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AUSTRALASIAN SOCIETY FOR GENERAL RELATIVITY AND GRAVITATION

Electronic Newsletter -- #26, Summer 2023-24

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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, 2024.

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CONTENTS:

* REPORT ON 11^{TH} AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG11): University
of Tasmania, Hobart, 2-4 February, 2022

* MINUTES OF THE 12^{TH} BIENNIAL GENERAL MEETING OF THE ASGRG

held at University of Tasmania, Hobart, 3 February, 2022

* SUSAN SCOTT AND MATT VISSER SELECTED AS FELLOWS OF THE ISGRG

* 12^{TH} AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG12): University of
Tasmania, Hobart, 27 November - 1 December, 2023

* 13^{TH} BIENNIAL GENERAL MEETING OF THE ASGRG, 29 November 2023

* MEMBERSHIP DETAILS ONLINE at

http://www.asgrg.org/membership/index.php

* FORTHCOMING MEETINGS

* MEMBERS' ABSTRACTS at gr-qc, December 2021 - October 2023

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REPORT ON 11^{TH} AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG11)

University of Tasmania, Hobart, 2-4 February, 2022

ACGRG11 was the eleventh in the series of biennial conferences run by the ASGRG. The venue was the University of Tasmania, although because of the lingering effects of the COVID-19 pandemic many people chose to attend online.

Keynote talks were given by Vaishali Adya (NEMO, a kHz gravitational wave detector), Hayley Macpherson (numerical relativity), Meg Millhouse (searches for gravitational waves beyond coalescences), Roger Penrose (quantized twistor theory), Ryan Shannon (pulsar timing arrays), Rory Smith (observations of gravitational waves from NS-BH coalescence), Chris Stevens (perturbations of black holes by gravitational waves), Mike Tobar (precision measurement to test quantum gravity) and Magdalena Zych (relativistic quantum clocks).

In addition, Roger Penrose gave a Public Talk (remotely) titled “From Black-Hole Singularities to Cyclic Cosmology” from 8 to 9 pm on Wednesday 2 February.

There were another 33 contributed talks (15 plenary and 18 parallel) on a range of mostly theoretical topics, covering black holes, neutron stars, high-frequency and continuous gravitational waves, modified theories of gravity, numerical simulations in cosmology, weak gravitational lensing, cosmological voids, axions, Bondi-Sachs energy-momentum, the number of spacetime dimensions, cosmological time, cosmological torsion, topological acceleration, non-Euclidean topologies, standing-wave spacetimes, fluid spike solutions, spontaneous tensorization and power-law cosmology.

The Kerr Prize for the best student talk at ACGRG11 was shared between Ben Grace of the Australian National University, who spoke on “Continuous Gravitational Waves from Young Neutron Stars”, and Lucy Strang of the University of Melbourne, who spoke on “Searches for Continuous Gravitational Waves from Young Supernova Remnants in the Early Third Observing Run of Advanced LIGO and Virgo”.

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MINUTES OF THE 12^{TH} BIENNIAL GENERAL MEETING OF THE ASGRG

held at the University of Tasmania and concurently online via Zoom,

Thursday 3 February, 2022

The meeting opened at 5.05 p.m. (AEDT)

Present: Jörg Frauendiener (President), Karl Wette (Treasurer), Malcolm Anderson (Secretary), Susan Scott, David Wiltshire, Krzysztof Bolejko, Andrew Norton, Woei Chet Lim, Tevian Dray, John Steele

Apologies: Ben Whale

1. The minutes of the 11th Biennial General Meeting, held at Victoria University of Wellington, New Zealand on Tuesday 10 December, 2019, were presented to the meeting. Jörg Frauendiener moved that the minutes be accepted, and Krzysztof Bolejko seconded. The motion was approved.

Matters Arising: None.

2. President's Report: Jörg Frauendiener informed the meeting that

(i) Due to the ongoing pandemic, no activities were organised by the ASGRG in 2020 or 2021.

(ii) David Blair, David McClelland, Susan Scott and Peter Veitch were awarded the 2020 Australian Prime Minister’s Prize for Science, while Susan Scott was also awarded the 2020 Dirac Medal for the Advancement of Physics (a prize given jointly by UNSW and the AIP).

(iii) The proposal submitted by David Wiltshire, Richard Easther and Jenny Adams for funding for the Kerr-Tinsley Centre, as a New Zealand Centre of Research Excellence in Astrophysics, was not successful.

(iv) Renate Meyer of the University of Auckland has received a grant of NZ3 million from the Royal Society Te Aparangi to study aspects of gravitational waves.

(v) A new working group on astro-statistics and general relativity has been established in New Zealand.

· Susan Scott asked if most of the GR theorists in New Zealand were involved in the working group. David Wiltshire affirmed that they were.

(vi) The next AIP Congress will be held in Adelaide from 11 to 16 December 2022. The nominated plenary speaker from the ASGRG is Kip Thorne, and the ASGRG can also nominate invited speakers for the GR sessions.

· Susan Scott informed the meeting that Kip Thorne has now accepted the invitation and will attend the AIP Congress.

(vii) The ASGRG currently has 63 members (59 life members and 4 ordinary members).

David Wiltshire moved that the President’s Report be accepted, and Karl Wette seconded. The motion was approved.

3. Treasurer's Report: The Treasurer, Karl Wette, reported that as of 31 August 2021 the Society's current account contained $10,702.93, and there was another $5,305.76 in Paypal. The net balance of funds was therefore unchanged from 2019 at $16,008.69.

Jörg Frauendiener moved that the Treasurer’s Report be accepted, and David Wiltshire seconded. The motion was approved.

4. Auditor's Report: The Auditor, John Schutz, certified in an email dated 3 February 2022 that he was satisfied with the Society's accounts.

5. Appointment of Auditor for the next session: John Schutz has agreed 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 2022-2023 session unopposed:

President: Krzysztof Bolejko (nominated: Susan Scott, seconded: Jörg Frauendiener)

Treasurer: Karl Wette (nominated: Susan Scott, seconded: David Wiltshire)

Secretary: Malcolm Anderson (nominated: Susan Scott, seconded: David Wiltshire)

Committee: Susan Scott (nominated: David Wiltshire, seconded: Malcolm Anderson)

Committee: David Wiltshire (nominated: Susan Scott, seconded: Malcolm Anderson)

7. Date and venue for ACGRG12: The date of the next Conference was set for December 2023, but the venue remained undecided. One possible venue was Auckland. David Wiltshire would investigate this.

8. Other business

· Resolutions: The following ordinary resolutions were submitted to the BGM for approval by the ASGRG Secretary Karl Wette.

1. The Society authorises the Treasurer to open a new bank account for the Society with Bank Australia.

2. The authorised signatories to the account with Bank Australia are the President, Secretary, & Treasurer of the Society.

3. For the sole purpose of opening a bank account with Bank Australia, the beneficial owners of the Society are its office holders (President, Secretary, Treasurer).

All three solutions were adopted unanimously by the members present at the meeting.

· ASGRG Climate Policy: Andrew Norton submitted a document to the meeting and proposed that it be adopted as the Society’s official climate policy. It was decided to submit the proposal to a remote ballot of the entire membership rather than have a small subgroup vote on it at the BGM. Andrew Norton and Krzysztof Bolejko would jointly refine the text of the proposed resolution for the remote ballot and send it to the Secretary.

· Susan Scott reported that the current term of the OzGrav Centre [the ARC Centre of Excellence for gravitational wave discovery, hosted by Swinburne University] would expire in 2024. A proposal has been submitted for the renewal of the Centre, which has been shortlisted as one of the top 17 candidates by the ARC. The renewed Centre would focus more on FRBs and the Hubble tension, and employ fewer theorists than currently.

The meeting closed at 6.20 p.m. (AEDT)

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SUSAN SCOTT AND MATT VISSER SELECTED AS FELLOWS OF THE ISGRG

In the 2022 selection round, Susan Scott and Matt Visser were admitted as Fellows of the International Society on General Relativity and Gravitations (ISGRG). Their citations read:

**Prof. Susan Scott**

Australian National University, Australia

Citation: For groundbreaking contributions to the understanding of the singularities and the global structure of spacetime.

** **

**Prof. Matt Visser**

Victoria University of Wellington, New Zealand

Citation: For pioneering research on Lorentzian wormholes and analogue gravity, and highly influential contributions to black-hole physics.

For more information about the ISGRG, see

http://www.isgrg.org/

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12^{TH} AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG12)

University of Tasmania, Hobart, 27 November – 1 December, 2023

ACGRG12 is the twelfth 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 Carl Blair (UWA), Jackie Bondell (Swinburne), Leonardo Giani (UQ), Jan Plefka (Humboldt), Jade Powell (Swinburne), Pedro Henrique Barboza Rossetto (Melbourne) and David Wiltshire (Canterbury).

ACGRG12 will be hosted by the University of Tasmania from 27 November to 1 December, 2023, and is open to anyone with an interest in general relativity. Attendance is possible both in person and via Zoom.

In addition, David Blair will give a Public Talk titled “Australia Tests Einstein 1922-2022” from 3.45 to 4.45 pm on Tuesday 28 November, in Chemistry Lecture Theatre (210), University of Tasmania.

Local Organising Committee: Krzysztof Bolejko, Earl Lester, Cassidy Mihalenko, Benjamin Riley, Karelle Siellez, Gawain Simpson, Samuel Verne

Scientific Organising Committee: Malcolm Anderson (Brunei), Krzysztof Bolejko (Tasmania), Joerg Frauendiener (Otago), Susan Scott (ANU), Karl Wette (ANU), David Wiltshire (Canterbury)

For further details, visit the conference website at acgrg12.org

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13^{TH} BIENNIAL GENERAL MEETING OF THE ASGRG

The 2023 Biennial General Meeting of the ASGRG will be held in conjunction with ACGRG12, at 11.35 am on Wednesday 29 November 2023.

All ASGRG Executive Committee positions will be filled by election at the BGM. The outgoing Executive Committee members are:

President – Krzysztof Bolejko

Treasurer - Karl Wette

Secretary - Malcolm Anderson

Officer - Susan Scott

Officer - David Wiltshire

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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.

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FORTHCOMING MEETINGS

December 4-7, 2023: Mathematical Relativity: Past, Present Future

Universität Wien

Vienna, Austria

https://www.esi.ac.at/events/e526/

December 6-9, 2023:
10^{th} International Conference on Gravitation and Cosmology (ICGC 2023):

New Horizons and Singularities in Gravity

Indian Institute of Technology, Guhawati

https://indico.cern.ch/event/1268737/

December 11-13, 2023: Future Perspectives on Primordial Black Holes

Botanical Garden of Sapienza University

Rome, Italy

https://agenda.infn.it/event/35854/

January 17-23, 2024: 1st Pacific Summer School:

Theory and Observations in Einstein's Theory of Gravity and its Modifications

Universidad Central de Chile, Santiago, Chile

http://univie.ac.at/cers/cers12/undex.html

January 29-31, 2024: International Conference on Gravitation and Cosmology (ICGC24)

Department of Mathematics and Statistics

University of Lahore, Pakistan

https://sites.uol.edu.pk/icgc24/

January 29-March 1, 2024: YITP Workshop: Gravity and Cosmology 2024

Yukawa Institute for Theoretical Physics

Kyoto University, Japan

https://www2.yukawa.kyoto-u.ac.jp/~gc2024/index.php

June 10-14, 2024: Estate Quantistica 2024

Interational School on Gravity, Cosmology and Mathematical Physics

Scalea, Italy

https://monge.u-bourgogne.fr/gdito/EQ2024/

June 17-21, 2024:
27^{th} Capra Meeting on Radiation Reaction in General Relativity

National University of Singapore

https://www.caprameeting.org/

July 1-5, 2024: New Horizons for Psi School and Workshop:

Black Holes and Fundamental Fields

Lisbon, Portugal

https://strong-gr.com/new-horizons-for-psi/about/

August 26-30, 2024: Black Holes Inside and Out

Niels Bohr Institute

Copenhagen, Denmark

https://strong-gr.com/black-holes-inside-and-out/

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MEMBERS' ABSTRACTS at gr-qc, December 2021 - October 2023

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.

In addition to the 143 papers listed here, 25 preprints by the LIGO, Virgo and NANOGrav Collaborations were posted or cross-linked to gr-qc between December 2021 and October 2023. These collaborations include ASGRG members Rana Adhikari, David Blair, Philip Charlton, Neil Cornish, Ju Li, David McClelland, John Miller, Susan Scott, Bram Slagmolen, Eric Thrane, Peter Veitch, Karl Wette and Bernard Whiting.

Note that these 168 total papers represent 1.39% of the 12092 papers posted or cross-linked to gr-qc between December 2021 and October 2023.

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arXiv:2203.08228 gr-qc astro-ph.IM hep-ex hep-th

**Snowmass2021 Cosmic Frontier White Paper: Future Gravitational-Wave Detector Facilities **

**Authors:** Stefan W. Ballmer, Rana Adhikari, Leonardo Badurina, Duncan A. Brown,
Swapan Chattopadhyay, Matthew Evans, Peter Fritschel, Evan Hall, Jason M. Hogan, Karan Jani, Tim Kovachy, Kevin Kuns, Ariel Schwartzman, Daniel Sigg, Bram Slagmolen, Salvatore Vitale, Christopher Wipf

Abstract: The next generation of gravitational-wave observatories can explore a wide range of fundamental physics phenomena throughout the history of the universe. These phenomena include access to the universe's binary black hole population throughout cosmic time, to the universe's expansion history independent of the cosmic distance ladders, to stochastic gravitational-waves from early-universe phase transitions, to warped space-time in the strong-field and high-velocity limit, to the equation of state of nuclear matter at neutron star and post-merger densities, and to dark matter candidates through their interaction in extreme astrophysical environments or their interaction with the detector itself. We present the gravitational-wave detector concepts than can drive the future of gravitational-wave astrophysics. We summarize the status of the necessary technology, and the research needed to be able to build these observatories in the 2030s.

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arXiv:2205.14197 gr-qc astro-ph.HE astro-ph.IM

**Exposing Gravitational Waves below the Quantum Shot Noise **

**Authors:** Hang Yu, Denis Martynov, Rana X Adhikari, Yanbei Chen

Abstract: The sensitivities of ground-based gravitational-wave (GW) detectors are limited
by quantum shot noise at a few hundred Hertz and above. Nonetheless, one can use a quantum-correlation technique proposed by Martynov, et al. [Phys. Rev. A 95, 043831 (2017)] to remove the expectation value of the shot noise, thereby exposing underlying classical
signals in the cross spectrum formed by cross-correlating the two outputs in a GW interferometer's anti-symmetric port. We explore here the prospects and analyze the sensitivity of using quantum correlation to detect astrophysical GW signals. Conceptually,
this technique is similar to the correlation of two different GW detectors as it utilizes the fact that a GW signal will be correlated in the two outputs but the shot noise will be uncorrelated. Quantum correlation also has its unique advantages as it requires
only a single interferometer to make a detection. Therefore, quantum correlation could increase the duty cycle, enhance the search efficiency, and enable the detection of highly polarized signals. In particular, we show that quantum correlation could be especially
useful for detecting post-merger remnants of binary neutron stars with both short (<1s) and intermediate (∼10−10^{4}s)
durations and setting upper limits on continuous emissions from unknown pulsars.

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arXiv:2209.02998 quant-ph gr-qc

**Quantum precision limits of displacement noise free interferometers **

**Authors:** Tuvia Gefen, Rajashik Tarafder, Rana X. Adhikari, Yanbei Chen

Abstract: Current laser-interferometric gravitational wave detectors suffer from a fundamental limit to their precision due to the displacement noise of optical elements contributed by various sources. Several schemes for Displacement-Noise Free Interferometers (DFI) have been proposed to mitigate their effects. The idea behind these schemes is similar to decoherence-free subspaces in quantum sensing i.e. certain modes contain information about the gravitational waves but are insensitive to the displacement noise. In this paper we derive quantum precision limits for general DFI schemes, including optimal measurement basis and optimal squeezing schemes. We introduce a triangular cavity DFI scheme and apply our general bounds to it. Precision analysis of this scheme with different noise models shows that the DFI property leads to interesting sensitivity profiles and improved precision due to noise mitigation and larger gain from squeezing. Further extensions of this scheme are presented.

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arXiv:2308.00150 astro-ph.IM gr-qc physics.optics

**On the effects of mirror birefringence and its fluctuations to laser interferometric
gravitational wave detectors **

**Authors:** Yuta Michimura, Haoyu Wang, Francisco Salces-Carcoba, Christopher Wipf,
Aidan Brooks, Koji Arai, Rana X Adhikari

Abstract: Crystalline materials are promising candidates as substrates or high-reflective
coatings of mirrors to reduce thermal noises in future laser interferometric gravitational wave detectors. However, birefringence of such materials could degrade the sensitivity of gravitational wave detectors, not only because it can introduce optical losses,
but also because its fluctuations create extra phase noise in the arm cavity reflected beam. In this paper, we analytically estimate the effects of birefringence and its fluctuations in the mirror substrate and coating for gravitational wave detectors. Our
calculations show that the requirements for the birefringence fluctuations in silicon substrate and AlGaAs coating will be in the order of 10^{−8} rad/√Hz and 10^{−10} rad/√Hz at 100 Hz, respectively, for future gravitational wave detectors.
We also point out that optical cavity response needs to be carefully taken into account to estimate optical losses from depolarization.

Report number: LIGO-P2300220, JGW-P231506

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arXiv:2203.14969 gr-qc

**Equatorial and polar quasinormal modes and quasiperiodic oscillations of quantum deformed
Kerr black hole **

** **

**Authors:** Kimet Jusufi, Mustapha Azreg-Aïnou, Mubasher Jamil, Qiang Wu

Abstract: In this paper we focus on the relation between quasinormal modes (QNMs) and
a rotating black hole shadows. As a specific example, we consider the quantum deformed Kerr black hole obtained via Newman--Janis--Azreg-Aïnou algorithm. In particular, using the geometric-optics correspondence between the parameters of a QNMs and the conserved
quantities along geodesics, we show that, in the eikonal limit, the real part of QNMs is related to the Keplerian frequency for equatorial orbits. To this end, we explore the typical shadow radius for the viewing angles, θ_{0}=π/2, and obtained an
interesting relation in the case of viewing angle θ_{0}=0 (or equivalently θ_{0}=π). Furthermore we have computed the corresponding equatorial and polar modes and the thermodynamical stability of the quantum deformed Kerr black hole. We also
investigate other astrophysical applications such as the quasiperiodic oscillations and the motion of S2 star to constrain the quantum deforming parameter.

Journal ref: Universe 2022, 8(4), 210

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arXiv:2304.08456 gr-qc

**A stationary axisymmetric vacuum solution for pure R ^{2} gravity **

** **

**Authors:** Mustapha Azreg-Aïnou, Hoang Ky Nguyen

Abstract: The closed-form expression for pure R^{2} vacuum solution obtained
in Phys. Rev. D 107, 104008 (2023) lends itself to a generalization to axisymmetric setup via the modified Newman-Janis algorithm. We adopt the procedure put forth in Phys. Rev. D 90, 064041 (2014) bypassing the complexification of the radial coordinate. The
procedure presumes the existence of Boyer-Lindquist coordinates. Using the Event Horizon Telescope Collaboration results, we model the central black hole M87{*} by the thus obtained exact rotating metric, depending on the mass, rotation parameter and a third
dimensionless parameter. The latter is constrained upon investigating the shadow angular size assuming mass and rotation parameters are those of M87{*}. Stability is investigated.

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arXiv:2305.04321 gr-qc

**Traversable Morris-Thorne-Buchdahl wormholes in quadratic gravity **

**Authors:** Hoang Ky Nguyen, Mustapha Azreg-Aïnou

Abstract: The special Buchdahl-inspired metric obtained in a recent paper [Phys. Rev.
D 107, 104008 (2023)] describes asymptotically flat spacetimes in pure R^{2} gravity. The metric depends on a new (Buchdahl) parameter k of higher-derivative characteristic, and recovers the Schwarzschild metric when k=0. It is shown that the special
Buchdahl-inspired metric supports a two-way traversable Morris-Thorne wormhole for k∈(−1,0)
in which case the Weak Energy Condition is formally violated, a naked singularity for k∈(−∞,−1)∪(0,+∞),
and a non-Schwarzschild structure for k=−1.

Journal ref: Eur.Phys.J.C 83 (2023) 7, 626

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arXiv:2305.15450 gr-qc hep-th

**Revisiting Weak Energy Condition and wormholes in Brans-Dicke gravity **

** **

**Authors:** Hoang Ky Nguyen, Mustapha Azreg-Aïnou

Abstract: It is known that the formation of a wormhole typically involves a violation of the Weak Energy Condition (WEC), but the reverse is not necessarily true. In the context of Brans-Dicke gravity, the generalized Campanelli-Lousto solution, which we shall unveil in this paper, demonstrates a WEC violation that coincides with the appearance of unbounded sheets of spacetime within the "interior" section. The emergence of a wormhole in the "exterior" section is thus only an indirect consequence of the WEC violation. Additionally, we use the generalized Campanelli-Lousto solution to construct a Kruskal-Szekeres diagram, which exhibits a "gulf" sandwiched between the four quadrants in the diagram, a novel feature in Brans-Dicke gravity. Overall, our findings shed new light onto a complex interplay between the WEC and wormholes in the Brans-Dicke theory.

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arXiv:2307.10804 gr-qc

**On the parameters of the spherically symmetric parametrized Rezzolla-Zhidenko spacetime
through solar system tests, orbit of S2 star about Sgr A**^{⋆}**,
and quasiperiodic oscillations**

**Authors:** Sanjar Shaymatov, Bobomurat Ahmedov, Mariafelicia De Laurentis, Mubasher
Jamil, Qiang Wu, Anzhong Wang, Mustapha Azreg-Aïnou

Abstract: In this paper, we find the higher order expansion parameters α and λ of spherically
symmetric parametrized Rezzolla--Zhidenko (PRZ) spacetime by using its functions of the radial coordinate. We subject the parameters of this spacetime to classical tests including weak gravitational field effects in Solar System, observations of the S2 star
located in the star cluster close to the Sgr A^{⋆}^{
}and of the frequencies of selected microquasars. Based on this spherically symmetric spacetime we perform the analytic calculations for Solar System effects like perihelion shift, light deflection, and gravitational time delay so as to determine limits
on the parameters by using observational data. We restrict our attention to the limits on the two higher order expansion parameters α and λ that survive at the horizon or near the horizon of spherically symmetric metrics. The properties of these two small
parameters expansion in PRZ parametrization are discussed. We further apply the Monte Carlo Markov Chain (MCMC) simulations to analyze and obtain the limits on the expansion parameters by using observations of phenomena of the S2 star. Finally, we consider
the epicyclic motions and derive analytic expressions of the epicyclic frequencies. Applying these expressions to the quasiperiodic oscillations (QPOs) of selected microquasars allows us to set further limits on the parameters of the PRZ spacetime. Our results
demonstrate that the higher order expansion parameters can be given in the range α,λ=(−0.09,0.09) and of order
∼10^{−2} as a consequence of three various tests and
observations.

