulated events to assess the prospects for future probes of anisotropy. Within a single year of operation, third-generation gravitational-wave observatories will probe anisotropies with an angular scale of 36at the level of 0.1%. Journal reference: Phys. Rev. D 102, 102004 (2020) ---------------------------------------------------------------- arXiv:2009.10193 astro-ph.IM astro-ph.HE gr-qc *Gravitational-wave astronomy with a physical calibration model * *Authors*: Ethan Payne, Colm Talbot, Paul D. Lasky, Eric Thrane, Jeffrey S. Kissel Abstract: We carry out astrophysical inference for compact binary merger events in LIGO-Virgo's first gravitational-wave transient catalog (GWTC-1) using a physically motivated calibration model. We demonstrate that importance sampling can be used to reduce the cost of what would otherwise be a computationally challenging analysis. We show that including the physical estimate for the calibration error distribution has negligible impact on the inference of parameters for the events in GWTC-1. Studying a simulated signal with matched filter signal-to-noise ratio SNR=200, we project that a calibration error estimate typical of GWTC-1 is likely to be negligible for the current generation of gravitational-wave detectors. We argue that other sources of systematic error---from waveforms, prior distributions, and noise modelling---are likely to be more important. Finally, using the events in GWTC-1 as standard sirens, we infer an astrophysically-informed improvement on the estimate of the calibration error in the LIGO interferometers. Journal reference: Phys. Rev. D 102, 122004 (2020) ---------------------------------------------------------------- arXiv:2010.01857 gr-qc *Black-hole spectroscopy, the no-hair theorem and GW150914: Kerr vs. Occam * *Authors*: Juan Calderón Bustillo, Paul D. Lasky, Eric Thrane Abstract: The "no-hair" theorem states that astrophysical black holes are fully characterised by just two numbers: their mass and spin. The gravitational-wave emission from a perturbed black-hole consists of a superposition of damped sinusoids, known as \textit{quasi-normal modes}. Quasi-normal modes are specified by three integers (,m,n): the (,m) integers describe the angular properties and (n) specifies the (over)tone. If the no-hair theorem holds, the frequencies and damping times of quasi-normal modes are determined uniquely by the mass and spin of the black hole, while phases and amplitudes depend on the particular perturbation. Current tests of the no-hair theorem, attempt to identify these modes in a semi-agnostic way, without imposing priors on the source of the perturbation. This is usually known as \textit{black-hole spectroscopy}. Applying this framework to GW150914, the measurement of the first overtone led to the confirmation of the theorem to 20% level. We show, however, that such semi-agnostic tests cannot provide strong evidence in favour of the no-hair theorem, even for extremely loud signals, given the increasing number of overtones (and free parameters) needed to fit the data. This can be solved by imposing prior assumptions on the origin of the perturbed black hole that can further constrain the explored parameters: in particular, our knowledge that the ringdown is sourced by a binary black hole merger. Applying this strategy to GW150914 we find a natural log Bayes factor of 6.5 in favour of the Kerr nature of its remnant, indicating that the hairy object hypothesis is disfavoured with <1:600 with respect to the Kerr black-hole one. Journal reference: Phys. Rev. D 103, 024041 (2021) ---------------------------------------------------------------- arXiv:2012.08172 astro-ph.HE gr-qc *The evolution of binary neutron star post-merger remnants: a review * *Authors*: Nikhil Sarin, Paul D. Lasky Abstract: Two neutron stars merge somewhere in the Universe approximately every 10 seconds, creating violent explosions observable in gravitational waves and across the electromagnetic spectrum. The transformative coincident gravitational-wave and electromagnetic observations of the binary neutron star merger GW170817 gave invaluable insights into these cataclysmic collisions, probing bulk nuclear matter at supranuclear densities, the jet structure of gamma-ray bursts, the speed of gravity, and the cosmological evolution of the local Universe, among other things. Despite the wealth of information, it is still unclear when the remnant of GW170817 collapsed to form a black hole. Evidence from other short gamma-ray bursts indicates a large fraction of mergers may form long-lived neutron stars. We review what is known observationally and theoretically about binary neutron star post-merger remnants. From a theoretical perspective, we review our understanding of the evolution of short- and long-lived merger remnants, including fluid, magnetic-field, and temperature evolution. These considerations impact prospects of detection of gravitational waves from either short- or long-lived neutron star remnants which potentially allows for new probes into the hot nuclear equation of state in conditions inaccessible in terrestrial experiments. We also review prospects for determining post-merger physics from current and future electromagnetic observations, including kilonovae and late-time x-ray and radio afterglow observations. ---------------------------------------------------------------- arXiv:2103.12274 gr-qc astro-ph.HE astro-ph.IM *Bayesian inference for gravitational waves from binary neutron star mergers in third-generation observatories * *Authors*: Rory Smith, Ssohrab Borhanian, Bangalore Sathyaprakash, Francisco Hernandez Vivanco, Scott Field, Paul Lasky, Ilya Mandel, Soichiro Morisaki, David Ottaway, Bram Slagmolen, Eric Thrane, Daniel Töyrä, Salvatore Vitale Abstract: Third-generation (3G) gravitational-wave detectors will observe thousands of coalescing neutron star binaries with unprecedented fidelity. Extracting the highest precision science from these signals is expected to be challenging owing to both high signal-to-noise ratios and long-duration signals. We demonstrate that current Bayesian inference paradigms can be extended to the analysis of binary neutron star signals without breaking the computational bank. We construct reduced order models for 90minute long gravitational-wave signals, covering the observing band (52048Hz), speeding up inference by a factor of 1.3×104 compared to the calculation times without reduced order models. The reduced order models incorporate key physics including the effects of tidal deformability, amplitude modulation due to the Earth's rotation, and spin-induced orbital precession. We show how reduced order modeling can accelerate inference on data containing multiple, overlapping gravitational-wave signals, and determine the speedup as a function of the number of overlapping signals. Thus, we conclude that Bayesian inference is computationally tractable for the long-lived, overlapping, high signal-to-noise-ratio events present in 3G observatories. Journal reference: Phys. Rev. Lett. 127, 081102 (2021) ---------------------------------------------------------------- arXiv:2105.02879 gr-qc astro-ph.HE *The Memory Remains (Undetected): Updates from the Second LIGO/Virgo Gravitational-Wave Transient Catalog * *Authors*: Moritz Hübner, Paul Lasky, Eric Thrane Abstract: The LIGO and Virgo observatories have reported 39 new gravitational-wave detections during the first part of the third observation run, bringing the total to 50. Most of these new detections are consistent with binary black-hole coalescences, making them suitable targets to search for gravitational-wave memory, a non-linear effect of general relativity. We extend a method developed in previous publications to analyse these events to determine a Bayes factor comparing the memory hypothesis to the no-memory hypothesis. Specifically, we calculate Bayes factors using two waveform models with higher-order modes that allow us to analyse events with extreme mass ratios and precessing spins, both of which have not been possible before. Depending on the waveform model we find a combined lnBF_mem =0.024 or lnBF_mem =0.049 in favour of memory. This result is consistent with recent predictions that indicate O(2000) binary black-hole detections will be required to confidently establish the presence or absence of memory. Journal reference: Phys. Rev. D 104, 023004 (2021) ---------------------------------------------------------------- arXiv:2106.04064 astro-ph.HE gr-qc *Can we measure the collapse time of a post-merger remnant for a future GW170817-like event? * *Authors*: Paul J. Easter, Paul D. Lasky, Andrew R. Casey Abstract: Measuring the collapse time of a binary neutron star merger remnant can inform the physics of extreme matter and improve modelling of short gamma-ray bursts and associated kilonova. The lifetime of the post-merger remnant directly impacts the mechanisms available for the jet launch of short gamma-ray bursts. We develop and test a method to measure the collapse time of post-merger remnants. We show that for a GW170817-like event at 40Mpc, a network of Einstein Telescope with Cosmic Explorer is required to detect collapse times of 10ms. For a two-detector network at A+ design sensitivity, post-merger remnants with collapse times of 10ms must be 10Mpc to be measureable. This increases to 1826Mpc if we include the proposed Neutron star Extreme Matter Observatory (NEMO), increasing the effective volume by a factor of 30. ---------------------------------------------------------------- arXiv:2106.09042 astro-ph.HE gr-qc *Implications of Eccentric Observations on Binary Black Hole Formation Channels * *Authors*: Michael Zevin, Isobel M. Romero-Shaw, Kyle Kremer, Eric Thrane, Paul D. Lasky Abstract: Orbital eccentricity is one of the most robust discriminators for distinguishing between dynamical and isolated formation scenarios of binary black hole mergers using gravitational-wave observatories such as LIGO and Virgo. Using state-of-the-art cluster models, we show how selection effects impact the detectable distribution of eccentric mergers from clusters. We show that the observation (or lack thereof) of eccentric binary black hole mergers can significantly constrain the fraction of detectable systems that originate from dynamical environments, such as dense star clusters. After roughly 150 observations, observing no eccentric binary signals would indicate that clusters cannot make up the majority of the merging binary black hole population in the local universe (95% credibility). However, if dense star clusters dominate the rate of eccentric mergers and a single system is confirmed to be measurably eccentric in the first and second gravitational-wave transient catalogs, clusters must account for at least 14% of detectable binary black hole mergers. The constraints on the fraction of detectable systems from dense star clusters become significantly tighter as the number of eccentric observations grows and will be constrained to within 0.5 dex once 10 eccentric binary black holes are observed. Journal reference: The Astrophysical Journal Letters, 921, L43 (2021) ---------------------------------------------------------------- arXiv:2107.11730 gr-qc astro-ph.HE *The imprint of superradiance on hierarchical black hole mergers * *Authors*: Ethan Payne, Ling Sun, Kyle Kremer, Paul D. Lasky, Eric Thrane Abstract: Ultralight bosons are a proposed solution to outstanding problems in cosmology and particle physics: they provide a dark-matter candidate while potentially explaining the strong charge-parity problem. If they exist, ultralight bosons can interact with black holes through the superradiant instability. In this work we explore the consequences of this instability on the evolution of hierarchical black holes within dense stellar clusters. By reducing the spin of individual black holes, superradiance reduce the recoil velocity of merging binary black holes, which, in turn, increases the retention fraction of hierarchical merger remnants. We show that the existence of ultralight bosons with mass 2×10^14 /eV2×10^13 would lead to an increased rate of hierarchical black hole mergers in nuclear star clusters. An ultralight boson in this energy range would result in up to 60% more present-day nuclear star clusters supporting hierarchical growth. The presence of an ultralight boson can also double the rate of intermediate mass black hole mergers to 0.08 Gpc^3 yr^1 in the local Universe. These results imply that a select range of ultralight boson mass can have far-reaching consequences for the population of black holes in dense stellar environments. Future studies into black hole cluster populations and the spin distribution of hierarchically formed black holes will test this scenario. ---------------------------------------------------------------- arXiv:2110.10892 astro-ph.HE gr-qc *Multimessenger astronomy with a kHz-band gravitational-wave observatory * *Authors*: Nikhil Sarin, Paul D. Lasky Abstract: Proposed next-generation networks of gravitational-wave observatories include dedicated kilohertz instruments that target neutron star science, such as the proposed Neutron Star Extreme Matter Observatory, NEMO. The original proposal for NEMO highlighted the need for it to exist in a network of gravitational-wave observatories to ensure detection confidence and sky localisation of sources. We show that NEMO-like observatories have significant utility on their own as coincident electromagnetic observations can provide the detection significance and sky localisation. We show that, with a single NEMO-like detector and expected electromagnetic observatories in the late 2020s and early 2030s such as the Vera C. Rubin observatory and SVOM, approximately 40% of all binary neutron star mergers detected with gravitational waves could be confidently identified as coincident multimessenger detections. We show that we expect 2^+10 _1 yr^1 coincident observations of gravitational-wave mergers with gamma-ray burst prompt emission, 13^+23 _10 yr^1 detections with kilonova observations, and 4^+18 _3 yr^1 with broadband afterglows and kilonovae, where the uncertainties are 90% confidence intervals arising from uncertainty in current merger-rate estimates. Combined, this implies a coincident detection rate of 14^+25 _11 yr^1 out to 300Mpc. These numbers indicate significant science potential for a single kilohertz gravitational-wave detector operating without a global network of other gravitational-wave observatories. ---------------------------------------------------------------- arXiv:2110.13319 gr-qc astro-ph.CO astro-ph.HE *Did Goryachev et al. detect megahertz gravitational waves? * *Authors*: Paul D. Lasky, Eric Thrane Abstract: Goryachev et al. [1] recently announced the detection of "two strongly significant events" in their Bulk Acoustic Wave High Frequency Gravitational Wave Antenna. They claim many possibilities for the cause of these events, including high-frequency megahertz gravitational waves. We demonstrate these events are not due to gravitational waves for two reasons. 1) The inferred stochastic gravitational-wave background from these events implies the gravitational-wave energy density of the Universe is _gw 10^8 , approximately 10^8 times the closure density of the Universe. 2) The low-frequency gravitational-wave memory signal that accompanies any high-frequency gravitational-wave source visible by the current generation of high-frequency detectors would have been visible by LIGO/Virgo as a transient burst with signal-to-noise ratio 10^6 . The non-detection of such loud memory bursts throughout the operation of LIGO/Virgo rules out the gravitational-wave explanation for the high-frequency events detected by Goryachev et al. We discuss broader implications of this work for the ongoing experimental search for ultra high-frequency (MHz-GHz) gravitational waves. Journal reference: Physical Review D 104, 103017 (2021) ---------------------------------------------------------------- arXiv:2102.12028 gr-qc *Cylindrical spikes * *Authors*: Muhammad Zubair Ali Moughal, Woei Chet Lim Abstract: The Geroch/Stephani transformation is a solution-generating transformation, and may generate spiky solutions. The spikes in solutions generated so far are either early-time permanent spikes or transient spikes. We want to generate a solution with a late-time permanent spike. We achieve this by applying Stephani's transformation with the rotational Killing vector field of the locally rotationally symmetric Jacobs solution. The late-time permanent spike occurs along the cylindrical axis. Using a mixed Killing vector field, the generated solution also features a rich variety of transient structures. We introduce a new technique to analyse these structures. Our findings lead us to discover a transient behaviour, which we call the overshoot transition. These discoveries compel us to revise the description of transient spikes. Journal reference: Class. Quantum Grav. 38 (2021) 075029 ---------------------------------------------------------------- arXiv:2111.15122 gr-qc *Transition analysis of the non-OT G2 stiff fluid spike solution * *Authors*: Woei Chet Lim, Muhammad Zubair Ali Moughal Abstract: We use the technique developed in Moughal's doctoral thesis to analyse the joint spike transition, revealing new groups of worldlines which undergo distinct transitions, and correcting misconceptions about spikes. ---------------------------------------------------------------- arXiv:2102.13551 gr-qc *Hawking-Ellis classification of stress-energy: test-fields versus back-reaction * *Authors*: Prado Martin-Moruno, Matt Visser Abstract: We consider the Hawking-Ellis (Segre-Plebanski) classification of stress-energy tensors, both in the test-field limit, and in the presence of back-reaction governed by the usual Einstein equations. For test fields it is not too difficult to get a type~IV stress-energy via quantum vacuum polarization effects. (For example, consider the Unruh quantum vacuum state for a massless scalar field in the Schwarzschild background.) However, in the presence of back-reaction driven by the ordinary Einstein equations the situation is often much more constrained. For instance: (1) in any static spacetime the stress-energy is always type I in the domain of outer communication, and on any horizon that might be present; (2) in any stationary axisymmetric spacetime the stress-energy is always type I on any horizon that might be present; (3) on any Killing horizon that is extendable to a bifurcation 2-surface the stress-energy is always type I; (4) in any stationary axisymmetric spacetime the stress-energy is always type I on the axis of symmetry; (5) some of the homogeneous Bianchi cosmologies are guaranteed to be Hawking-Ellis type I (for example, all the Bianchi type I cosmologies, all the FLRW cosmologies, and all the "single mode" Bianchi cosmologies). That is, in very many physically interesting situations once one includes back-reaction the more unusual stress-energy types are automatically excluded. Journal reference: Phys. Rev. D 103, 124003 (2021) ---------------------------------------------------------------- arXiv:2103.12786 gr-qc *The little rip in classical and quantum f(R) cosmology * *Authors*: Teodor Borislavov Vasilev, Mariam Bouhmadi-López, Prado Martín-Moruno Abstract: The little rip is a cosmological abrupt event predicted by some phantom dark energy models that could describe the future evolution of our Universe. This event can be interpreted as a big rip singularity delayed indefinitely, although in those models bounded structures will be destroyed in a finite cosmic time in the future. In this work, we analyse the little rip cosmology from a classical and quantum point of view within the scheme of alternative metric f(R) theories of gravity. The quantum analysis is performed in the framework of f(R) quantum geometrodynamics by means of the modified Wheeler-DeWitt equation. In this context, we show that the DeWitt criterion can be satisfied. Similar to what happens in general relativity, this result points towards the avoidance of the little rip in f(R) quantum cosmology. Journal reference: Phys. Rev. D 103, 124049 (2021) ---------------------------------------------------------------- arXiv:2105.12582 gr-qc *Modified Gravity and Cosmology: An Update by the CANTATA Network * *Authors*: Emmanuel N. Saridakis, Ruth Lazkoz, Vincenzo Salzano, Paulo Vargas Moniz, Salvatore Capozziello, Jose Beltrán Jiménez, Mariafelicia De Laurentis, Gonzalo J. Olmo, Yashar Akrami, Sebastian Bahamonde, Jose Luis Blázquez-Salcedo, Christian G. Böhmer, Camille Bonvin, Mariam Bouhmadi-López, Philippe Brax, Gianluca Calcagni, Roberto Casadio, Jose A. R. Cembranos, Álvaro de la Cruz-Dombriz, Anne-Christine Davis, Adrià Delhom, Eleonora Di Valentino, Konstantinos F. Dialektopoulos, Benjamin Elder, Jose María Ezquiaga , et al. (28 additional authors not shown) Abstract: General Relativity and the CDM framework are currently the standard lore and constitute the concordance paradigm. Nevertheless, long-standing open theoretical issues, as well as possible new observational ones arising from the explosive development of cosmology the last two decades, offer the motivation and lead a large amount of research to be devoted in constructing various extensions and modifications. All extended theories and scenarios are first examined under the light of theoretical consistency, and then are applied to various geometrical backgrounds, such as the cosmological and the spherical symmetric ones. Their predictions at both the background and perturbation levels, and concerning cosmology at early, intermediate and late times, are then confronted with the huge amount of observational data that astrophysics and cosmology are able to offer recently. Theories, scenarios and models that successfully and efficiently pass the above steps are classified as viable and are candidates for the description of Nature. We list the recent developments in the fields of gravity and cosmology, presenting the state of the art, high-lighting the open problems, and outlining the directions of future research. Its realization is performed in the framework of the COST European Action "Cosmology and Astrophysics Network for Theoretical Advances and Training Actions". ---------------------------------------------------------------- arXiv:2106.12050 gr-qc astro-ph.CO hep-th *Classical and quantum f(R) cosmology: The big rip, the little rip and the little sibling of the big rip * *Authors*: Teodor Borislavov Vasilev, Mariam Bouhmadi-López, Prado Martín-Moruno Abstract: The big rip, the little rip and the little sibling of the big rip are cosmological doomsdays predicted by some phantom dark energy models that could describe the future evolution of our Universe. When the universe evolves towards either of these future cosmic events all bounded structures and, ultimately, space-time itself are ripped apart. Nevertheless, it is commonly belief that quantum gravity effects may smooth or even avoid these classically predicted singularities. In this review, we discuss the classical and quantum occurrence of these riplike events in the scheme of metric f(R) theories of gravity. The quantum analysis is performed in the framework of f(R) quantum geometrodynamics. In this context, we analyse the fulfilment of the DeWitt criterion for the avoidance of these singular fates. Journal reference: Universe 7 (2021) no.8, 288 ---------------------------------------------------------------- arXiv:2111.05659 