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AUSTRALASIAN SOCIETY FOR GENERAL RELATIVITY AND GRAVITATION
Electronic Newsletter -- #23, Winter 2017
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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 June, 2018.
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CONTENTS:
* 9TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG9): Gingin, Western Australia, 27-30 November, 2017
* 10TH BIENNIAL GENERAL MEETING OF THE ASGRG, November 2017
* MEMBERSHIP DETAILS ONLINE at
http://www.asgrg.org/membership/index.php
* FORTHCOMING MEETINGS
* MEMBERS' ABSTRACTS at gr-qc, July 2016 - June 2017
* ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc, July 2016 - June 2017
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9TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG9)
Gingin, Western Australia, 27-30 November, 2017
ACGRG9 is the ninth 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 and a special workshop session on next generation gravitational wave detectors in the Asian region.
The conference is open to anyone with an interest in general relativity.
ACGRG9 will be hosted by the University of Western Australia at the Gravity Discovery Centre in Gingin, Western Australia, from 27 to 30 November, 2017.
Deadline for submission of abstracts: 27 October 2017
Local Organising Committee: Chunnong Zhao, David Blair, Eric Howell, Li Ju, Sergei Kuzenko, Xingjiang Zhu
Scientific Organising Committee: David Wiltshire (Canterbury), Hyung Mok Lee (Seoul NU), Leo Brewin (Monash), Leong Chuan Kwek (NU Singapore), Linqing Wen (UWA), Malcolm Anderson (Brunei), Matthew Bailes (Swinburne), Paul Lasky (Monash), Peter Veitch (Adelaide), Shiuh Chao (Tsing Hua), Susan Scott (ANU), Tjonnie Li (Chinese U Hong Kong), Zonghong Zhu (Beijing Normal)
For further details, visit the conference website at www.ecm.uwa.edu.au/acgrg9
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10TH BIENNIAL GENERAL MEETING OF THE ASGRG
The 2017 Biennial General Meeting of the ASGRG will be held in conjunction with ACGRG9, at a date in November 2017 to be determined.
All ASGRG Executive Committee positions will be filled by election at the BGM. The outgoing Executive Committee members are:
President - Leo Brewin
Treasurer - Todd Oliynyk
Secretary - Malcolm Anderson
Officer - Susan Scott
Officer - Paul Lasky
Co-Opted committee members: John Steele, Bram Slagmolen
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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
September 24-28, 2017: 4th Conference of the Polish Society on Relativity
Kazimierz Dolny, Poland
https://indico.cern.ch/event/588228/
October 11-14, 2017: European Einstein Toolkit Workshop 2017 (plus Edward Seidel Symposium)
Club Pollentia Resort
Mallorca, Spain
http://grg.uib.es/EinsteinToolkit2017/
October 12-14, 2017: 27th Midwest Relativity Meeting
University of Michigan
Ann Arbor, Michigan
https://sites.google.com/a/umich.edu/midwest-gravity-meeting/
October 16-20, 2017: Astro-GR@Barcelona 2017
“The capture of objects by supermassive black holes”
Barcelona, Spain
http://astro-gr.org/astro-gr-barcelona-2017/
November 12-18, 2017: XIIth School of Cosmology: “The CMB from A to Z – promises and challenges of the CMB as a cosmological probe”
Institut d’Études Scientifiques de Cargèse (IESC), Cargèse
Corsica, France
http://www.cpt.univ-mrs.fr/~cosmo/EC2017/EcoleLuminy17_a.html
November 27-30, 2017: 9th Australasian Conference on General Relativity (ACGRG9)
Gravity Discovery Centre
Gingin, Western Australia
http://www.ecm.uwa.edu.au/acgrg9
November 27-December 1, 2017: 27th Workshop on General Relativity and Gravitation in Japan (JGRG27)
Higashi Hiroshima Arts and Cultural Hall
Kurara, Japan
http://home.hiroshima-u.ac.jp/jgrg27/
December 3-8, 2017: 29th International Texas Symposium on Relativistic Astrophysics
CTICC
Cape Town, South Africa
http://www.texas2017.org
December 11-15, 2017: International Symposium on Cosmology and Particle Astrophysics (CosPA 2017)
Yukawa Institute for Theoretical Physics
Kyoto University, Spain
http://www.resceu.s.u-tokyo.ac.jp/symposium/cospa2017/
December 15-19, 2017: “Black Holes: From Classical to Quantum Gravity”
IIT Gandhinagar
Gandhinagar. India
http://events.iitgn.ac.in/2017/blackholes/programme.php
January 29-March 9, 2018: YITP long-term workshop “Gravity and Cosmology 2018”
Yukawa Institute for Theoretical Physics
Kyoto University, Japan
http://www2.yukawa.kyoto-u.ac.jp/~gc2018/
May 14-17, 2018: 5th International Conference on the Nature and Ontology of Spacetime
Hotel Laguna Garden
Albena, Bulgaria
http://www.minkoskiinstitute.org/conference/2018/
July 1-8, 2018: 15th Marcel Grossmann Meeting on General Relativity (MG15)
University of Rome “La Sapienza”
Rome, Italy
http://www.icra.it/mg/mg15/
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MEMBERS' ABSTRACTS at gr-qc, July 2016 - June 2017
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 Los Alamos archives which are not cross-linked to gr-qc.) If you do not send your papers to gr-qc but would like to have them noted in the newsletters, please send them to the Editor.