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arXiv:2310.08022 gr-qc

**JMN-1 singularity in weak magnetic field **

**Authors:** Mustapha Azreg-Aïnou, Kauntey Acharya, Pankaj S. Joshi

Abstract: The importance and significance of magnetic fields in the astrophysical scenario is well known. Many domains of astrophysical black hole physics such as polarized shadow image, high energy emitting processes and jet formation are dependent on the behavior of the magnetic fields in the vicinity of the compact objects. In light of this, we determine the master equation and master differential equation that determine the spatial behavior of the magnetic field inside a matter distribution or vacuum region, of general spherically symmetric metric, which is immersed in a test magnetic field. We also investigate here the case of JMN-1 singularity immersed in a uniform weak magnetic field and determine the behavior of magnetic fields by defining electromagnetic four potential vector. We find that the tangential component of the magnetic field is discontinuous at the matching surface of the JMN-1 singularity with the external Schwarzschild metric, resulting in surface currents. We briefly discuss the possible scenarios which would possess a discontinuous magnetic field and implications of the same and future possibilities in the realm of astrophysics are indicated.

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arXiv:2309.05336 gr-qc

**Vacuum Static Spherically Symmetric Spacetimes in Harada's Theory **

**Authors:** Alan Barnes

Abstract: Very recently Harada proposed a gravitational theory which is of third order in the derivatives of the metric tensor with the property that any solution of Einstein's field equations (EFEs) possibly with a cosmological constant is necessarily a solution of the new theory. He then applied his theory to derive a second-order ODE for the evolution of the scale factor of the FLRW metric. Remarkably he showed that, even in a matter-dominated universe with zero cosmological constant, there is a late-time transition from decelerating to accelerating expansion. Harada also derived a generalisation of the Schwarzschild solution. However, as his starting point he assumed an unnecessarily restricted form for a static spherically symmetric metric. In this note the most general spherically symmetric static vacuum solution of the theory is derived. Mantica and Molinari have shown that Harada's theory may be recast into the form of the EFEs with an additional source term in the form of a second-order conformal Killing tensor (CKT). Accordingly they have dubbed the theory conformal Killing gravity. Then, using a result in a previous paper of theirs on CKTs in generalised Robertson-Walker spacetimes, they rederived Harada's generalised evolution equation for the scale factor of the FLRW metric. However, Mantica and Molinari appear to have overlooked the fact that all solutions of the new theory (except those satisfying the EFEs) admit a non-trivial second-order Killing tensor. Such Killing tensors are invaluable when considering the geodesics of a metric as they lead to a second quadratic invariant of the motion in addition to that derived from the metric.

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arXiv:2209.14269 gr-qc

**Modeling Transit Dark Energy in f(R,L _{m})-gravity **

**Authors:** Anirudh Pradhan, Dinesh Chandra Maurya, Gopikant K. Goswami, Aroonkumar
Beesham

Abstract: This research paper deals with a transit dark energy cosmological model in
f(R,L_{m})-gravity with observational constraints. For this, we consider a flat FLRW space-time and have taken a cosmological cosntant-like parameter β in our field equations. The model has two energy parameters, Ω_{m0} and Ω_{β0},
which govern the mechanism of the universe, in particular its present accelerated phase. To make the model cope with the present observational scenario, we consider three types of observational data set: 46 Hubble parameter data set, SNe Ia 715 data sets of
distance modulus and apparent magnitude, and 40 datasets of SNe Ia Bined compilation in the redshift 0≤z<1.7. We have approximated the present values of the energy parameters by applying R^{2} and χ^{2}-test in the observational and theoretical
values of Hubble, distance modulus, and apparent magnitude parameters. Also, we have measured the approximate present values of cosmographic coefficients {H_{0},q_{0},j_{0},s_{0},l_{0},m_{0}}. It is found that
our approximated value-based model fits best with the observational module. We have found that as t→∞ (or z→0) then {q,j,s,l,m}→{−1,1,1,1,1}. The cosmic age of the present universe is also approximated and comes up to the expectation. Our model shows a transit
phase of the present accelerating universe with a deceleration in the past and has a transition point.

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arXiv:2210.15433 gr-qc

**An f(R,T) Gravity Based FLRW Model and Observational Constraints **

** **

**Authors:** Anirudh Pradhan, Gopikant Goswami, Rita Rani, Aroonkumar Beesham

Abstract: We attempt to construct a Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological
model in f(R,T) gravity which exhibits a phase transition from deceleration to acceleration at present. We take f(R,T)=R+2λT, λ being an arbitrary constant. In our model, the λ parameter develops a negative pressure in the universe whose Equation of state
is parameterized. The present values of model parameters such as density, Hubble, deceleration, Equation of state, and λ are estimated statistically by using the Chi-Square test. For this, we have used three different types of observational data sets: the
46 Hubble parameter data set, the SNeIa 715 data sets of distance modulus, and the 66 Pantheon data set (the latest compilation of SNeIa 40 bined plus 26 high red shift apparent magnitude mb data set in the red shift ranges from 0.014≤z≤2.26). We have calculated
the transitional red shift and time. The estimated results for the present values of various model parameters are found as per expectations and surveys. Interestingly, we get the present value of the density ρ_{0},
≃1.5ρc. The critical density is estimated as ρ_{c}≃1.88
h^{2}_{0} 10^{−29} gm/cm^{3} in the literature. The higher value of the present density is attributed to the presence of some additional energies in the universe apart from baryon energy. We have examined the behavior of the
pressure in our model. It is negative and produces acceleration in the universe. Its present value is obtained as p_{0}≃−0.7ρ_{0}.

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arXiv:2212.10413 gr-qc

**Observational constraints for an axially symmetric transitioning model with bulk viscosity
parameterization **

**Authors:** Archana Dixit, Anirudh Pradhan, Vinod Kumar Bhardwaj, A. Beesham

Abstract: In this paper, we have analyzed the significance of bulk viscosity in an axially symmetric Bianchi type-I model to study the accelerated expansion of the universe. We have considered four bulk viscosity parameterizations for the matter-dominated cosmological model. The function of the two significant Hubble H(z) and deceleration parameters are discussed in detail. The energy parameters of the universe are computed using the most recent observational Hubble data (57 data points) in the redshift range 0.07≤z≤2.36). In this model, we obtained all feasible solutions with the viscous component and analyzed the universe's expansion history. Finally, we analyzed the statefinder diagnostic and found some interesting results. The outcomes of our developed model now properly align with observational results.

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arXiv:2303.14136 gr-qc

**The Reconstruction of Constant Jerk Parameter with f(R,T) Gravity **

**Authors:** Anirudh Pradhan, Gopikant Goswami, Aroonkumar Beesham

Abstract: In this work, we have developed an FLRW type model of a universe which displays
transition from deceleration in the past to the acceleration at the present. For this, we have considered field equations of f(R,T) gravity and have taken f(R,T)=R+2λT, λ being an arbitrary constant. We have estimated the λ parameter in such a way that the
transition red shift is found similar in the deceleration parameter, pressure and the equation of state parameter ω. The present value of Hubble parameter is estimated on the basis of the three types of observational data set: latest compilation of 46 Hubble
data set, SNe Ia 580 data sets of distance modulus and 66 Pantheon data set of apparent magnitude which comprised of 40 SN Ia binned and 26 high redshift data's in the range 0.014≤z≤2.26. These data are compared with theoretical results through the χ^{2} statistical
test. Interestingly, the model satisfies all the three weak, strong and dominant energy conditions. The model fits well with observational findings. We have discussed some of the physical aspects of the model, in particular the age of the universe.

Journal ref: Journal of High Energy Astrophysics 2023

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arXiv:2304.09917 gr-qc

**Power law cosmology in modified theory with higher order curvature term **

**Authors:** J. K. Singh, Shaily, Anirudh Pradhan, Aroonkumar Beesham

Abstract: In this paper, we consider a cosmological model in f(R,G) gravity in flat space-time,
where R is the Ricci scalar and G is the Gauss-Bonnet invariant. Here, the function f(R,G) is taken as a linear combination of R and an exponential function of G. We analyze the observational constraints under a power law cosmology which depends on two parameters,
viz., the Hubble constant H_{0} and the deceleration parameter q. We examine the three sets of constraints H_{0}=68.119^{+0.028}_{−0.12} kmS^{−1}Mpc^{−1}, q=−0.109^{+0.014}_{−0.014}; H_{0}=70.5^{+1.3}_{−0.98}
kmS^{−1}Mpc^{−1}, q=−0.25^{+0.15}_{−0.15} and H_{0}=69.103^{+0.019}_{−0.10} KmS^{−1}Mpc^{−1}, q=−0.132^{+0.014}_{−0.014}, obtained by using the latest 77 points of the
H(z) data, 1048 points of the Pantheon data and the joint data of H(z)+Pantheon at the 1σ level, respectively, We compare our results with the results of the ΛCDM model. We find that our estimate of H_{0} is in very close agreement with some of the
latest results from the Planck Collaboration that assume the ΛCDM model. Our work in power law cosmology provides a better fit to the Pantheon data than the earlier analysis. We also discuss statefinder diagnostics and see that the power law models approach
the standard ΛCDM model (q→−0.5). Finally, we conclude that in f(R,G) gravity, power law cosmology explains most of the distinguished attributes of evolution in cosmology.

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arXiv:2304.11578 gr-qc

**A non-singular bouncing cosmology in f(R,T) gravity **

**Authors:** J. K. Singh, Shaily, Akanksha Singh, Aroonkumar Beesham, Hamid Shabani

Abstract: We investigate a bounce realization in the framework of higher order curvature in f(R,T) modified theory of gravity. We perform a detailed analysis of the cosmological parameters to explain the contraction phase, the bounce phase, and the expansion phase. Furthermore, we observe a violation of the null energy condition, instability of the model, and a singularity upon deceleration at the bouncing point, which are the supporting results for a bouncing cosmology. The outcome of the slow roll parameters is satisfactory to understand the inflation era and the equation of state parameter exhibits a ghost condensate behavior of the model near the bounce. Additionally, we discuss the stability of the model using linear perturbations in the Hubble parameter as well as the energy density.

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arXiv:2304.11616 gr-qc

**Reconstruction of an Observationally Constrained f(R,T) gravity model **

**Authors:** Anirudh Pradhan, Gopikant Goswami, Aroonkumar Beesham

Abstract: In this paper, an attempt is made to construct a Friedmann-Lemaitre-Robertson-Walker
model in f(R,T) gravity with a perfect fluid that yields acceleration at late times. We take f(R,T) as R + 8πμT. As in the ΛCDM model, we take the matter to consist of two components, viz., Ω_{m} and Ω_{μ} such that Ω_{m
}+ Ω_{μ}=1. The parameter Ω_{m} is the matter density (baryons + dark matter), and Ω_{μ} is the density associated with the Ricci scalar R and the trace T of the energy momentum tensor, which we shall call dominant matter. We find
that at present Ω_{μ} is dominant over Ω_{m}, and that the two are in the ratio 3:1 to 3:2 according to the three data sets: (i) 77 Hubble OHD data set (ii) 580 SNIa supernova distance modulus data set and (iii) 66 pantheon SNIa data which
include high red shift data in the range 0≤z≤2.36. We have also calculated the pressures and densities associated with the two matter densities, viz., p_{μ}, ρ_{μ}, p_{m} and ρ_{m}, respectively. It is also found that at present,
ρ_{μ} is greater than ρm. The negative dominant matter pressure pμ creates acceleration in the universe. Our deceleration and snap parameters show a change from negative to positive, whereas the jerk parameter is always positive. This means that the
universe is at present accelerating and in the past it was decelerating. State finder diagnostics indicate that our model is at present a dark energy quintessence model. The various other physical and geometric properties of the model are also discussed.

Journal ref: International Journal of Geometric Mathods in Modern Physics 2023

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arXiv:2306.09387 gr-qc

**Quintessences Universe in f(R,L _{m}) gravity with special form of deceleration
parameter **

** **

**Authors:** Bhupendra Kumar Shukla, Rishi KumarTiwari, Deger Sufuoglu, Aroonkumar
Beesham

Abstract: In this paper We have investigated a homogeneous and isotropic FRW cosmological
model with perfect fluid in the framework of f(R,L_{m}) gravity. We have explored for the non linear case of f(R,L_{m}) model, namely f(R,L_{m})=R/2+L_{m} and obtained the solution by using the condition that the deceleration
parameter is a linear function of the Hubble parameter. We employ 57 Hubble data points and 1048 Pantheon supernovae type Ia data samples to restrict the model parameters. Additionally, we employ Markoc Chain Monte Carlo (MCMC) simulation for our statistical
analysis. Additionally, we analyse the jerk and om diagnostic parameters for our model using the parameter values that were obtained.

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arXiv:2307.06969 physics.ed-ph gr-qc

**Maths for Einstein's Universe Tools for Understanding Modern Reality **

**Authors:
**Anastasia Popkova, David Blair, David Treagust

Abstract: Aversion to mathematics is a recognised and widespread problem. Following a review of the literature on this subject, this paper presents an education program which has been developed to test the hypothesis that transferring attention from traditional school arithmetic to a broad range of mathematical skills relevant to modern science at an early age (ages 7-12) will improve students' attitudes to mathematics, reduce the incidence of maths anxiety and prepare students for topics normally introduced at more senior levels. The program entitled Maths for Einstein's Universe includes five modules covering extreme numbers, estimation, probability, vectors and curved space geometry taught through group activities, games and plays. The modules complement appropriate early learning of modern physical concepts from the subatomic world to cosmology. While connected to science, the program aims to provide meaning and comprehension for socially relevant topics from national budgets to pandemics and opinion polls. The program has been trialled in multiple short workshops and extended learning programs as well as training programs for school teachers. Analysis of knowledge and attitude tests and questionnaires from about 170 participants demonstrate strong student enthusiasm and positive learning outcomes in areas normally considered beyond the ability of students in this age group. Trial results were used to identify strategies for enhancing school mathematics based on creation of stronger links between mathematics and science. We summarise results of pilot trials. In the paper we present the results of learning powers of ten and vectors. In total, around 700 participants have trialled Maths Einstein's Universe with nearly 200 hours of teaching for students and teachers.

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arXiv:2210.03322 astro-ph.CO gr-qc

**The Instantaneous Redshift Difference of Gravitationally Lensed Images: Theory and
Observational Prospects **

**Authors:** Chengyi Wang, Krzysztof Bolejko, Geraint F. Lewis

Abstract: Due to the expansion of our Universe, the redshift of distant objects changes with time. Although the amplitude of this redshift drift is small, it will be measurable with a decade-long campaigns on the next generation of telescopes. Here we present an alternative view of the redshift drift which captures the expansion of the universe in single epoch observations of the multiple images of gravitationally lensed sources. Considering a sufficiently massive lens, with an associated time delay of order decades, simultaneous photons arriving at a detector would have been emitted decades earlier in one image compared to another, leading to an instantaneous redshift difference between the images. We also investigate the effect of peculiar velocities on the redshift difference in the observed images. Whilst still requiring the observational power of the next generation of telescopes and instruments, the advantage of such a single epoch detection over other redshift drift measurements is that it will be less susceptible to systematic effects that result from requiring instrument stability over decade-long campaigns.

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arXiv:2304.00591 astro-ph.CO astro-ph.IM gr-qc

**Detecting cosmic voids via maps of geometric-optics parameters **

**Authors:** Marius Peper, Boudewijn F. Roukema, Krzysztof Bolejko

Abstract: Curved-spacetime geometric-optics maps derived from a deep photometric survey
should contain information about the three-dimensional matter distribution and thus about cosmic voids in the survey, despite projection effects. We explore to what degree sky-plane geometric-optics maps can reveal the presence of intrinsic three-dimensional
voids. We carry out a cosmological N-body simulation and place it further than a gigaparsec from the observer, at redshift 0.5. We infer three-dimensional void structures using the watershed algorithm. Independently, we calculate a surface overdensity map
and maps of weak gravitational lensing and geometric-optics scalars. We propose and implement a heuristic algorithm for detecting (projected) radial void profiles from these maps. We find in our simulation that given the sky-plane centres of the three-dimensional
watershed-detected voids, there is significant evidence of correlated void centres in the surface overdensity Σ, the averaged weak-lensing tangential shear γ_{⊥},
the Sachs expansion θ, and the Sachs shear modulus |σ|. Recovering the centres of the three-dimensional voids from the sky-plane information alone is significant given the weak-lensing shear γ_{⊥},
the Sachs expansion θ, or the Sachs shear |σ|, but not significant for the surface overdensity Σ. Void radii are uncorrelated between three-dimensional and two-dimensional voids; our algorithm is not designed to distinguish voids that are nearly concentric
in projection. This investigation shows preliminary evidence encouraging observational studies of gravitational lensing through individual voids, either blind or with spectroscopic/photometric redshifts. The former case - blind searches - should generate falsifiable
predictions of intrinsic three-dimensional void centres.

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arXiv:2303.15696 gr-qc astro-ph.IM

**pygwb: Python-based library for gravitational-wave background searches **

**Authors:** Arianna I. Renzini, Alba Romero-Rodrguez, Colm Talbot, Max Lalleman,
Shivaraj Kandhasamy, Kevin Turbang, Sylvia Biscoveanu, Katarina Martinovic, Patrick Meyers, Leo Tsukada, Kamiel Janssens, Derek Davis, Andrew Matas, Philip Charlton, Guo-Chin Liu, Irina Dvorkin, Sharan Banagiri, Sukanta Bose, Thomas Callister, Federico De
Lillo, Luca D'Onofrio, Fabio Garufi, Gregg Harry, Jessica Lawrence, Vuk Mandic, Adrian Macquet, Ioannis Michaloliakos, Sanjit Mitra, Kiet Pham, Rosa Poggiani, Tania Regimbau, Joseph D. Romano, Nick van Remortel, Haowen Zhong

Abstract: The collection of gravitational waves (GWs) that are either too weak or too numerous to be individually resolved is commonly referred to as the gravitational-wave background (GWB). A confident detection and model-driven characterization of such a signal will provide invaluable information about the evolution of the Universe and the population of GW sources within it. We present a new, user-friendly Python--based package for gravitational-wave data analysis to search for an isotropic GWB in ground--based interferometer data. We employ cross-correlation spectra of GW detector pairs to construct an optimal estimator of the Gaussian and isotropic GWB, and Bayesian parameter estimation to constrain GWB models. The modularity and clarity of the code allow for both a shallow learning curve and flexibility in adjusting the analysis to one's own needs. We describe the individual modules which make up {\tt pygwb}, following the traditional steps of stochastic analyses carried out within the LIGO, Virgo, and KAGRA Collaboration. We then describe the built-in pipeline which combines the different modules and validate it with both mock data and real GW data from the O3 Advanced LIGO and Virgo observing run. We successfully recover all mock data injections and reproduce published results.

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arXiv:2202.09382 gr-qc astro-ph.HE

**Probing neutron stars with the full pre-merger and post-merger gravitational wave
signal from binary coalescences **

**Authors:** Marcella Wijngaarden, Katerina Chatziioannou, Andreas Bauswein, James
A. Clark, Neil J. Cornish

Abstract: The gravitational wave signal emitted during the coalescence of two neutron stars carries information about the stars' internal structure. During the long inspiral phase the main matter observable is the tidal interaction between the binary components, an effect that can be parametrically modeled with compact-binary solutions to General Relativity. After the binary merger the main observable is frequency modes of the remnant, most commonly giving rise to a short-duration signal accessible only through numerical simulations. The complicated morphology and the decreasing detector sensitivity in the relevant frequencies currently hinder detection of the post-merger signal and motivate separate analyses for the pre-merger and post-merger data. However, planned and ongoing detector improvements could soon put the post-merger signal within reach. In this study we target the whole pre-merger and post-merger signal without an artificial separation at the binary merger. We construct a hybrid analysis that models the inspiral with templates based on analytical calculations and calibrated to numerical relativity and the post-merger signal with a flexible morphology-independent analysis. Applying this analysis to GW170817 we find, as expected, that the post-merger signal remains undetected. We further study simulated signals and find that we can reconstruct the full signal and simultaneously estimate both the pre-merger tidal deformation and the post-merger signal frequency content. Our analysis allows us to study neutron star physics using all the data available and directly test the pre-merger and post-merger signal for consistency thus probing effects such as the onset of the hadron-quark phase transition.

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arXiv:2204.07160 gr-qc astro-ph.HE

**Fast Bayesian analysis of individual binaries in pulsar timing array data **

**Authors:** Bence Bécsy, Neil J. Cornish, Matthew C. Digman

Abstract: Searching for gravitational waves in pulsar timing array data is computationally intensive. The data is unevenly sampled, and the noise is heteroscedastic, necessitating the use of a time-domain likelihood function with attendant expensive matrix operations. The computational cost is exacerbated when searching for individual supermassive black hole binaries, which have a large parameter space due to the additional pulsar distance, phase offset and noise model parameters needed for each pulsar. We introduce a new formulation of the likelihood function which can be used to make the Bayesian analysis significantly faster. We divide the parameters into projection and shape parameters. We then accelerate the exploration of the projection parameters by more than four orders of magnitude by precomputing the expensive inner products for each set of shape parameters. The projection parameters include nuisance parameters such as the gravitational wave phase offset at each pulsar. In the new scheme, these troublesome nuisance parameters are efficiently marginalized over using multiple-try Markov chain Monte Carlo sampling as part of a Metropolis-within-Gibbs scheme. The acceleration provided by our method will become increasingly important as pulsar timing datasets rapidly grow. Our method also makes sophisticated analyses more tractable, such as searches for multiple binaries, or binaries with non-negligible eccentricities.

Journal ref: Phys. Rev. D 105, 122003 (2022)

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arXiv:2205.03461 astro-ph.EP astro-ph.IM astro-ph.SR gr-qc

**Bayesian Characterisation of Circumbinary Exoplanets with LISA **

**Authors:** Michael L. Katz, Camilla Danielski, Nikolaos Karnesis, Valeriya Korol,
Nicola Tamanini, Neil J. Cornish, Tyson B. Littenberg

Abstract: The Laser Interferometer Space Antenna (LISA) will detect and characterize ∼104 Galactic Binaries consisting predominantly of two White Dwarfs (WD). An interesting prospect within this population is a third object--another WD star, a Circumbinary Exoplanet (CBP), or a Brown Dwarf (BD)--in orbit about the inner WD pair. We present the first fully Bayesian detection and posterior analysis of sub-stellar objects with LISA, focusing on the characterization of CBPs. We used an optimistic astrophysically motivated catalogue of these CBP third-body sources, including their orbital eccentricity around the inner binary for the first time. We examined Bayesian Evidence computations for detectability, as well as the effects on the posterior distributions for both the inner binary parameters and the third-body parameters. We find that the posterior behavior bifurcates based on whether the third-body period is above or below half the observation time. Additionally, we find that undetectable third-body sources can bias the inner binary parameters whether or not the correct template is used. We used the information retrieved from the study of the CBP population to make an initial conservative prediction for the number of detectable BD systems in the original catalogue. We end with commentary on the predicted qualitative effects on LISA global fitting and Galactic Binary population analysis. The procedure used in this work is generic and can be directly applied to other astrophysical effects expected within the Galactic Binary population.