hep-ph astro-ph.HE gr-qc hep-th *Quantum gravity phenomenology at the dawn of the multi-messenger era -- A review * *Authors*: A. Addazi, J. Alvarez-Muniz, R. Alves Batista, G. Amelino-Camelia, V. Antonelli, M. Arzano, M. Asorey, J. -L. Atteia, S. Bahamonde, F. Bajardi, A. Ballesteros, B. Baret, D. M. Barreiros, S. Basilakos, D. Benisty, O. Birnholtz, J. J. Blanco-Pillado, D. Blas, J. Bolmont, D. Boncioli, P. Bosso, G. Calcagni, S. Capozziello, J. M. Carmona, S. Cerci , et al. (135 additional authors not shown) Abstract: The exploration of the universe has recently entered a new era thanks to the multi-messenger paradigm, characterized by a continuous increase in the quantity and quality of experimental data that is obtained by the detection of the various cosmic messengers (photons, neutrinos, cosmic rays and gravitational waves) from numerous origins. They give us information about their sources in the universe and the properties of the intergalactic medium. Moreover, multi-messenger astronomy opens up the possibility to search for phenomenological signatures of quantum gravity. On the one hand, the most energetic events allow us to test our physical theories at energy regimes which are not directly accessible in accelerators; on the other hand, tiny effects in the propagation of very high energy particles could be amplified by cosmological distances. After decades of merely theoretical investigations, the possibility of obtaining phenomenological indications of Planck-scale effects is a revolutionary step in the quest for a quantum theory of gravity, but it requires cooperation between different communities of physicists (both theoretical and experimental). This review is aimed at promoting this cooperation by giving a state-of-the art account of the interdisciplinary expertise that is needed in the effective search of quantum gravity footprints in the production, propagation and detection of cosmic messengers. ---------------------------------------------------------------- arXiv:2111.06986 gr-qc astro-ph.HE astro-ph.IM *Expanding the Reach of Gravitational Wave Astronomy to the Edge of the Universe: The Gravitational-Wave International Committee Study Reports on Next Generation Ground-based Gravitational-Wave Observatories * *Authors*: David Reitze, Michele Punturo, Peter Couvares, Stavros Katsanevas, Takaaki Kajita, Vicky Kalogera, Harald Lueck, David McClelland, Sheila Rowan, Gary Sanders, B. S. Sathyaprakash, David Shoemaker, Jo van den Brand Abstract: The first direct detection of gravitational waves emitted from a pair of merging black holes in 2015 has been heralded as one of most significant scientific breakthroughs in physics and astronomy of the 21st century. Motivated by the tremendous scientific opportunities now opened by gravitational-wave observatories and recognizing that to fully exploit the new field will require new observatories that may take 15 to 20 years from conception until operations begin, the Gravitational Wave International Committee (GWIC) convened a subcommittee to examine the path to build and operate a network of future ground-based observatories, capable of extending the observational GW horizon well beyond that currently attainable with the current generation of detectors. This report is the first in a six part series of reports by the GWIC 3G Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the Edge of the Universe (this report), ii) The Next Generation Global Gravitational Wave Observatory: The Science Book, iii) 3G R&D: R&D for the Next Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based Gravitational-wave Observatories: Synergies with Other Scientific Communities, and vi) An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network. ---------------------------------------------------------------- arXiv:2111.06987 gr-qc astro-ph.HE astro-ph.IM *Gravitational Wave Data Analysis: Computing Challenges in the 3G Era * *Authors*: Peter Couvares, Ian Bird, Ed Porter, Stefano Bagnasco, Michele Punturo, David Reitze, Stavros Katsanevas, Takaaki Kajita, Vicky Kalogera, Harald Lueck, David McClelland, Sheila Rowan, Gary Sanders, B. S. Sathyaprakash, David Shoemaker, Jo van den Brand Abstract: Cyber infrastructure will be a critical consideration in the development of next generation gravitational-wave detectors. The demand for data analysis computing in the 3G era will be driven by the high number of detections as well as the expanded search parameter space for compact astrophysical objects and the subsequent parameter estimation follow-up required to extract the nature of the sources. Additionally, there will be an increased need to develop appropriate and scalable computing cyberinfrastructure, including data access and transfer protocols, and storage and management of software tools, that have sustainable development, support, and management processes. This report identifies the major challenges and opportunities facing 3G gravitational-wave observatories and presents recommendations for addressing them. This report is the fourth in a six part series of reports by the GWIC 3G Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the Edge of the Universe, ii) The Next Generation Global Gravitational Wave Observatory: The Science Book, iii) 3G R&D: R&D for the Next Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave Data Analysis: Computing Challenges in the 3G Era (this report), v) Future Ground-based Gravitational-wave Observatories: Synergies with Other Scientific Communities, and vi) An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network. ---------------------------------------------------------------- arXiv:2111.06988 gr-qc astro-ph.HE astro-ph.IM *Future Ground-Based Gravitational-Wave Observatories: Synergies with Other Scientific Communities * *Authors*: Michele Punturo, David Reitze, Peter Couvares, Stavros Katsanevas, Takaaki Kajita, Vicky Kalogera, Harald Lueck, David McClelland, Sheila Rowan, Gary Sanders, B. S. Sathyaprakash, David Shoemaker, Jo van den Brand Abstract: Planning for the development of a 3rd generation global gravitational-wave detector array is a multifaceted and complex effort that will necessarily need a high level of community input. Interfacing to extant and new stakeholders in the broader scientific constituencies is necessary to keep them aware of the activities taking place in the ground-based gravitational-wave community and receive input to inform and evolve the planning. In this report, we present the approaches GWIC and gravitational-wave collaborations and projects should consider taking to engage with broader community. This report is the fifth in a six part series of reports by the GWIC 3G Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the Edge of the Universe, ii) The Next Generation Global Gravitational Wave Observatory: The Science Book, iii) 3G R&D: R&D for the Next Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based Gravitational-wave Observatories: Synergies with Other Scientific Communities (this report), and vi) An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network. ---------------------------------------------------------------- arXiv:2111.06989 gr-qc astro-ph.HE astro-ph.IM *An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network * *Authors*: Stavros Katsanevas, Gary Sanders, Beverly Berger, Gabriela González, James Hough, Ajit K. Kembhavi, David McClelland, Masatake Ohashi, Fulvio Ricci, Stan Whitcomb, Michele Punturo, David Reitze, Peter Couvares, Takaaki Kajita, Vicky Kalogera, Harald Lueck, David McClelland, Sheila Rowan, B. S. Sathyaprakash, David Shoemaker, Jo van den Brand Abstract: The construction of a global network of detectors is the cornerstone to scientific success for 3rd generation gravitational wave astronomy. If carried out with a vision to the future, the third generation implementation, in its infrastructure, technology base and governance can provide the point of departure for subsequent developments beyond the third generation, a path to the long and revolutionary future of gravitational wave astrophysics. The goal of this document is to lay out considerations that influence the optimal choice of governance and to lay out a possible path that can lead the community to an optimal governance model. This report is the six in a six part series of reports by the GWIC 3G Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the Edge of the Universe, ii) The Next Generation Global Gravitational Wave Observatory: The Science Book, iii) 3G R&D: R&D for the Next Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based Gravitational-wave Observatories: Synergies with Other Scientific Communities, and vi) An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network (this report). ---------------------------------------------------------------- arXiv:2111.06990 gr-qc *The Next Generation Global Gravitational Wave Observatory: The Science Book * *Authors*: Vicky Kalogera, B. S. Sathyaprakash, Matthew Bailes, Marie-Anne Bizouard, Alessandra Buonanno, Adam Burrows, Monica Colpi, Matt Evans, Stephen Fairhurst, Stefan Hild, Mansi M. Kasliwal, Luis Lehner, Ilya Mandel, Vuk Mandic, Samaya Nissanke, Maria Alessandra Papa, Sanjay Reddy, Stephan Rosswog, Chris Van Den Broeck, P. Ajith, Shreya Anand, Igor Andreoni, K. G. Arun, Enrico Barausse, Masha Baryakhtar , et al. (66 additional authors not shown) Abstract: The next generation of ground-based gravitational-wave detectors will observe coalescences of black holes and neutron stars throughout the cosmos, thousands of them with exceptional fidelity. The Science Book is the result of a 3-year effort to study the science capabilities of networks of next generation detectors. Such networks would make it possible to address unsolved problems in numerous areas of physics and astronomy, from Cosmology to Beyond the Standard Model of particle physics, and how they could provide insights into workings of strongly gravitating systems, astrophysics of compact objects and the nature of dense matter. It is inevitable that observatories of such depth and finesse will make new discoveries inaccessible to other windows of observation. In addition to laying out the rich science potential of the next generation of detectors, this report provides specific science targets in five different areas in physics and astronomy and the sensitivity requirements to accomplish those science goals. This report is the second in a six part series of reports by the GWIC 3G Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the Edge of the Universe, ii) The Next Generation Global Gravitational Wave Observatory: The Science Book (this report), iii) 3G R&D: R&D for the Next Generation of Ground-based Gravitational Wave Detectors, iv) Gravitational Wave Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based Gravitational-wave Observatories: Synergies with Other Scientific Communities, and vi) An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network. ---------------------------------------------------------------- arXiv:2111.06991 gr-qc *3G R&D: R&D for the Next Generation of Ground-based Gravitational-wave Detectors * *Authors*: David McClelland, Harald Lueck, Rana Adhikari, Masaki Ando, GariLynn Billingsley, Geppo Cagnoli, Matt Evans, Martin Fejer, Andreas Freise, Paul Fulda, Eric Genin, Gabriela González, Jan Harms, Stefan Hild, Giovanni Losurdo, Ian Martin, Anil Prabhakar, Stuart Reid, Fulvio Ricci, Norna Robertson, Jo van den Brand, Benno Willke, Michael Zucker, Alessandro Bertolini, Stefan Danilishin , et al. (21 additional authors not shown) Abstract: To deliver on the promise of next generation gravitational-wave observatories, a sustained and coordinated detector research and development program is required. This report examines in detail the wide range of nearer- and longer-term detector R&D programs needed for next generation GW detectors commensurate with the key science targets presented in "The Next Generation Global Gravitational Wave Observatory: The Science Book", including considerations of site selection and large-scale vacuum infrastructure. The report makes a series of detailed recommendations on the needed advances in detector technology and the timescales needed to achieve those advances. It also identifies areas where larger-scale globally coordinated R&D efforts will be critical to ensuring success while minimizing costs. This report is the third in a six part series of reports by the GWIC 3G Subcommittee: i) Expanding the Reach of Gravitational Wave Observatories to the Edge of the Universe, ii) The Next Generation Global Gravitational Wave Observatory: The Science Book, iii) 3G R&D: R&D for the Next Generation of Ground-based Gravitational Wave Detectors (this report), iv) Gravitational Wave Data Analysis: Computing Challenges in the 3G Era, v) Future Ground-based Gravitational-wave Observatories: Synergies with Other Scientific Communities, and vi) An Exploration of Possible Governance Models for the Future Global Gravitational-Wave Observatory Network. ---------------------------------------------------------------- arXiv:2003.08588 gr-qc astro-ph.HE astro-ph.IM *Search for gravitational waves from twelve young supernova remnants with a hidden Markov model in Advanced LIGO's second observing run * *Authors*: Margaret Millhouse, Lucy Strang, Andrew Melatos Abstract: Persistent gravitational waves from rapidly rotating neutron stars, such as those found in some young supernova remnants, may fall in the sensitivity band of the advanced Laser Interferometer Gravitational-wave Observatory (aLIGO). Searches for these signals are computationally challenging, as the frequency and frequency derivative are unknown and evolve rapidly due to the youth of the source. A hidden Markov model (HMM), combined with a maximum-likelihood matched filter, tracks rapid frequency evolution semi-coherently in a computationally efficient manner. We present the results of an HMM search targeting 12 young supernova remnants in data from Advanced LIGO's second observing run. Six targets produce candidates that are above the search threshold and survive pre-defined data quality vetoes. However, follow-up analyses of these candidates show that they are all consistent with instrumental noise artefacts. Journal reference: Phys. Rev. D 102, 083025 (2020) ---------------------------------------------------------------- arXiv:2006.06907 astro-ph.HE astro-ph.IM gr-qc *Search for gravitational waves from five low mass X-ray binaries in the second Advanced LIGO observing run with an improved hidden Markov model * *Authors*: Hannah Middleton, Patrick Clearwater, Andrew Melatos, Liam Dunn Abstract: Low mass X-ray binaries are prime targets for continuous gravitational wave searches by ground-based interferometers. Results are presented from a search for five low-mass X-ray binaries whose spin frequencies and orbital elements are measured accurately from X-ray pulsations: HETE J1900.1-2455, IGR J00291+5934, SAX J1808.4-3658, XTE J0929-314, and XTE J1814-338. Data are analysed from Observing Run 2 of the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO). The search algorithm uses a hidden Markov model to track spin wandering, the J-statistic maximum likelihood matched filter to track orbital phase, and a suite of five vetoes to reject artefacts from non-Gaussian noise. The search yields a number of low-significance, above threshold candidates consistent with the selected false-alarm probability. The candidates will be followed up in subsequent observing runs. ---------------------------------------------------------------- arXiv:2102.06334 astro-ph.HE gr-qc *Search for continuous gravitational waves from ten H.E.S.S. sources using a hidden Markov model * *Authors*: Deeksha Beniwal, Patrick Clearwater, Liam Dunn, Andrew Melatos, David Ottaway Abstract: Isolated neutron stars are prime targets for continuous-wave (CW) searches by ground-based gravitationalwave interferometers. Results are presented from a CW search targeting ten pulsars. The search uses a semicoherent algorithm, which combines the maximum-likelihood F-statistic with a hidden Markov model (HMM) to efficiently detect and track quasimonochromatic signals which wander randomly in frequency. The targets, which are associated with TeV sources detected by the High Energy Stereoscopic System (H.E.S.S.), are chosen to test for gravitational radiation from young, energetic pulsars with strong -ray emission, and take maximum advantage of the frequency tracking capabilities of HMM compared to other CW search algorithms. The search uses data from the second observing run of the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO). It scans 1Hz sub-bands around f_ , 4f_ /3, and 2f_ , where f_ denotes the star's rotation frequency, in order to accommodate a physically plausible frequency mismatch between the electromagnetic and gravitational-wave emission. The 24 sub-bands searched in this study return 5,256 candidates above the Gaussian threshold with a false alarm probability of 1% per sub-band per target. Only 12 candidates survive the three data quality vetoes which are applied to separate nonGaussian artifacts from true astrophysical signals. CW searches using the data from subsequent observing runs will clarify the status of the remaining candidates. Journal reference: Phys.Rev.D 103 (2021) 083009 ---------------------------------------------------------------- arXiv:2102.10816 gr-qc *Enhancing gravitational-wave burst detection confidence in expanded detector networks with the BayesWave pipeline * *Authors*: Yi Shuen C. Lee, Margaret Millhouse, Andrew Melatos Abstract: The global gravitational-wave detector network achieves higher detection rates, better parameter estimates, and more accurate sky localisation, as the number of detectors, I increases. This paper quantifies network performance as a function of I for BayesWave, a source-agnostic, wavelet-based, Bayesian algorithm which distinguishes between true astrophysical signals and instrumental glitches. Detection confidence is quantified using the signal-to-glitch Bayes factor, B_S,G . An analytic scaling is derived for B_S,G versus I, the number of wavelets, and the network signal-to-noise ratio, SNR_net , which is confirmed empirically via injections into detector noise of the Hanford-Livingston (HL), Hanford-Livingston-Virgo (HLV), and Hanford-Livingston-KAGRA-Virgo (HLKV) networks at projected sensitivities for the fourth observing run (O4). The empirical and analytic scalings are consistent; B_S,G increases with I. The accuracy of waveform reconstruction is quantified using the overlap between injected and recovered waveform, O_net . The HLV and HLKV network recovers 87% and 86% of the injected waveforms with O_net >0.8 respectively, compared to 81% with the HL network. The accuracy of BayesWave sky localisation is 10 times better for the HLV network than the HL network, as measured by the search area, A, and the sky areas contained within 50% and 90% confidence intervals. Marginal improvement in sky localisation is also observed with the addition of KAGRA. ---------------------------------------------------------------- arXiv:2103.12976 gr-qc *Deep exploration for continuous gravitational waves at 171--172 Hz in LIGO second observing run data * *Authors*: Karl Wette, Liam Dunn, Patrick Clearwater, Andrew Melatos Abstract: We pursue a novel strategy towards a first detection of continuous gravitational waves from rapidly-rotating deformed neutron stars. Computational power is focused on a narrow region of signal parameter space selected by a strategically-chosen benchmark. We search data from the 2nd observing run of the LIGO Observatory with an optimised analysis run on graphics processing units. While no continuous waves are detected, the search achieves a sensitivity to gravitational wave strain of h_0 =1.01×10^25 at 90% confidence, 24% to 69% better than past searches of the same parameter space. Constraints on neutron star deformity are within theoretical maxima, thus a detection by this search was not inconceivable. Journal reference: Physical Review D 103, 083020 (2021) ---------------------------------------------------------------- arXiv:2107.12822 gr-qc *Hidden Markov model tracking of continuous gravitational waves from a neutron star with wandering spin. III. Rotational phase tracking * *Authors*: A. Melatos, P. Clearwater, S. Suvorova, L. Sun, W. Moran, R. J. Evans Abstract: A hidden Markov model (HMM) solved recursively by the Viterbi algorithm can be configured to search for persistent, quasimonochromatic gravitational radiation from an isolated or accreting neutron star, whose rotational frequency is unknown and wanders stochastically. Here an existing HMM analysis pipeline is generalized to track rotational phase and frequency simultaneously, by modeling the intra-step rotational evolution according to a phase-wrapped Ornstein-Uhlenbeck process, and by calculating the emission probability using a phase-sensitive version of the Bayesian matched filter known as the B-statistic. The generalized algorithm tracks signals from isolated and binary sources with characteristic wave strain h_0 1.3×10^26 in Gaussian noise with amplitude spectral density 4×10^24 Hz^1/2 , for a simulated observation composed of N_T =37 data segments, each T_drift =10days long, the typical duration of a search for the low-mass X-ray binary (LMXB) Sco X1 with the Laser Interferometer Gravitational Wave Observatory (LIGO). It is equally sensitive to isolated and binary sources and 1.5 times more sensitive than the previous pipeline. Receiver operating characteristic curves and errors in the recovered parameters are presented for a range of practical h_0 and N_T values. The generalized algorithm successfully detects every available synthetic signal in Stage I of the Sco X1 Mock Data Challenge convened by the LIGO Scientific Collaboration, recovering the frequency and orbital semimajor axis with accuracies of better than 9.5×10^7 Hz and 1.6×10^3 lts respectively. The Viterbi solver runs in 2×10^3 CPU-hr for an isolated source and 10^5 CPU-hr for a LMXB source in a typical, broadband (0.5-kHz) search. ---------------------------------------------------------------- arXiv:2108.11338 gr-qc *Scalar perturbations of Galileon cosmologies in the mechanical approach within the late Universe * *Authors*: Jan Novák Abstract: We investigate the Universe at the late stage of its evolution and inside the cell of uniformity 150-370 Mpc. We consider the Universe to be filled at these scales with dust like matter, a minimally coupled Galileon field and radiation. We use the mechanical approach. Therefore, the peculiar velocities of the inhomogeneities as well as the fluctuations of the other perfect fluids can be considered nonrelativistic. Such fluids are said to be coupled because they are concentrated around the inhomogeneities. We investigate the conditions under which the physical Galileon field, i.e. compatible with results of the latest gravitational wave experiments GW150914, GW151226, GW170104, GW170814, GW170817 and GW170608, can become coupled. We know that at the background level coupled scalar fields behave as a two-component perfect fluids: one which mimics a network of frustrated cosmic string and an another one which corresponds to an effective cosmological constant. We found a correction for the Galileon field, which behaves like a matter component. Journal reference: Gravitation and Cosmology, 2020, Vol.26, No.4, pp. 351-357 ---------------------------------------------------------------- arXiv:2002.02119 