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arXiv:2205.13580 gr-qc astro-ph.IM

**Accurate modeling and mitigation of overlapping signals and glitches in gravitational-wave
data **

**Authors:** Sophie Hourihane, Katerina Chatziioannou, Marcella Wijngaarden, Derek
Davis, Tyson Littenberg, Neil Cornish

Abstract: The increasing sensitivity of gravitational-wave detectors has brought about an increase in the rate of astrophysical signal detections as well as the rate of "glitches"; transient and non-Gaussian detector noise. Temporal overlap of signals and glitches in the detector presents a challenge for inference analyses that typically assume the presence of only Gaussian detector noise. In this study we perform an extensive exploration of the efficacy of a recently proposed method that models the glitch with sine-Gaussian wavelets while simultaneously modeling the signal with compact-binary waveform templates. We explore a wide range of glitch families and signal morphologies and demonstrate that the joint modeling of glitches and signals (with wavelets and templates respectively) can reliably separate the two. We find that the glitches that most affect parameter estimation are also the glitches that are well modeled by such wavelets due to their compact time-frequency signature. As a further test, we investigate the robustness of this analysis against waveform systematics like those arising from the exclusion of higher-order modes and spin-precession effects. Our analysis provides an estimate of the signal parameters; the glitch waveform to be subtracted from the data; and an assessment of whether some detected excess power consists of a glitch, signal, or both. We analyze the low-significance triggers (191225_215715 and 200114_020818) and find that they are both consistent with glitches overlapping high-mass signals.

Journal ref: Phys. Rev. D 106, 042006 15 August 2022

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arXiv:2206.14813 astro-ph.IM gr-qc

**LISA Gravitational Wave Sources in A Time-Varying Galactic Stochastic Background **

**Authors:** Matthew C. Digman, Neil J. Cornish

Abstract: A unique challenge for data analysis with the Laser Interferometer Space Antenna (LISA) is that the noise backgrounds from instrumental noise and astrophysical sources will change significantly over both the year and the entire mission. Variations in the noise levels will be on time scales comparable to, or shorter than, the time most signals spend in the detector's sensitive band. The variation in the amplitude of the galactic stochastic GW background from galactic binaries as the antenna pattern rotates relative to the galactic center is a particularly significant component of the noise variation. LISA's sensitivity to different source classes will therefore vary as a function of sky location and time. The variation will impact both overall signal-to-noise and the efficiency of alerts to EM observers to search for multi-messenger counterparts.

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arXiv:2207.01607 astro-ph.HE gr-qc

**Exploring realistic nanohertz gravitational-wave backgrounds **

**Authors:** Bence Bécsy, Neil J. Cornish, Luke Zoltan Kelley

Abstract: Hundreds of millions of supermassive black hole binaries are expected to contribute to the gravitational-wave signal in the nanohertz frequency band. Their signal is often approximated either as an isotropic Gaussian stochastic background with a power-law spectrum, or as an individual source corresponding to the brightest binary. In reality, the signal is best described as a combination of a stochastic background and a few of the brightest binaries modeled individually. We present a method that uses this approach to efficiently create realistic pulsar timing array datasets using synthetic catalogs of binaries based on the Illustris cosmological hydrodynamic simulation. We explore three different properties of such realistic backgrounds which could help distinguish them from those formed in the early universe: i) their characteristic strain spectrum; ii) their statistical isotropy; and iii) the variance of their spatial correlations. We also investigate how the presence of confusion noise from a stochastic background affects detection prospects of individual binaries. We calculate signal-to-noise ratios of the brightest binaries in different realizations for a simulated pulsar timing array based on the NANOGrav 12.5-year dataset extended to a time span of 15 years. We find that ∼6% of the realizations produce systems with signal-to-noise ratios larger than 5, suggesting that individual systems might soon be detected (the fraction increases to ∼41% at 20 years). These can be taken as a pessimistic prediction for the upcoming NANOGrav 15-year dataset, since it does not include the effect of potentially improved timing solutions and newly added pulsars.

Journal ref: ApJ 941 119 (2022)

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arXiv:2212.04600 gr-qc astro-ph.HE astro-ph.IM

**Parameter Estimation for Stellar-Origin Black Hole Mergers In LISA **

**Authors:** Matthew C. Digman, Neil J. Cornish

Abstract: The population of stellar origin black hole binaries (SOBHBs) detected by existing ground-based gravitational wave detectors is an exciting target for the future space-based Laser Interferometer Space Antenna (LISA). LISA is sensitive to signals at significantly lower frequencies than ground-based detectors. SOBHB signals will thus be detected much earlier in their evolution, years to decades before they merge. The mergers will then occur in the frequency band covered by ground-based detectors. Observing SOBHBs years before merger can help distinguish between progenitor models for these systems. We present a new Bayesian parameter estimation algorithm for LISA observations of SOBHBs that uses a time-frequency (wavelet) based likelihood function. Our technique accelerates the analysis by several orders of magnitude compared to the standard frequency domain approach and allows for an efficient treatment of non-stationary noise.

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arXiv:2301.03673 gr-qc astro-ph.HE astro-ph.IM

**Prototype Global Analysis of LISA Data with Multiple Source Types **

**Authors:** Tyson B. Littenberg, Neil J. Cornish

Abstract: The novel data analysis challenges posed by the Laser Interferometer Space Antenna (LISA) arise from the overwhelmingly large number of astrophysical sources in the measurement band and the density with which they are found in the data. Robust detection and characterization of the numerous gravitational wave sources in LISA data can not be done sequentially, but rather through a simultaneous global fit of a data model containing the full suite of astrophysical and instrumental features present in the data. While previous analyses have focused on individual source types in isolation, here we present the first demonstration of a LISA global fit analysis containing combined astrophysical populations. The prototype pipeline uses a blocked Metropolis Hastings algorithm to alternatingly fit to a population of ultra compact galactic binaries, known ``verification binaries'' already identified by electromagnetic observations, a population of massive black hole mergers, and an instrument noise model. The Global LISA Analysis Software Suite (GLASS) is assembled from independently developed samplers for the different model components. The modular design enables flexibility to future development by defining standard interfaces for adding new, or updating additional, components to the global fit without being overly prescriptive for how those modules must be internally designed. The GLASS pipeline is demonstrated on data simulated for the LISA Data Challenge 2b. Results of the analysis and a road-map for continued development are described in detail.

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arXiv:2301.11941 gr-qc

**Waveform accuracy and systematic uncertainties in current gravitational wave observations **

**Authors:** Caroline B. Owen, Carl-Johan Haster, Scott Perkins, Neil J. Cornish,
Nicolás Yunes

Abstract: The post-Newtonian formalism plays an integral role in the models used to extract information from gravitational wave data, but models that incorporate this formalism are inherently approximations. Disagreement between an approximate model and nature will produce mismodeling biases in the parameters inferred from data, introducing systematic error. We here carry out a proof-of-principle study of such systematic error by considering signals produced by quasi-circular, inspiraling black hole binaries through an injection and recovery campaign. In particular, we study how unknown, but calibrated, higher-order post-Newtonian corrections to the gravitational wave phase impact systematic error in recovered parameters. As a first study, we produce injected data of non-spinning binaries as detected by a current, second-generation network of ground-based observatories and recover them with models of varying PN order in the phase. We find that the truncation of higher order (>3.5) post-Newtonian corrections to the phase can produce significant systematic error even at signal-to-noise ratios of current detector networks. We propose a method to mitigate systematic error by marginalizing over our ignorance in the waveform through the inclusion of higher-order post-Newtonian coefficients as new model parameters. We show that this method can reduce systematic error greatly at the cost of increasing statistical error.

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arXiv:2306.17130 gr-qc astro-ph.HE astro-ph.IM

**Searching for gravitational wave burst in PTA data with piecewise linear functions **

**Authors:** Heling Deng, Bence Bécsy, Xavier Siemens, Neil J. Cornish, Dustin R.
Madison

Abstract: Transient gravitational waves (aka gravitational wave bursts) within the nanohertz frequency band could be generated by a variety of astrophysical phenomena such as the encounter of supermassive black holes, the kinks or cusps in cosmic strings, or other as-yet-unknown physical processes. Radio-pulses emitted from millisecond pulsars could be perturbed by passing gravitational waves, hence the correlation of the perturbations in a pulsar timing array can be used to detect and characterize burst signals with a duration of O(1-10) years. We propose a fully Bayesian framework for the analysis of the pulsar timing array data, where the burst waveform is generically modeled by piecewise straight lines, and the waveform parameters in the likelihood can be integrated out analytically. As a result, with merely three parameters (in addition to those describing the pulsars' intrinsic and background noise), one is able to efficiently search for the existence and the sky location of {a burst signal}. If a signal is present, the posterior of the waveform can be found without further Bayesian inference. We demonstrate this model by analyzing simulated data sets containing a stochastic gravitational wave background {and a burst signal generated by the parabolic encounter of two supermassive black holes.

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arXiv:2308.12827 gr-qc

**The LISA Data Challenge Radler Analysis and Time-dependent Ultra-compact Binary Catalogues **

**Authors:** Kristen Lackeos, Tyson B. Littenberg, Neil J. Cornish, James I. Thorpe

Abstract: Context. Galactic binaries account for the loudest combined continuous gravitational wave signal in the Laser Interferometer Space Antenna (LISA) band, which spans a frequency range of 0.1 mHz to 1 Hz. Aims. A superposition of low frequency Galactic and extragalactic signals and instrument noise comprise the LISA data stream. Resolving as many Galactic binary signals as possible and characterising the unresolved Galactic foreground noise after their subtraction from the data are a necessary step towards a global fit solution to the LISA data. Methods. We analyse a simulated gravitational wave time series of tens of millions of ultra-compact Galactic binaries hundreds of thousands of years from merger. This data set is called the Radler Galaxy and is part of the LISA Data challenges. We use a Markov Chain Monte Carlo search pipeline specifically designed to perform a global fit to the Galactic binaries and detector noise. Our analysis is performed for increasingly larger observation times of 1.5, 3, 6 and 12 months. Results. We show that after one year of observing, as many as ten thousand ultra-compact binary signals are individually resolvable. Ultra-compact binary catalogues corresponding to each observation time are presented. The Radler Galaxy is a training data set, with binary parameters for every signal in the data stream included. We compare our derived catalogues to the LISA Data challenge Radler catalogue to quantify the detection efficiency of the search pipeline. Included in the appendix is a more detailed analysis of two corner cases that provide insight into future improvements to our search pipeline.

Journal ref: A&A 678, A123 (2023)

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arXiv:2205.05235 gr-qc astro-ph.CO

**The Cosmic Web Crystal: Ising model for Large Scale Structures **

**Authors:** Leonardo Giani, Tamara M. Davis

Abstract: If Dark Matter halos possess the gravitational equivalent of an intrinsic magnetic spin, a formal analogy exists between the low redshift behaviour of the Cosmic Web in a flat FLRW background, and a crystal of spins submerged in a thermal reservoir with temperature T∝H(t). We argue that, within the use of the Bianchi type IX geometry to describe the gravitational collapse of matter inhomogeneities, the spins are nothing but the heritage of its underlying SU(2) symmetry. Therefore, just like electrons in quantum mechanics, these structures may have spin independently from their orbital angular momentum. We explore the phenomenological implications on cosmological scales of a possible late time phase transition of the Cosmic Web towards (the gravitational equivalent of) a ferromagnetic state, described qualitatively using the Ising model in the mean field approximation.

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arXiv:2301.08381 astro-ph.CO gr-qc

**Cross-correlating radial peculiar velocities and CMB lensing convergence **

**Authors:** Leonardo Giani, Cullan Howlett, Rossana Ruggeri, Federico Bianchini,
Khaled Said, Tamara M. Davis

Abstract: We study, for the first time, the cross correlation between the angular distribution
of radial peculiar velocities (PV) and the lensing convergence of cosmic microwave background (CMB) photons. We derive theoretical expectations for the signal and its covariance and assess its detectability with existing and forthcoming surveys. We find that
such cross-correlations are expected to improve constraints on different gravitational models by partially breaking degeneracies with the matter density. We identify in the distance-scaling dispersion of the peculiar velocities the most relevant source of
noise in the cross correlation. For this reason, we also study how the above picture changes assuming a redshift-independent scatter for the PV, obtained for example using a reconstruction technique. Our results show that the cross correlation might be detected
in the near future combining PV measurements from DESI and the convergence map from CMB-S4. Using realistic direct PV measurements we predict a cumulative signal-to-noise ratio of approximately 3.8σ using data on angular scales 3≤ℓ≤200. For an idealized reconstructed
peculiar velocity map extending up to redshift z=0.15 and a smoothing scale of 4 Mpc h^{−1} we predict a cumulative signal-to-noise ratio of approximately 27σ from angular scales 3≤ℓ≤200. We conclude that currently reconstructed peculiar velocities
have more constraining power than directly observed ones, even though they are more cosmological-model dependent.

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arXiv:2302.04878 astro-ph.HE astro-ph.IM gr-qc

**Designing an Optimal Kilonova Search using DECam for Gravitational Wave Events **

**Authors:** C. R. Bom, J. Annis, A. Garcia, A. Palmese, N. Sherman, M. Soares-Santos,
L. Santana-Silva, R. Morgan, K. Bechtol, T. Davis, H.T. Diehl, S. S. Allam, T. G. Bachmann, B. M. O. Fraga, J. Garcıa-Bellido, M. S. S. Gill, K. Herner, C. D. Kilpatrick, M. Makler, F. Olivares E., M. E. S. Pereira, J. Pineda, A. Santos, D. L. Tucker, M. P.
Wiesner, M. Aguena, O. Alves, D. Bacon, P. H. Bernardinelli, E. Bertin, S. Bocquet, D. Brooks, M. Carrasco Kind, J. Carretero, C. Conselice, M. Costanzi, L. N. da Costa, J. De Vicente, S. Desai, P. Doel, S. Everett, I. Ferrero, J. Frieman, M. Gatti, D. W.
Gerdes, D. Gruen, R. A. Gruendl, G. Gutierrez, S. R. Hinton, D. L. Hollowood, K. Honscheid, D. J. James, K. Kuehn, N. Kuropatkin, P. Melchior, J. Mena-Fernandez, F. Menanteau, A. Pieres, A. A. Plazas Malagon, M. Raveri, M. Rodriguez-Monroy, E. Sanchez, B.
Santiago, I. Sevilla-Noarbe, M. Smith, E. Suchyta, M. E. C. Swanson, G. Tarle, C. To, N. Weaverdyck

Abstract: We address the problem of optimally identifying all kilonovae detected via gravitational wave emission in the upcoming LIGO/Virgo/KAGRA Collaboration observing run, O4, which is expected to be sensitive to a factor of ∼7 more Binary Neutron Stars alerts than previously. Electromagnetic follow-up of all but the brightest of these new events will require >1 meter telescopes, for which limited time is available. We present an optimized observing strategy for the Dark Energy Camera during O4. We base our study on simulations of gravitational wave events expected for O4 and wide-prior kilonova simulations. We derive the detectabilities of events for realistic observing conditions. We optimize our strategy for confirming a kilonova while minimizing telescope time. For a wide range of kilonova parameters, corresponding to a fainter kilonova compared to GW170817/AT2017gfo we find that, with this optimal strategy, the discovery probability for electromagnetic counterparts with the Dark Energy Camera is ∼80% at the nominal binary neutron star gravitational wave detection limit for the next LVK observing run (190 Mpc), which corresponds to a ∼30% improvement compared to the strategy adopted during the previous observing run. For more distant events (∼330 Mpc), we reach a ∼60% probability of detection, a factor of ∼2 increase. For a brighter kilonova model dominated by the blue component that reproduces the observations of GW170817/AT2017gfo, we find that we can reach ∼90% probability of detection out to 330 Mpc, representing an increase of ∼20%, while also reducing the total telescope time required to follow-up events by ∼20%.

Report number: DES-2022-0714, FERMILAB-PUB-23-048-PPD

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arXiv:2302.10472 astro-ph.CO gr-qc

**Can Einstein (rings) surf Gravitational Waves? **

**Authors:** Leonardo Giani, Cullan Howlett, Tamara M. Davis

Abstract: How does the appearance of a strongly lensed system change if a gravitational wave is produced by the lens? In this work we address this question by considering a supermassive black hole binary at the center of the lens emitting gravitational waves propagating either colinearly or orthogonally to the line of sight. Specializing to an Einstein ring configuration (where the source, the lens and the observer are aligned), we show that the gravitational wave induces changes on the ring's angular size and on the optical path of photons. The changes are the same for a given pair of antipodal points on the ring, but maximally different for any pair separated by 90∘. For realistic lenses and binaries, we find that the change in the angular size of the Einstein ring is dozens of orders of magnitude smaller than the precision of current experiments. On the other hand, the difference in the optical path induced on a photon by a gravitational wave propagating \textit{orthogonally} to the line of sight triggers, at peak strain, time delays in the range ∼0.01−1 seconds, making the chance of their detection (and thus the use of Einstein rings as gravitational wave detectors) less hopeless.

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arXiv:2311.00215 astro-ph.CO gr-qc

**An effective description of Laniakea and its backreaction: Impact on Cosmology and
the local determination of the Hubble constant **

**Authors:** L. Giani, C. Howlett, K. Said, T. Davis, S. Vagnozzi

Abstract: We propose an effective model to describe the backreaction on cosmological observables induced by Laniakea, the gravitational supercluster hosting the Milky Way, which was defined using peculiar velocity data from Cosmicflows-4 (CF4). The structure is well described by an ellipsoidal shape exhibiting triaxial expansion, reasonably approximated by a constant expansion rate along the principal axes. Our best fits suggest that the ellipsoid, after subtracting the background expansion, contracts along the two smaller axes and expands along the longest one, predicting an average expansion of ∼−1.1 km/s/Mpc. The different expansion rates within the region, relative to the mean cosmological expansion, induce line-of-sight-dependent corrections in the computation of luminosity distances. We apply these corrections to two low-redshift datasets: the Pantheon+ catalog of type Ia Supernovae (SN~Ia), and 63 measurements of Surface Brightness Fluctuations (SBF) of early-type massive galaxies from the MASSIVE survey. We find corrections on the distances of order ∼2−3%, resulting in a shift in the inferred best-fit values of the Hubble constant H0 of order ΔHSN Ia0≈0.5 km/s/Mpc and ΔHSBF0≈1.1 km/s/Mpc, seemingly worsening the Hubble tension.

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arXiv:2306.08199 astro-ph.CO astro-ph.GA astro-ph.HE gr-qc

**Observational implications of cosmologically coupled black holes **

**Authors:** Sohan Ghodla, Richard Easther, M. M. Briel, J. J. Eldridge

Abstract: It was recently suggested that "cosmologically coupled" black holes with masses that increase in proportion to the volume of the Universe might constitute the physical basis of dark energy. We take this claim at face value and discuss its potential astrophysical implications. We show that the gravitational wave emission in binary systems would be significantly enhanced so that the number of black hole mergers would exceed the observed rate by orders of magnitude, with typical masses much larger than those seen by the LIGO-Virgo-KAGRA network. Separately, if the mass growth happens at fixed angular momentum, the supermassive black holes in matter-deficient elliptical galaxies should be slowly rotating. Finally, cosmological coupling would stabilize small black holes against Hawking radiation-induced evaporation.

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arXiv:2306.15939 astro-ph.GA astro-ph.SR gr-qc

**Inside MOND: Testing Gravity with Stellar Accelerations **

**Authors:** Maxwell Finan-Jenkin, Richard Easther

Abstract: We quantify the differences between stellar accelerations in disk galaxies
formed in a MONDian universe relative to galaxies with the identical baryonic matter distributions and a fitted cold dark matter halo. In a Milky Way-like galaxy the maximal transverse acceleration is O(10^{−9}) arcseconds per year per decade, well
beyond even the most optimistic extrapolations of current capabilities. Conversely, the maximum difference in the line-of-sight acceleration is O(1) centimetre per second per decade at solar distances from the galactic centre. This level of precision is within
reach of plausible future instruments.

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arXiv:2211.13276 gr-qc

**The non-linear perturbation of a black hole by gravitational waves. II. Quasinormal
modes and the compactification problem **

**Authors:** Jörg Frauendiener, Chris Stevens

Abstract: Recently, Friedrich's Generalized Conformal Field Equations (GCFE) have been implemented numerically and global quantities such as the Bondi energy and the Bondi-Sachs mass loss have been successfully calculated directly on null-infinity. Although being an attractive option for studying global quantities by way of local differential geometrical methods, how viable are the GCFE for study of quantities arising in the physical space-time? In particular, how long can the evolution track phenomena that need a constant proper physical timestep to be accurately resolved? We address this question by studying the curvature oscillations induced on the Schwarzschild space-time by a non-linear gravitational perturbation. For small enough amplitudes, these are the well approximated by the linear quasinormal modes, where each mode rings at a frequency determined solely by the Schwarzschild mass. We find that the GCFE can indeed resolve these oscillations, which quickly approach the linear regime, but only for a short time before the compactification becomes ``too fast'' to handle numerically.

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arXiv:2301.05268 gr-qc

**The non-linear perturbation of a black hole by gravitational waves. III. Newman-Penrose
constants **

**Authors:** Jörg Frauendiener, Alex Goodenbour, Chris Stevens

Abstract: In this paper we continue our study of the non-linear response of a Schwarzschild
black hole to an ingoing gravitational wave by computing the Newman-Penrose (NP) constants. The NP constants are five complex, supertranslation-invariant quantities defined on null infinity I^{+} and although put forward in the 60's, they have never
been computed in a non-stationary setting. We accomplish this through a numerical implementation of Friedrich's generalized conformal field equations whose semi-global evolution yields direct access to I^{+}. Generalizations of the NP constants' integral
expressions are made to allow their computation in a more general gauge that better suits the output of a numerical evolution. Canonical methods of fixing inherent degrees of freedom in their definitions are discussed. The NP constants are then computed for
a variety of different ingoing wave profiles in axisymmetry, and then with no symmetry assumptions in 3+1 for which all five are non-zero.

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arXiv:2309.09519 astro-ph.HE gr-qc

**Magnetically Confined Mountains on Accreting Neutron Stars in General Relativity **

**Authors:** Pedro H. B. Rossetto, Jörg Frauendiener, Ryan Brunet, Andrew Melatos

Abstract: The general relativistic formulation of the problem of magnetically confined
mountains on neutron stars is presented, and the resulting equations are solved numerically, generalising previous Newtonian calculations. The hydromagnetic structure of the accreted matter and the subsequent magnetic burial of the star's magnetic dipole moment
are computed. Overall, it is observed that relativistic corrections reduce the hydromagnetic deformation associated with the mountain. The magnetic field lines are curved more gently than in previous calculations, and the screening of the dipole moment is
reduced. Quantitatively, it is found that the dimensionless dipole moment (m_{d}) depends on the accreted mass (M_{a}) as m_{d}=−3.2×10^{3}M_{a}/M_{⊙}+1.0,
implying approximately three times less screening compared to the Newtonian theory. Additionally, the characteristic scale height of the mountain, governing the gradients of quantities like pressure, density, and magnetic field strength, reduces by approximately
40% for an isothermal equation of state.

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arXiv:2206.11617 gr-qc physics.ed-ph

**Simple precession calculation for Mercury: a linearization approach **

**Authors:** Michael J. W. Hall

Abstract: The additional precession of Mercury due to general relativity can be calculated by a method that is no more difficult than solving for the Newtonian orbit. The method relies on linearizing the relativistic orbit equation, is simpler than standard textbook methods, and is closely related to Newton's theorem on revolving orbits. The main result is accurate to all orders in 1/c for near-circular orbits.

Journal ref: Am. J. Phys. 90 (2022) 857-860

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arXiv:2204.11809 gr-qc astro-ph.HE

**Simulating bulk viscosity in neutron stars. I. Formalism **

**Authors:** Giovanni Camelio, Lorenzo Gavassino, Marco Antonelli, Sebastiano Bernuzzi,
Brynmor Haskell

Abstract: The faithful inclusion of the effects of bulk viscosity induced by the presence of chemical reactions is an important issue for simulations of core-collapse supernovae, binary neutron star mergers, and neutron star oscillations, where particle abundances are locally pushed out of chemical equilibrium by rarefaction and compression of the fluid elements. In this work, we discuss three different approaches that can be used to implement bulk viscosity in general relativistic hydrodynamic simulations of neutron stars: the exact multi-component reacting fluid, and two Müller-Israel-Stewart theories, namely the second order Hiscock-Lindblom model and its linear limit, the Maxwell-Cattaneo model. After discussing the theory behind the three approaches, we specialize their dynamics equations to spherical symmetry in the radial gauge-polar slicing (i.e., Schwarzschild) coordinates. We also discuss a particular choice for the equation of state of the fluid and the associated neutrino emission rates, which are used in a companion paper for the numerical comparison of the three frameworks, and we obtain the effective sound speed for the Hiscock-Lindblom theory in the non-linear regime.

Journal ref: Phys. Rev. D 107, 103031 (2023)

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arXiv:2204.11810 gr-qc astro-ph.HE

**Simulating bulk viscosity in neutron stars. II. Evolution in spherical symmetry **

**Authors:** Giovanni Camelio, Lorenzo Gavassino, Marco Antonelli, Sebastiano Bernuzzi,
Brynmor Haskell

Abstract: Out-of-equilibrium reactions between different particle species are the main processes contributing to bulk viscosity in neutron stars. In this work, we numerically compare three different approaches to the modeling of bulk viscosity: the multi-component fluid with reacting particle species and two bulk stress formalism based on the Müller-Israel-Stewart theory, namely the Hiscock-Lindblom and the Maxwell-Cattaneo models, whose flux-conservative formulation in radial gauge-polar slicing coordinates and spherical symmetry is derived in a companion paper. To our knowledge, this is the first time that a neutron star is simulated with the complete Hiscock-Lindblom model of bulk viscosity. We find that the Hiscock-Lindblom and Maxwell-Cattaneo models are good approximations of the multi-component fluid for small perturbations and when the non-equilibrium equation of state of the fluid depends on only one independent particle fraction. For more than one independent particle fraction and for large perturbations, the bulk stress approximation is still valid but less accurate. In addition, we include the energy loss due to the luminosity of the reactions in the bulk stress formulation. We find that the energy loss due to bulk viscosity has a larger effect on the dynamics than the bulk stress or the variation in particle composition per se. The new one-dimensional, general-relativistic hydrodynamic code developed for this work, hydro-bulk-1D, is publicly available.

Journal ref: Phys. Rev. D 107, 103032 (2023)

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arXiv:2207.14778 gr-qc hep-th physics.flu-dyn

**Symmetric-hyperbolic quasi-hydrodynamics **

**Authors:** Lorenzo Gavassino, Marco Antonelli, Brynmor Haskell

Abstract: We set up a general framework for systematically building and classifying, in the linear regime, causal and stable dissipative hydrodynamic theories that, alongside with the usual hydrodynamic modes, also allow for an arbitrary number of non-hydrodynamic modes with complex dispersion relation (such theories are often referred to as "quasi-hydrodynamic"). To increase the number of non-hydrodynamic modes one needs to add more effective fields to the model. The system of equations governing this class of quasi-hydrodynamic theories is symmetric hyperbolic, thermodynamically consistent (i.e. the entropy is a Lyapunov function) and can be derived from an action principle. As a first application of the formalism, we prove that, in the linear regime, the Israel-Stewart theory in the Eckart frame and the Israel-Stewart theory in the Landau frame are exactly the same theory. In addition, with an Onsager-Casimir analysis, we show that in strongly coupled plasmas the non-equilibrium degrees of freedom typically appear in pairs, whose members acquire opposite phase under time reversal. We use this insight to modify Cattaneo's model for diffusion, in a way to make its initial transient consistent with the transient dynamics of holographic plasmas.

Journal ref: Phys. Rev. D 106, 056010 (2022)

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arXiv:2211.15507 astro-ph.HE gr-qc

**Continuous gravitational wave emission from neutron stars with pinned superfluids
in the core **

**Authors:** Brynmor Haskell, Marco Antonelli, Pierre Pizzochero

Abstract: We investigate the effect of a pinned superfluid component on the gravitational wave emission of a rotating neutron star. Pinning of superfluid vortices to the flux-tubes in the outer core (where the protons are likely to form a type-II superconductor) is a possible mechanism to sustain long-lived and non-axisymmetric neutron currents in the interior, that break the axial symmetry of the unperturbed hydrostatic configuration. We consider pinning-induced perturbations to a stationary corotating configuration, and determine upper limits on the strength of gravitational wave emission due to the pinning of vortices with a strong toroidal magnetic field of the kind predicted by recent magneto-hydrodynamic simulations of neutron star interiors. We estimate the contributions to gravitational wave emission from both the mass and current multipole generated by the pinned vorticity in the outer core, and find that the mass quadrupole can be large enough for gravitational waves to provide the dominant spindown torque in millisecond pulsars.

Journal ref: Universe 2022, 8(12), 619

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arXiv:2303.12191 gr-qc

**The characteristic initial value problem for the conformally invariant wave equation
on a Schwarzschild background **

**Authors:** Jörg Hennig

Abstract: We resume former discussions of the conformally invariant wave equation on a Schwarzschild background, with a particular focus on the behaviour of solutions near the 'cylinder', i.e. Friedrich's representation of spacelike infinity. This analysis can be considered a toy model for the behaviour of the full Einstein equations and the resulting logarithmic singularities that appear to be characteristic for massive spacetimes. The investigation of the Cauchy problem for the conformally invariant wave equation (Frauendiener and Hennig 2018, Class. Quantum Grav. 35, 065015) showed that solutions generically develop logarithmic singularities at infinitely many expansion orders at the cylinder, but an arbitrary finite number of these singularities can be removed by appropriately restricting the initial data prescribed at t=0. From a physical point of view, any data at t=0 are determined from the earlier history of the system and hence not exactly 'free data'. Therefore, it is appropriate to ask what happens if we 'go further back in time' and prescribe initial data as early as possible, namely at a portion of past null infinity, and on a second past null hypersurface to complete the initial value problem. Will regular data at past null infinity automatically lead to a regular evolution up to future null infinity? Or does past regularity restrict the solutions too much, and regularity at both null infinities is mutually exclusive? Or do we still have suitable degrees of freedom for the data that can be chosen to influence regularity of the solutions to any desired degree? In order to answer these questions, we study the corresponding characteristic initial value problem. In particular, we investigate in detail the appearance of singularities at expansion orders n=0,…,4 for angular modes ℓ=0,…,4.

Journal ref: Class. Quantum Grav. 40, 085006 (2023)

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arXiv:2203.03471 astro-ph.HE gr-qc

**Gravitomagnetic interaction of a Kerr black hole with a magnetic field as the source
of the jetted GeV radiation of gamma-ray bursts **

**Authors:** J. A. Rueda, R. Ruffini, R. P. Kerr

Abstract: We show that the gravitomagnetic interaction of a Kerr black hole (BH) with a surrounding magnetic field induces an electric field that accelerates charged particles to ultra-relativistic energies in the vicinity of the BH. Along the BH rotation axis, these electrons/protons can reach energies of even thousands of PeV, so stellar-mass BHs in long gamma-ray bursts (GRBs) and supermassive BHs in active galactic nuclei (AGN) can contribute to the ultrahigh-energy cosmic rays (UHECRs) thorough this mechanism. At off-axis latitudes, the particles accelerate to energies of hundreds of GeV and emit synchrotron radiation at GeV energies. This process occurs within 60∘ around the BH rotation axis, and due to the equatorial-symmetry, it forms a double-cone emission. We outline the theoretical framework describing these acceleration and radiation processes, how they extract the rotational energy of the Kerr BH and the consequences for the astrophysics of GRBs.

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arXiv:2201.08491 astro-ph.HE gr-qc

**Linking the rates of neutron star binaries and short gamma-ray bursts **

**Authors:** Nikhil Sarin, Paul D. Lasky, Francisco H. Vivanco, Simon P. Stevenson,
Debatri Chattopadhyay, Rory Smith, Eric Thrane

Abstract: Short gamma-ray bursts are believed to be produced by both binary neutron star
(BNS) and neutron star-black hole (NSBH) mergers. We use current estimates for the BNS and NSBH merger rates to calculate the fraction of observable short gamma-ray bursts produced through each channel. This allows us to constrain merger rates of BNS to RBNS=384^{+431}_{−213}Gpc^{−3}yr^{−1} (90%
credible interval), a 16% decrease in the rate uncertainties from the second LIGO--Virgo Gravitational-Wave Transient Catalog, GWTC-2. Assuming a top-hat emission profile with a large Lorentz factor, we constrain the average opening angle of gamma-ray burst
jets produced in BNS mergers to ≈15^{∘}^{.} We
also measure the fraction of BNS and NSBH mergers that produce an observable short gamma-ray burst to be 0.02^{+0.02}_{−0.01} and 0.01±0.01, respectively and find that
≳40% of BNS mergers launch jets (90% confidence). We forecast
constraints for future gravitational-wave detections given different modelling assumptions, including the possibility that BNS and NSBH jets are different. With 24 BNS and 55 NSBH observations, expected within six months of the LIGO-Virgo-KAGRA network operating
at design sensitivity, it will be possible to constrain the fraction of BNS and NSBH mergers that launch jets with 10% precision. Within a year of observations, we can determine whether the jets launched in NSBH mergers have a different structure than those
launched in BNS mergers and rule out whether ≳80% of binary
neutron star mergers launch jets. We discuss the implications of future constraints on understanding the physics of short gamma-ray bursts and binary evolution.

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arXiv:2202.05479 astro-ph.IM gr-qc

**When models fail: an introduction to posterior predictive checks and model misspecification
in gravitational-wave astronomy **

**Authors:** Isobel M. Romero-Shaw, Eric Thrane, Paul D. Lasky

Abstract: Bayesian inference is a powerful tool in gravitational-wave astronomy. It enables us to deduce the properties of merging compact-object binaries and to determine how these mergers are distributed as a population according to mass, spin, and redshift. As key results are increasingly derived using Bayesian inference, there is increasing scrutiny on Bayesian methods. In this review, we discuss the phenomenon of \textit{model misspecification}, in which results obtained with Bayesian inference are misleading because of deficiencies in the assumed model(s). Such deficiencies can impede our inferences of the true parameters describing physical systems. They can also reduce our ability to distinguish the "best fitting" model: it can be misleading to say that Model A is preferred over Model B if both models are manifestly poor descriptions of reality. Broadly speaking, there are two ways in which models fail: models that fail to adequately describe the data (either the signal or the noise) have misspecified likelihoods. Population models -- designed, for example, to describe the distribution of black hole masses -- may fail to adequately describe the true population due to a misspecified prior. We recommend tests and checks that are useful for spotting misspecified models using examples inspired by gravitational-wave astronomy. We include companion python notebooks to illustrate essential concepts.

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arXiv:2203.07905 astro-ph.HE gr-qc

**Detectability of the gravitational-wave background produced by magnetar giant flares **

**Authors:** Nikolaos Kouvatsos, Paul D. Lasky, Ryan Quitzow-James, Mairi Sakellariadou

Abstract: We study the gravitational-wave background produced by f-mode oscillations of neutron stars triggered by magnetar giant flares. For the gravitational-wave energy, we use analytic formulae obtained via general relativistic magnetohydrodynamic simulations of strongly magnetized neutron stars. Assuming the magnetar giant flare rate is proportional to the star-formation rate, we show the gravitational-wave signal is likely undetectable by third-generation detectors such as the Einstein Telescope and Cosmic Explorer. We calculate the minimum value of the magnetic field and the magnetar giant flare rate necessary for such a signal to be detectable, and discuss these in the context of our current understanding of magnetar flares throughout the Universe.

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arXiv:2204.13267 gr-qc astro-ph.HE astro-ph.IM

**GWCloud: a searchable repository for the creation and curation of gravitational-wave
inference results **

**Authors:** A. Makai Baker, Paul D. Lasky, Eric Thrane, Gregory Ashton, Jesmigel
Cantos, Lewis Lakerink, Asher Leslie, Gregory B. Poole, Thomas Reichardt

Abstract: There are at present O(100) gravitational-wave candidates from compact binary
mergers reported in the astronomical literature. As detector sensitivities are improved, the catalog will swell in size: first to O(1000) events in the A+ era and then to O(10^{6}) events in the era of third-generation observatories like Cosmic Explorer
and the Einstein Telescope. Each event is analyzed using Bayesian inference to determine properties of the source including component masses, spins, tidal parameters, and the distance to the source. These inference products are the fodder for some of the most
exciting gravitational-wave science, enabling us to measure the expansion of the Universe with standard sirens, to characterise the neutron star equation of state, and to unveil how and where gravitational-wave sources are assembled. In order to maximize the
science from the coming deluge of detections, we introduce GWCloud, a searchable repository for the creation and curation of gravitational-wave inference products. It is designed with five pillars in mind: uniformity of results, reproducibility of results,
stability of results, access to the astronomical community, and efficient use of computing resources. We describe how to use GWCloud with examples, which readers can replicate using the companion code to this paper. We describe our long-term vision for GWCloud.

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arXiv:2206.14006 gr-qc astro-ph.HE

**Gravitational-wave inference for eccentric binaries: the argument of periapsis **

**Authors:** Teagan A. Clarke, Isobel M. Romero-Shaw, Paul D. Lasky, Eric Thrane

Abstract: Gravitational waves from binary black hole mergers have allowed us to directly observe stellar-mass black hole binaries for the first time, and therefore explore their formation channels. One of the ways to infer how a binary system is assembled is by measuring the system's orbital eccentricity. Current methods of parameter estimation do not include all physical effects of eccentric systems such as spin-induced precession, higher-order modes, and the initial argument of periapsis: an angle describing the orientation of the orbital ellipse. We explore how varying the argument of periapsis changes gravitational waveforms and study its effect on the inference of astrophysical parameters. We use the eccentric spin-aligned waveforms TEOBResumS and SEOBNRE to measure the change in the waveforms as the argument of periapsis is changed. We find that the argument of periapsis could already be impacting analyses performed with TEOBResumS. However, it is likely to be well-resolvable in the foreseeable future only for the loudest events observed by LIGO--Virgo--KAGRA. The systematic error in previous, low-eccentricity analyses that have not considered the argument of periapsis is likely to be small.

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arXiv:2207.00207 astro-ph.IM astro-ph.HE gr-qc

**Generating transient noise artifacts in gravitational-wave detector data with generative
adversarial networks **

**Authors:** Jade Powell, Ling Sun, Katinka Gereb, Paul D. Lasky, Markus Dollmann

Abstract: Transient noise glitches in gravitational-wave detector data limit the sensitivity of searches and contaminate detected signals. In this Paper, we show how glitches can be simulated using generative adversarial networks. We produce hundreds of synthetic images for the 22 most common types of glitches seen in the LIGO, KAGRA, and Virgo detectors. The artificial glitches can be used to improve the performance of searches and parameter-estimation algorithms. We perform a neural network classification to show that our artificial glitches are an excellent match for real glitches, with an average classification accuracy across all 22 glitch types of 99.0%.

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arXiv:2207.14346 gr-qc astro-ph.HE

**A Rosetta Stone for eccentric gravitational waveform models **

**Authors:** Alan M. Knee, Isobel M. Romero-Shaw, Paul D. Lasky, Jess McIver, Eric
Thrane

Abstract: Orbital eccentricity is a key signature of dynamical binary black hole formation. The gravitational waves from a coalescing binary contain information about its orbital eccentricity, which may be measured if the binary retains sufficient eccentricity near merger. Dedicated waveforms are required to measure eccentricity. Several models have been put forward, and show good agreement with numerical relativity at the level of a few percent or better. However, there are multiple ways to define eccentricity for inspiralling systems, and different models internally use different definitions of eccentricity, making it difficult to directly compare eccentricity measurements. In this work, we systematically compare two eccentric waveform models, SEOBNRE and TEOBResumS, by developing a framework to translate between different definitions of eccentricity. This mapping is constructed by minimizing the relative mismatch between the two models over eccentricity and reference frequency, before evolving the eccentricity of one model to the same reference frequency as the other model. We show that for a given value of eccentricity passed to SEOBNRE, one must input a 20-50% smaller value of eccentricity to TEOBResumS in order to obtain a waveform with the same empirical eccentricity. We verify this mapping by repeating our analysis for eccentric numerical relativity simulations, demonstrating that TEOBResumS reports a correspondingly smaller value of eccentricity than SEOBNRE.

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arXiv:2210.14938 astro-ph.HE gr-qc

**Missed opportunities: GRB 211211A and the case for continual gravitational-wave coverage
with a single observatory **

**Authors:** Nikhil Sarin, Paul D. Lasky, Rowina S. Nathan

Abstract: Gamma-ray burst GRB 211211A may have been the result of a neutron star merger at ≈350 Mpc. However, none of the LIGO-Virgo detectors were operating at the time. We show that the gravitational-wave signal from a \grb-like binary neutron star inspiral in the next LIGO-Virgo-KAGRA observing run (O4) would be below the conventional detection threshold, however a coincident gamma-ray burst observation would provide necessary information to claim a statistically-significant multimessenger observation. We calculate that with O4 sensitivity, approximately 11% of gamma-ray bursts within 600 Mpc will produce a confident association between the gravitational-wave binary neutron star inspiral signature and the prompt gamma-ray signature. This corresponds to a coincident detection rate of $\unit[0.22^{+8.3}_{-0.22}]{yr^{-1}}$, where the uncertainties are the 90% confidence intervals arising from uncertainties in the absolute merger rate, beaming and jet-launching fractions. These increase to approximately 34% and $\unit[0.71^{+26.8}_{-0.70}]{yr^{-1}}$ with proposed O5 sensitivity. We show that the above numbers do not depend significantly on the number of gravitational-wave observatories operating with the specific sensitivity. That is, the number of confident joint gamma-ray burst and gravitational-wave detections is only marginally improved with two or three detectors operating compared to a single detector. It is therefore worth considering whether one detector with sufficient sensitivity (post O4) should remain in sky-watch mode at all times to elucidate the true nature of GRB 211211A-like events, a proposal we discuss in detail.

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arXiv:2302.09711 gr-qc astro-ph.HE

**Nuclear physics with gravitational waves from neutron stars disrupted by black holes **

**Authors:** Teagan A. Clarke, Lani Chastain, Paul D. Lasky, Eric Thrane

Abstract: Gravitational waves from neutron star-black hole (NSBH) mergers that undergo tidal disruption provide a potential avenue to study the equation of state of neutron stars and hence the behaviour of matter at its most extreme densities. We present a phenomenological model for the gravitational-wave signature of tidal disruption, which allows us to measure the disruption time. We carry out a study with mock data, assuming an optimistically nearby NSBH event with parameters optimised for measuring the tidal disruption. We show that a two-detector network of 40 km Cosmic Explorer instruments can measure the time of disruption with a precision of 0.5 ms, which corresponds to a constraint on the neutron star radius of 0.7 km (90% credibility). This radius constraint is wider than the constraint obtained by measuring the tidal deformability of the neutron star of the same system during the inspiral. Moreover, the neutron star radius is likely to be more tightly constrained using binary neutron star mergers. While NSBH mergers are important for the information they provide about stellar and binary astrophysics, they are unlikely to provide insights into nuclear physics beyond what we will already know from binary neutron star mergers.

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arXiv:2303.10847 gr-qc

**The effect of noise artefacts on gravitational-wave searches for neutron star post-merger
remnants **

**Authors:** Fiona H. Panther, Paul D. Lasky

Abstract: Gravitational waves from binary neutron star post-merger remnants have the
potential to uncover the physics of the hot nuclear equation of state. These gravitational-wave signals are high frequency (∼ kHz)
and short lived (O(10ms)), which introduces potential problems for data-analysis algorithms due to the presence of non-stationary and non-Gaussian noise artefacts in gravitational-wave observatories. We quantify the degree to which these noise features in
LIGO data may affect our confidence in identifying post-merger gravitational-wave signals. We show that the combination of vetoing data with non-stationary glitches and the application of the Allen χ^{2} veto (usually reserved for long-lived lower-frequency
gravitational-wave signals), allows one to confidently detect post-merger signals with signal-to-noise ratio ρ≳8.
We discuss the need to incorporate the data-quality checks and vetos into realistic post-merger gravitational-wave searches, and describe how one can incorporate them to calculate realistic false-alarm and false-dismissal rates.

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arXiv:2304.02793 astro-ph.IM astro-ph.HE gr-qc

**Improving pulsar-timing solutions through dynamic pulse fitting **

**Authors:** Rowina S. Nathan, Matthew T. Miles, Gregory Ashton, Paul D. Lasky, Eric
Thrane, Daniel J. Reardon, Ryan M. Shannon, Andrew D. Cameron

Abstract: Precision pulsar timing is integral to the detection of the nanohertz stochastic gravitational-wave background as well as understanding the physics of neutron stars. Conventional pulsar timing often uses fixed time and frequency-averaged templates to determine the pulse times of arrival, which can lead to reduced accuracy when the pulse profile evolves over time. We illustrate a dynamic timing method that fits each observing epoch using basis functions. By fitting each epoch separately, we allow for the evolution of the pulse shape epoch to epoch. We apply our method to PSR J1103−5403 and demonstrate that it undergoes mode changing, making it the fourth millisecond pulsar to exhibit such behaviour. Our method, which is able to identify and time a single mode, yields a timing solution with a root-mean-square error of 1.343 μs, a factor of 1.78 improvement over template fitting on both modes. In addition, the white-noise amplitude is reduced 4.3 times, suggesting that fitting the full data set causes the mode changing to be incorrectly classified as white noise. This reduction in white noise boosts the signal-to-noise ratio of a gravitational-wave background signal for this particular pulsar by 32%. We discuss the possible applications for this method of timing to study pulsar magnetospheres and further improve the sensitivity of searches for nanohertz gravitational waves.