gr-qc math.AP *Stabilizing relativistic fluids on spacetimes with non-accelerated expansion * *Authors*: David Fajman, Todd A. Oliynyk, Zoe Wyatt Abstract: We establish global regularity and stability for the irrotational relativistic Euler equations with equation of state p¯=K¯, where 02 are non-linearly stable, and all sufficiently small perturbations exhibit asymptotically velocity term dominated (AVTD) behavior and blow-up of the Kretschmann scalar. ---------------------------------------------------------------- arXiv:2108.02886 gr-qc math.AP *Stability of Asymptotic Behavior Within Polarised T^2 -Symmetric Vacuum Solutions with Cosmological Constant * *Authors*: Ellery Ames, Florian Beyer, James Isenberg, Todd Oliynyk Abstract: We prove the nonlinear stability of the asymptotic behavior of perturbations of subfamilies of Kasner solutions in the contracting time direction within the class of polarised T^2 -symmetric solutions of the vacuum Einstein equations with arbitrary cosmological constant . This stability result generalizes the results proven in [3], which focus on the =0 case, and as in that article, the proof relies on an areal time foliation and Fuchsian techniques. Even for =0, the results established here apply to a wider class of perturbations of Kasner solutions within the family of polarised T^2 -symmetric vacuum solutions than those considered in [3] and [26]. Our results establish that the areal time coordinate takes all values in (0,T_0 ] for some T_0 >0, for certain families of polarised T^2 -symmetric solutions with cosmological constant. ---------------------------------------------------------------- arXiv:1912.02173 gr-qc astro-ph.CO *Black Hole Genesis and origin of cosmic acceleration * *Authors*: Nikodem J. Popawski Abstract: We consider a hypothesis that the closed Universe was formed on the other side of the event horizon of a black hole existing in another universe. That black hole appears in the Universe as a boundary white hole, and its rest frame in comoving coordinates is a frame of reference in which the cosmic microwave background is isotropic. We consider the Lagrangian density for the gravitational field that is proportional to the curvature scalar, and use the metric-affine variational principle in which the symmetric affine connection and the metric tensor are variables. The white hole appears in the Lagrangian through a simplest, generally covariant and linear term: the four-divergence of the four-velocity of the white hole in comoving coordinates. We show that the variation of the action with respect to the connection generates the nonmetricity, which creates a term in the Lagrangian that is equivalent to a positive cosmological constant. The current cosmic acceleration may therefore be a manifestation of the boundary of the closed Universe. We also show that the equation of motion of a test particle deviates from the geodesic equation by a term that depends on the four-velocities of the particle and the white hole. The rest frame of the white hole in comoving coordinates is the only absolutely inertial frame of reference. This deviation might be observed on galactic scales. ---------------------------------------------------------------- arXiv:2007.11556 gr-qc astro-ph.CO *The universe as a closed anisotropic universe born in a black hole * *Authors*: Nikodem Popawski Abstract: We consider a universe formed in a black hole in the Einstein--Cartan theory of gravity. The interior of a Schwarzschild black hole can be represented by the Kantowski--Sachs metric that describes a closed anisotropic universe. We use this metric to derive the field equations with a relativistic spin fluid as a source. We show that torsion may prevent a singularity and replace it with a nonsingular bounce if particle production dominates over shear. Particle production after the last bounce can generate a finite period of inflation, during which the universe expands and isotropizes to the currently observed state. Our universe might have therefore originated from a black hole. Journal reference: Gen. Relativ. Gravit. 53, 18 (2021) ---------------------------------------------------------------- arXiv:2008.02136 gr-qc astro-ph.CO *Gravitational collapse of a fluid with torsion into a universe in a black hole * *Authors*: Nikodem Popawski Abstract: We consider gravitational collapse of a spherically symmetric sphere of a fluid with spin and torsion into a black hole. We use the Tolman metric and the Einstein--Cartan field equations with a relativistic spin fluid as a source. We show that gravitational repulsion of torsion prevents a singularity and replaces it with a nonsingular bounce. Quantum particle production during contraction helps torsion to dominate over shear. Particle production during expansion can generate a finite period of inflation and produce enormous amounts of matter. The resulting closed universe on the other side of the event horizon may have several bounces. Such a universe is oscillatory, with each cycle larger in size then the previous cycle, until it reaches the cosmological size and expands indefinitely. Our universe might have therefore originated from a black hole. Journal reference: Zh. Eksp. Teor. Fiz. 159, 448 (2021) ---------------------------------------------------------------- arXiv:2101.04212 gr-qc quant-ph *Relativistic wave--particle duality for spinors * *Authors*: Nikodem Popawski Abstract: We propose that relativistic wave--particle duality can be embodied in a relation u^i =¯^i /¯, which determines the mean four-velocity of the fermion particle associated with a Dirac wave function. We use the Einstein--Cartan theory of gravity with torsion, which incorporates the spin-orbit interaction in curved spacetime. This relation is satisfied by a spinor plane wave and it is consistent with the energy-momentum tensor for particles. We suggest that spacetime guides the evolution of a wave function, that in turn guides the mean motion of the associated particle. Consequently, spacetime guides the motion of particles. The exact motion is limited by the uncertainty principle. ---------------------------------------------------------------- arXiv:2107.01612 hep-th gr-qc *Torsional Regularization of Self-Energy and Bare Mass of Electron * *Authors*: Michael Del Grosso, Nikodem Popawski Abstract: In the presence of spacetime torsion, the momentum components do not commute; therefore, in quantum field theory, summation over the momentum eigenvalues will replace integration over the momentum. In the Einstein--Cartan theory of gravity, in which torsion is coupled to spin, the separation between the eigenvalues increases with the magnitude of the momentum. Consequently, this replacement regularizes divergent integrals in Feynman diagrams with loops by turning them into convergent sums. In this article, we apply torsional regularization to the self-energy of a charged lepton in quantum electrodynamics. We show that this procedure eliminates the ultraviolet divergence. We also show that torsion gives a photon a small nonzero mass, which regularizes the infrared divergence. In the end, we calculate the finite bare masses of the electron, muon, and tau lepton: 0.4329MeV, 90.95MeV, and 1543MeV, respectively. These values constitute about 85% of the observed, re-normalized masses. ---------------------------------------------------------------- arXiv:2102.07353 astro-ph.IM *SkyMapper Optical Follow-up of Gravitational Wave Triggers: Alert Science Data Pipeline and LIGO/Virgo O3 Run * *Authors*: Seo-Won Chang, Christopher A. Onken, Christian Wolf, Lance Luvaul, Anais Möller, Richard Scalzo, Brian P. Schmidt, Susan M. Scott, Nikunj Sura, Fang Yuan Abstract: We present an overview of the SkyMapper optical follow-up program for gravitational-wave event triggers from the LIGO/Virgo observatories, which aims at identifying early GW170817-like kilonovae out to 200 Mpc distance. We describe our robotic facility for rapid transient follow-up, which can target most of the sky at <+10° to a depth of iAB20 mag. We have implemented a new software pipeline to receive LIGO/Virgo alerts, schedule observations and examine the incoming real-time data stream for transient candidates. We adopt a real-bogus classifier using ensemble-based machine learning techniques, attaining high completeness (98%) and purity (91%) over our whole magnitude range. Applying further filtering to remove common image artefacts and known sources of transients, such as asteroids and variable stars, reduces the number of candidates by a factor of more than 10. We demonstrate the system performance with data obtained for GW190425, a binary neutron star merger detected during the LIGO/Virgo O3 observing campaign. In time for the LIGO/Virgo O4 run, we will have deeper reference images allowing transient detection to iAB21 mag. ---------------------------------------------------------------- arXiv:2103.15984 gr-qc *The Endpoint Theorem * *Authors*: Susan M Scott, Ben E Whale Abstract: The Endpoint Theorem links the existence of a sequence (curve), without accumulation points, in a manifold to the existence of an open embedding of that manifold so that the image of the given sequence (curve) has a unique endpoint. It plays a fundamental role in the theory of the Abstract Boundary as it implies that there is always an Abstract Boundary boundary point to represent the endpoint of such sequences and curves. The Endpoint Theorem will be of interest to researchers analysing specific spacetimes as it shows how to construct a chart in the original manifold which contains the sequence (curve). In particular, it has application to the study of singularities predicted by the singularity theorems. Journal reference: Class. Quantum Grav. 38 065012 (2021) ---------------------------------------------------------------- arXiv:2110.10870 gr-qc *Enhanced Frequency noise suppression for LISA by combining cavity and arm locking control systems * *Authors*: Jobin Thomas Valliyakalayil, Andrew J. H. Sutton, Robert E. Spero, Daniel A. Shaddock, Kirk McKenzie Abstract: This paper presents a novel method for laser frequency stabilisation in the Laser Interferometer Space Antenna (LISA) mission by locking a laser to two stable length references - the arms of the interferometer and an on-board optical cavity. The two references are digitally fused using carefully designed control systems, ensuring no changes to the baseline LISA mission hardware are required. The interferometer arm(s) provides the most stable reference available in the LISA science band (0.1 mHz - 1 Hz), while the cavity sensor's wide-band and linear readout enables additional control system gain below and above the LISA band. The main technical issue with this dual sensor approach is the undesirable slow laser frequency pulling which couples into the control system with the imperfect knowledge of the Doppler shift of the light due to relative spacecraft motion along the LISA arm. This paper outlines requirements on the Doppler shift knowledge to maintain the cavity well within the resonance when activating the fused control system. Two Doppler shift estimation methods are presented that use the already on-board measurements, the inter-spacecraft interferometer link (the main science measurement), and the absolute inter-spacecraft laser ranging system. Both methods reach the required precision after a few thousand seconds of measurement integration. The paper demonstrates an approach to initialise and engage the proposed laser stabilization system, starting from free-running laser and ending with the dual sensor frequency control system. The results show that the technique lowers the residual laser frequency noise in the LISA science band by over 3 orders of magnitude, potentially allowing the requirements on Time-Delay-Interferometry (TDI) to be relaxed - possibly to the point where first-generation TDI may be sufficient. ---------------------------------------------------------------- arXiv:2001.10889 gr-qc *Observation of a potential future sensitivity limitation from ground motion at LIGO Hanford * *Authors*: Jan Harms, Edgard Bonilla, Michael Coughlin, Jenne Driggers, Sheila Dwyer, David McManus, Michael Ross, Bram Slagmolen, Krishna Venkateswara Abstract: A first detection of terrestrial gravity noise in gravitational-wave detectors is a formidable challenge. With the help of environmental sensors, it can in principle be achieved before the noise becomes dominant by estimating correlations between environmental sensors and the detector. The main complication is to disentangle different coupling mechanisms between the environment and the detector. In this paper, we analyze the relations between physical couplings and correlations that involve ground motion and LIGO strain data h(t) recorded during its second science run in 2016 and 2017. We find that all noise correlated with ground motion was more than an order of magnitude lower than dominant low-frequency instrument noise, and the dominant coupling over part of the spectrum between ground and h(t) was residual coupling through the seismic-isolation system. We also present the most accurate gravitational coupling model so far based on a detailed analysis of data from a seismic array. Despite our best efforts, we were not able to unambiguously identify gravitational coupling in the data, but our improved models confirm previous predictions that gravitational coupling might already dominate linear ground-to-h(t) coupling over parts of the low-frequency, gravitational-wave observation band. Journal reference: Phys. Rev. D 101, 102002 (2020) ---------------------------------------------------------------- arXiv:2012.03608 gr-qc astro-ph.IM *Gravitational-wave physics with Cosmic Explorer: limits to low-frequency sensitivity * *Authors*: Evan D. Hall, Kevin Kuns, Joshua R. Smith, Yuntao Bai, Christopher Wipf, Sebastien Biscans, Rana X Adhikari, Koji Arai, Stefan Ballmer, Lisa Barsotti, Yanbei Chen, Matthew Evans, Peter Fritschel, Jan Harms, Brittany Kamai, Jameson Graef Rollins, David Shoemaker, Bram Slagmolen, Rainer Weiss, Hiro Yamamoto Abstract: Cosmic Explorer (CE) is a next-generation ground-based gravitational-wave observatory concept, envisioned to begin operation in the 2030s, and expected to be capable of observing binary neutron star and black hole mergers back to the time of the first stars. Cosmic Explorer's sensitive band will extend below 10 Hz, where the design is predominantly limited by geophysical, thermal, and quantum noises. In this work, thermal, seismic, gravity-gradient, quantum, residual gas, scattered-light, and servo-control noises are analyzed in order to motivate facility and vacuum system design requirements, potential test mass suspensions, Newtonian noise reduction strategies, improved inertial sensors, and cryogenic control requirements. Our analysis shows that with improved technologies, Cosmic Explorer can deliver a strain sensitivity better than 10^23 /Hz1/2 down to 5 Hz. Our work refines and extends previous analysis of the Cosmic Explorer concept and outlines the key research areas needed to make this observatory a reality. Journal reference: Phys. Rev. D 103, 122004 (2021) ---------------------------------------------------------------- arXiv:2109.09882 astro-ph.IM astro-ph.HE gr-qc *A Horizon Study for Cosmic Explorer: Science, Observatories, and Community * *Authors*: Matthew Evans, Rana X Adhikari, Chaitanya Afle, Stefan W. Ballmer, Sylvia Biscoveanu, Ssohrab Borhanian, Duncan A. Brown, Yanbei Chen, Robert Eisenstein, Alexandra Gruson, Anuradha Gupta, Evan D. Hall, Rachael Huxford, Brittany Kamai, Rahul Kashyap, Jeff S. Kissel, Kevin Kuns, Philippe Landry, Amber Lenon, Geoffrey Lovelace, Lee McCuller, Ken K. Y. Ng, Alexander H. Nitz, Jocelyn Read, B. S. Sathyaprakash , et al. (7 additional authors not shown) Abstract: This Horizon Study describes a next-generation ground-based gravitational-wave observatory: Cosmic Explorer. With ten times the sensitivity of Advanced LIGO, Cosmic Explorer will push gravitational-wave astronomy towards the edge of the observable universe (z100). The goals of this Horizon Study are to describe and evaluate design concepts for Cosmic Explorer; to plan for the United States' leadership in gravitational-wave astronomy; and to envisage the role of Cosmic Explorer in the international effort to build a "Third-Generation" (3G) observatory network that will make discoveries transformative across astronomy, physics, and cosmology. ---------------------------------------------------------------- arXiv:2007.04022 gr-qc *Spherical orbits around a Kerr black hole * *Authors*: Edward Teo Abstract: A special class of orbits known to exist around a Kerr black hole are spherical orbits -- orbits with constant coordinate radii that are not necessarily confined to the equatorial plane. Spherical time-like orbits were first studied by Wilkins almost 50 years ago. In the present paper, we perform a systematic and thorough study of these orbits, encompassing and extending previous works on them. We first present simplified forms for the parameters of these orbits. The parameter space of these orbits is then analysed in detail; in particular, we delineate the boundaries between stable and unstable orbits, bound and unbound orbits, and prograde and retrograde orbits. Finally, we provide analytic solutions of the geodesic equations, and illustrate a few representative examples of these orbits. Journal reference: Gen. Rel. Grav. 53 (2021) 10 ---------------------------------------------------------------- arXiv:2004.06984 physics.ins-det astro-ph.IM gr-qc hep-ex *Broadband Electrical Action Sensing Techniques with conducting wires for low-mass dark matter axion detection * *Authors*: Michael Edmund Tobar, Ben T. McAllister, Maxim Goryachev Abstract: Due to the inverse Primakoff effect it has been shown that when axions interact with a DC magnetic B-field the resulting electrical action will produce an AC electromotive force which oscillates at the Compton frequency of the axion, and may be modeled as an oscillating effective impressed magnetic current boundary source. We use this result to calculate the sensitivity of new experiments to low-mass axions using the quasi-static technique. First, we calculate the current induced in an electric dipole antenna (straight conducting wire) when the DC B-field is spatially constant and show that it has a sensitivity proportional to the axion mass. Following this we extend the topology by making use of the full extent of the spatially varying DC B-field. This extension is achieved by transforming the 1D conducting wire to a 2D winding, to fully link the effective magnetic current boundary source and thus couple to the full axion induced electrical action. In this case the conductor becomes a coil winding where the voltage induced across the winding increases proportionally to the number of windings. We investigate two different topologies: The 1st uses a single winding, and couples to the effective short circuit current generated in the winding, which is read out using a sensitive low impedance SQUID amplifier: The 2nd uses multiple windings, with every turn effectively increasing the the voltage output proportional to the winding number. The read out of this configuration is optimised by implementing a cryogenic low-noise high input impedance voltage amplifier. The end result is a new Broadband Electrical Action Sensing Techniques with orders of magnitude improved sensitivity, which is linearly proportional to the axion photon coupling and capable of detecting QCD dark matter axions. Journal reference: Physics of the Dark Universe 30 (2020) 100624 ---------------------------------------------------------------- arXiv:2011.12414 gr-qc astro-ph.CO astro-ph.IM hep-ex hep-ph *Challenges and Opportunities of Gravitational Wave Searches at MHz to GHz Frequencies * *Authors*: N. Aggarwal, O. D. Aguiar, A. Bauswein, G. Cella, S. Clesse, A. M. Cruise, V. Domcke, D. G. Figueroa, A. Geraci, M. Goryachev, H. Grote, M. Hindmarsh, F. Muia, N. Mukund, D. Ottaway, M. Peloso, F. Quevedo, A. Ricciardone, J. Steinlechner, S. Steinlechner, S. Sun, M. E. Tobar, F. Torrenti, C. Unal, G. White Abstract: The first direct measurement of gravitational waves by the LIGO and Virgo collaborations has opened up new avenues to explore our Universe. This white paper outlines the challenges and gains expected in gravitational wave searches at frequencies above the LIGO/Virgo band, with a particular focus on Ultra High-Frequency Gravitational Waves (UHF-GWs), covering the MHz to GHz range. The absence of known astrophysical sources in this frequency range provides a unique opportunity to discover physics beyond the Standard Model operating both in the early and late Universe, and we highlight some of the most promising gravitational sources. We review several detector concepts which have been proposed to take up this challenge, and compare their expected sensitivity with the signal strength predicted in various models. This report is the summary of the workshop "Challenges and opportunities of high-frequency gravitational wave detection" held at ICTP Trieste, Italy in October 2019, that set up the stage for the recently launched Ultra-High-Frequency Gravitational Wave (UHF-GW) initiative. Journal reference: Living Reviews in Relativity volume 24, Article number: 4 (2021) ---------------------------------------------------------------- arXiv:2102.05859 gr-qc astro-ph.HE hep-ex physics.ins-det *Rare Events Detected with a Bulk Acoustic Wave High Frequency Gravitational Wave Antenna * *Authors*: Maxim Goryachev, William M. Campbell, Ik Siong Heng, Serge Galliou, Eugene N. Ivanov, Michael E. Tobar Abstract: This work describes the operation of a High Frequency Gravitational Wave detector based on a cryogenic Bulk Acoustic Wave (BAW) cavity and reports observation of rare events during 153 days of operation over two seperate experimental runs (Run 1 and Run 2). In both Run 1 and Run 2 two modes were simultaneously monitored. Across both runs, the 3rd overtone of the fast shear mode (3B) operating at 5.506 MHz was monitored, while in Run 1 the second mode was chosen to be the 5th OT of the slow shear mode (5C) operating at 8.392 MHz. However, in Run 2 the second mode was selected to be closer in frequency to the first mode, and chosen to be the 3rd overtone of the slow shear mode (3C) operating at 4.993 MHz. Two strong events were observed as transients responding to energy deposition within acoustic modes of the cavity. The first event occurred during Run 1 on the 12/05/2019 (UTC), and was observed in the 5.506 MHz mode, while the second mode at 8.392 MHz observed no event. During Run 2, a second event occurred on the 27/11/2019(UTC) and was observed by both modes. Timing of the events were checked against available environmental observations as well as data from other detectors. Various possibilities explaining the origins of the events are discussed. Journal reference: Phys. Rev. Lett. 127, 071102 (2021) ---------------------------------------------------------------- arXiv:2109.04056 hep-ph astro-ph.IM gr-qc hep-ex *Abraham and Minkowski Poynting vector controversy in axion modified electrodynamics * *Authors*: Michael E Tobar, Ben T McAllister, Maxim Goryachev Abstract: The most sensitive haloscopes that search for axion dark matter through the two photon electromagnetic anomaly, convert axions into photons through the mixing of axions with a large DC magnetic