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arXiv:2307.15194 gr-qc astro-ph.CO

**Inhomogeneous Cosmology using General Relativistic Smoothed Particle Hydrodynamics
coupled to Numerical Relativity **

**Authors:** Spencer J. Magnall, Daniel J. Price, Paul D. Lasky, Hayley J. Macpherson

Abstract: We perform three-dimensional simulations of homogeneous and inhomogeneous cosmologies
via the coupling of a numerical relativity code for spacetime evolution and smoothed particle hydrodynamics (SPH) code. Evolution of a flat dust and radiation dominated Friedmann-Lemaître-Roberston-Walker (FLRW) spacetime shows an agreement of exact solutions
with residuals on the order 10^{−6} and 10^{−3} respectively, even at low grid resolutions. We demonstrate evolution of linear perturbations of density, velocity and metric quantities to the FLRW with residuals of only 10^{−2} compared
to exact solutions. Finally, we demonstrate the evolution of non-linear perturbations of the metric past shell-crossing, such that dark matter halo formation is possible. We show that numerical relativistic smoothed particle hydrodynamics is a viable method
for understanding non-linear effects in cosmology.

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arXiv:2201.07596 gr-qc math-ph

**Cosmological global dynamical systems analysis **

**Authors:** Artur Alho, Woei Chet Lim, Claes Uggla

Abstract: We consider a dynamical systems formulation for models with an exponential scalar field and matter with a linear equation of state in a spatially flat and isotropic spacetime. In contrast to earlier work, which only considered linear hyperbolic fixed point analysis, we do a center manifold analysis of the non-hyperbolic fixed points associated with bifurcations. More importantly though, we construct monotonic functions and a Dulac function. Together with the complete local fixed point analysis this leads to proofs that describe the entire global dynamics of these models, thereby complementing previous local results in the literature.

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arXiv:2202.02480 gr-qc

**Numerical confirmations of joint spike transitions in G _{2} cosmologies **

**Authors:** Woei Chet Lim

Abstract: We produce numerical evidence that the joint spike transitions between Kasner
eras of G_{2} cosmologies are described by the non-orthogonally transitive G_{2} spike solution. A new matching procedure is developed for this purpose.

Journal ref: Class. Quantum Grav. 39 (2022) 065010

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arXiv:2210.07276 gr-qc

**Big rip in shift-symmetric Kinetic Gravity Braiding theories **

**Authors:** Teodor Borislavov Vasilev, Mariam Bouhmadi-López, Prado Martín-Moruno

Abstract: We revise the future fate of a group of scalar-tensor theories known as kinetic gravity braiding models. As it is well-known, these theories can safely drive the expansion of the universe towards a future de Sitter state if the corresponding Lagrangian is invariant under constant shifts in the scalar field. However, this is not the only possible future state of these shift-symmetric models as we show in this letter. In fact, future cosmic singularities characterized by a divergence of the energy density can also appear in this framework. We present an explicit example where a big rip singularity is the only possible future fate of the cosmos.

Journal ref: Phys. Lett. B 838 (2023) 137711

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arXiv:2212.02547 gr-qc astro-ph.CO

**Phantom attractors in Kinetic Gravity Braiding theories: a dynamical system approach **

**Authors:** Teodor Borislavov Vasilev, Mariam Bouhmadi-López, Prado Martín-Moruno

Abstract: We revise the expansion history of the scalar field theories known as Kinetic Gravity Braiding. These theories are well-known for the possibility of driving the expansion of the cosmos towards a future self-tuning de Sitter state when the corresponding Lagrangian is invariant under constant shifts in the scalar field. Nevertheless, this is not the only possible future fate of these shift-symmetric models. Using a dynamical system formulation we show that future cosmological singularities can also appear in this framework. Moreover, we present explicit examples where the future attractor in the configuration space of the theory corresponds to a big rip singularity.

Journal ref: JCAP 06 (2023) 026

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arXiv:2302.08186 gr-qc astro-ph.CO hep-th

**Graviton-photon oscillation in a cosmic background for a general theory of gravity **

**Authors:** José A. R. Cembranos, Miguel González Ortiz, Prado Martín-Moruno

Abstract: Graviton-photon oscillation is the conversion of gravitational waves to electromagnetic waves and vice versa in the presence of a background electromagnetic field. We investigate this phenomenon in a cosmological scenario considering a background cosmic magnetic field and assuming different gravitational frameworks. We obtain the damping term that characterizes the attenuation of the conversion probability in cosmological backgrounds. This is a general feature that is present even for standard General Relativity. Furthermore, we show that the effects of decoherence, which are due to the interaction with the cosmological expansion and with the additional degrees of freedom of alternative theories of gravity, can be relevant to the phenomenon of graviton-photon mixing.

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arXiv:2307.14861 gr-qc astro-ph.CO

**TDiff in the Dark: Gravity with a scalar field invariant under transverse diffeomorphisms **

**Authors:** Darío Jaramillo-Garrido, Antonio L. Maroto, Prado Martín-Moruno

Abstract: We reflect on the possibility of having a matter action that is invariant only under transverse diffeomorphisms. This possibility is particularly interesting for the dark sector, where no restrictions arise based on the weak equivalence principle. In order to implement this idea we consider a scalar field which couples to gravity minimally but via arbitrary functions of the metric determinant. We show that the energy-momentum tensor of the scalar field takes the perfect fluid form when its velocity vector is time-like. We analyze the conservation of this tensor in detail, obtaining a seminal novel result for the energy density of this field in the kinetic dominated regime. Indeed, in this regime the fluid is always adiabatic and we obtain an explicit expression for the speed of sound. Furthermore, to get insight in the gravitational properties of these theories, we consider the fulfillment of the energy conditions, concluding that nontrivial physically reasonable matter violates the strong energy condition in the potential domination regime. On the other hand, we present some shift-symmetric models of particular interest. These are: constant equation of state models (which may provide us with a successful description of dark matter or dark radiation) and models presenting different gravitational domains (characterized by the focusing or possible defocusing of time-like geodesics), as it happens in unified dark matter-energy models.

Report number: IPARCOS-UCM-23-064

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arXiv:2308.13103 gr-qc astro-ph.HE

**Multi-messenger astronomy with a Southern-Hemisphere gravitational-wave observatory **

**Authors:** James W. Gardner, Ling Sun, Ssohrab Borhanian, Paul D. Lasky, Eric Thrane,
David E. McClelland, Bram J. J. Slagmolen

Abstract: Joint observations of gravitational waves and electromagnetic counterparts
will answer questions about cosmology, gamma-ray bursts, and the behaviour of matter at supranuclear densities. The addition of a Southern-Hemisphere gravitational-wave observatory to proposed global networks creates a longer baseline, which is beneficial
for sky localisation. We analyse how an observatory in Australia can enhance the multi-messenger astronomy capabilities of future networks. We estimate the number of binary neutron star mergers with joint observations of gravitational waves and kilonova counterparts
detectable by the Vera C. Rubin Observatory. First, we consider a network of upgrades to current observatories. Adding an Australian observatory to a three-observatory network (comprising two observatories in the USA and one in Europe) boosts the rate of joint
observations from 2.5^{+4.5}_{−2.0} per year to 5.6^{+10}_{−4.5} per year (a factor of two improvement). Then, we consider a network of next-generation observatories. Adding a 20 km Australian observatory to a global network
of a Cosmic Explorer 40 km in the USA and an Einstein Telescope in Europe only marginally increases the rate from 40^{+71}_{−32} per year to 44^{+79}_{−35} per year (a factor of 1.1 improvement). The addition of an Australian
observatory, however, ensures that at least two observatories are online far more often. When the Cosmic Explorer 40 km is offline for a major upgrade, the Australian observatory increases the joint observation rate from 0.5^{+0.8}_{−0.4} per
year to 38^{+68}_{−30} per year (a factor of 82 improvement). When the Einstein Telescope is offline, the joint observation rate increases from 0.2^{+0.3}_{−0.1} per year to 19^{+34}_{−15} per year (a factor
of 113 improvement). We sketch out the broader science case for a Southern-Hemisphere gravitational-wave observatory.

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arXiv:2206.06529 quant-ph astro-ph.IM gr-qc

**Nondegenerate internal squeezing: an all-optical, loss-resistant quantum technique
for gravitational-wave detection **

**Authors:** James W. Gardner, Min Jet Yap, Vaishali Adya, Sheon Chua, Bram J. J.
Slagmolen, David E. McClelland

Abstract: The detection of kilohertz-band gravitational waves promises discoveries in astrophysics, exotic matter, and cosmology. To improve the kilohertz quantum noise-limited sensitivity of interferometric gravitational-wave detectors, we investigate nondegenerate internal squeezing: optical parametric oscillation inside the signal-recycling cavity with distinct signal-mode and idler-mode frequencies. We use an analytic Hamiltonian model to show that this stable, all-optical technique is tolerant to decoherence from optical detection loss and that it, with its optimal readout scheme, is feasible for broadband sensitivity enhancement.

Accepted on July 11 2022 as a Letter in Phys. Rev. D

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arXiv:2307.10421 gr-qc astro-ph.CO astro-ph.HE astro-ph.IM

**Characterizing Gravitational Wave Detector Networks: From A ^{♯
}to Cosmic Explorer **

**Authors:** Ish Gupta, Chaitanya Afle, K. G. Arun, Ananya Bandopadhyay, Masha Baryakhtar,
Sylvia Biscoveanu, Ssohrab Borhanian, Floor Broekgaarden, Alessandra Corsi, Arnab Dhani, Matthew Evans, Evan D. Hall, Otto A. Hannuksela, Keisi Kacanja, Rahul Kashyap, Sanika Khadkikar, Kevin Kuns, Tjonnie G. F. Li, Andrew L. Miller, Alexander Harvey Nitz,
Benjamin J. Owen, Cristiano Palomba, Anthony Pearce, Hemantakumar Phurailatpam, Binod Rajbhandari, Jocelyn Read, Joseph D. Romano, Bangalore S. Sathyaprakash, David H. Shoemaker, Divya Singh, Salvatore Vitale, Lisa Barsotti, Emanuele Berti, Craig Cahillane,
Hsin-Yu Chen, Peter Fritschel, Carl-Johan Haster, Philippe Landry, Geoffrey Lovelace, David McClelland, Bram J J Slagmolen, Joshua Smith, Marcelle Soares-Santos, Ling Sun, David Tanner, Hiro Yamamoto, Michael Zucker

Abstract: Gravitational-wave observations by the Laser Interferometer Gravitational-Wave
Observatory (LIGO) and Virgo have provided us a new tool to explore the universe on all scales from nuclear physics to the cosmos and have the massive potential to further impact fundamental physics, astrophysics, and cosmology for decades to come. In this
paper we have studied the science capabilities of a network of LIGO detectors when they reach their best possible sensitivity, called A^{#}, and a new generation of observatories that are factor of 10 to 100 times more sensitive (depending on the frequency),
in particular a pair of L-shaped Cosmic Explorer observatories (one 40 km and one 20 km arm length) in the US and the triangular Einstein Telescope with 10 km arms in Europe. We use a set of science metrics derived from the top priorities of several funding
agencies to characterize the science capabilities of different networks. The presence of one or two A^{#} observatories in a network containing two or one next generation observatories, respectively, will provide good localization capabilities for
facilitating multimessenger astronomy and precision measurement of the Hubble parameter. A network of two Cosmic Explorer observatories and the Einstein Telescope is critical for accomplishing all the identified science metrics including the nuclear equation
of state, cosmological parameters, growth of black holes through cosmic history, and make new discoveries such as the presence of dark matter within or around neutron stars and black holes, continuous gravitational waves from rotating neutron stars, transient
signals from supernovae, and the production of stellar-mass black holes in the early universe. For most metrics the triple network of next generation terrestrial observatories are a factor 100 better than what can be accomplished by a network of three A^{#
}observatories.

Report number: CE Document No. P2300019-v2

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arXiv:2201.00451 gr-qc

**Graphics processing unit implementation of the F-statistic for continuous gravitational
wave searches **

**Authors:** Liam Dunn, Patrick Clearwater, Andrew Melatos, Karl Wette

Abstract: The F-statistic is a detection statistic used widely in searches for continuous gravitational waves with terrestrial, long-baseline interferometers. A new implementation of the F-statistic is presented which accelerates the existing "resampling" algorithm using graphics processing units (GPUs). The new implementation runs between 10 and 100 times faster than the existing implementation on central processing units without sacrificing numerical accuracy. The utility of the GPU implementation is demonstrated on a pilot narrowband search for four newly discovered millisecond pulsars in the globular cluster Omega Centauri using data from the second Laser Interferometer Gravitational-Wave Observatory observing run. The computational cost is 17.2 GPU-hours using the new implementation, compared to 1092 core-hours with the existing implementation.

Journal ref: Class. Quantum Grav. 39 (2022) 045003

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arXiv:2201.03683 gr-qc astro-ph.IM physics.ed-ph

**Continuous gravitational waves in the lab: recovering audio signals with a table-top
optical microphone **

**Authors:** James W. Gardner, Hannah Middleton, Changrong Liu, Andrew Melatos, Robin
Evans, William Moran, Deeksha Beniwal, Huy Tuong Cao, Craig Ingram, Daniel Brown, Sebastian Ng

Abstract: Gravitational-wave observatories around the world are searching for continuous waves: persistent signals from sources such as spinning neutron stars. These searches use sophisticated statistical techniques to look for weak signals in noisy data. In this paper, we demonstrate these techniques using a table-top model gravitational-wave detector: a Michelson interferometer where sound is used as an analog for gravitational waves. Using signal processing techniques from continuous-wave searches, we demonstrate the recovery of tones with constant and wandering frequencies. We also explore the use of the interferometer as a teaching tool for educators in physics and electrical engineering by using it as an "optical microphone" to capture music and speech. A range of filtering techniques used to recover signals from noisy data are detailed in the Supplementary Material. Here, we present highlights of our results using a combined notch plus Wiener filter and the statistical log minimum mean-square error (logMMSE) estimator. Using these techniques, we easily recover recordings of simple chords and drums, but complex music and speech are more challenging. This demonstration can be used by educators in undergraduate laboratories and can be adapted for communicating gravitational-wave and signal-processing topics to non-specialist audiences.

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arXiv:2202.05410 astro-ph.GA astro-ph.HE gr-qc

**An estimate of the stochastic gravitational wave background from the MassiveBlackII
simulation **

**Authors:** Bailey Sykes, Hannah Middleton, Andrew Melatos, Tiziana Di Matteo, Colin
DeGraf, Aklant Bhowmick

Abstract: A population of super-massive black hole binaries is expected to generate a
stochastic gravitational wave background (SGWB) in the pulsar timing array (PTA) frequency range of 10^{−9}--10^{−7} Hz. Detection of this signal is a current observational goal and so predictions of its characteristics are of significant interest.
In this work we use super-massive black hole binary mergers from the MassiveBlackII simulation to estimate the characteristic strain of the stochastic background. We examine both a gravitational wave driven model of binary evolution and a model which also
includes the effects of stellar scattering and a circumbinary gas disk. Results are consistent with PTA upper limits and similar to estimates in the literature. The characteristic strain at a reference frequency of 1yr^{−1} is found to be A_yr^{−1}=6.9×10^{−16} and
A_yr^{−1}=6.4×10^{−16} in the gravitational-wave driven and stellar scattering/gas disk cases, respectively. Using the latter approach, our models show that the SGWB is mildly suppressed compared to the purely gravitational wave driven model
as frequency decreases inside the PTA frequency band.

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arXiv:2203.14468 gr-qc astro-ph.IM

**Validating continuous gravitational-wave candidates from a semicoherent search using
Doppler modulation and an effective point spread function **

**Authors:** Dana Jones, Ling Sun, Julian Carlin, Liam Dunn, Meg Millhouse, Hannah
Middleton, Patrick Meyers, Patrick Clearwater, Deeksha Beniwal, Lucy Strang, Andrés Vargas, Andrew Melatos

Abstract: Following up large numbers of candidates in continuous gravitational wave searches presents a challenge, particularly in regard to computational power and the time required to manually scrutinize each of the candidates. It is important to design and test good follow-up procedures that are safe (i.e., minimize false dismissals) and computationally efficient across many search configurations. We investigate two follow-up procedures, or "vetoes," both of which exploit the Doppler modulation predicted in astrophysical signals. In particular, we introduce the concept of using an effective point spread function as part of our veto criteria. We take advantage of a well-established semicoherent search algorithm based on a hidden Markov model to study various search configurations and to generalize the veto criteria by considering the overall veto performance in terms of efficiency and safety. The results can serve as a guideline for follow-up studies in future continuous gravitational wave searches using a hidden Markov model algorithm. The results also apply qualitatively to other semicoherent search algorithms.

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arXiv:2205.15026 astro-ph.HE gr-qc

**Mountain formation by repeated, inhomogeneous crustal failure in a neutron star **

**Authors:** A. D. Kerin, A. Melatos

Abstract: The elastic crust of a neutron star fractures repeatedly as it spins down electromagnetically. An idealised, macroscopic model of inhomogeneous crustal failure is presented based on a cellular automaton with nearest-neighbour tectonic interactions involving strain redistribution and thermal dissipation. Predictions are made of the size and waiting-time distributions of failure events, as well as the rate of failure as the star spins down. The last failure event typically occurs when the star spins down to approximately 1% of its birth frequency with implications for rotational glitch activity. Neutron stars are commonly suggested as sources of continuous gravitational waves. The output of the automaton is converted into predictions of the star's mass ellipticity and gravitational wave strain as functions of its age, with implications for future observations with instruments such as the Laser Interferometer Gravitational Wave Observatory (LIGO), the Virgo interferometer, or the Kamioka Gravitational Wave Detector (KAGRA).

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arXiv:2208.03932 gr-qc

**Search for continuous gravitational waves from PSR J0437−4715 with a hidden Markov
model in O3 LIGO data **

**Authors:** Andrés F. Vargas, Andrew Melatos

Abstract: Results are presented for a semi-coherent search for continuous gravitational waves from the millisecond pulsar PSR J0437−4715, using a hidden Markov model to track spin wandering, in LIGO data from the third LIGO-Virgo observing run. This is the first search for PSR J0437−4715 to cover a wide frequency range from 60 Hz to 500 Hz and simultanously accommodate random spin deviations from the secular radio ephemeris. Two searches are performed with plausible coherence times of 10 days and 30 days, as the frequency wandering time-scale of the gravitational-wave-emitting quadrupole is unknown. The former analysis yields no surviving candidates, while the latter yields five candidates after the veto procedure. The detection statistic of each of the five survivors is mapped as a function of sky position, in preparation for follow-up analyses in the future, e.g. during LIGO-Virgo-KAGRA fourth observing run.

Journal ref: Phys. Rev. D 107 (2022) 064062

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arXiv:2209.10981 gr-qc

**Inferring neutron star properties with continuous gravitational waves **

**Authors:** Neil Lu, Karl Wette, Susan M. Scott, Andrew Melatos

Abstract: Detection of continuous gravitational waves from rapidly-spinning neutron stars opens up the possibility of examining their internal physics. We develop a framework that leverages a future continuous gravitational wave detection to infer a neutron star's moment of inertia, equatorial ellipticity, and the component of the magnetic dipole moment perpendicular to its rotation axis. We assume that the neutron star loses rotational kinetic energy through both gravitational wave and electromagnetic radiation, and that the distance to the neutron star can be measured, but do not assume electromagnetic pulsations are observable or a particular neutron star equation of state. We use the Fisher information matrix and Monte Carlo simulations to estimate errors in the inferred parameters, assuming a population of gravitational-wave-emitting neutron stars consistent with the typical parameter domains of continuous gravitational wave searches. After an observation time of one year, the inferred errors for many neutron stars are limited chiefly by the error in the distance to the star. The techniques developed here will be useful if continuous gravitational waves are detected from a radio, X-ray, or gamma-ray pulsar, or else from a compact object with known distance, such as a supernova remnant.

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arXiv:2210.09592 astro-ph.HE gr-qc

**Search for continuous gravitational waves from HESS~J1427-608 with a hidden Markov
model **

**Authors:** Deeksha Beniwal, Patrick Clearwater, Liam Dunn, Lucy Strang, Gavin Rowell,
Andrew Melatos, David Ottaway

Abstract: We present a search for continuous gravitational wave signals from an unidentified
pulsar potentially powering HESS~J1427-608, a spatially unresolved TeV point source detected by the High Energy Stereoscopic System (HESS). The search uses a semi-coherent algorithm, which combines the maximum likelihood F-statistic with a hidden Markov model
to efficiently detect and track quasi-monochromatic signals that wander randomly in frequency. It uses data from the second observing run of the Advanced Laser Interferometer Gravitational-Wave Observatory. Multi-wavelength observations of the HESS source
are combined with the proprieties of the population of TeV-bright pulsar wind nebulae to constrain the search parameters. We find no evidence of gravitational wave emission from this target. We set upper limits on the characteristic wave strain h^{95%}_{0}
(for circularly polarized signals) at 95% confidence level in sample sub-bands and interpolate it to estimate the sensitivity in the full band. We find h^{95%}_{0}=1.3×10^{−25} near 185 Hz. The implied constraints on the ellipticity
and \textit{r}-mode amplitude reach ε≤10^{−5} and α≤10^{−3} at 200 Hz, respectively.

Report number: LIGO-P2200181-v1

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arXiv:2211.07884 eess.SP astro-ph.IM gr-qc

**Estimating and detecting random processes on the unit circle **

**Authors:** Changrong Liu, S. Suvorova, R. J. Evans, B. Moran, A. Melatos

Abstract: The problem of detecting a sinusoidal signal with randomly varying frequency has a long history. It is one of the core problems in signal processing, arising in many applications including, for example, underwater acoustic frequency line tracking, demodulation of FM radio communications, laser phase drift in optical communications and, recently, continuous gravitational wave astronomy. In this paper we describe a Markov Chain Monte Carlo based procedure to compute a specific detection posterior density. We demonstrate via simulation that our approach results in an up to 25 percent higher detection rate than Hidden Markov Model based solutions, which are generally considered to be the leading techniques for these problems.

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arXiv:2310.19183 gr-qc

**Search for gravitational waves from Scorpius X-1 with a hidden Markov model in O3
LIGO data with a corrected orbital ephemeris **

**Authors:** Andrés F. Vargas, Andrew Melatos

Abstract: Results are presented for a semi-coherent search for gravitational waves from the low-mass X-ray binary Scorpius X-1 in Observing Run 3 (O3) data from the Laser Interferometer Gravitational Wave Observatory, using an updated orbital parameter ephemeris and a hidden Markov model (HMM) to allow for spin wandering. The new orbital ephemeris corrects errors in previously published orbital measurements and implies a new search domain. This search domain does not overlap with the one used in the original Scorpius X-1 HMM O3 search. The corrected domain is approximately three times smaller by area in the Tasc--P plane than the original domain, where Tasc and P denote the time of passage through the ascending node and the orbital period respectively, reducing the trials factor and computing time. No evidence is found for gravitational radiation in the search band from 60 Hz to 500 Hz. Upper limits are computed for the characteristic gravitational wave strain. They are consistent with the values from the original Scorpius X-1 HMM O3 search.

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arXiv:2112.07730 gr-qc math.AP

**Localized big bang stability for the Einstein-scalar field equations **

**Authors:** Florian Beyer, Todd A. Oliynyk

Abstract: We prove the nonlinear stability in the contracting direction of Friedmann-Lemaître-Robertson-Walker
(FLRW) solutions to the Einstein-scalar field equations in n≥3 spacetime dimensions that are defined on spacetime manifolds of the form (0,t_{0}]×T^{n−1}, t_{0}>0. Stability is established under the assumption that the initial data
is \textit{synchronized}, which means that on the initial hypersurface Σ={t_{0}}× T^{n−1} the scalar field τ=exp(√[2(n−2)/(n−1)]φ) is constant, that is, Σ=τ^{−1}({t_{0}}). As we show that all initial data sets that are sufficiently
close to FRLW ones can be evolved via the Einstein-scalar field equation into new initial data sets that are \textit{synchronized}, no generality is lost by this assumption. By using τ as a time coordinate, we establish that the perturbed FLRW spacetime manifolds
are of the form M=⋃_{t}_{∈}_{(0,t0}]τ^{−1}({t})≅(0,t_{0}]×T^{n−1},
the perturbed FLRW solutions are asymptotically pointwise Kasner as τ↘0,
and a big bang singularity, characterised by the blow up of the scalar curvature, occurs at τ=0. An important aspect of our stability proof is that we use a hyperbolic gauge reduction of the Einstein-scalar field equations. As a consequence, all of the estimates
used in the stability proof can be localized and we employ this property to establish a corresponding localized stability result for the FLRW solutions.

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arXiv:2202.05432 gr-qc math.AP

**Global existence and stability of de Sitter-like solutions to the Einstein-Yang-Mills
equations in spacetime dimensions n≥4 **

**Authors:** Chao Liu, Todd A. Oliynyk, Jinhua Wang

Abstract: We establish the global existence and stability to the future of non-linear perturbation of de Sitter-like solutions to the Einstein-Yang-Mills system in n≥4 spacetime dimension. This generalizes Friedrich's Einstein-Yang-Mills stability results in dimension n=4 [11] to all higher dimensions.

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arXiv:2208.12009 math.NA gr-qc

**A polyhedral discrete de Rham numerical scheme for the Yang-Mills equations **

**Authors:** Jérôme Droniou, Todd A. Oliynyk, Jia Jia Qian

Abstract: We present a discretisation of the 3+1 formulation of the Yang-Mills equations in the temporal gauge, using a Lie algebra-valued extension of the discrete de Rham (DDR) sequence, that preserves the non-linear constraint exactly. In contrast to Maxwell's equations, where the preservation of the analogous constraint only depends on reproducing some complex properties of the continuous de Rham sequence, the preservation of the non-linear constraint relies for the Yang-Mills equations on a constrained formulation, previously proposed in [10]. The fully discrete nature of the DDR method requires to devise appropriate constructions of the non-linear terms, adapted to the discrete spaces and to the need for replicating the crucial Ad-invariance property of the L2-product. We then prove some energy estimates, and provide results of 3D numerical simulations based on this scheme.

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arXiv:2209.06982 gr-qc math.AP

**On the stability of relativistic perfect fluids with linear equations of state p=Kρ
where 1/3<K<1 **

**Authors:** Elliot Marshall, Todd A. Oliynyk

Abstract: For 1/3<K<1, we consider the stability of two distinct families of spatially
homogeneous solutions to the relativistic Euler equations with a linear equation of state p=Kρ on exponentially expanding FLRW spacetimes. The two families are distinguished by one being spatially isotropic while the other is not. We establish the future stability
of nonlinear perturbations of the non-isotropic family for the full range of parameter values 1/3<K<1, which improves a previous stability result established by the second author that required K to lie in the restricted range (1/3,1/2). As a first step towards
understanding the behaviour of nonlinear perturbations of the isotropic family, we construct numerical solutions to the relativistic Euler equations under a T^{2}-symmetry assumption. These solutions are generated from initial data at a fixed time
that is chosen to be suitably close to the initial data of an isotropic solution. Our numerical results reveal that, for the full parameter range 1/3<K<1, the density contrast ∂xρρ associated to a nonlinear perturbation of an isotropic solution develops steep
gradients near a finite number of spatial points where it becomes unbounded at future timelike infinity. This behaviour, anticipated by Rendall in \cite{Rendall:2004}, is of particular interest since it is not consistent with the standard picture for inflation
in cosmology.

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arXiv:2301.11191 math.AP gr-qc

**The Stability of Relativistic Fluids in Linearly Expanding Cosmologies **

**Authors:** David Fajman, Maximilian Ofner, Todd A. Oliynyk, Zoe Wyatt

Abstract: In this paper we study cosmological solutions to the Einstein--Euler equations. We first establish the future stability of nonlinear perturbations of a class of homogeneous solutions to the relativistic Euler equations on fixed linearly expanding cosmological spacetimes with a linear equation of state p=Kρ for the parameter values K∈(0,1/3). This removes the restriction to irrotational perturbations in earlier work, and relies on a novel transformation of the fluid variables that is well-adapted to Fuchsian methods. We then apply this new transformation to show the global regularity and stability of the Milne spacetime under the coupled Einstein--Euler equations, again with a linear equation of state p=Kρ, K∈(0,1/3). Our proof requires a correction mechanism to account for the spatially curved geometry. In total, this is indicative that structure formation in cosmological fluid-filled spacetimes requires an epoch of decelerated expansion.

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arXiv:2302.03159 gr-qc

**Future instability of FLRW fluid solutions for linear equations of state p=Kρ with
1/3<K<1 **

**Authors:** Florian Beyer, Elliot Marshall, Todd A. Oliynyk

Abstract: Using numerical methods, we examine the dynamics of nonlinear perturbations in the expanding time direction, under a Gowdy symmetry assumption, of FLRW fluid solutions to the Einstein-Euler equations with a positive cosmological constant Λ>0 and a linear equation of state p=Kρ for the parameter values 1/3<K<1. This paper builds upon the numerical work in \cite{Marshalloliynyk:2022} in which the simpler case of a fluid on a fixed FLRW background spacetime was studied. The numerical results presented here confirm that the instabilities observed in \cite{Marshalloliynyk:2022} are also present when coupling to gravity is included as was previously conjectured in \cite{Rendall:2004,Speck:2013}. In particular, for the full parameter range 1/3<K<1, we find that the density contrast of the nonlinear perturbations develop steep gradients near a finite number of spatial points and becomes unbounded there at future timelike infinity. This instability is of particular interest since it is not consistent with the standard picture for late time expansion in cosmology.

Journal ref: Phys. Rev. D 107 (2023), 104030

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arXiv:2308.07475 gr-qc

**Past stability of FLRW solutions to the Einstein-Euler-scalar field equations and
their big bang singularites **

**Authors:** Florian Beyer, Todd A. Oliynyk

Abstract: We establish, in spacetime dimensions n≥3, the nonlinear stability in the contracting
direction of Friedmann-Lemaître-Robertson-Walker (FLRW) solutions to the Einstein-Euler-scalar field equations with linear equations of state P=c^{2}_{s}ρ for sounds speeds c_{s} satisfying 1/(n−1)<c^{2}_{s}<1. We further
show that nonlinear perturbations of the FLRW solutions are asymptotically pointwise Kasner and terminate in crushing, asymptotically velocity term dominated (AVTD) big bang singularities characterised by curvature blow-up.

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arXiv:2310.19184 gr-qc

**On the fractional density gradient blow-up conjecture of Rendall **

**Authors:** Todd A. Oliynyk

Abstract: On exponentially expanding Friedmann-Lemaître-Robertson-Walker (FLRW) spacetimes,
there is a distinguished family of spatially homogeneous and isotropic solutions to the relativistic Euler equations with a linear equation of state of the form p=σρ, where σ∈[0,1]
is the square of the sound speed. Restricting these solutions to a constant time hypersurface yields initial data that uniquely generates them. In this article, we show, for sound speeds satisfying 13<σ<(k+1)/(3k) with k∈Z>32,
that T^{2}-symmetric initial data that is chosen sufficiently close to spatially homogeneous and isotropic data uniquely generates a T^{2}-symmetric solution of the relativistic Euler equations that exists globally to the future. Moreover,
provided k∈Z>52, we show that there exist open sets of T^{2}-symmetric
initial data for which the fractional density gradient becomes unbounded at timelike infinity. This rigorously confirms, in the restricted setting of relativistic fluids on exponentially expanding FLRW spacetimes, the fractional density gradient blow-up scenario
conjectured by Rendall in \cite{Rendall:2004}.

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arXiv:2211.03234 gr-qc hep-th quant-ph

**General-relativistic pilot-wave quantum mechanics with torsion **

**Authors:** Francisco Ribeiro Benard Guedes, Nikodem Janusz Popławski

Abstract: We propose that the four-velocity of a Dirac particle is related to its relativistic
wave function by u_{i}=ψ¯γ_{i}ψ/ψ¯ψ. We associate the four-momentum of a spinor with a generator of translation, given by a covariant derivative. We associate the intrinsic angular momentum four-tensor of a spinor with a generator of rotation
in the spinor representation of the Lorentz group. We use the covariant conservation laws for the spin and energy−momentum tensors for a spinor field in the presence of the Einstein−Cartan torsion to show that if the wave satisfies the Dirac equation, then
the four-velocity, four-momentum, and spin four-tensor satisfy the classical Mathisson−Papapetrou equations of motion. We show that these equations reduce to the geodesic equation of motion. Consequently, the motion of a particle guided by the four-velocity
in the pilot-wave quantum mechanics coincides with the geodesic motion of the particle determined by the geometry of spacetime, representing a relativistic wave−particle duality.

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arXiv:2307.12190 gr-qc astro-ph.CO

**Gravitational Collapse with Torsion and Universe in a Black Hole **

**Authors:** Nikodem Popławski

Abstract: We consider gravitational collapse of a sphere of a fluid with torsion generated by spin, which forms 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, replacing it with a nonsingular bounce. Quantum particle creation during contraction prevents shear from overcoming torsion. Particle creation during expansion can generate a finite period of inflation and produce large 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 than the preceding cycle, until it reaches a size at which dark energy dominates and expands indefinitely. Our universe might have therefore originated from a black hole existing in another universe.

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arXiv:2305.00401 gr-qc astro-ph.HE hep-ph

**Methods and prospects for gravitational wave searches targeting ultralight vector
boson clouds around known black holes **

**Authors:** Dana Jones, Ling Sun, Nils Siemonsen, William E. East, Susan M. Scott,
Karl Wette

Abstract: Ultralight bosons are predicted in many extensions to the Standard Model and
are popular dark matter candidates. The black hole superradiance mechanism allows for these particles to be probed using only their gravitational interaction. In this scenario, an ultralight boson cloud may form spontaneously around a spinning black hole and
extract a non-negligible fraction of the black hole's mass. These oscillating clouds produce quasi-monochromatic, long-duration gravitational waves that may be detectable by ground-based or space-based gravitational wave detectors. We discuss the capability
of a new long-duration signal tracking method, based on a hidden Markov model, to detect gravitational wave signals generated by ultralight vector boson clouds, including cases where the signal frequency evolution timescale is much shorter than that of a typical
continuous wave signal. We quantify the detection horizon distances for vector boson clouds with current- and next-generation ground-based detectors. We demonstrate that vector clouds hosted by black holes with mass
≳60M_{⊙} and
spin ≳0.6 are within the reach of current-generation detectors
up to a luminosity distance of ∼1 Gpc. This search method enables
one to target vector boson clouds around remnant black holes from compact binary mergers detected by gravitational-wave detectors. We discuss the impact of the sky localization of the merger events and demonstrate that a typical remnant black hole reasonably
well-localized by the current generation detector network is accessible in a follow-up search.

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arXiv:2305.06606 gr-qc astro-ph.HE

**What are neutron stars made of? Gravitational waves may reveal the answer **

**Authors:** Neil Lu, Susan M. Scott, Karl Wette

Abstract: Neutron stars are one of the most mysterious wonders in the Universe. Their extreme densities hint at new and exotic physics at work within. Gravitational waves could be the key to unlocking their secrets. In particular, a first detection of gravitational waves from rapidly-spinning, deformed neutron stars could yield new insights into the physics of matter at extreme densities and under strong gravity. Once a first detection is made, a critical challenge will be to robustly extract physically interesting information from the detected signals. In this essay, we describe initial research towards answering this challenge, and thereby unleashing the full power of gravitational waves as an engine for the discovery of new physics.

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arXiv:2309.01133 gr-qc

**Population synthesis and parameter estimation of neutron stars with continuous gravitational
waves and third-generation detectors **

**Authors:** Yuhan Hua, Karl Wette, Susan M. Scott, Matthew D. Pitkin

Abstract: Precise measurement of stellar properties through the observation of continuous gravitational waves from spinning non-axisymmetric neutron stars can shed light onto new physics beyond terrestrial laboratories. Although hitherto undetected, prospects for detecting continuous gravitational waves improve with longer observation periods and more sensitive gravitational wave detectors. We study the capability of the Advanced Laser Interferometer Gravitational-Wave Observatory, and the Einstein Telescope to measure the physical properties of neutron stars through continuous gravitational wave observations. We simulate a population of Galactic neutron stars, assume continuous gravitational waves from the stars have been detected, and perform parameter estimation of the detected signals. Using the estimated parameters, we infer the stars' moments of inertia, ellipticities, and the components of the magnetic dipole moment perpendicular to the rotation axis. The estimation of the braking index proved challenging and is responsible for the majority of the uncertainties in the inferred parameters. Using the Einstein Telescope with an observation period of 5 yrs, point estimates using median can be made with errors of ~ 10 - 100% and ~ 5 - 50% respectively, subject to the inference of the braking index. The perpendicular magnetic dipole moment could not be accurately inferred for neutron stars that emit mainly gravitational waves.

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arXiv:2310.12463 gr-qc astro-ph.IM

**Piecewise frequency model for searches for long-transient gravitational waves from
young neutron stars **

**Authors:** Benjamin Grace, Karl Wette, Susan M. Scott, Ling Sun

Abstract: Previous searches for a gravitational-wave signal from a possible neutron star
remnant of the binary neutron star merger event GW170817 have focused on short (<500 s) and long duration (2.5 hr -- 8 day) signals. To date, no such post-merger signal has been detected. We introduce a new piecewise model which has the flexibility to accurately
follow gravitational-wave signals which are rapidly evolving in frequency, such as those which may be emitted from young neutron stars born from binary neutron star mergers or supernovae. We investigate the sensitivity and computational cost of this piecewise
model when used in a fully coherent 1800-second F-statistic search on simulated data containing possible signals from the GW170817 remnant. The sensitivity of the search using the piecewise model is determined using simulated data, with noise consistent with
the LIGO second observing run. Across a 100--2000 Hz frequency band, the model achieves a peak sensitivity of h^{50%}_{rss}=4.4×10^{−23}Hz^{−1/2} at 200 Hz, competitive with other methods. The computational cost of conducting
the search, over a bank of 1.1×10^{12} templates, is estimated at 10 days running on 100 CPU's.

Report number: LIGO-P2300314

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arXiv:2212.06914 gr-qc

**Toward fixing a framework for conformal cyclic cosmology **

**Authors:** Chris Stevens, Oliver Markwell

Abstract: Conformal Cyclic Cosmology (CCC) is a cyclic model of the universe put forward by Sir Roger Penrose. A conformal invariance assumption in the neighbourhood of the crossover region between cycles (which Penrose calls aeons) allows successive space-times to be related by a conformal rescaling. A major open problem is how to choose the conformal factor in a unique way, and is a fundamental hurdle to further study. Proposals have been put forward by Newman, Tod and Nurowski, but they disagree in one way or another with Penrose's original assumptions as well as each other. In this paper we compare these different models in detail and rule out certain choices for the conformal factor that have been put forward by Penrose. We extend the results of Newman and fix inconsistencies that arose in his calculations. A new class of solutions are put forward which agree with Penrose's assumptions exactly so long as a certain additional relation is satisfied.

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arXiv:2203.12152 hep-ex astro-ph.IM gr-qc physics.ins-det

**Direct Search for Dark Matter Axions Excluding ALP Cogenesis in the 63-67 micro-eV
Range, with The ORGAN Experiment **

**Authors:** Aaron P. Quiskamp, Ben T. McAllister, Paul Altin, Eugene N. Ivanov, Maxim
Goryachev, Michael E. Tobar

Abstract: The standard model axion seesaw Higgs portal inflation (SMASH) model is a well
motivated, self-contained description of particle physics over a range of energy scales that predicts axion dark matter particles to exist within the mass range of 50−200μeV. To scan these masses an axion haloscope under a strong constant magnetic field must
operate between 12 to 48 GHz. The ORGAN experiment (situated in Perth, Australia) is a microwave cavity axion haloscope that aims to search the majority of the mass range predicted by the SMASH model. Here we present results of Phase 1a, the first experiment
to scan and search for axions in the microwave Ku Band. Our initial scan sets a new limit on the coupling of axions to two photons of g_{aγγ}≥3×10^{−12}GeV^{−1} over the mass range 63.2 to 67.1 μeV with 95% confidence. This result is
the most sensitive to date in this mass range, sufficient to exclude the well motivated ALP (Axion Like Particle) cogenesis model for dark matter, which adds ALPs to the standard model in the early universe to simultaneously explain the observed baryon and
dark matter densities. To attain this level of sensitivity we utilised a TM010 cylindrical cavity resonator, scanned between 15.28 to 16.23 GHz through the utilisation of a tuning rod. Measurements were performed over a duration of 3.5 weeks with a 74% duty
cycle, with the resonator coupled to a low noise HEMT amplifier and placed inside a superconducting solenoidal electromagnet of 11.5 Tesla in magnetic field strength.

Journal ref: Science Advances, Vol 8, Issue 27, eabq3765, 6 Jul 2022

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arXiv:2207.14437 hep-ph astro-ph.CO gr-qc

**Searching for Scalar Field Dark Matter using Cavity Resonators and Capacitors **

**Authors:** V. V. Flambaum, B. T. McAllister, I. B. Samsonov, M. E. Tobar

Abstract: We establish new experiments to search for dark matter based on a model of a light scalar field with a dilaton-like coupling to the electromagnetic field, which is strongly motivated by superstring theory. We estimate the power of the photon signal in the process of a non-resonant scalar-photon transition and in a cavity resonator permeated by electric and magnetic fields. We show that existing cavity resonators employed in the experiments like ADMX have a low but non-vanishing sensitivity to the scalar-photon coupling. As a result, by re-purposing the results of the ADMX experiment, we find new limits on the scalar-photon coupling in the range of the scalar field masses from 2.7 to 4.2 μeV. We discuss possible modifications of this experiment, which enhance the sensitivity to the scalar field dark matter. We also propose a broadband experiment for scalar field dark matter searches based on a high-voltage capacitor. The estimated sensitivity of this experiment exceeds by nearly two orders in magnitude the sensitivity of the experiment based on molecular spectroscopy.

Journal ref: Phys. Rev. D 106, 055037, 2022

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arXiv:2209.03004 physics.ins-det astro-ph.IM gr-qc hep-ex

**Comparing Instrument Spectral Sensitivity of Dissimilar Electromagnetic Haloscopes
to Axion Dark Matter and High Frequency Gravitational Waves **

**Authors:** Michael E. Tobar, Catriona A. Thomson, William M. Campbell, Aaron Quiskamp,
Jeremy F. Bourhill, Benjamin T. McAllister, Eugene N. Ivanov, Maxim Goryachev

Abstract: It is known that haloscopes that search for dark matter axions via the axion-photon anomaly are also sensitive to gravitational radiation through the inverse Gertsenshtein effect. Recently this way of searching for high frequency gravitational waves has gained momentum as it has been shown that the strain sensitivities of such detectors are of the same order of sensitivity to the axion-photon theta angle. Thus, after calculating the sensitivity of a haloscope to an axion signal, we also have calculated the order of magnitude sensitivity to a gravitational wave signal of the same spectral and temporal form. However, it is unlikely that a gravitational wave and an axion signal will be of the same form, since physically the way the signals are generated are completely different. For gravitational wave detection, the spectral strain sensitivity is in units strain per square root Hz, is the natural way to compare the sensitivity of gravitational wave detectors due to its independence on the gravitational wave signal. In this work, we introduce a systematic way to calculate the spectral sensitivity of an axion haloscope, so instrument comparison may be achieved independent of signal assumptions and only depends on the axion to signal transduction sensitivity and noise in the instrument. Thus, the calculation of the spectral sensitivity not only allows the comparison of dissimilar axion detectors independent of signal, but also allows us to compare the order of magnitude gravitational wave sensitivity in terms of spectral strain sensitivity, allowing comparisons to standard gravitational wave detectors based on optical interferometers and resonant-mass technology.

Journal ref: Symmetry vol. 14, no. 10: 2165, 2022

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arXiv:2211.09637 hep-ph gr-qc hep-ex physics.ins-det quant-ph

**Sensitivity of Resonant Axion Haloscopes to Quantum Electromagnetodynamics **

**Authors:** Michael E. Tobar, Catriona A. Thomson, Benjamin T. McAllister, Maxim
Goryachev, Anton Sokolov, Andreas Ringwald

Abstract: Recently interactions between putative axions and magnetic monopoles have been
revisited by two of us [arXiv:2205.02605 [hep-ph]]. It has been shown that significant modifications to conventional axion electrodynamics arise due to these interactions, so that the axion-photon coupling parameter space is expanded from one parameter g_{aγγ}
to three (g_{aγγ},g_{aAB},g_{aBB}). We implement Poynting theorem to determine how to exhibit sensitivity to g_{aAB} and g_{aBB} using resonant haloscopes, allowing new techniques to search for axions and a possible
indirect way to determine if magnetically charged matter exists.

Journal ref: Ann. Phys.(Berlin) 2023, 2200594

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arXiv:2304.00688 gr-qc hep-th quant-ph

**Improved Constraints on the Minimum Length with a Macroscopic Low Loss Phonon Cavity **

**Authors:** William M. Campbell, Michael E. Tobar, Serge Galliou, Maxim Goryachev

Abstract: Many theories that attempt to formulate a quantum description of gravity suggest the existence of a fundamental minimum length scale. A popular method for incorporating this minimum length is through a modification of the Heisenberg uncertainty principle known as the generalised uncertainty principle (GUP). Experimental tests of the GUP applied to composite systems can be performed by searching for the induced frequency perturbations of the modes of mechanical resonators, thus constraining the degree of minimum length in certain scenarios. In this work previous constraints made with mechanical resonators are improved upon by three orders of magnitude, via the utilisation of a cryogenic quartz bulk acoustic wave resonator. As well as purely mechanical resonant modes; hybrid electromechanical anti-resonant modes are investigated, and shown to be sensitive to the same GUP induced effects.