field. In this work we apply Poynting theorem to the resulting axion modified electrodynamics and identify two possible Poynting vectors, one similar to the Abraham Poynting vector and the other to the Minkowski Poynting vector in electrodynamics. The latter picks up the extra non-conservative terms while the former does not. To understand the source of energy conversion and power flow in the detection systems, we apply the two Poynting theorems to axion modified electrodynamics, for both the resonant cavity and broadband low-mass axion detectors. We show that both Poynting theorems give the same sensitivity for a resonant cavity axion haloscope, but predict markedly different sensitivity for a low-mass broadband capacitive haloscope. Hence we ask the question, can understanding which one is the correct one for axion dark matter detection, be considered under the framework of the Abraham-Minkowski controversy? In reality, this should be confirmed by experiment when the axion is detected. However, many electrodynamic experiments have ruled in favour of the Minkowski Poynting vector when considering the canonical momentum in dielectric media. In light of this, we show that the axion modified Minkowski Poynting vector should indeed be taken seriously for sensitivity calculation for low-mass axion haloscope detectors in the quasi static limit, and predict orders of magnitude better sensitivity than the Abraham Poynting vector equivalent. ---------------------------------------------------------------- arXiv:2009.06517 astro-ph.CO gr-qc hep-ph hep-th *Screening anisotropy via energy-momentum squared gravity: CDM model with hidden anisotropy * *Authors*: Ozgur Akarsu, John D. Barrow, N. Merve Uzun Abstract: We construct a generalization of the standard CDM model, wherein we simultaneously replace the spatially flat Robertson-Walker metric with its simplest anisotropic generalization (LRS Bianchi I metric), and couple the cold dark matter to the gravity in accordance with the energy-momentum squared gravity (EMSG) of the form f(T_ T^ )T_ T^ . These two modifications -- namely, two new stiff fluid-like terms of different nature -- can mutually cancel out, i.e., the shear scalar can be screened completely, and reproduce mathematically exactly the same Friedmann equation of the standard CDM model. This evades the BBN limits on the anisotropy, and thereby provides an opportunity to manipulate the cosmic microwave background quadrupole temperature fluctuation at the desired amount. We further discuss the consequences of the model on the very early times and far future of the Universe. This study presents also an example of that the EMSG of the form f(T_ T^ )T_ T^ , as well as similar type other constructions, is not necessarily relevant only to very early Universe but may even be considered in the context of a major problem of the current cosmology related to the present-day Universe, the so-called H_0 problem. Journal reference: Phys. Rev. D 102, 124059 (2020) ---------------------------------------------------------------- arXiv:2001.11964 gr-qc astro-ph.CO *Cosmographic analysis of redshift drift * *Authors*: Francisco S. N. Lobo, Jose Pedro Mimoso, Matt Visser Abstract: Redshift drift is the phenomenon whereby the observed redshift between an emitter and observer comoving with the Hubble flow in an expanding FLRW universe will slowly evolve --- on a timescale comparable to the Hubble time. There are nevertheless serious astrometric proposals for actually observing this effect. We shall however pursue a more abstract theoretical goal, and perform a general cosmographic analysis of this effect, eschewing (for now) dynamical considerations in favour of purely kinematic symmetry considerations based on FLRW spacetimes. We shall develop various exact results and series expansions for the redshift drift in terms of the present day Hubble, deceleration, jerk, snap, crackle, and pop parameters, as well as the present day redshift of the source. In particular, potential observation of this redshift drift effect is intimately related to the universe exhibiting a nonzero deceleration parameter. Journal reference: JCAP 04 (2020) 043 ---------------------------------------------------------------- arXiv:2002.10659 gr-qc hep-th *Relativistic combination of non-collinear 3-velocities using quaternions * *Authors*: Thomas Berry, Matt Visser Abstract: Quaternions have an (over a century-old) extensive and quite complicated interaction with special relativity. Since quaternions are intrinsically 4-dimensional, and do such a good job of handling 3-dimensional rotations, the hope has always been that the use of quaternions would simplify some of the algebra of the Lorentz transformations. Herein we report a relatively nice result for the relativistic combination of non-collinear 3-velocities. If we work with the relativistic half-velocities w defined by v=2w/(1+w^2 ), and promote them to quaternions using w=wn^, where n^ is a unit quaternion, then we shall show w_1 _ _2= w_1 w_2 =(1w_1 w_2 )^1 (w_1 +w_2 )=(w_1 +w_2 )(1w_2 w_1 )^1. All of the complicated angular dependence for relativistic combination of non-collinear 3-velocities is now encoded in the quaternion multiplication of w1 with w2. This result can furthermore be extended to obtain an elegant and compact formula for the associated Wigner angle: e^ =e^^ =(1w_1 w_2 )^1 (1w_2 w_1 ), in terms of which n^_1 _ _2 =e^/2 (w1+w_2 )/|w_1 +w_2 |;n^_2 _ _1 =e^/2 (w_1 +w_2 )/|w_1 +w_2 |. Thus, we would argue, many key results that are ultimately due to the non-commutativity of non-collinear boosts can be easily rephrased in terms of the algebra of quaternions. ---------------------------------------------------------------- arXiv:2003.09419 gr-qc *Dynamic thin-shell black-bounce traversable wormholes * *Authors*: Francisco S. N. Lobo, Alex Simpson, Matt Visser Abstract: Based on the recently introduced black-bounce spacetimes, we shall consider the construction of the related spherically symmetric thin-shell traversable wormholes within the context of standard general relativity. All of the really unusual physics is encoded in one simple parameter a which characterizes the scale of the bounce. Keeping the discussion as close as possible to standard general relativity is the theorist's version of only adjusting one feature of the model at a time. We shall modify the standard thin-shell traversable wormhole construction, each bulk region now being a black-bounce spacetime, and with the physics of the thin shell being (as much as possible) derivable from the Einstein equations. Furthermore, we shall apply a dynamical analysis to the throat by considering linearized radial perturbations around static solutions, and demonstrate that the stability of the wormhole is equivalent to choosing suitable properties for the exotic material residing on the wormhole throat. The construction is sufficiently novel to be interesting, and sufficiently straightforward to be tractable Journal reference: Phys. Rev. D 101, 124035 (2020) ---------------------------------------------------------------- arXiv:2005.08533 gr-qc *Causal hierarchy in modified gravity * *Authors*: Raul Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Matt Visser Abstract: We investigate the causal hierarchy in various modified theories of gravity. In general relativity the standard causal hierarchy, (key elements of which are chronology, causality, strong causality, stable causality, and global hyperbolicity), is well-established. In modified theories of gravity there is typically considerable extra structure, (such as: multiple metrics, aether fields, modified dispersion relations, Horava-like gravity, parabolic propagation, etcetera), requiring a reassessment and rephrasing of the usual causal hierarchy. We shall show that in this extended framework suitable causal hierarchies can indeed be established, and discuss the implications for the interplay between "superluminal" propagation and causality. The key distinguishing feature is whether the signal velocity is finite or infinite. Preserving even minimal notions of causality in the presence of infinite signal velocity requires the aether field to be both unique and hypersurface orthogonal, leading us to introduce the notion of global parabolicity. ---------------------------------------------------------------- arXiv:2006.14258 gr-qc *Painleve-Gullstrand form of the Lense-Thirring spacetime * *Authors*: Joshua Baines, Thomas Berry, Alex Simpson, Matt Visser Abstract: The standard Lense-Thirring metric is a century-old slow-rotation large-distance approximation to the gravitational field outside a rotating massive body, depending only on the total mass and angular momentum of the source. Although it is not an exact solution to the vacuum Einstein equations, asymptotically the Lense-Thirring metric approaches the Kerr metric at large distances. Herein we shall discuss a specific variant of the standard Lense-Thirring metric, carefully chosen for simplicity, clarity, and various forms of "improved" behaviour, (to be more carefully defined in the body of the article). In particular we shall construct an explicit unit-lapse Painleve-Gullstrand variant of the Lense-Thirring spacetime, that has flat spatial slices, a very simple and physically intuitive tetrad, and extremely simple curvature tensors. We shall verify that this variant of the Lense-Thirring spacetime is Petrov type I, (so it is not algebraically special), but nevertheless possesses some very straightforward timelike geodesics, (the "rain" geodesics). We shall also discuss on-axis and equatorial geodesics, ISCOs and circular photon orbits. Finally, we wrap up by discussing some astrophysically relevant estimates, and analyze what happens if we extrapolate down to small values of r. ---------------------------------------------------------------- arXiv:2008.03817 gr-qc *Unit-lapse versions of the Kerr spacetime * *Authors*: Joshua Baines, Thomas Berry, Alex Simpson, Matt Visser Abstract: The Kerr spacetime is perhaps the most astrophysically important of the currently known exact solutions to the Einstein field equations. Whenever spacetimes can be put in unit-lapse form it becomes possible to identify some very straightforward timelike geodesics, (the "rain" geodesics), making the physical interpretation of these spacetimes particularly clean and elegant. The most well-known of these unit-lapse formulations is the Painleve-Gullstrand form of the Schwarzschild spacetime, though there is also a Painleve-Gullstrand form of the Lense-Thirring (slow rotation) spacetime. More radically there are also two known unit-lapse forms of the Kerr spacetime -- the Doran and Natario metrics -- though these are not precisely in Painleve-Gullstrand form. Herein we shall seek to explicate the most general unit-lapse form of the Kerr spacetime. While at one level this is "merely" a choice of coordinates, it is a strategically and tactically useful choice of coordinates, thereby making the technically challenging but astrophysically crucial Kerr spacetime somewhat easier to deal with. ---------------------------------------------------------------- arXiv:2008.07046 gr-qc *Thin-shell traversable wormhole crafted from a regular black hole with asymptotically Minkowski core * *Authors*: Thomas Berry, Francisco S. N. Lobo, Alex Simpson, Matt Visser Abstract: Recently, a novel model for a regular black hole was advocated which possesses an asymptotically Minkowski core implemented via an exponential suppression (in the core region) of the Misner-Sharp quasi-local mass. Using this regular black hole as a template, we shall construct a spherically symmetric thin-shell traversable wormhole using the "cut-and-paste" technique, thereby constructing yet another black hole mimicker. The surface stress-energy at the wormhole throat is calculated, and the stability of the wormhole is analyzed. An important result is that, (as compared to their Schwarzschild thin-shell counterparts), increasing the exponential suppression of the Misner-Sharp quasi-local mass by increasing the suppression parameter "a", also considerably increases the stability regions for these thin-shell wormholes, and furthermore minimizes the amount of energy condition violating exotic matter required to keep the wormhole throat open. Journal reference: Phys. Rev. D 102, 064054 (2020) ---------------------------------------------------------------- arXiv:2008.13308 gr-qc *Photon spheres, ISCOs, and OSCOs: Astrophysical observables for regular black holes with asymptotically Minkowski cores * *Authors*: Thomas Berry, Alex Simpson, Matt Visser Abstract: Classical black holes contain a singularity at their core. This has prompted various researchers to propose a multitude of modified spacetimes that mimic the physically observable characteristics of classical black holes as best as possible, but that crucially do not contain singularities at their cores. Due to recent advances in near-horizon astronomy, the ability to observationally distinguish between a classical black hole and a potential black hole mimicker is becoming increasingly feasible. Herein, we calculate some physically observable quantities for a recently proposed regular black hole with an asymptotically Minkowski core -- the radius of the photon sphere and the extremal stable timelike circular orbit (ESCO). The manner in which the photon sphere and ESCO relate to the presence (or absence) of horizons is much more complex than for the Schwarzschild black hole. We find situations in which photon spheres can approach arbitrarily close to (near extremal) horizons, situations in which some photon spheres become stable, and situations in which the locations of both photon spheres and ESCOs become multi-valued, with both ISCOs (innermost stable circular orbits) and OSCOs (outermost stable circular orbits). This provides an extremely rich phenomenology of potential astrophysical interest. ---------------------------------------------------------------- arXiv:2009.01397 gr-qc *Darboux diagonalization of the spatial 3-metric in Kerr spacetime * *Authors*: Joshua Baines, Thomas Berry, Alex Simpson, Matt Visser Abstract: The astrophysical importance of the Kerr spacetime cannot be overstated. Of the currently known exact solutions to the Einstein field equations, the Kerr spacetime stands out in terms of its direct applicability to describing astronomical black hole candidates. In counterpoint, purely mathematically, there is an old classical result of differential geometry, due to Darboux, that all 3-manifolds can have their metrics recast into diagonal form. In the case of the Kerr spacetime the Boyer-Lindquist coordinates provide an explicit example of a diagonal spatial 3-metric. Unfortunately, as we demonstrate herein, Darboux diagonalization of the spatial 3-slices of the Kerr spacetime is incompatible with simultaneously putting the Kerr metric into unit-lapse form while retaining manifest axial symmetry. This no-go theorem is somewhat reminiscent of the no-go theorem to the effect that the spatial 3-slices of the Kerr spacetime cannot be chosen to be conformally flat. ---------------------------------------------------------------- arXiv:2009.12057 gr-qc *Novel black-bounce spacetimes: wormholes, regularity, energy conditions, and causal structure * *Authors*: Francisco S. N. Lobo, Manuel E. Rodrigues, Marcos V. de S. Silva, Alex Simpson, Matt Visser Abstract: We develop a number of novel "black-bounce" spacetimes. These are specific regular black holes where the "area radius" always remains non-zero, thereby leading to a "throat" that is either timelike (corresponding to a traversable wormhole), spacelike (corresponding to a "bounce" into a future universe), or null (corresponding to a "one-way wormhole"). We shall first perform a general analysis of the regularity conditions for such a spacetime, and then consider a number of specific examples. The examples are constructed using a mass function similar to that of Fan--Wang, and fall into several particular cases, such as the original Simpson--Visser model, a Bardeen-type model, and other generalizations thereof. We shall analyse the regularity, the energy conditions, and the causal structure of these models. The main results are several new geometries, more complex than before, with two or more horizons, with the possibility of an extremal case. We shall derive a general theorem regarding static space-time regularity, and another general theorem regarding (non)-satisfaction of the classical energy conditions. Journal reference: Phys. Rev. D 103, 084052 (2021) ---------------------------------------------------------------- arXiv:2101.05006 gr-qc *Inner horizon instability and the unstable cores of regular black holes * *Authors*: Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Costantino Pacilio, Matt Visser Abstract: Regular black holes with nonsingular cores have been considered in several approaches to quantum gravity, and as agnostic frameworks to address the singularity problem and Hawking's information paradox. While in a recent work we argued that the inner core is destabilized by linear perturbations, opposite claims were raised that regular black holes have in fact stable cores. To reconcile these arguments, we discuss a generalization of the geometrical framework, originally applied to Reissner--Nordtsröm black holes by Ori, and show that regular black holes have an exponentially growing Misner--Sharp mass at the inner horizon. This result can be taken as an indication that stable nonsingular black hole spacetimes are not the definitive endpoint of a quantum gravity regularization mechanism, and that nonperturbative backreaction effects must be taken into account in order to provide a consistent description of the quantum-gravitational endpoint of gravitational stellar collapse. ---------------------------------------------------------------- arXiv:2101.05971 gr-qc math-ph *Lorentz boosts and Wigner rotations: self-adjoint complexified quaternions * *Authors*: Thomas Berry, Matt Visser Abstract: Herein we shall consider Lorentz boosts and Wigner rotations from a (complexified) quaternionic point of view. We shall demonstrate that for a suitably defined self-adjoint complex quaternionic 4-velocity, pure Lorentz boosts can be phrased in terms of the quaternion square root of the relative 4-velocity connecting the two inertial frames. Straightforward computations then lead to quite explicit and relatively simple algebraic formulae for the composition of 4-velocities and the Wigner angle. We subsequently relate the Wigner rotation to the generic non-associativity of the composition of three 4-velocities, and develop a necessary and sufficient condition for associativity to hold. Finally, we relate the composition of 4-velocities to a specific implementation of the Baker-Campbell-Hausdorff theorem. As compared to ordinary 4x4 Lorentz transformations, the use of self-adjoint complexified quaternions leads, from a computational view, to storage savings and more rapid computations, and from a pedagogical view to to relatively simple and explicit formulae. ---------------------------------------------------------------- arXiv:2102.01831 gr-qc *Counterexamples to the maximum force conjecture * *Authors*: Aden Jowsey, Matt Visser Abstract: Dimensional analysis shows that the speed of light and Newton's constant of gravitation can be combined to define a quantity F_ =c^4 /G_N with the dimensions of force (equivalently, tension). Then in any physical situation we must have F_physical =fF_ , where the quantity f is some dimensionless function of dimensionless parameters. In many physical situations explicit calculation yields f=O(1), and quite often f1/4. This has lead multiple authors to suggest a (weak or strong) maximum force/maximum tension conjecture. Working within the framework of standard general relativity, we will instead focus on counter-examples to this conjecture, paying particular attention to the extent to which the counter-examples are physically reasonable. The various counter-examples we shall explore strongly suggest that one should not put too much credence into any universal maximum force/maximum tension conjecture. Specifically, fluid spheres on the verge of gravitational collapse will generically violate the weak (and strong) maximum force conjectures. If one wishes to retain any general notion of "maximum force" then one will have to very carefully specify precisely which forces are to be allowed within the domain of discourse. ---------------------------------------------------------------- arXiv:2102.02471 gr-qc hep-th *General class of "quantum deformed" regular black holes * *Authors*: Thomas Berry, Alex Simpson, Matt Visser Abstract: We discuss the "quantum deformed Schwarzschild spacetime" as originally introduced by Kazakov and Solodukhin in 1993, and investigate the precise sense in which it does and does not satisfy the desiderata for being a "regular black hole". We shall carefully distinguish (i) regularity of the metric components, (ii) regularity of the Christoffel components, and (iii) regularity of the curvature. We shall then embed the Kazakov-Solodukhin spacetime in a more general framework where these notions are clearly and cleanly separated. Finally we analyze aspects of the classical physics of these "quantum deformed Schwarzschild spacetimes". We shall discuss the surface gravity, the classical energy conditions, null and timelike geodesics, and the appropriate variant of Regge--Wheeler equation. ---------------------------------------------------------------- arXiv:2104.11376 gr-qc *Charged black-bounce spacetimes * *Authors*: Edgardo Franzin, Stefano Liberati, Jacopo Mazza, Alex Simpson, Matt Visser Abstract: Given the recent development of rotating black-bounce-Kerr spacetimes, for both theoretical and observational purposes it becomes interesting to see whether it might be possible to construct black-bounce variants of the entire Kerr-Newman family. Specifically, herein we shall consider black-bounce-Reissner-Nordström and black-bounce-Kerr-Newman spacetimes as particularly simple and clean everywhere-regular black hole "mimickers" that deviate from the Kerr-Newman family in a precisely controlled and minimal manner, and smoothly interpolate between regular black holes and traversable wormholes. While observationally the electric charges on astrophysical black holes are likely to be extremely low, |Q|/m1, introducing any non-zero electric charge has a significant theoretical impact. In particular, we verify the existence of a Killing tensor (and associated Carter-like constant) but without the full Killing tower of principal tensor and Killing-Yano tensor, also we discuss how, assuming general relativity, the black-bounce-Kerr-Newman solution requires an interesting, non-trivial matter/energy content. Journal reference: JCAP07(2021)036 ---------------------------------------------------------------- arXiv:2105.03079 gr-qc *Generic warp drives violate the null energy condition * *Authors*: Jessica Santiago, Sebastian Schuster, Matt Visser Abstract: Three very recent articles have claimed that it is possible to, at least in theory, either set up positive energy warp drives satisfying the weak energy condition (WEC), or at the very least, to minimize the WEC violations. These claims are at best incomplete, since the arguments presented only demonstrate the existence of one set of timelike observers, the co-moving Eulerian observers, who see "nice" physics. While