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arXiv:2306.13320 hep-ph astro-ph.IM gr-qc physics.ins-det

**Searching for GUT-scale QCD Axions and Monopoles with a High Voltage Capacitor **

**Authors:** Michael E. Tobar, Anton V. Sokolov, Andreas Ringwald, Maxim Goryachev

Abstract: The QCD axion has been postulated to exist because it solves the strong CP
problem. Furthermore, if it exists axions should be created in the early Universe and could account for all the observed dark matter. In particular, axion masses of order 10^{−10} to 10^{−7} eV correspond to axions in the vicinity of the GUT-scale.
In this mass range many experiments have been proposed to search for the axion through the standard QED coupling parameter gaγγ. Recently axion electrodynamics has been expanded to include two more coupling parameters, g_{aEM} and g_{aMM},
which could arise if heavy magnetic monopoles exist. In this work we show that both g_{aMM} and g_{aEM} may be searched for using a high voltage capacitor. Since the experiment is not sensitive to g_{aγγ}, it gives a new way to search
for effects of heavy monopoles if the GUT-scale axion is shown to exist, or to simultaneously search for both the axion and the monopole at the same time.

Journal ref: Physical Review D 108, 035024 (2023)

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arXiv:2307.00715 gr-qc astro-ph.HE astro-ph.IM hep-ph

**The Multi-mode Acoustic Gravitational Wave Experiment: MAGE **

**Authors:** William M. Campbell, Maxim Goryachev, Michael E. Tobar

Abstract: The Multi-mode Acoustic Gravitational wave Experiment (MAGE) is a high frequency
gravitational wave detection experiment. In its first stage, the experiment features two near-identical quartz bulk acoustic wave resonators that act as strain antennas with spectral sensitivity as low as 6.6×10^{−21}[strain]/ √Hz in multiple narrow
bands across MHz frequencies. MAGE is the successor to the initial path-finding experiments; GEN 1 and GEN 2. These precursor runs demonstrated the successful use of the technology, employing a single quartz gravitational wave detector that found significantly
strong and rare transient features. As the next step to this initial experiment, MAGE will employ further systematic rejection strategies by adding an additional quartz detector such that localised strains incident on just a single detector can be identified.
The primary goals of MAGE will be to target signatures arising from objects and/or particles beyond that of the standard model, as well as identifying the source of the rare events seen in the predecessor experiment. The experimental set-up, current status
and future directions for MAGE are discussed. Calibration procedures of the detector and signal amplification chain are presented. The sensitivity of MAGE to gravitational waves is estimated from knowledge of the quartz resonators. Finally, MAGE is assembled
and tested in order to determine the thermal state of its new components.

Journal ref: Sci Rep 13, 10638 (2023)

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arXiv:2112.02699 gr-qc math-ph

**Symplectic evolution of an observed light bundle **

**Authors:** Nezihe Uzun

Abstract: Each and every observational information we obtain from the sky regarding the brightnesses, distances or image distortions resides on the deviation of a null geodesic bundle. In this talk, we present the symplectic evolution of this bundle on a reduced phase space. The resulting formalism is analogous to the one in paraxial Newtonian optics. It allows one to identify any spacetime as an optical device and distinguish its thin lens, pure magnifier and rotator components. We will discuss the fact that the distance reciprocity in relativity results from the symplectic evolution of this null bundle. Other potential applications like wavization and its importance for both electromagnetic and gravitational waves will also be summarized.

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arXiv:2112.04647 gr-qc

**Astrophysically viable Kerr-like spacetime -- into the eye of the storm **

**Authors:** Alex Simpson, Matt Visser

Abstract: We analyse a rotating regular black hole with asymptotically Minkowski core. This Kerr-like geometry possesses the full "Killing tower" of nontrivial Killing tensor, Killing-Yano tensor, and principal tensor. The Hamilton-Jacobi equation, the Klein-Gordon equation, and Maxwell's equations are separable. Energy-condition-violating physics is pushed into an arbitrarily small region in the deep core. The geometry has a very high level of mathematical tractability; extraction of astrophysical observables falsifiable/verifiable by the observational community is straightforward.

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arXiv:2112.05228 gr-qc

**Geodesics for the Painleve-Gullstrand form of Lense-Thirring spacetime **

**Authors:** Joshua Baines, Thomas Berry, Alex Simpson, Matt Visser

Abstract: Recently, the current authors have formulated and extensively explored a rather novel Painleve-Gullstrand variant of the slow-rotation Lense-Thirring spacetime, a variant which has particularly elegant features -- including unit lapse, intrinsically flat spatial 3-slices, and a separable Klein-Gordon equation (wave operator). This spacetime also possesses a non-trivial Killing tensor, implying separability of the Hamilton-Jacobi equation, the existence of a Carter constant, and complete formal integrability of the the geodesic equations. Herein we investigate the geodesics in some detail, in the general situation demonstrating the occurrence of "ultra-elliptic" integrals. Only in certain special cases can the complete geodesic integrability be explicitly cast in terms of elementary functions. The model is potentially of astrophysical interest both in the asymptotic large-distance limit and as an example of a "black hole mimic", a controlled deformation of the Kerr spacetime that can be contrasted with ongoing astronomical observations.

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arXiv:2202.09010 gr-qc

**Constant-r geodesics in the Painleve-Gullstrand form of Lense-Thirring spacetime **

**Authors:** Joshua Baines, Thomas Berry, Alex Simpson, Matt Visser

Abstract: Herein we explore the non-equatorial constant-r ("quasi-circular") geodesics (both timelike and null) in the Painleve-Gullstrand variant of the Lense-Thirring spacetime recently introduced by the current authors. Even though the spacetime is not spherically symmetric, shells of constant-r geodesics still exist. Whereas the radial motion is (by construction) utterly trivial, determining the allowed locations of these constant-r geodesics is decidedly non-trivial, and the stability analysis is equally tricky. Regarding the angular motion, these constant-r orbits will be seen to exhibit both precession and nutation -- typically with incommensurate frequencies. Thus this constant-r geodesic motion, though integrable in the precise technical sense, is generically surface-filling, with the orbits completely covering a symmetric equatorial band which is a segment of a spherical surface, (a so-called "spherical zone"), and whose latitudinal extent is governed by delicate interplay between the orbital angular momentum and the Carter constant. The situation is qualitatively similar to that for the (exact) Kerr spacetime -- but we now see that any physical model having the same slow-rotation weak-field limit as general relativity will still possess non-equatorial constant-r geodesics.

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arXiv:2203.14516 gr-qc

**On the inner horizon instability of non-singular black holes **

**Authors:** Francesco Di Filippo, Raúl Carballo-Rubio, Stefano Liberati, Costantino
Pacilio, Matt Visser

Abstract: Regular black holes represent a conservative model in which the classical singularity is replaced by a non-singular core without necessarily modifying the spacetime outside the trapping horizon. Given the possible lack of phenomenological signatures, it is crucial to study the consistency of the model. In this short work, we review the physical mechanism leading to the instability of the central core, arguing that non-perturbative backreaction is non-negligible and must be taken into account to provide a meaningful description of physical black holes.

Report number: YITP-22-22

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arXiv:2205.13555 astro-ph.HE gr-qc

**Constraints on horizonless objects after the EHT observation of Sagittarius A* **

**Authors:** Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Matt Visser

Abstract: The images of Sagittarius A^{∗} recently
released by the Event Horizon Telescope collaboration have been accompanied [Ap.J.Lett.\,{\bf 930\,\#2}\,(2022)\,L17] by an analysis of the constraints on the possible absence of a trapping horizon, i.e.~on the possibility that the object at the center of
our galaxy is an ultra-compact object with a surface re-emitting incident radiation. Indeed, using the observed image size and the broadband spectrum of Sgr A^{∗},
it is claimed that the radius of any such thermal surface is strongly bounded from above by these latest observations. Herein, we discuss how the reported constraint relies on the extremely strong assumption of perfect balance in the energy exchange between
the accretion disk and the central object, and show that this is violated whenever the surface is endowed with any non-zero absorption coefficient. We discuss in detail the upper-bound constraints that can be cast on the radius and dimensionless absorption
coefficient of the surface. We show that the conclusions of the analysis presented by the EHT collaboration hold only for unnaturally small values of the absorption coefficient (i.e. much lower than 10^{−14}), and thus have to be significantly revised
in scenarios with physical significance.

Report number: YITP-22-54

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arXiv:2205.13556 gr-qc

**Regular black holes without mass inflation instability **

**Authors:** Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Costantino
Pacilio, Matt Visser

Abstract: Generic models of regular black holes have separate outer and inner horizons, both with nonzero surface gravity. It has been shown that a nonzero inner horizon surface gravity results in exponential instability at the inner horizon controlled by this parameter. This phenomenon takes the name of "mass inflation instability", and its presence has put in question the physical viability of regular black holes as alternatives to their (singular) general relativity counterparts. In this paper, we show that it is possible to make the inner horizon surface gravity vanish, while maintaining the separation between horizons, and a non-zero outer horizon surface gravity. We construct specific geometries satisfying these requirements, and analyze their behavior under different kinds of perturbations, showing that the exponential growth characteristic of mass inflation instability is not present for these geometries. These "inner-extremal" regular black holes are thereby better behaved than singular black holes and generic regular black holes, thus providing a well-motivated alternative of interest for fundamental and phenomenological studies.

Report number: YITP-22-53

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arXiv:2205.15950 gr-qc

**ADM mass in warp drive spacetimes**

**Authors:** Sebastian Schuster, Jessica Santiago, Matt Visser

Abstract: What happens when a warp bubble has mass? This seemingly innocent question forces one to carefully formalize exactly what one means by a warp bubble, exactly what one means by having the warp bubble "move" with respect to the fixed stars, and forces one to more carefully examine the notion of mass in warp-drive spacetimes. This is the goal of the present article. In this process, we will see that often-made throw-away comments regarding "payloads" are even simpler than commonly assumed, while there are two further, distinct yet subtle ways in which a mass can appear in connection with a warp drive space-time: One, that the warp bubble (not its payload) has the mass; two, that the mass is a background feature in front of which the warp drive moves. For simplicity, we consider generic Natário warp drives with zero-vorticity flow field. The resulting spacetimes are sufficiently simple to allow an exact and fully explicit computation of all of the stress-energy components, and verify that (as expected) the null energy condition (NEC) is violated. Likewise the weak, strong, and dominant energy conditions (WEC, SEC, DEC) are violated. Indeed, this confirms the community's folk wisdom, and recent (fully general, but implicit) results of the present authors which closed previous gaps in the argument. However, folk wisdom should be carefully and critically examined before being believed, and the present examples for general results will greatly aid physical intuition.

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arXiv:2207.02465 gr-qc

**General-relativistic thin-shell Dyson mega-spheres **

**Authors:** Thomas Berry, Alex Simpson, Matt Visser

Abstract: Loosely inspired by the somewhat fanciful notion of detecting an arbitrarily advanced alien civilization, we consider a general-relativistic thin-shell Dyson mega-sphere completely enclosing a central star-like object, and perform a full general-relativistic analysis using the Israel--Lanczos--Sen junction conditions. We focus attention on the surface mass density, the surface stress, the classical energy conditions, and the forces between hemispheres. We find that in the physically acceptable region the NEC, WEC, and SEC are always satisfied, while the DEC can be violated if the Dyson mega-sphere is sufficiently close to forming a black hole. We also demonstrate that the original quasi-local version of the maximum force conjecture, F <= {1/4} F_{Stoney} = {1/4} F_{Planck}, can easily be violated if the Dyson mega-sphere is sufficiently compact, that is, sufficiently close to forming a black hole. Interestingly there is a finite region of parameter space where one can violate the original quasi-local version of the maximum force conjecture without violating the DEC. Finally, we very briefly discuss the possibility of nested thin-shell mega-spheres (Matrioshka configurations) and thick-shell Dyson mega-spheres.

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arXiv:2207.08375 gr-qc astro-ph.CO

**Cosmology in Painleve-Gullstrand coordinates **

**Authors:** Rudeep Gaur, Matt Visser

Abstract: Cosmology is most typically analyzed using standard co-moving coordinates, in which the galaxies are (on average, up to presumably small peculiar velocities) "at rest", while "space" is expanding. But this is merely a specific coordinate choice; and it is important to realise that for certain purposes other, (sometimes radically different) coordinate choices might also prove useful and informative, but without changing the underlying physics. Specifically, herein we shall consider the k=0 spatially flat FLRW cosmology but in Painleve-Gullstrand coordinates -- these coordinates are very explicitly not co-moving: "space" is now no longer expanding, although the distance between galaxies is still certainly increasing. Working in these Painleve-Gullstrand coordinates provides an alternate viewpoint on standard cosmology, and the symmetries thereof, and also makes it somewhat easier to handle cosmological horizons. With a longer view, we hope that investigating these Painleve-Gullstrand coordinates might eventually provide a better framework for understanding large deviations from idealized FLRW spacetimes. We illustrate these issues with a careful look at the Kottler and McVittie spacetimes.

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arXiv:2207.09034 gr-qc

**Physically motivated ansatz for the Kerr spacetime **

**Authors:** Joshua Baines, Matt Visser

Abstract: Despite some 60 years of work on the subject of the Kerr rotating black hole
there is as yet no widely accepted physically based and pedagogically viable ansatz suitable for deriving the Kerr solution without significant computational effort. (Typically involving computer-aided symbolic algebra.) Perhaps the closest one gets in this
regard is the Newman-Janis trick; a trick which requires several physically unmotivated choices in order to work. Herein we shall try to make some progress on this issue by using a non-ortho-normal tetrad based on oblate spheroidal coordinates to absorb as
much of the messy angular dependence as possible, leaving one to deal with a relatively simple angle-independent tetrad-component metric. That is, we shall write g_{ab}=g_{AB
}e^{A}_{a}e^{B}_{b} seeking to keep both the tetrad-component metric g_{AB} and the non-ortho-normal co-tetrad e^{A}_{a} relatively simple but non-trivial. We shall see that it is possible to put
all the mass dependence into g_{AB}, while the non-ortho-normal co-tetrad e^{A}_{a} can be chosen to be a mass-independent representation of flat Minkowski space in oblate spheroidal coordinates: (g_{Minkowski})_{ab}=η_{AB
}e^{A}_{a}e^{B}_{b}. This procedure separates out, to the greatest extent possible, the mass dependence from the rotational dependence, and makes the Kerr solution perhaps a little less mysterious.

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arXiv:2210.11057 gr-qc

**Painleve-Gullstrand coordinates versus Kerr spacetime geometry **

**Authors:
**Matt Visser, Stefano Liberati

Abstract: We discuss the tension between the possible existence of Painleve-Gullstrand coordinate systems versus the explicit geometrical features of the Kerr spacetime; a subject of interest to Professor Thanu Padmanabhan in the weeks immediately preceding his unexpected death. We shall carefully distinguish strong and weak Painleve-Gullstrand coordinate systems, and conformal variants thereof, cataloguing what we know can and cannot be done -- sometimes we can make explicit global statements, sometimes we must resort to implicit local statements. For the Kerr spacetime the best that seems to be achievable is to set the lapse function to unity and represent the spatial slices with a 3-metric in factorized unimodular form; this arises from considering the Doran version of Kerr spacetime in Cartesian coordinates. We finish by exploring the (limited) extent to which this construction might possibly lead to implementing an "analogue spacetime" model suitable for laboratory simulations of the Kerr spacetime.

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arXiv:2210.13946 gr-qc astro-ph.CO

**Dynamical analysis of the redshift drift in FLRW universes **

**Authors:** Francisco S. N. Lobo, José Pedro Mimoso, Jessica Santiago, 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. In a previous article [JCAP 04 (2020) 043; \arXiv{2001.11964}] three of the current authors had performed a cosmographic analysis of the redshift drift in a FLRW universe, temporarily putting aside the issue of dynamics (the Friedmann equations). In the current article we now add dynamics, still within the framework of an exact FLRW universe. We shall develop a suitable generic matter model and apply it to both standard FLRW and various dark energy models. Furthermore, we shall also present a section analyzing the utility of using alternative cosmographic variables to describe the redshift drift data.

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arXiv:2211.05817 gr-qc hep-th

**A connection between regular black holes and horizonless ultracompact stars **

**Authors:** Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Matt Visser

Abstract: We illustrate that regular black holes and horizonless stars, typically considered as quite distinct families of black hole mimickers, are intimately intertwined. We show that any spherically symmetric regular black hole can be continuously deformed into a horizonless star under the mild conditions of non-negativity of gravitational energy (Misner--Sharp quasi-local mass), and an assumed linear relation between the latter and the Arnowitt--Deser--Misner (ADM) mass. We illustrate this general result by considering the family of geometries proposed by Hayward as the description of regular black holes, and we also describe the properties of the corresponding horizonless stars. The form of the associated effective stress-energy tensor shows that these horizonless stars can be identified as anisotropic gravastars with a soft surface and inner/outer light rings. We also construct dynamical geometries that could describe the evolution of regular black holes towards horizonless stars, and show that semiclassical physics contains the necessary ingredients to trigger the early stages of such dynamical evolution.

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arXiv:2211.07835 gr-qc

**Null affine parameter in spacetimes conformal to spacetimes exhibiting a timelike
conformal Killing vector **

**Authors:** Matt Visser

Abstract: Finding affine parameters for null geodesics is often quite tedious, and can sometimes even be somewhat tricky. Herein we shall demonstrate that the existence of a conformally related spacetime containing a conformal Killing vector, timelike in the domain of outer communication, is sufficient to define a preferred set of spatial 3-slices -- on which a well-defined "affine" 3-metric can be introduced to capture the notion of affine null parameter -- before explicitly finding the null geodesics. The construction depends on the properties of conformal transformations, and on the conserved quantity associated with the conformal Killing vector. Having the affine null parameter in hand before attempting to find the actual null geodesics often simplifies other parts of the analysis.

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arXiv:2212.07458 gr-qc hep-th

**Comment on "Stability properties of Regular Black Holes" **

**Authors:** Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Costantino
Pacilio, Matt Visser

Abstract: Regular black holes are generically unstable because of the phenomenon that goes by the name of "mass inflation" which destabilizes the inner horizon. In recent works, [arXiv:2209.10612v1 and arXiv:2211.09192v1], it is argued that Hawking radiation can cure this instability and some concerns are raised against the validity of the previous analyses showing its existence in first place. In this short comment, we explain our reservations regarding these recent claims and reiterate the relevance of the mass inflation instability for regular black holes of astrophysical interest.

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arXiv:2302.00028 gr-qc hep-th

**Singularity-free gravitational collapse: From regular black holes to horizonless objects **

**Authors:** Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Matt Visser

Abstract: Penrose's singularity theorem implies that if a trapped region forms in a gravitational collapse, then a singularity must form as well within such region. However, it is widely expected that singularities should be generically avoided by quantum gravitational effects. Here we shall explore both the minimum requirements to avoid singularities in a gravitational collapse as well as discuss, without relying on a specific quantum gravity model, the possible regular spacetimes associated to such regularization of the spacetime fabric. In particular, we shall expose the intimate and quite subtle relationship between regular black holes, black bounces and their corresponding horizonless object limits. In doing so, we shall devote specific attention to those critical (extremal) black hole configurations lying at the boundary between horizonful and horizonless geometries. While these studies are carried out in stationary configurations, the presence of generic instabilities strongly suggest the need for considering more realistic time-dependent dynamical spacetimes. Missing specific dynamical models, much less rigorous statements can be made for evolving geometries. We shall nonetheless summarize here their present understanding and discuss their implications for future phenomenological studies.

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arXiv:2303.07380 gr-qc

**Killing horizons and surface gravities for a well-behaved three-function generalisation
of the Kerr spacetime **

**Authors:** Joshua Baines, Matt Visser

Abstract: Thanks to the recent advent of the event horizon telescope (EHT), we now have the opportunity to test the physical ramifications of the strong-field near-horizon regime for astrophysical black holes. Herein, emphasizing the trade-off between tractability and generality, the authors discuss a particularly powerful three-function distortion of the Kerr spacetime, depending on three arbitrary functions of the radial coordinate r, which on the one hand can be fit to future observational data, and on the other hand is sufficiently general so as to encompass an extremely wide class of theoretical models. In all of these spacetimes, both the timelike Hamilton--Jacobi (geodesic) and massive Klein--Gordon (wave) equations separate, and the spacetime geometry is asymptotically Kerr; hence these spacetimes are well-suited to modelling real astrophysical black holes. The authors then prove the existence of Killing horizons for this entire class of spacetimes, and give tractable expressions for the angular velocities, areas, and surface gravities of these horizons. We emphasize the validity of rigidity results and zeroth laws for these horizons.

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arXiv:2304.10692 gr-qc

**Black holes, white holes, and near-horizon physics **

**Authors:** Rudeep Gaur, Matt Visser

Abstract: Black and white holes play remarkably contrasting roles in general relativity versus observational astrophysics. While there is overwhelming observational evidence for the existence of compact objects that are "cold, dark, and heavy", which thereby are natural candidates for black holes, the theoretically viable time-reversed variants -- the "white holes" -- have nowhere near the same level of observational support. Herein we shall explore the possibility that the connection between black and white holes is much more intimate than commonly appreciated. We shall first construct "horizon penetrating" coordinate systems that differ from the standard curvature coordinates only in a small near-horizon region, thereby emphasizing that ultimately the distinction between black and white horizons depends only on near-horizon physics. We shall then construct an explicit model for a "black-to-white transition" where all of the nontrivial physics is confined to a compact region of spacetime -- a finite-duration finite-thickness, (in principle arbitrarily small), region straddling the naive horizon. Moreover we shall show that it is possible to arrange the "black-to-white transition" to have zero action -- so that it will not be subject to destructive interference in the Feynman path integral. This then raises the very intriguing possibility that astrophysical black holes might be interpratable in terms of a quantum superposition of black and white horizons.

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arXiv:2305.08910 gr-qc

**Explicit formulae for surface gravities in stationary circular axi-symmetric spacetimes **

**Authors:
**Joshua Baines, Matt Visser

Abstract: Using minimalist assumptions we develop a natural functional decomposition for the spacetime metric, and explicit tractable formulae for the surface gravities, in arbitrary stationary circular (PT symmetric) axisymmetric spacetimes. We relate rigidity results, (the existence of a Killing horizon), and the zeroth law to the absence of curvature singularities at the would-be horizons. These observations are of interest to both observational astrophysicists (modelling the cold, dark, heavy objects at the centre of most spiral galaxies), and to the analogue spacetime community, (wherein the presence of naked singularities is not necessarily deprecated, and the occurrence of non-Killing horizons is relatively common).

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arXiv:2306.17480 astro-ph.HE gr-qc

**Constraints on thermalizing surfaces from infrared observations of supermassive black
holes **

**Authors:** Raúl Carballo-Rubio, Francesco Di Filippo, Stefano Liberati, Matt Visser

Abstract: Infrared observations of Sgr A^{∗} and
M87^{∗} are incompatible with the assumption that these
sources have physical surfaces in thermal equilibrium with their accreting environments. In this paper we discuss a general parametrization of the energy balance in a horizonless object, which permits to quantify how close a horizonless object is in its behavior
to a black hole, and analyze the timescale in which its surface can thermalize. We show that the thermalization timescale is unbounded, growing large for objects that mimic closely the behavior of a black hole (and being infinite for the latter). In particular,
the thermalization timescale is proportional to the time that energy spends inside the horizonless object due to propagation and interactions with the bulk. Hence, these observations can be used to quantitatively restrict the dynamical behavior of horizonless
objects, without being able to discard the existence of a physical surface.