these observers might see a positive energy density, the WEC requires all timelike observers to see positive energy density. Therefore, one should revisit this issue. A more careful analysis shows that the situation is actually much grimmer than advertised -- all physically reasonable warp drives will violate the null energy condition, and so also automatically violate the WEC, and both the strong and dominant energy conditions. While warp drives are certainly interesting examples of speculative physics, the violation of the energy conditions, at least within the framework of standard general relativity, is unavoidable. Even in modified gravity, physically reasonable warp drives will still violate the purely geometrical null convergence condition and the timelike convergence condition which, in turn, will place very strong constraints on any modified-gravity warp drive. ---------------------------------------------------------------- arXiv:2105.06650 gr-qc *Reconsidering maximum luminosity * *Authors*: Aden Jowsey, Matt Visser Abstract: The suggestion that there is a maximum luminosity (maximum power) in nature has a long and somewhat convoluted history. Though this idea is commonly attributed to Freeman Dyson, he was actually much more circumspect in his views. What is certainly true is that dimensional analysis shows that the speed of light and Newton's constant of gravitation can be combined to define a quantity P_* = c^5/G_N with the dimensions of luminosity (equivalently, power). Then in any physical situation we must have P_{physical} =P_*, where the quantity is some dimensionless function of dimensionless parameters. This has lead some authors to suggest a maximum luminosity/maximum power conjecture. Working within the framework of standard general relativity, we will re-assess this conjecture, paying particular attention to the extent to which various examples and counter-examples are physically reasonable. We focus specifically on Vaidya spacetimes, and on an evaporating version of Schwarzschild's constant density star. For both of these spacetimes luminosity can be arbitrarily large. We argue that any luminosity bound must depend on delicate internal features of the radiating object. ---------------------------------------------------------------- arXiv:2106.05002 gr-qc *Tractor beams, pressor beams, and stressor beams in general relativity * *Authors*: Jessica Santiago, Sebastian Schuster, Matt Visser Abstract: The metrics of general relativity generally fall into two categories: Those which are solutions of the Einstein equations for a given source energy-momentum tensor, and the "reverse engineered" metrics -- metrics bespoke for a certain purpose. Their energy-momentum tensors are then calculated by inserting these into the Einstein equations. This latter approach has found frequent use when confronted with creative input from fiction, wormholes and warp drives being the most famous examples. In this paper, we shall again take inspiration from fiction, and see what general relativity can tell us about the possibility of a gravitationally induced tractor beam. We will base our construction on warp drives and show how versatile this ansatz alone proves to be. Not only can we easily find tractor beams (attracting objects); repulsor/pressor beams are just as attainable, and a generalization to "stressor" beams is seen to present itself quite naturally. We show that all of these metrics would violate various energy conditions. This will provide an opportunity to ruminate on the meaning of energy conditions as such, and what we can learn about whether an arbitrarily advanced civilization might have access to such beams. ---------------------------------------------------------------- arXiv:2107.00343 math-ph gr-qc *Explicit Baker-Campbell-Hausdorff-Dynkin formula for Spacetime via Geometric Algebra * *Authors*: Joseph Wilson, Matt Visser Abstract: We present a compact Baker-Campbell-Hausdorff-Dynkin formula for the composition of Lorentz transformations ei in the spin representation (a.k.a. Lorentz rotors) in terms of their generators _i : ln(e^1 e^2 )=tanh^1 (tanh1+tanh_2 +(1/2)[tanh_1 ,tanh_2 ]1+(1/2){tanh_1 ,tanh_2 }) This formula is general to geometric algebras (a.k.a. real Clifford algebras) of dimension 4, naturally generalising Rodrigues' formula for rotations in R^3 . In particular, it applies to Lorentz rotors within the framework of Hestenes' spacetime algebra, and provides an efficient method for composing Lorentz generators. Computer implementations are possible with a complex 2×2 matrix representation realised by the Pauli spin matrices. The formula is applied to the composition of relativistic 3-velocities yielding simple expressions for the resulting boost and the concomitant Wigner angle. ---------------------------------------------------------------- arXiv:2110.01814 gr-qc *Killing tensor and Carter constant for Painleve-Gullstrand form of Lense-Thirring spacetime * *Authors*: Joshua Baines, Thomas Berry, Alex Simpson, Matt Visser Abstract: Recently, the authors have formulated and explored a novel Painleve-Gullstrand variant of the Lense-Thirring spacetime, which has some particularly elegant features -- including unit-lapse, intrinsically flat spatial 3-slices, and some particularly simple geodesics, the "rain" geodesics. At linear level in the rotation parameter this spacetime is indistinguishable from the usual slow-rotation expansion of Kerr. Herein, we shall show that this spacetime possesses a nontrivial Killing tensor, implying separability of the Hamilton-Jacobi equation. Furthermore, we shall show that the Klein-Gordon equation is also separable on this spacetime. However, while the Killing tensor has a 2-form square root, we shall see that this 2-form square root of the Killing tensor is not a Killing-Yano tensor. Finally, the Killing-tensor-induced Carter constant is easily extracted, and now, with a fourth constant of motion, the geodesics become (in principle) explicitly integrable. Journal reference: Universe 7 (2021) 473 ---------------------------------------------------------------- arXiv:2110.14926 gr-qc *Tractor beams, pressor beams, and stressor beams within the context of general relativity * *Authors*: Matt Visser, Jessica Santiago, Sebastian Schuster Abstract: Both traversable wormholes and warp drives, concepts originally developed within the context of science fiction, have now (for some 30 odd years) been studied, debated, and carefully analyzed within the framework of general relativity. An overarching theme of the general relativistic analysis is unavoidable violations of the classical point-wise energy conditions. Another science fiction trope, now over 80 years old, is the tractor beam and/or pressor beam. We shall discuss how to formulate both tractor beams and/or pressor beams, and a variant to be called a stressor beam, within the context of reverse engineering the spacetime metric. (While such reverse engineering is certainly well beyond our civilization's current capabilities, we shall be more interested in asking what an arbitrarily advanced civilization might be able to accomplish.) We shall see that tractor beams and/or pressor beams can be formulated by suitably modifying the notion of warp drives, and that, as for wormholes and warp drives, violations of the classical point-wise energy conditions are utterly unavoidable. ---------------------------------------------------------------- arXiv:2111.03113 gr-qc hep-th *Geodesically complete black holes in Lorentz-violating gravity * *Authors*: Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Matt Visser Abstract: We present a systematic study of the geometric structure of non-singular spacetimes describing black holes in Lorentz-violating gravity. We start with a review of the definition of trapping horizons, and the associated notions of trapped and marginally trapped surfaces, and then study their significance in frameworks with modified dispersion relations. This leads us to introduce the notion of universally marginally trapped surfaces, as the direct generalization of marginally trapped surfaces for frameworks with infinite signal velocities (Hoava-like frameworks), which then allows us to define universal trapping horizons. We find that trapped surfaces cannot be generalized in the same way, and discuss in detail why this does not prevent using universal trapping horizons to define black holes in Hoava-like frameworks. We then explore the interplay between the kinematical part of Penrose's singularity theorem, which implies the existence of incomplete null geodesics in the presence of a focusing point, and the existence of multiple different metrics. This allows us to present a complete classification of all possible geometries that neither display incomplete physical trajectories nor curvature singularities. Our main result is that not all classes that exist in frameworks in which all signal velocities are realized in Hoava-like frameworks. However, the taxonomy of geodesically complete black holes in Ho\v rava-like frameworks includes diverse scenarios such as evaporating regular black holes, regular black holes bouncing into regular white holes, and hidden wormholes. ---------------------------------------------------------------- arXiv:2111.12329 gr-qc *The eye of the storm: A regular Kerr black hole * *Authors*: Alex Simpson, Matt Visser Abstract: We present a highly tractable non-singular modification of the Kerr geometry, dubbed the "eye of the storm" -- a rotating regular black hole with an asymptotically Minkowski core. This is achieved by "exponentially suppressing" the mass parameter in the Kerr spacetime: mme^/r . The single parameter quantifies the deviation from the usual Kerr spacetime. Some of the classical energy conditions are globally satisfied, whilst certain choices for force any energy-condition-violating physics into the deep core. The geometry possesses the full "Killing tower'" of principal tensor, Killing--Yano tensor, and nontrivial Killing tensor, with associated Carter constant; hence the Hamilton--Jacobi equations are separable, and the geodesics integrable. The Klein--Gordon equation is also separable on this candidate spacetime. The tightly controlled deviation from Kerr renders the physics extraordinarily tractable when compared with analogous results in the current literature. This spacetime will be amenable to straightforward extraction of astrophysical observables falsifiable/ verifiable by the experimental community. ---------------------------------------------------------------- arXiv:2111.14016 gr-qc hep-th *Feynman's i-epsilon prescription, almost real spacetimes, and acceptable complex spacetimes * *Authors*: Matt Visser Abstract: Feynman's i-epsilon prescription for quantum field theoretic propagators has a quite natural reinterpretation in terms of a slight complex deformation of the Minkowski spacetime metric. Though originally a strictly flat-space result, once reinterpreted in this way, these ideas can be naturally extended first to semi-classical curved-spacetime QFT on a fixed background geometry and then, (with more work), to fluctuating spacetime geometries. There are intimate connections with variants of the weak energy condition. We shall take the Lorentzian signature metric as primary, but note that allowing the complex deformation to become large leads to a variant of Wick rotation, and more importantly leads to physically motivated constraints on the configuration space of acceptable off-shell geometries to include in Feynman's functional integral when attempting to quantize gravity. Ultimately this observation allows one to connect the discussion back to recent ideas on "acceptable" complex metrics, in the Louko-Sorkin and Kontsevich-Segal-Witten sense, with Lorentzian signature spacetimes occurring exactly on the boundary of the set of "acceptable" complex metrics. By adopting the tetrad formalism we explicitly construct the most general set of acceptable complex metrics satisfying the 0-form, 1-form, and 2-form acceptability conditions. ---------------------------------------------------------------- arXiv:2005.04027 gr-qc cond-mat.other quant-ph *The next generation of analogue gravity experiments * *Authors*: Maxime J Jacquet, Silke Weinfurtner, Friedrich Koenig Abstract: This article is an introduction for a theme issue following a Scientific Discussion Meeting on \emph{The next generation of analogue gravity experiments} held at the Royal Society in December 2019. This theme issue comprises a collection of recent advances of the research programme, as well as their philosophical implications, that were presented at the meeting. ---------------------------------------------------------------- arXiv:1802.09807 gr-qc *Boyer-Lindquist space-times and beyond: Meta-material analogues * * * *Authors:* Sebastian Schuster , Matt Visser Abstract: Physically reasonable stationary axisymmetric spacetimes can (under very mild technical conditions) be put into Boyer-Lindquist form. Unfortunately a metric presented in Boyer-Lindquist form is not well-adapted to the "quasi-Cartesian" meta-material analysis we developed in our previous article on "bespoke analogue spacetimes" (arXiv:1801.05549 [gr-qc]). In the current article we first focus specifically on spacetime metrics presented in Boyer-Lindquist form, and determine the equivalent meta-material susceptibility tensors in a laboratory setting. We then turn to analyzing generic stationary spacetimes, again determining the equivalent meta-material susceptibility tensors. While the background laboratory metric is always taken to be Riemann-flat, we now allow for arbitrary curvilinear coordinate systems. Finally, we reconsider static spherically symmetric spacetimes, but now in general spherical polar rather than quasi-Cartesian coordinates. The article provides a set of general tools for mimicking various interesting spacetimes by using non-trivial susceptibility tensors in general laboratory settings. ---------------------------------------------------------------- arXiv:2007.03769 gr-qc physics.flu-dyn *Superradiance in dispersive black hole analogues * *Authors*: Sam Patrick, Silke Weinfurtner Abstract: Wave equations containing spatial derivatives which are higher than second order arise naturally in the context of condensed matter systems. The solutions of such equations contain more than two modes and consequently, the range of possible interactions between the different modes is significantly enhanced compared to the two mode case. We develop a framework for analysing the different mode interactions based on the classical turning points of the dispersion relation. We then apply this framework to the scattering of deep water gravity waves with a draining bathtub vortex, a system which constitutes the analogue of a rotating black hole in the non-dispersive limit. In particular, we show that the different scattering outcomes are controlled by the light-ring frequencies, a concept routinely applied in black hole physics, and two new frequencies which are related to the strength of dispersion. We find that the frequency range in which the reflected wave is superradiantly amplified appears as a simple modification to the non-dispersive case. However, the condition to observe this amplification is complicated by the fact that a superradiant mode can be reflected back into the system by scattering with one of the additional modes. We provide estimates for the reflection coefficients in the full dispersive regime. Journal reference: Phys. Rev. D 102, 084041 (2020) ---------------------------------------------------------------- arXiv:2007.07160 gr-qc cond-mat.quant-gas *Interferometric Unruh detectors for Bose-Einstein condensates * *Authors*: Cisco Gooding, Steffen Biermann, Sebastian Erne, Jorma Louko, William G. Unruh, Joerg Schmiedmayer, Silke Weinfurtner Abstract: The Unruh effect predicts a thermal response for an accelerated detector moving through the vacuum. Here we propose an interferometric scheme to observe an analogue of the circular Unruh effect using a localized laser coupled to a Bose-Einstein condensate (BEC). Quantum fluctuations in the condensate are governed by an effective relativistic field theory, and as demonstrated, the coupled laser field acts as an effective Unruh-DeWitt detector thereof. The effective speed of light is lowered by 12 orders of magnitude to the sound velocity in the BEC. For detectors traveling close to the sound speed, observation of the Unruh effect in the analogue system becomes experimentally feasible. Journal reference: Phys. Rev. Lett. 125, 213603 (2020) ---------------------------------------------------------------- arXiv:2007.09523 gr-qc cond-mat.quant-gas *Unruh and analogue Unruh temperatures for circular motion in 3+1 and 2+1 dimensions * *Authors*: Steffen Biermann, Sebastian Erne, Cisco Gooding, Jorma Louko, Jörg Schmiedmayer, William G. Unruh, Silke Weinfurtner Abstract: The Unruh effect states that a uniformly linearly accelerated observer with proper acceleration a experiences Minkowski vacuum as a thermal state in the temperature T_lin =a/(2), operationally measurable via the detailed balance condition between excitation and de-excitation probabilities. An observer in uniform circular motion experiences a similar Unruh-type temperature Tcirc, operationally measurable via the detailed balance condition, but T_circ depends not just on the proper acceleration but also on the orbital radius and on the excitation energy. We establish analytic results for T_circ for a massless scalar field in 3+1 and 2+1 spacetime dimensions in several asymptotic regions of the parameter space, and we give numerical results in the interpolating regions. In the ultrarelativistic limit, we verify that in 3+1 dimensions T_circ is of the order of T_lin uniformly in the energy, as previously found by Unruh, but in 2+1 dimensions T_circ is significantly lower at low energies. We translate these results to an analogue spacetime nonrelativistic field theory in which the circular acceleration effects may become experimentally testable in the near future. We establish in particular that the circular motion analogue Unruh temperature grows arbitrarily large in the near-sonic limit, encouragingly for the experimental prospects, but the growth is weaker in effective spacetime dimension 2+1 than in 3+1. Journal reference: Phys. Rev. D 102, 085006 (2020) ---------------------------------------------------------------- arXiv:2105.11509 gr-qc cond-mat.quant-gas physics.flu-dyn *The sound-ring radiation of expanding vortex clusters * *Authors*: August Geelmuyden, Sebastian Erne, Sam Patrick, Carlo Barenghi, Silke Weinfurtner Abstract: We investigate wave-vortex interaction emerging from an expanding compact vortex cluster in a two-dimensional Bose-Einstein condensate. We adapt techniques developed for compact gravitational objects to derive the characteristic modes of the wave-vortex interaction perturbatively around an effective vortex flow field. We demonstrate the existence of orbits or sound-rings, in analogy to gravitational light-rings, and compute the characteristic spectrum for the out-of-equilibrium vortex cluster. The spectrum obtained from numerical simulations of a stochastic Gross-Pitaevskii equation exhibiting an expanding vortex cluster is in excellent agreement with analytical predictions. Our findings are relevant for 2d-quantum turbulence, the semi-classical limit around fluid flows, and rotating compact objects exhibiting discrete circulation. ---------------------------------------------------------------- arXiv:2011.06787 gr-qc *The SPIIR online coherent pipeline to search for gravitational waves from compact binary coalescences * *Authors*: Qi Chu, Manoj Kovalam, Linqing Wen, Teresa Slaven-Blair, Joel Bosveld, Yanbei Chen, Patrick Clearwater, Alex Codoreanu, Zhihui Du, Xiangyu Guo, Xiaoyang Guo, Kyungmin Kim, Tjonnie G. F. Li, Victor Oloworaran, Fiona Panther, Jade Powell, Anand S. Sengupta, Karl Wette, Xingjiang Zhu Abstract: This paper presents the SPIIR pipeline used for public alerts during the third advanced LIGO and Virgo observation run (O3 run). The SPIIR pipeline uses infinite impulse response (IIR) filters to perform extremely low-latency matched filtering and this process is further accelerated with graphics processing units (GPUs). It is the first online pipeline to select candidates from multiple detectors using a coherent statistic based on the maximum network likelihood ratio statistic principle. Here we simplify the derivation of this statistic using the singular-value-decomposition (SVD) technique and show that single-detector signal-to-noise ratios from matched filtering can be directly used to construct the statistic for each sky direction. Coherent searches are in general more computationally challenging than coincidence searches due to extra search over sky direction parameters. The search over sky directions follows an embarrassing parallelization paradigm and has been accelerated using GPUs. The detection performance is reported using a segment of public data from LIGO-Virgo's second observation run. We demonstrate that the median latency of the SPIIR pipeline is less than 9 seconds, and present an achievable roadmap to reduce the latency to less than 5 seconds. During the O3 online run, SPIIR registered triggers associated with 38 of the 56 non-retracted public alerts. The extreme low-latency nature makes it a competitive choice for joint time-domain observations, and offers the tantalizing possibility of making public alerts prior to the merger phase of binary coalescence systems involving at least one neutron star. ---------------------------------------------------------------- arXiv:2012.09552 stro-ph.IM gr-qc *SWIGLAL: Python and Octave interfaces to the LALSuite gravitational-wave data analysis libraries * *Authors*: Karl Wette Abstract: The LALSuite data analysis libraries, written in C, implement key routines critical to the successful detection of gravitational waves, such as the template waveforms describing the merger of two black holes or two neutron stars. SWIGLAL is a component of LALSuite which provides interfaces for Python and Octave, making LALSuite routines accessible directly from scripts written in those languages. It has enabled modern gravitational-wave data analysis software, used in the first detection of gravitational waves, to be written in Python, thereby benefiting from its ease of development and rich feature set, while still having access to the computational speed and scientific trustworthiness of the routines provided by LALSuite. Journal reference: SoftwareX 12 (2020) 100634 ---------------------------------------------------------------- arXiv:2104.14829 gr-qc astro-ph.IM *Geometric Approach to Analytic Marginalisation of the Likelihood Ratio for Continuous Gravitational Wave Searches * *Authors*: Karl Wette Abstract: The likelihood ratio for a continuous gravitational wave signal is viewed geometrically as a function of the orientation of two vectors; one representing the optimal signal-to-noise ratio, the other representing the maximised likelihood ratio or F-statistic. Analytic marginalisation over the angle between the vectors yields a marginalised likelihood ratio which is a function of the F-statistic. Further analytic marginalisation over the optimal signal-to-noise ratio is explored using different choices of prior. Monte-Carlo simulations show that the marginalised likelihood ratios have identical