Report number: YITP-22-84

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arXiv:2308.07374 gr-qc astro-ph.CO

**Black holes embedded in FLRW cosmologies **

**Authors:** Rudeep Gaur, Matt Visser

Abstract: There has recently been some considerable interest expressed in a highly speculative model of black hole evolution -- allegedly by a postulated direct coupling between black holes and cosmological expansion independently of accretion or mergers. We wish to make several cautionary comments in this regard -- at least three exact solutions corresponding to black holes embedded in a FLRW background are known, (Kottler, McVittie, Kerr-de Sitter), and they show no hint of this claimed effect -- thereby implying that this claimed effect (if it exists at all) is certainly nowhere near ubiquitous.

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arXiv:2308.13766 gr-qc

**Photon escape cones, physical and optical metrics, asymptotic and near-horizon physics **

**Authors:** Joshua Baines, Matt Visser

Abstract: We consider the explicit analytic behaviour of photon escape cones in generic static spherically symmetric spacetimes, emphasizing the interplay between the physical spacetime metric and the optical metric, and the interplay between large-distance asymptotic and near-horizon physics. The circular photon orbits (photon spheres) are shown to be given by wormhole throats in the optical metric, (not the physical metric), and the escape cone solid angle is easily calculable in terms of the capture cross section, sigma_{capture}, the area of the spherical 2-surfaces, and the norm of the timelike Killing vector. Under appropriate circumstances, for near-horizon photon emission the escape cone solid angle can be related to the surface gravity kappa_H. We provide a number of illustrative examples, involving both black holes and wormholes, including situations with multiple photon spheres.

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arXiv:2308.16624 gr-qc

**Defect wormholes are defective **

**Authors:** Joshua Baines, Rudeep Gaur, Matt Visser

Abstract: The various "defect wormholes" developed by Klinkhamer have recently attracted
considerable attention -- especially in view of the fact that the simplest example, the so-called "vacuum defect wormhole", was claimed to be an everywhere-vacuum everywhere-Ricci-flat exact solution to the Einstein equations. This claim has been conclusively
refuted by Feng, and in the current article we take a deeper look at exactly what goes wrong. The central issue is this: Although Klinkhamer's specific representation of the metric gab is smooth (C^{∞}) his inverse metric gab is not even everywhere
continuous (C^{0}), being undefined at the wormhole throat. This situation implies that one should very carefully investigate curvature tensors at the throat using the Israel--Lanczos--Sen thin-shell formalism. Doing so reveals the presence of a delta-function
energy-condition-violating thin shell of matter at the wormhole throat. The "defect wormholes" are thus revealed to be quite ordinary "cut-and-paste" thin-shell wormholes, but represented in a coordinate system which is unfortunately pathological at exactly
the same place that all the interesting physics occurs. To help clarify the situation, we shall focus on the behaviour of suitable coordinate invariants -- the Ricci scalar, the eigenvalues of the mixed Rab Ricci tensor, and the eigenvalues of the mixed Rabcd
Riemann tensor.

Journal ref: Universe 9 (2023) 452

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arXiv:2112.12266 cond-mat.quant-gas gr-qc hep-ph hep-th

**Quantum vortex instability and black hole superradiance **

**Authors:** Sam Patrick, August Geelmuyden, Sebastian Erne, Carlo F. Barenghi, Silke
Weinfurtner

Abstract: Vortices and black holes set the scene for many interesting dynamical processes in physics. Here, we study the dynamical instability of quantised vortices and rotational superradiance around rotating black holes, illustrating in the process that the same physics is at play in these two seemingly disparate phenomena. We also compare the instability of the vortex to the black hole bomb instability, which occurs for massive scalar fields in the Kerr spacetime. Taking inspiration from the analogy between black hole bomb modes and the hydrogen spectrum, the vortex instability is compared with nuclear resonances involved in α-decay.

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arXiv:2202.05926 physics.flu-dyn gr-qc hep-th

**Wave focusing by submerged islands and gravitational analogues **

**Authors:** Theo Torres, Max Lloyd, Sam R. Dolan, Silke Weinfurtner

Abstract: We study water waves propagating over a smooth obstacle in a fluid of varying depth, motivated by the observation that submerged islands in the ocean act as effective lenses that increase the amplitude and destructive power of tsunami waves near focal points. We show that islands of substantial height (compared to the water depth) lead to strong focusing in their immediate vicinity, and generate caustics of either cusp or butterfly type. We highlight similarities and differences with focusing of (high-frequency) gravitational waves by a neutron star. In the linear regime, the comparison is made precise through an effective-spacetime description of the island-fluid system. This description is then put to practical use: we identify caustics by solving the Raychaudhuri equation (a transport equation) along rays of the effective metric. Next, the island-fluid scattering processes are examined in detail (i.e.~deflection angle, phase shifts, scattering amplitudes) using numerical simulations and analytical techniques, including the eikonal approximation and its generalisation in the form of the Gaussian beam approximation. We show that the techniques capture the key features of the simulations. Finally, we extend the eikonal approximation to the dispersive regime, demonstrating that the essential features are robust in dispersive settings. This paves the way for future exploration in a controlled laboratory set-up.

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arXiv:2204.11867 hep-th astro-ph.CO gr-qc hep-ph

**Mass Renormalization in Lattice Simulations of False Vacuum Decay **

**Authors:** Jonathan Braden, Matthew C. Johnson, Hiranya V. Peiris, Andrew Pontzen,
Silke Weinfurtner

Abstract: False vacuum decay, a quantum mechanical first-order phase transition in scalar field theories, is an important phenomenon in early universe cosmology. Recently, real-time semi-classical techniques based on ensembles of lattice simulations were applied to the problem of false vacuum decay. In this context, or any other lattice simulation, the effective potential experienced by long-wavelength modes is not the same as the bare potential. To make quantitative predictions using the real-time semi-classical techniques, it is therefore necessary to understand the redefinition of model parameters and the corresponding deformation of the vacuum state, as well as stochastic contributions that require modeling of unresolved subgrid modes. In this work, we focus on the former corrections and compute the expected modification of the true and false vacuum effective mass, which manifests as a modified dispersion relationship for linear fluctuations about the vacuum. We compare these theoretical predictions to numerical simulations and find excellent agreement. Motivated by this, we use the effective masses to fix the shape of a parameterized effective potential, and explore the modeling uncertainty associated with non-linear corrections. We compute the decay rates in both the Euclidean and real-time formalisms, finding qualitative agreement in the dependence on the UV cutoff. These calculations further demonstrate that a quantitative understanding of the rates requires additional corrections.

Journal ref: Phys. Rev. D 107 (2023) 8, 083509

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arXiv:2207.02199 gr-qc astro-ph.CO physics.flu-dyn

**Primary thermalisation mechanism of Early Universe observed from Faraday-wave scattering
on liquid-liquid interfaces **

**Authors:** Vitor S. Barroso, August Geelmuyden, Zack Fifer, Sebastian Erne, Anastasios
Avgoustidis, Richard J. A. Hill, Silke Weinfurtner

Abstract: For the past two hundred years, parametric instabilities have been studied in various physical systems, such as fluids, mechanical devices and even inflationary cosmology. It was not until a few decades ago that this subharmonic unstable response arose as a central mechanism for the thermalisation of the Early Universe, in a theory known as preheating. Here we study a parametrically driven two-fluid interface to simulate the key aspects of inflationary preheating dynamics through the onset of nonlinear Faraday waves. We present a detailed analysis of the effective field theory description for interfacial waves through the factorization properties of higher-order correlations. Despite the intricacies of a damped and highly interacting hydrodynamical system, we show that the scattering of large amplitude Faraday waves is connected to a broadening of primary resonance bands and the subsequent appearance of secondary instabilities as predicted in preheating dynamics.

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arXiv:2302.12023 gr-qc hep-th

**Third sound detectors in accelerated motion **

**Authors:** Cameron R. D. Bunney, Steffen Biermann, Vitor S. Barroso, August Geelmuyden,
Cisco Gooding, Grégoire Ithier, Xavier Rojas, Jorma Louko, Silke Weinfurtner

Abstract: An accelerated observer moving through empty space sees particles appearing and disappearing, while an observer with a constant velocity does not register any particles. This phenomenon, generally known as the Unruh effect, relies on an initial vacuum state, thereby unifying the experience of all inertial observers. We propose an experiment to probe this observer-dependent detector response, using a laser beam in circular motion as a local detector of superfluid helium-4 surface modes or third sound waves. To assess experimental feasibility, we develop a theoretical framework to include a non-zero temperature initial state. We find that an acceleration-dependent signal persists, independent of the initial temperature. By introducing a signal-to-noise measure we show that observing this signal is within experimental reach.

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arXiv:2305.00226 gr-qc physics.flu-dyn

**Non-linear effective field theory simulators in two-fluid interfaces **

**Authors:** Vitor S. Barroso, Cameron R. D. Bunney, Silke Weinfurtner

Abstract: Analogue gravity offers an approach for testing the universality and robustness of quantum field theories in curved spacetimes and validating them using down-to-earth, laboratory-based experiments. Fluid interfaces are a promising framework for creating these gravity simulators and have successfully replicated phenomena such as Hawking radiation and black hole superradiance. Recent work has shown that hydrodynamical instabilities on the interface between two fluids can capture features of the post-inflationary thermalisation of the Early Universe. In this study, we extend fluid dynamics methods to develop an effective field theory for the interface between two fluids, demonstrating the equivalence between the governing equations and a relativistic scalar field in an analogue spacetime. We also show that the interfacial height field serves as the analogue relativistic field even in a nonlinear, interacting field theory. We propose that these mathematical equivalences can be extrapolated to probe regimes where calculations are challenging or impractical. Our work provides a new framework for simulating far-from-equilibrium cosmological and gravitational scenarios in the laboratory.

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arXiv:2307.02549 cond-mat.quant-gas astro-ph.CO gr-qc hep-ph hep-th

**From the tabletop to the Big Bang: Analogue vacuum decay from vacuum initial conditions **

**Authors:** Alexander C. Jenkins, Jonathan Braden, Hiranya V. Peiris, Andrew Pontzen,
Matthew C. Johnson, Silke Weinfurtner

Abstract: Ultracold atomic gases can undergo phase transitions that mimic relativistic vacuum decay, allowing us to empirically test early-Universe physics in tabletop experiments. We investigate the physics of these analogue systems, going beyond previous analyses of the classical equations of motion to study quantum fluctuations in the cold-atom false vacuum. We show that the fluctuation spectrum of this vacuum state agrees with the usual relativistic result in the regime where the classical analogy holds, providing further evidence for the suitability of these systems for studying vacuum decay. Using a suite of semiclassical lattice simulations, we simulate bubble nucleation from this analogue vacuum state in a 1D homonuclear potassium-41 mixture, finding qualitative agreement with instanton predictions. We identify realistic parameters for this system that will allow us to study vacuum decay with current experimental capabilities, including a prescription for efficiently scanning over decay rates, and show that this setup will probe the quantum (rather than thermal) decay regime at temperatures T≲10nK. Our results help lay the groundwork for using upcoming cold-atom experiments as a new probe of nonperturbative early-Universe physics.

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arXiv:2308.07892 quant-ph gr-qc hep-th

**Vacuum entanglement probes for ultra-cold atom systems **

**Authors:** Cisco Gooding, Allison Sachs, Robert B. Mann, Silke Weinfurtner

Abstract: This study explores the transfer of nonclassical correlations from an ultra-cold atom system to a pair of pulsed laser beams. Through nondestructive local probe measurements, we introduce an alternative to destructive techniques for mapping BEC entanglement. Operating at ultralow temperatures, the setup emulates a relativistic vacuum field, using lasers as Unruh-DeWitt detectors for phonons. The vacuum holds intrinsic entanglement, transferable to distant probes briefly interacting with it - a phenomenon termed ``entanglement harvesting''. Our study accomplishes two primary objectives: first, establishing a mathematical equivalence between a pair of pulsed laser probes interacting with an effective relativistic field and the entanglement harvesting protocol; and second, to closely examine the potential and persisting obstacles for realising this protocol in an ultra-cold atom experiment.

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arXiv:2308.10773 gr-qc physics.flu-dyn

**Exploring the Quantum-to-Classical Vortex Flow: Quantum Field Theory Dynamics in Rotating
Curved Spacetimes **

**Authors:** Patrik Švančara, Pietro Smaniotto, Leonardo Solidoro, James F. MacDonald,
Sam Patrick, Ruth Gregory, Carlo F. Barenghi, Silke Weinfurtner

Abstract: Gravity simulators are laboratory systems where small excitations like sound or surface waves behave as fields propagating on a curved spacetime geometry. The analogy between gravity and fluids requires vanishing viscosity, a feature naturally realised in superfluids like liquid helium or cold atomic clouds. Such systems have been successful in verifying key predictions of quantum field theory in curved spacetime. In particular, quantum simulations of rotating curved spacetimes indicative of astrophysical black holes require the realisation of an extensive vortex flow in superfluid systems. Despite the inherent instability of multiply quantised vortices, here we demonstrate that a stationary giant quantum vortex can be stabilised in superfluid 4He. Its compact core carries thousands of circulation quanta, prevailing over current limitations in other physical systems such as magnons, atomic clouds and polaritons. We introduce a minimally invasive way to characterise the vortex flow by exploiting the interaction of micrometre-scale waves on the superfluid interface with the background velocity field. Intricate wave-vortex interactions, including the detection of bound states and distinctive analogue black hole ringdown signatures, have been observed. These results open new avenues to explore quantum-to-classical vortex transitions and utilise superfluid helium as a finite temperature quantum field theory simulator for rotating curved spacetimes.

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arXiv:2207.09326 gr-qc astro-ph.HE physics.data-an

**Implementation of a new weave-based search pipeline for continuous gravitational waves
from known binary systems **

**Authors:** Arunava Mukherjee, Reinhard Prix, Karl Wette

Abstract: Scorpius X-1 (Sco X-1) has long been considered one of the most promising targets for detecting continuous gravitational waves with ground-based detectors. Observational searches for Sco X-1 have achieved substantial sensitivity improvements in recent years, to the point of starting to rule out emission at the torque-balance limit in the low-frequency range \sim 40--180 Hz. In order to further enhance the detection probability, however, there is still much ground to cover for the full range of plausible signal frequencies \sim 20--1500 Hz, as well as a wider range of uncertainties in binary orbital parameters. Motivated by this challenge, we have developed BinaryWeave, a new search pipeline for continuous waves from a neutron star in a known binary system such as Sco X-1. This pipeline employs a semi-coherent StackSlide F-statistic using efficient lattice-based metric template banks, which can cover wide ranges in frequency and unknown orbital parameters. We present a detailed timing model and extensive injection-and-recovery simulations that illustrate that the pipeline can achieve high detection sensitivities over a significant portion of the parameter space when assuming sufficiently large (but realistic) computing budgets. Our studies further underline the need for stricter constraints on the Sco X-1 orbital parameters from electromagnetic observations, in order to be able to push sensitivity below the torque-balance limit over the entire range of possible source parameters.

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arXiv:2305.07106 gr-qc astro-ph.IM

**Searches for continuous gravitational waves from neutron stars: A twenty-year retrospective **

**Authors:** Karl Wette

Abstract: Seven years after the first direct detection of gravitational waves, from the collision of two black holes, the field of gravitational wave astronomy is firmly established. A first detection of continuous gravitational waves from rapidly-spinning neutron stars could be the field's next big discovery. I review the last twenty years of efforts to detect continuous gravitational waves using the LIGO and Virgo gravitational wave detectors. I summarise the model of a continuous gravitational wave signal, the challenges to finding such signals in noisy data, and the data analysis algorithms that have been developed to address those challenges. I present a quantitative analysis of 297 continuous wave searches from 80 papers, published from 2003 to 2022, and compare their sensitivities and coverage of the signal model parameter space.

Journal ref: Astroparticle Physics 153 (2023) 102880

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arXiv:2207.01653 gr-qc hep-th

**Mode Stability For Massless Scalars In Five-Dimensional Black Hole Backgrounds **

**Authors:** M. Cvetic, C. N. Pope, B. F. Whiting, Haoyu Zhang

Abstract: The mode stability of the Kerr black hole in four dimensions was demonstrated by Whiting in 1989, by separating the Teukolsky equation that describes gravitational perturbations and then transforming the radial and angular equations in such a way that the problem can be reformulated as a wave equation in an auxiliary spacetime in which the proof of stability is greatly simplified, owing to the absence of an ergoregion. As a preliminary step towards extending these ideas to higher-dimensional black holes, we study the mode stability of the massless scalar wave equation in the five-dimensional black hole solutions of Einstein gravity and supergravity. We show how the wave equation can again be mapped into one in an auxiliary spacetime in which there is no ergoregion, allowing us to give a proof of the mode stability of the solutions of the scalar wave equation.

Report number: UPR-1320-T, CERN-TH-2022-102, MI-HET-779

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arXiv:2207.12952 math.AP gr-qc math-ph

**Mode analysis for the linearized Einstein equations on the Kerr metric : the large
a case **

**Authors:** Lars Andersson, Dietrich Häfner, Bernard F. Whiting

Abstract: We give a complete analysis of mode solutions for the linearized Einstein equations
and the 1−form wave operator on the Kerr metric in the large a case. By mode solutions we mean solutions of the form e^{−it}^{∗}^{σ}h(r,θ,φ)
where t_{∗} is a suitable time variable. The corresponding
Fourier transformed 1−form wave operator and linearized Einstein operator are shown to be Fredholm between suitable function spaces and h has to lie in the domain of these operators. These spaces are constructed following the general framework of Vasy. No
mode solutions exist for Iσ≥0,σ≠0. For σ=0 mode solutions are Coulomb solutions for the 1−form wave operator and linearized Kerr solutions plus pure gauge terms in the case of the linearized Einstein equations. If we fix a De Turck/wave map gauge, then the
zero mode solutions for the linearized Einstein equations lie in a fixed 7−dimensional space. The proof relies on the absence of modes for the Teukolsky equation shown by the third author and a complete classification of the gauge invariants of linearized
gravity on the Kerr spacetime due to Aksteiner et al.

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arXiv:2309.09891 gr-qc

**A Classical Firewall Transformation as a Canonical Transformation **

**Authors:** Nathaniel A. Strauss, Bernard F. Whiting

Abstract: The firewall transformation put forward by 't Hooft in recent years has made ambitious claims of solving the firewall problem and the black hole information paradox while maintaining unitary evolution. However, the theory has received limited attention from the community, especially in regards to its foundations in purely classical gravitational physics. This paper investigates the underlying assumptions of 't Hooft's firewall transformation before quantization. We find that the limiting procedure used by 't Hooft in order to obtain an identification of the quantum operators for ingoing and outgoing particles near a black hole is not consistent. We propose a correction, which involves a more relaxed approximation regime. In the new approximation regime, we find a new classical analog for the firewall transformation for spherical shells, which allows evolving the spherical shells' dynamics past their point of collision. In the classical theory, no firewall is removed, as both ingoing and outgoing matter is present on every spacelike hypersurface, and it does not appear that any firewalls will be removed after a canonical quantization.

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arXiv:2309.09905 gr-qc

**An Exact, Coordinate Independent Classical Firewall Transformation **

**Authors:** Nathaniel A. Strauss, Bernard F. Whiting

Abstract: A proposal for resolving the black hole information paradox was recently put forward by 't Hooft in the form of his firewall transformation. Although this proposal has begun to gain some limited traction, its physical foundation is still somewhat obscure. Here we develop a classical Hamiltonian analog, which is oriented towards quantization, by using the canonical formalism developed by Arnowitt, Deser, and Misner (ADM). We use a model of two null, spherical shells in a Schwarzschild black hole background, and within our ADM formalism we are able to characterize the dynamics of the entire system, especially at the point of collision, and we reproduce the related Dray-'t Hooft-Redmount formula. Finally, we are able to find a classical analog for 't Hooft's firewall transformation. Unlike 't Hooft's firewall transformation and previous classical analogs, the classical firewall transformation we obtain is free from approximation and maintains the coordinate independence of the ADM formalism. We leave to future work the quantization of the theory.

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arXiv:2207.05765 astro-ph.CO gr-qc hep-ph hep-th physics.hist-ph

**Is the Observable Universe Consistent with the Cosmological Principle? **

**Authors:** Pavan Kumar Aluri, Paolo Cea, Pravabati Chingangbam, Ming-Chung Chu,
Roger G. Clowes, Damien Hutsemékers, Joby P. Kochappan, Alexia M. Lopez, Lang Liu, Niels C. M. Martens, C. J. A. P. Martins, Konstantinos Migkas, Eoin Ó Colgáin, Pratyush Pranav, Lior Shamir, Ashok K. Singal, M. M. Sheikh-Jabbari, Jenny Wagner, Shao-Jiang
Wang, David L. Wiltshire, Shek Yeung, Lu Yin, Wen Zhao

Abstract: The Cosmological Principle (CP) -- the notion that the Universe is spatially isotropic and homogeneous on large scales -- underlies a century of progress in cosmology. It is conventionally formulated through the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive Λ-Cold-Dark-Matter (ΛCDM) model. Yet, tensions have emerged within the ΛCDM model, most notably a statistically significant discrepancy in the value of the Hubble constant, H0. Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the H0 tension may extend beyond ΛCDM to the CP itself. This review surveys current observational hints for deviations from the expectations of the CP, highlighting synergies and disagreements that warrant further study. Setting aside the debate about individual large structures, potential deviations from the CP include variations of cosmological parameters on the sky, discrepancies in the cosmic dipoles, and mysterious alignments in quasar polarizations and galaxy spins. While it is possible that a host of observational systematics are impacting results, it is equally plausible that precision cosmology may have outgrown the FLRW paradigm, an extremely pragmatic but non-fundamental symmetry assumption.

Journal ref: Classical and Quantum Gravity, Vol. 40, Issue No. 9, Page No. 094001 (2023)

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arXiv:2206.13224 gr-qc astro-ph.IM quant-ph

**Boosting the sensitivity of high frequency gravitational wave detectors by PT-symmetry **

**Authors:** Chuming Wang, Chunnong Zhao, Xiang Li, Enping Zhou, Haixing Miao, Yanbei
Chen, Yiqiu Ma

Abstract: The kilo-Hertz gravitational waves radiated by the neutron star merger remnants carry rich information about the physics of high-density nuclear matter states, and many important astrophysical phenomena such as gamma-ray bursts and black hole formation. Current laser interferometer gravitational wave detectors, such as LIGO, VIRGO, and KAGRA have limited signal response at the kilo-Hertz band, thereby unable to capture these important physical phenomena. This work proposes an alternative protocol for boosting the sensitivity of the gravitational wave detectors at high frequency by implementing an optomechanical quantum amplifier. With the auxiliary quantum amplifier, this design has the feature of Parity-Time (PT) symmetry so that the detection band will be significantly broadened within the kilo-Hertz range. In this work, we carefully analyze the quantum-noise-limited sensitivity and the dynamical stability of this design. Based on our protocol, our result shows that the quantum-noise-limited sensitivity will be improved by one order of magnitude around 3kHz, which indicates the potential of our design for a future search of neutron star merger signals.

Journal ref: Phys. Rev. D 106, 082002 (2022)

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