detection power to the F-statistic. This approach demonstrates a route to viewing the F-statistic in a Bayesian context, while retaining the advantages of its efficient computation. Journal reference: Universe 2021, 7(6), 174 ---------------------------------------------------------------- arXiv:2106.16142 gr-qc astro-ph.HE *Template Lattices for a Cross-Correlation Search for Gravitational Waves from Scorpius X-1 * *Authors*: Katelyn J. Wagner, John T. Whelan, Jared K. Wofford, Karl Wette Abstract: We describe the application of the lattice covering problem to the placement of templates in a search for continuous gravitational waves from the low-mass X-Ray binary Scorpius X-1. Efficient placement of templates to cover the parameter space at a given maximum mismatch is an application of the sphere covering problem, for which an implementation is available in the LatticeTiling software library. In the case of Sco X-1, potential correlations, in both the prior uncertainty and the mismatch metric, between the orbital period and orbital phase, lead to complications in the efficient construction of the lattice. We define a shearing coordinate transformation which simultaneously minimizes both of these sources of correlation, and allows us to take advantage of the small prior orbital period uncertainty. The resulting lattices have a factor of about 3 fewer templates than the corresponding parameter space grids constructed by the prior straightforward method, allowing a more sensitive search at the same computing cost and maximum mismatch. ---------------------------------------------------------------- arXiv:1912.08988 gr-qc hep-th *Positive Energy Functional for Massless Scalars in Rotating Black Hole Backgrounds of Maximal Ungauged Supergravity * *Authors*: M. Cvetic, G. W. Gibbons, C. N. Pope, B. F. Whiting Abstract: We outline a proof of the stability of a massless neutral scalar field in the background of a wide class of four dimensional asymptotically flat rotating and ``electrically charged'' solutions of supergravity, and the low energy limit of string theory, known as STU metrics. Despite their complexity, we find it possible to circumvent the difficulties presented by the existence of ergo-regions and the related phenomenon of super-radiance in the original metrics by following a strategy due to Whiting, and passing to an auxiliary metric admitting an everywhere lightlike Killing field and constructing a scalar field (related to a possible unstable mode by a non-local transformation) which satisfies the massless wave equation with respect to the auxiliary metric. By contrast with the case for , the associated energy density of is not only conserved but is also non-negative. Journal reference: Phys. Rev. Lett. 124, 231102 (2020) ---------------------------------------------------------------- arXiv:2002.02501 gr-qc quant-ph *Classical Tools for Antipodal Identification in Reissner-Nordström Spacetime * *Authors*: Nathaniel A. Strauss, Bernard F. Whiting, Anne T. Franzen Abstract: We extend the discussion of the antipodal identification of black holes to the Reissner-Nordström (RN) spacetime by developing the classical tools necessary to define the corresponding quantum field theory (QFT). We solve the massless Klein-Gordon equation in the RN background in terms of scattering coefficients and provide a procedure for constructing a solution for an arbitrary analytic extension of RN. The behavior of the maximally extended solution is highly dependent upon the coefficients of scattering between the inner and outer horizons, so we present the low-frequency behavior of, and numerical solutions for, these quantities. We find that, for low enough frequency, field amplitudes of solutions with purely positive or negative frequency at each horizon will acquire only a phase after passing both the inner and outer horizons, while at higher frequencies the amplitudes will tend to grow exponentially either to the future or to the past, and decay exponentially in the other direction. Regardless, we can always construct a basis of globally antipodal symmetric and antisymmetric solutions for any finite analytic extension of RN. We have characterized this basis in terms of positive and negative frequency solutions for future use in constructing the corresponding QFT. Journal reference: Class. Quant. Grav. 37 (2020) 18, 185006 ---------------------------------------------------------------- arXiv:2101.00856 gr-qc math-ph *On the geometry of Petrov type II spacetimes * *Authors*: Steffen Aksteiner, Lars Andersson, Bernardo Araneda, Bernard Whiting Abstract: In general, geometries of Petrov type II do not admit symmetries in terms of Killing vectors or spinors. We introduce a weaker form of Killing equations which do admit solutions. In particular, there is an analog of the Penrose-Walker Killing spinor. Some of its properties, including associated conservation laws, are discussed. Perturbations of Petrov type II Einstein geometries in terms of a complex scalar Debye potential yield complex solutions to the linearized Einstein equations. The complex linearized Weyl tensor is shown to be half Petrov type N. The remaining curvature component on the algebraically special side is reduced to a first order differential operator acting on the potential. ---------------------------------------------------------------- arXiv:2106.05163 gr-qc *The First-Order Velocity Memory Effect from Compact Binary Coalescing Sources * *Authors*: Atul K. Divakarla, Bernard F. Whiting Abstract: It has long been known that gravitational waves from compact binary coalescing sources are responsible for a first-order displacement memory effect experienced by a pair of freely falling test masses. This constant displacement is sourced from the non-vanishing final gravitational-wave strain present in the wave's after-zone, often referred to as the non-linear memory effect, and is of the same order of magnitude as the strain from the outgoing quadrupole radiation. Hence, this prediction of general relativity is verifiable experimentally by measurement of the final relative separation between test masses that comprise gravitational-wave detectors. In a separate context, independent calculations have demonstrated that exact, sandwich, plane wave spacetimes exhibit a velocity memory effect: a non-zero relative velocity, gained by a pair of test masses in free fall, after the passage of a gravitational wave. In this paper, we find that in addition to the known constant displacement memory effect test masses experience, a velocity memory effect at leading order arises due to the non-linear nature of gravitational waves from compact binary sources. We discuss the magnitude of the first-order velocity memory effect in the context of observing gravitational-wave radiation from super massive binary black hole mergers in LISA. Journal reference: Phys. Rev. D 104, 064001 (2021) ---------------------------------------------------------------- arXiv:2109.03254 gr-qc hep-th Supergravity Black Holes, Love Numbers and Harmonic Coordinates Authors: M. Cvetic, G. W. Gibbons, C. N. Pope, B. F. Whiting Abstract: To perform realistic tests of theories of gravity, we need to be able to look beyond general relativity and evaluate the consistency of alternative theories with observational data from, especially, gravitational wave detections using, for example, an agnostic Bayesian approach. In this paper we further examine properties of one class of such viable, alternative theories, based on metrics arising from ungauged supergravity. In particular, we examine the massless, neutral, minimally coupled scalar wave equation in a general stationary, axisymmetric background metric such as that of a charged rotating black hole, when the scalar field is either time independent or in the low-frequency, near-zone limit, with a view to calculating the Love numbers of tidal perturbations, and of obtaining harmonic coordinates for the background metric. For a four-parameter family of charged asymptotically flat rotating black hole solutions of ungauged supergravity theory known as STU black holes, which includes Kaluza-Klein black holes and the Kerr-Sen black hole as special cases, we find that all time-independent solutions, and hence the harmonic coordinates of the metrics, are identical to those of the Kerr solution. In the low-frequency limit we find the scalar fields exhibit the same SL(2,R) symmetry as holds in the case of the Kerr solution. We point out extensions of our results to a wider class of metrics, which includes solutions of Einstein-Maxwell-Dilaton theory. ****************************************************************************** ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc, December 2019 - November 2021 The LIGO Scientific Collaboration is a consortium of scientific institutions doing work on the Laser Interferometer Gravitational-Wave Observatory (LIGO), which consists of two laser interferometers 3030 km apart, one at Hanford, Washington State and the other at Livingston, Louisiana. The LIGO Scientific Collaboration includes ASGRG members David Blair, Philip Charlton, Neil Cornish, Brynmor Haskell, Jörg Hennig, Paul Lasky, Ju Li, David McClelland, Andrew Melatos, Susan Scott, Daniel Shaddock, Bram Slagmolen, Peter Veitch, Karl Wette, Bernard Whiting and Chunnong Zhao. Listed below are all the abstracts listed on gr-qc from December 2019 to November 2021 from consortia that include at leastone ASGRG member as a co-author these are mostly LIGO abstracts, but there are occasionally some from eLISA and Virgo. ---------------------------------------------------------------- arXiv:1912.11716 gr-qc astro-ph.IM *Open data from the first and second observing runs of Advanced LIGO and Advanced Virgo * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: Advanced LIGO and Advanced Virgo are actively monitoring the sky and collecting gravitational-wave strain data with sufficient sensitivity to detect signals routinely. In this paper we describe the data recorded by these instruments during their first and second observing runs. The main data products are the gravitational-wave strain arrays, released as time series sampled at 16384 Hz. The datasets that include this strain measurement can be freely accessed through the Gravitational Wave Open Science Center at http://gw-openscience.org, together with data-quality information essential for the analysis of LIGO and Virgo data, documentation, tutorials, and supporting software. Journal reference: SoftwareX 13 (2021) 100658 ---------------------------------------------------------------- arXiv:2001.01761 astro-ph.HE gr-qc *GW190425: Observation of a Compact Binary Coalescence with Total Mass ****3.4M**** * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: On 2019 April 25, the LIGO Livingston detector observed a compact binary coalescence with signal-to-noise ratio 12.9. The Virgo detector was also taking data that did not contribute to detection due to a low signal-to-noise ratio, but were used for subsequent parameter estimation. The 90% credible intervals for the component masses range from 1.12 to 2.52 M(1.45 to 1.88 Mif we restrict the dimensionless component spin magnitudes to be smaller than 0.05). These mass parameters are consistent with the individual binary components being neutron stars. However, both the source-frame chirp mass 1.44^+0.02 _0.02 Mand the total mass 3.4^+0.3 _0.1 Mof this system are significantly larger than those of any other known binary neutron star system. The possibility that one or both binary components of the system are black holes cannot be ruled out from gravitational-wave data. We discuss possible origins of the system based on its inconsistency with the known Galactic binary neutron star population. Under the assumption that the signal was produced by a binary neutron star coalescence, the local rate of neutron star mergers is updated to 2502810Gpc^3 yr^1 . Journal reference: Astrophysical Journal Letters 892 (2020) L3 ---------------------------------------------------------------- arXiv:2001.00923 astro-ph.HE *A Joint Fermi-GBM and LIGO/Virgo Analysis of Compact Binary Mergers From the First and Second Gravitational-wave Observing Runs * *Authors*: The Fermi Gamma-ray Burst Monitor Team, the LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We present results from offline searches of Fermi Gamma-ray Burst Monitor (GBM) data for gamma-ray transients coincident with the compact binary coalescences observed by the gravitational-wave (GW) detectors Advanced LIGO and Advanced Virgo during their first and second observing runs. In particular, we perform follow-up for both confirmed events and low significance candidates reported in the LIGO/Virgo catalog GWTC-1. We search for temporal coincidences between these GW signals and GBM triggered gamma-ray bursts (GRBs). We also use the GBM Untargeted and Targeted subthreshold searches to find coincident gamma-rays below the on-board triggering threshold. This work implements a refined statistical approach by incorporating GW astrophysical source probabilities and GBM visibilities of LIGO/Virgo sky localizations to search for cumulative signatures of coincident subthreshold gamma-rays. All search methods recover the short gamma-ray burst GRB 170817A occurring ~1.7 s after the binary neutron star merger GW170817. We also present results from a new search seeking GBM counterparts to LIGO single-interferometer triggers. This search finds a candidate joint event, but given the nature of the GBM signal and localization, as well as the high joint false alarm rate of 1.1×10^6 Hz, we do not consider it an astrophysical association. We find no additional coincidences. Journal reference: The Astrophysical Journal, 893:100 (14pp), 2020 April 20 ---------------------------------------------------------------- arXiv:2004.08342 astro-ph.HE gr-qc *GW190412: Observation of a Binary-Black-Hole Coalescence with Asymmetric Masses * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We report the observation of gravitational waves from a binary-black-hole coalescence during the first two weeks of LIGO's and Virgo's third observing run. The signal was recorded on April 12, 2019 at 05:30:44 UTC with a network signal-to-noise ratio of 19. The binary is different from observations during the first two observing runs most notably due to its asymmetric masses: a ~30 solar mass black hole merged with a ~8 solar mass black hole companion. The more massive black hole rotated with a dimensionless spin magnitude between 0.22 and 0.60 (90% probability). Asymmetric systems are predicted to emit gravitational waves with stronger contributions from higher multipoles, and indeed we find strong evidence for gravitational radiation beyond the leading quadrupolar order in the observed signal. A suite of tests performed on GW190412 indicates consistency with Einstein's general theory of relativity. While the mass ratio of this system differs from all previous detections, we show that it is consistent with the population model of stellar binary black holes inferred from the first two observing runs. Journal reference: Phys. Rev. D 102, 043015 (2020) ---------------------------------------------------------------- arXiv:2006.00714 astro-ph.IM gr-qc *Bayesian inference for compact binary coalescences with BILBY: Validation and application to the first LIGO--Virgo gravitational-wave transient catalogue * *Authors*: I. M. Romero-Shaw, C. Talbot, S. Biscoveanu, V. D'Emilio, G. Ashton, C. P. L. Berry, S. Coughlin, S. Galaudage, C. Hoy, M. Huebner, K. S. Phukon, M. Pitkin, M. Rizzo, N. Sarin, R. Smith, S. Stevenson, A. Vajpeyi, M. Arene, K. Athar, S. Banagiri, N. Bose, M. Carney, K. Chatziioannou, J. A. Clark, M. Colleoni , et al. (34 additional authors not shown) Abstract: Gravitational waves provide a unique tool for observational astronomy. While the first LIGO--Virgo catalogue of gravitational-wave transients (GWTC-1) contains eleven signals from black hole and neutron star binaries, the number of observations is increasing rapidly as detector sensitivity improves. To extract information from the observed signals, it is imperative to have fast, flexible, and scalable inference techniques. In a previous paper, we introduced BILBY: a modular and user-friendly Bayesian inference library adapted to address the needs of gravitational-wave inference. In this work, we demonstrate that BILBY produces reliable results for simulated gravitational-wave signals from compact binary mergers, and verify that it accurately reproduces results reported for the eleven GWTC-1 signals. Additionally, we provide configuration and output files for all analyses to allow for easy reproduction, modification, and future use. This work establishes that BILBY is primed and ready to analyse the rapidly growing population of compact binary coalescence gravitational-wave signals. ---------------------------------------------------------------- arXiv:2006.12611 astro-ph.HE gr-qc *GW190814: Gravitational Waves from the Coalescence of a 23 M****Black Hole with a 2.6 M****Compact Object * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We report the observation of a compact binary coalescence involving a 22.2 - 24.3 Mblack hole and a compact object with a mass of 2.50 - 2.67 M(all measurements quoted at the 90% credible level). The gravitational-wave signal, GW190814, was observed during LIGO's and Virgo's third observing run on August 14, 2019 at 21:10:39 UTC and has a signal-to-noise ratio of 25 in the three-detector network. The source was localized to 18.5 deg^2 at a distance of 241^+41 _45 Mpc; no electromagnetic counterpart has been confirmed to date. The source has the most unequal mass ratio yet measured with gravitational waves, 0.112^+0.008 _0.009 , and its secondary component is either the lightest black hole or the heaviest neutron star ever discovered in a double compact-object system. The dimensionless spin of the primary black hole is tightly constrained to 0.07. Tests of general relativity reveal no measurable deviations from the theory, and its prediction of higher-multipole emission is confirmed at high confidence. We estimate a merger rate density of 1-23 Gpc^3 yr^1 for the new class of binary coalescence sources that GW190814 represents. Astrophysical models predict that binaries with mass ratios similar to this event can form through several channels, but are unlikely to have formed in globular clusters. However, the combination of mass ratio, component masses, and the inferred merger rate for this event challenges all current models for the formation and mass distribution of compact-object binaries. Accepted by ApJ Letters ---------------------------------------------------------------- arXiv:2007.14251 astro-ph.HE gr-qc *Gravitational-wave constraints on the equatorial ellipticity of millisecond pulsars * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We present a search for continuous gravitational waves from five radio pulsars, comprising three recycled pulsars (PSR J0437-4715, PSR J0711-6830, and PSR J0737-3039A) and two young pulsars: the Crab pulsar (J0534+2200) and the Vela pulsar (J0835-4510). We use data from the third observing run of Advanced LIGO and Virgo combined with data from their first and second observing runs. For the first time we are able to match (for PSR J0437-4715) or surpass (for PSR J0711-6830) the indirect limits on gravitational-wave emission from recycled pulsars inferred from their observed spin-downs, and constrain their equatorial ellipticities to be less than 10^8 . For each of the five pulsars, we perform targeted searches that assume a tight coupling between the gravitational-wave and electromagnetic signal phase evolution. We also present constraints on PSR J0711-6830, the Crab pulsar and the Vela pulsar from a search that relaxes this assumption, allowing the gravitational-wave signal to vary from the electromagnetic expectation within a narrow band of frequencies and frequency derivatives. Journal reference: 2020 ApJL 902 L21 ---------------------------------------------------------------- arXiv:2009.01190 astro-ph.HE gr-qc *Properties and astrophysical implications of the 150 Msun binary black hole merger GW190521 * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: The gravitational-wave signal GW190521 is consistent with a binary black hole merger source at redshift 0.8 with unusually high component masses, 85^+21 _14 Mand 66^+17 _18 M, compared to previously reported events, and shows mild evidence for spin-induced orbital precession. The primary falls in the mass gap predicted by (pulsational) pair-instability supernova theory, in the approximate range 65120M. The probability that at least one of the black holes in GW190521 is in that range is 99.0%. The final mass of the merger (142^+28 _16 M) classifies it as an intermediate-mass black hole. Under the assumption of a quasi-circular binary black hole coalescence, we detail the physical properties of GW190521's source binary and its post-merger remnant, including component masses and spin vectors. Three different waveform models, as well as direct comparison to numerical solutions of general relativity, yield consistent estimates of these properties. Tests of strong-field general relativity targeting the merger-ringdown stages of coalescence indicate consistency of the observed signal with theoretical predictions. We estimate the merger rate of similar systems to be 0.13^+0.30 _0.11 Gpc^3 yr^1 . We discuss the astrophysical implications of GW190521 for stellar collapse, and for the possible formation of black holes in the pair-instability mass gap through various channels: via (multiple) stellar coalescence, or via hierarchical merger of lower-mass black holes in star clusters or in active galactic nuclei. We find it to be unlikely that GW190521 is a strongly lensed signal of a lower-mass black hole binary merger. We also discuss more exotic possible sources for GW190521, including a highly eccentric black hole binary, or a primordial black hole binary. Journal reference: Astrophys. J. Lett. 900, L13 (2020) ---------------------------------------------------------------- arXiv:2010.14527 gr-qc astro-ph.HE *GWTC-2: Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run * *Authors*: R. Abbott, T. D. Abbott, S. Abraham, F. Acernese, K. Ackley, A. Adams, C. Adams, R. X. Adhikari, V. B. Adya, C. Affeldt, M. Agathos, K. Agatsuma, N. Aggarwal, O. D. Aguiar, L. Aiello, A. Ain, P. Ajith, S. Akcay, G. Allen, A. Allocca, P. A. Altin, A. Amato, S. Anand, A. Ananyeva, S. B. Anderson , et al. (1327 additional authors not shown) Abstract: We report on gravitational wave discoveries from compact binary coalescences detected by Advanced LIGO and Advanced Virgo in the first half of the third observing run (O3a) between 1 April 2019 15:00 UTC and 1 October 2019 15:00. By imposing a false-alarm-rate threshold of two per year in each of the four search pipelines that constitute our search, we present 39 candidate gravitational wave events. At this threshold, we expect a contamination fraction of less than 10%. Of these, 26 candidate events were reported previously in near real-time through GCN Notices and Circulars; 13 are reported here for the first time. The catalog contains events whose sources are black hole binary mergers up to a redshift of ~0.8, as well as events whose components could not be unambiguously identified as black holes or neutron stars. For the latter group, we are unable to determine the nature based on estimates of the component masses and spins from gravitational wave data alone. The range of candidate events which are unambiguously identified as binary black holes (both objects 3 M) is increased compared to GWTC-1, with total masses from 14 Mfor GW190924_021846 to 150 Mfor GW190521. For the first time, this catalog includes binary systems with significantly asymmetric mass ratios, which had not been observed in data taken before April 2019. We also find that 11 of the 39 events detected since April 2019 have positive effective inspiral spins under our default prior (at 90% credibility), while none exhibit negative effective inspiral spin. Given the increased sensitivity of Advanced LIGO and Advanced Virgo, the detection of 39 candidate events in ~26 weeks of data (~1.5 per week) is consistent with GWTC-1. Journal reference: Phys. Rev. X 11, 021053 (2021) ---------------------------------------------------------------- arXiv:2010.14529 gr-qc astro-ph.HE *Tests of General Relativity with Binary Black Holes from the second LIGO-Virgo Gravitational-Wave Transient Catalog * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: Gravitational waves enable tests of general relativity in the highly dynamical and strong-field regime. Using events detected by LIGO-Virgo up to 1 October 2019, we evaluate the consistency of the data with predictions from the theory. We first establish that residuals from the best-fit waveform are consistent with detector noise, and that the low- and high-frequency parts of the signals are in agreement. We then consider parametrized modifications to the waveform by varying post-Newtonian and phenomenological coefficients, improving past constraints by factors of 2; we also find consistency with Kerr black holes when we specifically target signatures of the spin-induced quadrupole moment. Looking for gravitational-wave dispersion, we tighten constraints on Lorentz-violating coefficients by a factor of 2.6 and bound the mass of the graviton to mg1.76×10^23 eV/c^2 with 90% credibility. We also analyze the properties of the merger remnants by measuring ringdown frequencies and damping times, constraining fractional deviations away from the Kerr frequency to f_220 =0.03^+0.38 _0.35 for the fundamental quadrupolar mode, and f_221 =0.04^+0.27 _0.32 for the first overtone; additionally, we find no evidence for postmerger echoes. Finally, we determine that our data are consistent with tensorial polarizations through a template-independent method. When possible, we assess the validity of general relativity based on collections of events analyzed jointly. We find no evidence for new physics beyond general relativity, for black hole mimickers, or for any unaccounted systematics. Journal reference: Phys. Rev. D 103, 122002 (2021) ---------------------------------------------------------------- arXiv:2010.14533 astro-ph.HE gr-qc *Population Properties of Compact Objects from the Second LIGO-Virgo Gravitational-Wave Transient Catalog * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We report on the population of the 47 compact binary mergers detected with a false-alarm rate 1/yr in the second LIGO--Virgo Gravitational-Wave Transient Catalog, GWTC-2. We observe several characteristics of the merging binary black hole (BBH) population not discernible until now. First, we find that the primary mass spectrum contains structure beyond a power-law with a sharp high-mass cut-off; it is more consistent with a broken power law with a break at 39.7^+20.3 _9.1 M, or a power law with a Gaussian feature peaking at 33.1^+4.0 _5.6 M(90% credible interval). While the primary mass distribution must extend to 65Mor beyond, only 2.9^+3.5 _-1.7 % of systems have primary masses greater than 45M. Second, we find that a fraction of BBH systems have component spins misaligned with the orbital angular momentum, giving rise to precession of the orbital plane. Moreover, 12% to 44% of BBH systems have spins tilted by more than 90, giving rise to a negative effective inspiral spin parameter _eff . Under the assumption that such systems can only be formed by dynamical interactions, we infer that between 25% and 93% of BBH with non-vanishing |_eff |>0.01 are dynamically assembled. Third, we estimate merger rates, finding RBBH=23.9^+14.3 _-8.6 Gpc^3 yr^1 for BBH and RBNS=320^+490 _240 Gpc^3 yr^1 for binary neutron stars. We find that the BBH rate likely increases with redshift (85% credibility), but not faster than the star-formation rate (86% credibility). Additionally, we examine recent exceptional events in the context of our population models, finding that the asymmetric masses of GW190412 and the high component masses of GW190521 are consistent with our models, but the low secondary mass of GW190814 makes it an outlier. ---------------------------------------------------------------- arXiv:2010.14550 astro-ph.HE gr-qc *Search for Gravitational Waves Associated with Gamma-Ray Bursts Detected by Fermi and Swift During the LIGO-Virgo Run O3a * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We search for gravitational-wave transients associated with gamma-ray bursts detected by the Fermi and Swift satellites during the first part of the third observing run of Advanced LIGO and Advanced Virgo (1 April 2019 15:00 UTC - 1 October 2019 15:00 UTC). 105 gamma-ray bursts were analyzed using a search for generic gravitational-wave transients; 32 gamma-ray bursts were analyzed with a search that specifically targets neutron star binary mergers as short gamma-ray burst progenitors. We describe a method to calculate the probability that triggers from the binary merger targeted search are astrophysical and apply that method to the most significant gamma-ray bursts in that search. We find no significant evidence for gravitational-wave signals associated with the gamma-ray bursts that we followed up, nor for a population of unidentified subthreshold signals. We consider several source types and signal morphologies, and report for these lower bounds on the distance to each gamma-ray burst. Journal reference: Astrophys. J. 915, 86 (2021) ---------------------------------------------------------------- arXiv:2012.12128 gr-qc astro-ph.HE *All-sky search in early O3 LIGO data for continuous gravitational-wave signals from unknown neutron stars in binary systems * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: Rapidly spinning neutron stars are promising sources of persistent, continuous gravitational waves. Detecting such a signal would allow probing of the physical properties of matter under extreme conditions. A significant fraction of the known pulsar population belongs to binary systems. Searching for unknown neutron stars in binary systems requires specialized algorithms to address unknown orbital frequency modulations. We present a search for continuous gravitational waves emitted by neutron stars in binary systems in early data from the third observing run of the Advanced LIGO and Advanced Virgo detectors using the semicoherent, GPU-accelerated, BinarySkyHough pipeline. The search analyzes the most sensitive frequency band of the LIGO detectors, 50 - 300 Hz. Binary orbital parameters are split into four regions, comprising orbital periods of 3 - 45 days and projected semimajor axes of 2 - 40 light-seconds. No detections are reported. We estimate the sensitivity of the search using simulated continuous wave signals, achieving the most sensitive results to date across the analyzed parameter space. Journal reference: Phys. Rev. D 103, 064017 (2021) ---------------------------------------------------------------- arXiv:2012.12926 astro-ph.HE gr-qc Diving below the spin-down limit: Constraints on gravitational waves from the energetic young pulsar PSR J0537-6910 *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We present a search for continuous gravitational-wave signals from the young, energetic X-ray pulsar PSR J0537-6910 using data from the second and third observing runs of LIGO and Virgo. The search is enabled by a contemporaneous timing ephemeris obtained using NICER data. The NICER ephemeris has also been extended through 2020 October and includes three new glitches. PSR J0537-6910 has the largest spin-down luminosity of any pulsar and is highly active with regards to glitches. Analyses of its long-term and inter-glitch braking indices provided intriguing evidence that its spin-down energy budget may include gravitational-wave emission from a time-varying mass quadrupole moment. Its 62 Hz rotation frequency also puts its possible gravitational-wave emission in the most sensitive band of LIGO/Virgo detectors. Motivated by these considerations, we search for gravitational-wave emission at both once and twice the rotation frequency. We find no signal, however, and report our upper limits. Assuming a rigidly rotating triaxial star, our constraints reach below the gravitational-wave spin-down limit for this star for the first time by more than a factor of two and limit gravitational waves from the l=m=2 mode to account for less than 14% of the spin-down energy budget. The fiducial equatorial ellipticity is limited to less than about 3e-5, which is the third best constraint for any young pulsar. ---------------------------------------------------------------- arXiv:2101.11673 astro-ph.IM gr-qc *LIGO Detector Characterization in the Second and Third Observing Runs * *Authors*: D. Davis, J. S. Areeda, B. K. Berger, R. Bruntz, A. Effler, R. C. Essick, R. P. Fisher, P. Godwin, E. Goetz, A. F. Helmling-Cornell, B. Hughey, E. Katsavounidis, A. P. Lundgren, D. M. Macleod, Z. Márka, T. J. Massinger, A. Matas, J. McIver, G. Mo, K. Mogushi, P. Nguyen, L. K. Nuttall, R. M. S. Schofield, D. H. Shoemaker, S. Soni , et al. (262 additional authors not shown) Abstract: The characterization of the Advanced LIGO detectors in the second and third observing runs has increased the sensitivity of the instruments, allowing for a higher number of detectable gravitational-wave signals, and provided confirmation of all observed gravitational-wave events. In this work, we present the methods used to characterize the LIGO detectors and curate the publicly available datasets, including the LIGO strain data and data quality products. We describe the essential role of these datasets in LIGO-Virgo Collaboration analyses of gravitational-waves from both transient and persistent sources and include details on the provenance of these datasets in order to support analyses of LIGO data by the broader community. Finally, we explain anticipated changes in the role of detector characterization and current efforts to prepare for the high rate of gravitational-wave alerts and events in future observing runs. Journal reference: Class. Quantum Grav. 38 135014 (2021) ---------------------------------------------------------------- arXiv:2101.12130 gr-qc astro-ph.CO *Upper Limits on the Isotropic Gravitational-Wave Background from Advanced LIGO's and Advanced Virgo's Third Observing Run * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We report results of a search for an isotropic gravitational-wave background (GWB) using data from Advanced LIGO's and Advanced Virgo's third observing run (O3) combined with upper limits from the earlier O1 and O2 runs. Unlike in previous observing runs in the advanced detector era, we include Virgo in the search for the GWB. The results are consistent with uncorrelated noise, and therefore we place upper limits on the strength of the GWB. We find that the dimensionless energy density GW5.8×109 at the 95% credible level for a flat (frequency-independent) GWB, using a prior which is uniform in the log of the strength of the GWB, with 99% of the sensitivity coming from the band 20-76.6 Hz; 3.4×109 at 25 Hz for a power-law GWB with a spectral index of 2/3 (consistent with expectations for compact binary coalescences), in the band 20-90.6 Hz; and 3.9×1010 at 25 Hz for a spectral index of 3, in the band 20-291.6 Hz. These upper limits improve over our previous results by a factor of 6.0 for a flat GWB. We also search for a GWB arising from scalar and vector modes, which are predicted by alternative theories of gravity; we place upper limits on the strength of GWBs with these polarizations. We demonstrate that there is no evidence of correlated noise of magnetic origin by performing a Bayesian analysis that allows for the presence of both a GWB and an effective magnetic background arising from geophysical Schumann resonances. We compare our upper limits to a fiducial model for the GWB from the merger of compact binaries. Finally, we combine our results with observations of individual mergers andshow that, at design sensitivity, this joint approach may yield stronger constraints on the merger rate of binary black holes at z2 than can be achieved with individually resolved mergers alone. [abridged] Journal reference: Phys. Rev. D 104, 022004 (2021) ---------------------------------------------------------------- arXiv:2101.12248 gr-qc astro-ph.CO hep-th *Constraints on cosmic strings using data from the third Advanced LIGO-Virgo observing run * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We search for gravitational-wave signals produced by cosmic strings in the Advanced LIGO and Virgo full O3 data set. Search results are presented for gravitational waves produced by cosmic string loop features such as cusps, kinks and, for the first time, kink-kink collisions.cA template-based search for short-duration transient signals does not yield a detection. We also use the stochastic gravitational-wave background energy density upper limits derived from the O3 data to constrain the cosmic string tension, G, as a function of the number of kinks, or the number of cusps, for two cosmic string loop distribution models.cAdditionally, we develop and test a third model which interpolates between these two models. Our results improve upon the previous LIGO-Virgo constraints on G by one to two orders of magnitude depending on the model which is tested. In particular, for one loop distribution model, we set the most competitive constraints to date, G4×10^15 . Journal reference: Phys. Rev. Lett. 126, 241102 (2021) ---------------------------------------------------------------- arXiv:2103.08520 gr-qc *Search for anisotropic gravitational-wave backgrounds using data from Advanced LIGO and Advanced Virgo's first three observing runs * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We report results from searches for anisotropic stochastic gravitational-wave backgrounds using data from the first three observing runs of the Advanced LIGO and Advanced Virgo detectors. For the first time, we include Virgo data in our analysis and run our search with a new efficient pipeline called {\tt PyStoch} on data folded over one sidereal day. We use gravitational-wave radiometry (broadband and narrow band) to produce sky maps of stochastic gravitational-wave backgrounds and to search for gravitational waves from point sources. A spherical harmonic decomposition method is employed to look for gravitational-wave emission from spatially-extended sources. Neither technique found evidence of gravitational-wave signals. Hence we derive 95\% confidence-level upper limit sky maps on the gravitational-wave energy flux from broadband point sources, ranging from F_, <(0.0137.6)×10^8 ergcm^2 s^1 Hz^1 , and on the (normalized) gravitational-wave energy density spectrum from extended sources, ranging from _, <(0.579.3)×10^9 sr^1 , depending on direction () and spectral index (). These limits improve upon previous limits by factors of 2.93.5. We also set 95% confidence level upper limits on the frequency-dependent strain amplitudes of quasimonochromatic gravitational waves coming from three interesting targets, Scorpius X-1, SN 1987A and the Galactic Center, with best upper limits range from h_0 <(1.72.1)×10^25 , a factor of 2.0 improvement compared to previous stochastic radiometer searches. Journal reference: Phys. Rev. D 104, 022005 (2021) ---------------------------------------------------------------- arXiv:2104.14417 astro-ph.HE gr-qc *Constraints from LIGO O3 data on gravitational-wave emission due to r-modes in the glitching pulsar PSR J0537-6910 * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We present a search for continuous gravitational-wave emission due to r-modes in the pulsar PSR J0537-6910 using data from the LIGO-Virgo Collaboration observing run O3. PSR J0537-6910 is a young energetic X-ray pulsar and is the most frequent glitcher known. The inter-glitch braking index of the pulsar suggests that gravitational-wave emission due to r-mode oscillations may play an important role in the spin evolution of this pulsar. Theoretical models confirm this possibility and predict emission at a level that can be probed by ground-based detectors. In order to explore this scenario, we search for r-mode emission in the epochs between glitches by using a contemporaneous timing ephemeris obtained from NICER data. We do not detect any signals in the theoretically expected band of 86-97 Hz, and report upper limits on the amplitude of the gravitational waves. Our results improve on previous amplitude upper limits from r-modes in J0537-6910 by a factor of up to 3 and place stringent constraints on theoretical models for r-mode driven spin-down in PSR J0537-6910, especially for higher frequencies at which our results reach below the spin-down limit defined by energy conservation. Journal reference: ApJ 922 71 (2021) ---------------------------------------------------------------- arXiv:2105.06384 gr-qc astro-ph.HE *Search for lensing signatures in the gravitational-wave observations from the first half of LIGO-Virgo's third observing run * Authors: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We search for signatures of gravitational lensing in the gravitational-wave signals from compact binary coalescences detected by Advanced LIGO and Advanced Virgo during O3a, the first half of their third observing run. We study: 1) the expected rate of lensing at current detector sensitivity and the implications of a non-observation of strong lensing or a stochastic gravitational-wave background on the merger-rate density at high redshift; 2) how the interpretation of individual high-mass events would change if they were found to be lensed; 3) the possibility of multiple images due to strong lensing by galaxies or galaxy clusters; and 4) possible wave-optics effects due to point-mass microlenses. Several pairs of signals in the multiple-image analysis show similar parameters and, in this sense, are nominally consistent with the strong lensing hypothesis. However, taking into account population priors, selection effects, and the prior odds against lensing, these events do not provide sufficient evidence for lensing. Overall, we find no compelling evidence for lensing in the observed gravitational-wave signals from any of these analyses. ---------------------------------------------------------------- arXiv:2105.07455 gr-qc *An all-sky search in early O3 LIGO data for continuous gravitational-wave signals from unknown neutron stars in binary systems * Authors: LIGO Scientific Collaboration, Virgo Collaboration Abstract: We present a search for continuous gravitational waves emitted by neutron stars in binary systems conducted on data from the early third observing run of the Advanced LIGO and Advanced Virgo detectors using the semicoherent, GPU-accelerated, BinarySkyHough pipeline. The search analyzes the most sensitive frequency band of the LIGO detectors, 50 - 300 Hz. Binary orbital parameters are split into four regions, comprising orbital periods of 3 - 45 days and projected semimajor axes of 2 - 40 light-seconds. No detections are reported. We estimate the sensitivity of the search using simulated continuous wave signals, achieving the most sensitive results to date across the analyzed parameter space. ---------------------------------------------------------------- arXiv:2105.13085 astro-ph.CO gr-qc hep-ph *Constraints on dark photon dark matter using data from LIGO's and Virgo's third observing run * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We present a search for dark photon dark matter that could couple to gravitational-wave interferometers using data from Advanced LIGO and Virgo's third observing run. To perform this analysis, we use two methods, one based on cross-correlation of the strain channels in the two nearly aligned LIGO detectors, and one that looks for excess power in the strain channels of the LIGO and Virgo detectors. The excess power method optimizes the Fourier Transform coherence time as a function of frequency, to account for the expected signal width due to Doppler modulations. We do not find any evidence of dark photon dark matter with a mass between m_A 10^14 10^11 eV/c^2 , which corresponds to frequencies between 10-2000 Hz, and therefore provide upper limits on the square of the minimum coupling of dark photons to baryons, i.e. U(1)B dark matter. For the cross-correlation method, the best median constraint on the squared coupling is 1.31×10^47 at m_A 4.2×10^13 eV/c^2 ; for the other analysis, the best constraint is 1.2×10^47 at m_A 5.7×10^13 eV/c^2 . These limits improve upon those obtained in direct dark matter detection experiments by a factor of 100 for m_A [24]×10^13 eV/c^2 . ---------------------------------------------------------------- arXiv:2105.15120 astro-ph.HE gr-qc *Search for intermediate mass black hole binaries in the third observing run of Advanced LIGO and Advanced Virgo * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: Intermediate-mass black holes (IMBHs) span the approximate mass range 100--105M, between black holes (BHs) formed by stellar collapse and the supermassive BHs at the centers of galaxies. Mergers of IMBH binaries are the most energetic gravitational-wave sources accessible by the terrestrial detector network. Searches of the first two observing runs of Advanced LIGO and Advanced Virgo did not yield any significant IMBH binary signals. In the third observing run (O3), the increased network sensitivity enabled the detection of GW190521, a signal consistent with a binary merger of mass 150Mproviding direct evidence of IMBH formation. Here we report on a dedicated search of O3 data for further IMBH binary mergers, combining both modelled (matched filter) and model independent search methods. We find some marginal candidates, but none are sufficiently significant to indicate detection of further IMBH mergers. We quantify the sensitivity of the individual search methods and of the combined search using a suite of IMBH binary signals obtained via numerical relativity, including the effects of spins misaligned with the binary orbital axis, and present the resulting upper limits on astrophysical merger rates. Our most stringent limit is for equal mass and aligned spin BH binary of total mass 200Mand effective aligned spin 0.8 at 0.056Gpc^3 yr^1 (90 % confidence), a factor of 3.5 more constraining than previous LIGO-Virgo limits. We also update the estimated rate of mergers similar to GW190521 to 0.08Gpc^3 yr^1 . ---------------------------------------------------------------- arXiv:2107.00600 gr-qc astro-ph.HE *All-sky Search for Continuous Gravitational Waves from Isolated Neutron Stars in the Early O3 LIGO Data * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We report on an all-sky search for continuous gravitational waves in the frequency band 20-2000\,Hz and with a frequency time derivative in the range of [1.0,+0.1]×10^8 Hz/s. Such a signal could be produced by a nearby, spinning and slightly non-axisymmetric isolated neutron star in our galaxy. This search uses the LIGO data from the first six months of Advanced LIGO's and Advanced Virgo's third observational run, O3. No periodic gravitational wave signals are observed, and 95%\ confidence-level (CL) frequentist upper limits are placed on their strengths. The lowest upper limits on worst-case (linearly polarized) strain amplitude h_0 are 1.7×10^25 near 200 Hz. For a circularly polarized source (most favorable orientation), the lowest upper limits are 6.3×10^26 . These strict frequentist upper limits refer to all sky locations and the entire range of frequency derivative values. For a population-averaged ensemble of sky locations and stellar orientations, the lowest 95\%\ CL upper limits on the strain amplitude are 1.×10^25 . These upper limits improve upon our previously published all-sky results, with the greatest improvement (factor of 2) seen at higher frequencies, in part because quantum squeezing has dramatically improved the detector noise level relative to the second observational run, O2. These limits are the most constraining to date over most of the parameter space searched. Journal reference: Phys. Rev. D 104, 082004 (2021) ---------------------------------------------------------------- arXiv:2107.03294 gr-qc astro-ph.IM *Calibration of Advanced Virgo and reconstruction of detector strain h(t) during the Observing Run O3 * *Authors*: Virgo Collaboration. Abstract: The three Advanced Virgo and LIGO gravitational wave detectors participated to the third observing run (O3) between 1 April 2019 15:00 UTC and 27 March 2020 17:00 UTC,leading to several gravitational wave detections per month. This paper describes the Advanced Virgo detector calibration and the reconstruction of the detector strain h(t) during O3, as well as the estimation of the associated uncertainties. For the first time, the photon calibration technique as been used as reference for Virgo calibration, which allowed to cross-calibrate the strain amplitude of the Virgo and LIGO detectors. The previous reference, so-called free swinging Michelson technique, has still been used but as an independent cross-check. h(t) reconstruction and noise subtraction were processed online, with good enough quality to prevent the need for offline reprocessing, except for the two last weeks of September 2019. The uncertainties for the reconstructed h(t) strain, estimated in this paper in a 20-2000~Hz frequency band, are frequency independent: 5% in amplitude, 35 mrad in phase and 10 s in timing, with the exception of larger uncertainties around 50 Hz. ---------------------------------------------------------------- arXiv:2107.03701 gr-qc astro-ph.HE *All-sky search for short gravitational-wave bursts in the third Advanced LIGO and Advanced Virgo run * Authors: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: This paper presents the results of a search for generic short-duration gravitational-wave transients in data from the third observing run of Advanced LIGO and Advanced Virgo. Transients with durations of milliseconds to a few seconds in the 24--4096 Hz frequency band are targeted by the search, with no assumptions made regarding the incoming signal direction, polarization or morphology. Gravitational waves from compact binary coalescences that have been identified by other targeted analyses are detected, but no statistically significant evidence for other gravitational wave bursts is found. Sensitivities to a variety of signals are presented. These include updated upper limits on the source rate-density as a function of the characteristic frequency of the signal, which are roughly an order of magnitude better than previous upper limits. This search is sensitive to sources radiating as little as 10^10 Mc^2 in gravitational waves at 70 Hz from a distance of 10 kpc, with 50% detection efficiency at a false alarm rate of one per century. The sensitivity of this search to two plausible astrophysical sources is estimated: neutron star f-modes, which may be excited by pulsar glitches, as well as selected core-collapse supernova models. ---------------------------------------------------------------- arXiv:2107.13796 gr-qc astro-ph.HE *All-sky search for long-duration gravitational-wave bursts in the third Advanced LIGO and Advanced Virgo run * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: After the detection of gravitational waves from compact binary coalescences, the search for transient gravitational-wave signals with less well-defined waveforms for which matched filtering is not well-suited is one of the frontiers for gravitational-wave astronomy. Broadly classified into "short" 1 s and "long" 1 s duration signals, these signals are expected from a variety of astrophysical processes, including non-axisymmetric deformations in magnetars or eccentric binary black hole coalescences. In this work, we present a search for long-duration gravitational-wave transients from Advanced LIGO and Advanced Virgo's third observing run from April 2019 to March 2020. For this search, we use minimal assumptions for the sky location, event time, waveform morphology, and duration of the source. The search covers the range of 2 500 s in duration and a frequency band of 242048 Hz. We find no significant triggers within this parameter space; we report sensitivity limits on the signal strength of gravitational waves characterized by the root-sum-square amplitude hrss as a function of waveform morphology. These hrss limits improve upon the results from the second observing run by an average factor of 1.8. ---------------------------------------------------------------- arXiv:2108.01045 gr-qc *GWTC-2.1: Deep Extended Catalog of Compact Binary Coalescences Observed by LIGO and Virgo During the First Half of the Third Observing Run * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, R. Abbott, T. D. Abbott, F. Acernese, K. Ackley, C. Adams, N. Adhikari, R. X. Adhikari, V. B. Adya, C. Affeldt, D. Agarwal, M. Agathos, K. Agatsuma, N. Aggarwal, O. D. Aguiar, L. Aiello, A. Ain, P. Ajith, S. Albanesi, A. Allocca, P. A. Altin, A. Amato, C. Anand, S. Anand , et al. (1407 additional authors not shown) Abstract: The second gravitational-wave transient catalog, GWTC-2, reported on 39 compact binary coalescences observed by the Advanced LIGO and Advanced Virgo detectors between 1 April 2019 15:00 UTC and 1 October 2019 15:00 UTC. Here, we present GWTC-2.1, which reports on a deeper list of candidate events observed over the same period. We analyze the final version of the strain data over this period, which is now publicly released. We employ three matched-filter search pipelines for candidate identification, and estimate the probability of astrophysical origin for each candidate event. While GWTC-2 used a false alarm rate threshold of 2 per year, we include in GWTC-2.1, 1201 candidates that pass a false alarm rate threshold of 2 per day. We calculate the source properties of a subset of 44 high-significance candidates that have a probability of astrophysical origin greater than 0.5, using the default priors. Of these candidates, 36 have been reported in GWTC-2. If the 8 additional high-significance candidates presented here are astrophysical, the mass range of candidate events that are unambiguously identified as binary black holes (both objects 3M) is increased compared to GWTC-2, with total masses from 14Mfor GW190924_021846 to 184Mfor GW190426_190642. The primary components of two new candidate events (GW190403_051519 and GW190426_190642) fall in the mass gap predicted by pair-instability supernova theory. We also expand the population of binaries with significantly asymmetric mass ratios reported in GWTC-2 by an additional two events (q<0.61 and q<0.62 at 90% credibility for GW190403_051519 and GW190917_114630 respectively), and find that 2 of the 8 new events have effective inspiral spins _eff >0 (at 90% credibility), while no binary is consistent with _eff <0 at the same significance. ---------------------------------------------------------------- arXiv:2109.09255 astro-ph.HE gr-qc *Search for continuous gravitational waves from 20 accreting millisecond X-ray pulsars in O3 LIGO data * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: Results are presented of searches for continuous gravitational waves from 20 accreting millisecond X-ray pulsars with accurately measured spin frequencies and orbital parameters, using data from the third observing run of the Advanced LIGO and Advanced Virgo detectors. The search algorithm uses a hidden Markov model, where the transition probabilities allow the frequency to wander according to an unbiased random walk, while the J-statistic maximum-likelihood matched filter tracks the binary orbital phase. Three narrow sub-bands are searched for each target, centered on harmonics of the measured spin frequency. The search yields 16 candidates, consistent with a false alarm probability of 30% per sub-band and target searched. These candidates, along with one candidate from an additional target-of-opportunity search done for SAX J1808.43658, which was in outburst during one month of the observing run, cannot be confidently associated with a known noise source. Additional follow-up does not provide convincing evidence that any are a true astrophysical signal. When all candidates are assumed non-astrophysical, upper limits are set on the maximum wave strain detectable at 95% confidence, h^95% _0 . The strictest constraint is h^95% _0 =4.7×10^26 from IGR J170626143. Constraints on the detectable wave strain from each target lead to constraints on neutron star ellipticity and r-mode amplitude, the strictest of which are ^95% =3.1×10^7 and ^95% =1.8×10^5 respectively. This analysis is the most comprehensive and sensitive search of continuous gravitational waves from accreting millisecond X-ray pulsars to date. ---------------------------------------------------------------- arXiv:2109.12197 astro-ph.CO astro-ph.HE gr-qc *Search for subsolar-mass binaries in the first half of Advanced LIGO and Virgo's third observing run * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We report on a search for compact binary coalescences where at least one binary component has a mass between 0.2 Mand 1.0 Min Advanced LIGO and Advanced Virgo data collected between 1 April 2019 1500 UTC and 1 October 2019 1500 UTC. We extend previous analyses in two main ways: we include data from the Virgo detector and we allow for more unequal mass systems, with mass ratio q0.1. We do not report any gravitational-wave candidates. The most significant trigger has a false alarm rate of 0.14 yr^1 . This implies an upper limit on the merger rate of subsolar binaries in the range [22024200]Gpc^3 yr^1 , depending on the chirp mass of the binary. We use this upper limit to derive astrophysical constraints on two phenomenological models that could produce subsolar-mass compact objects. One is an isotropic distribution of equal-mass primordial black holes. Using this model, we find that the fraction of dark matter in primordial black holes is f_PBH _PBH /_DM 6%. The other is a dissipative dark matter model, in which fermionic dark matter can collapse and form black holes. The upper limit on the fraction of dark matter black holes depends on the minimum mass of the black holes that can be formed: the most constraining result is obtained at M_min =1M, where f_DBH _PBH /_DM 0.003%. These are the tightest limits on spinning subsolar-mass binaries to date. ---------------------------------------------------------------- arXiv:2110.09834 gr-qc *All-sky, all-frequency directional search for persistent gravitational-waves from Advanced LIGO's and Advanced Virgo's first three observing runs * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We present the first results from an all-sky all-frequency (ASAF) search for an anisotropic stochastic gravitational-wave background using the data from the first three observing runs of the Advanced LIGO and Advanced Virgo detectors. Upper limit maps on broadband anisotropies of a persistent stochastic background were published for all observing runs of the LIGO-Virgo detectors. However, a broadband analysis is likely to miss narrowband signals as the signal-to-noise ratio of a narrowband signal can be significantly reduced when combined with detector output from other frequencies. Data folding and the computationally efficient analysis pipeline, {\tt PyStoch}, enable us to perform the radiometer map-making at every frequency bin. We perform the search at 3072 {\tt{HEALPix}} equal area pixels uniformly tiling the sky and in every frequency bin of width 1/32~Hz in the range 201726~Hz, except for bins that are likely to contain instrumental artefacts and hence are notched. We do not find any statistically significant evidence for the existence of narrowband gravitational-wave signals in the analyzed frequency bins. Therefore, we place 95% confidence upper limits on the gravitational-wave strain for each pixel-frequency pair, the limits are in the range (0.0309.6)×10^24 . In addition, we outline a method to identify candidate pixel-frequency pairs that could be followed up by a more sensitive (and potentially computationally expensive) search, e.g., a matched-filtering-based analysis, to look for fainter nearly monochromatic coherent signals. The ASAF analysis is inherently independent of models describing any spectral or spatial distribution of power. We demonstrate that the ASAF results can be appropriately combined over frequencies and sky directions to successfully recover the broadband directional and isotropic results. ---------------------------------------------------------------- arXiv:2111.03604 astro-ph.CO gr-qc *Constraints on the cosmic expansion history from GWTC-3 * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We use 47 gravitational-wave sources from the Third LIGO-Virgo-KAGRA Gravitational-Wave Transient Catalog (GWTC-3) to estimate the Hubble parameter H(z), including its current value, the Hubble constant H_0 . Each gravitational-wave (GW) signal provides the luminosity distance to the source and we estimate the corresponding redshift using two methods: the redshifted masses and a galaxy catalog. Using the binary black hole (BBH) redshifted masses, we simultaneously infer the source mass distribution and H(z). The source mass distribution displays a peak around 34M, followed by a drop-off. Assuming this mass scale does not evolve with redshift results in a H(z) measurement, yielding H_0 =68^+12 _7 kms^1 Mpc^1 (68% credible interval) when combined with the H_0 measurement from GW170817 and its electromagnetic counterpart. This represents an improvement of 17% with respect to the H_0 estimate from GWTC-1. The second method associates each GW event with its probable host galaxy in the catalog GLADE+, statistically marginalizing over the redshifts of each event's potential hosts. Assuming a fixed BBH population, we estimate a value of H_0 =68^+8 _6 kms^1 Mpc^1 with the galaxy catalog method, an improvement of 42% with respect to our GWTC-1 result and 20% with respect to recent H_0 studies using GWTC-2 events. However, we show that this result is strongly impacted by assumptions about the BBH source mass distribution; the only event which is not strongly impacted by such assumptions (and is thus informative about H_0 ) is the well-localized event GW190814. ---------------------------------------------------------------- arXiv:2111.03606 gr-qc astro-ph.HE *GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo During the Second Part of the Third Observing Run * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: The third Gravitational-wave Transient Catalog (GWTC-3) describes signals detected with Advanced LIGO and Advanced Virgo up to the end of their third observing run. Updating the previous GWTC-2.1, we present candidate gravitational waves from compact binary coalescences during the second half of the third observing run (O3b) between 1 November 2019, 15:00 UTC and 27 March 2020, 17:00 UTC. There are 35 compact binary coalescence candidates identified by at least one of our search algorithms with a probability of astrophysical origin p_astro >0.5. Of these, 18 were previously reported as low-latency public alerts, and 17 are reported here for the first time. Based upon estimates for the component masses, our O3b candidates with p_astro >0.5 are consistent with gravitational-wave signals from binary black holes or neutron star-black hole binaries, and we identify none from binary neutron stars. However, from the gravitational-wave data alone, we are not able to measure matter effects that distinguish whether the binary components are neutron stars or black holes. The range of inferred component masses is similar to that found with previous catalogs, but the O3b candidates include the first confident observations of neutron star-black hole binaries. Including the 35 candidates from O3b in addition to those from GWTC-2.1, GWTC-3 contains 90 candidates found by our analysis with p_astro >0.5 across the first three observing runs. These observations of compact binary coalescences present an unprecedented view of the properties of black holes and neutron stars. ---------------------------------------------------------------- arXiv:2111.03608 astro-ph.HE gr-qc *Search for Gravitational Waves Associated with Gamma-Ray Bursts Detected by Fermi and Swift During the LIGO-Virgo Run O3b * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We search for gravitational-wave signals associated with gamma-ray bursts detected by the Fermi and Swift satellites during the second half of the third observing run of Advanced LIGO and Advanced Virgo (1 November 2019 15:00 UTC-27 March 2020 17:00 UTC).We conduct two independent searches: a generic gravitational-wave transients search to analyze 86 gamma-ray bursts and an analysis to target binary mergers with at least one neutron star as short gamma-ray burst progenitors for 17 events. We find no significant evidence for gravitational-wave signals associated with any of these gamma-ray bursts. A weighted binomial test of the combined results finds no evidence for sub-threshold gravitational wave signals associated with this GRB ensemble either. We use several source types and signal morphologies during the searches, resulting in lower bounds on the estimated distance to each gamma-ray burst. Finally, we constrain the population of low luminosity short gamma-ray bursts using results from the first to the third observing runs of Advanced LIGO and Advanced Virgo. The resulting population is in accordance with the local binary neutron star merger rate. ---------------------------------------------------------------- arXiv:2111.03634 astro-ph.HE gr-qc *The population of merging compact binaries inferred using gravitational waves through GWTC-3 * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We report on the population properties of 76 compact binary mergers detected with gravitational waves below a false alarm rate of 1 per year through GWTC-3. The catalog contains three classes of binary mergers: BBH, BNS, and NSBH mergers. We infer the BNS merger rate to be between 13 Gpc^3 yr^1 and 1900 Gpc^3 yr^1 and the NSBH merger rate to be between 7.4 Gpc^3 yr^1 and 320 Gpc^3 yr^1 , assuming a constant rate density versus comoving volume and taking the union of 90% credible intervals for methods used in this work. Accounting for the BBH merger rate to evolve with redshift, we find the BBH merger rate to be between 17.3 Gpc^3 yr^1 and 45 Gpc^3 yr^1 at a fiducial redshift (z=0.2). We obtain a broad neutron star mass distribution extending from 1.2^+0.1 _0.2 Mto 2.0^+0.3 _0.2 M. We can confidently identify a rapid decrease in merger rate versus component mass between neutron star-like masses and black-hole-like masses, but there is no evidence that the merger rate increases again before 10 M. We also find the BBH mass distribution has localized over- and under-densities relative to a power law distribution. While we continue to find the mass distribution of a binary's more massive component strongly decreases as a function of primary mass, we observe no evidence of a strongly suppressed merger rate above 60M. The rate of BBH mergers is observed to increase with redshift at a rate proportional to (1+z)^|| with =2.7^+1.8 _1.9 for z1. Observed black hole spins are small, with half of spin magnitudes below _i 0.26. We observe evidence of negative aligned spins in the population, and an increase in spin magnitude for systems with more unequal mass ratio. ---------------------------------------------------------------- arXiv:2111.13106 astro-ph.HE gr-qc *Searches for Gravitational Waves from Known Pulsars at Two Harmonics in the Second and Third LIGO-Virgo Observing Runs * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration. Abstract: We present a targeted search for continuous gravitational waves (GWs) from 236 pulsars using data from the third observing run of LIGO and Virgo (O3) combined with data from the second observing run (O2). Searches were for emission from the l=m=2 mass quadrupole mode with a frequency at only twice the pulsar rotation frequency (single harmonic) and the l=2,m=1,2 modes with a frequency of both once and twice the rotation frequency (dual harmonic). No evidence of GWs was found so we present 95% credible upper limits on the strain amplitudes h_0 for the single harmonic search along with limits on the pulsars' mass quadrupole moments Q_22 and ellipticities . Of the pulsars studied, 23 have strain amplitudes that are lower than the limits calculated from their electromagnetically measured spin-down rates. These pulsars include the millisecond pulsars J0437-4715 and J0711-6830 which have spin-down ratios of 0.87 and 0.57 respectively. For nine pulsars, their spin-down limits have been surpassed for the first time. For the Crab and Vela pulsars our limits are factors of 100 and 20 more constraining than their spin-down limits, respectively. For the dual harmonic searches, new limits are placed on the strain amplitudes C_21 and C_22 . For 23 pulsars we also present limits on the emission amplitude assuming dipole radiation as predicted by Brans-Dicke theory. ---------------------------------------------------------------- arXiv:2111.15116 gr-qc *Search of the Early O3 LIGO Data for Continuous Gravitational Waves from the Cassiopeia A and Vela Jr. Supernova Remnants * *Authors*: The LIGO Scientific Collaboration, the Virgo Collaboration. Abstract: We present directed searches for continuous gravitational waves from the neutron stars in the Cassiopeia A (Cas A) and Vela Jr. supernova remnants. We carry out the searches in the LIGO data from the first six months of the third Advanced LIGO and Virgo observing run, using the Weave semi-coherent method, which sums matched-filter detection-statistic values over many time segments spanning the observation period. No gravitational wave signal is detected in the search band of 20--976 Hz for assumed source ages greater than 300 years for Cas A and greater than 700 years for Vela Jr. Estimates from simulated continuous wave signals indicate we achieve the most sensitive results to date across the explored parameter space volume, probing to strain magnitudes as low as ~6.3×10^26 for Cas A and ~5.6×10^26 for Vela Jr. at frequencies near 166 Hz at 95% efficiency. ******************************************************************************