Note that the 66 papers listed here and in the LIGO section represent 1.49% of the 4420 papers posted or cross-linked to gr-qc between July 2016 and June 2017.
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arXiv:1702.00964 [gr-qc]
Author: M. M. Akbar
(Submitted on 3 Feb 2017)
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Author: M. M. Akbar
(Submitted on 3 Feb 2017)
Abstract: Contrary to the general consensus in the literature that Friedmann--Lema\^{i}tre--Robertson--Walker (FLRW) geometries are of embedding class one (i.e.,\ embeddable in one higher dimensional pseudo-Euclidean spaces), we show that the most general $k=0$ and $k=-1$ FLRW geometries are of embedding class two, and their corresponding pseudo-Euclidean spaces have strictly one and two negative eigenvalues, respectively. These are particular results that follow from the new perspective on FLRW embedding that we develop in this paper, namely that these embeddings are equivalent to unit-speed parametrized curves in two or three dimensions. A careful analysis of appropriate tensor fields then gives identical results and further explains why the class-two geometries remained hidden. However, the signatures of the embedding spaces, as well as the explicit embedding formulae, follow only from the curve picture. This also streamlines the comparatively difficult $k=0$ class and provides new explicit embedding formulae for it and reproduces known embedding formulae for the $k=1,-1$ classes. Embedding into anti-de Sitter space in one higher dimension can likewise be done by constructing associated curves. In particular, we find that all $k=1$ and mildly restricted subclasses of $k=0, -1$ geometries are embeddable in anti-de Sitter space in one higher dimension.
Journal reference: Phys. Rev. D 95, 064058 (2017)
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Authors: Miftachul Hadi, Malcolm Anderson, Andri Husein
(Submitted on 9 Nov 2016)
Abstract: We study nonlinear sigma model, especially Skyrme model with twist: twisted Skyrmion string where twist term, $mkz$, is indicated in vortex solution. To add gravity, we replace $\eta^{\mu\nu}$ in Lagrangian system with a space-time metric tensor, $g^{\mu\nu}$, which in view of the time-independence and cylindrical symmetry of the assumed vortex solution is taken to be a function of $r$ alone. We use ode45 for numerical calculation, i.e. a tool box in Matlab to solve coupled Einstein field equations which have ordinary differential equations (ODE) form.
Comments: 8 pages, 5 figures, submitted to ICSAS 2016
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Author: Estelle Asmodelle
(Submitted on 11 May 2017)
Abstract: This report is a literature review of significant and successful tests of general relativity [GR]. The GR predicted value for the perihelion advance of Mercury was Deltaf = 43.03 arcsec century^-1 and fit well with observation, being the first success of GR. The GR result for the bending of light around the Sun dGR = 1.75 arcsec, confirmed by observation, marked the second successful validation of GR. Gravitational Redshift [GvR] was first detected 1925 and is the third successful classical test. The parametrized post-Newtonian PPN employs b and g for GR testing. Shapiro delay was also confirmed with g = 1.000021 +/- 0.000023, against GR value g = 1, some consider this to be the fourth classical test. So too gravitational time dilation GvT was experimentally confirmed in 1971, while GvT for GPS is a daily validation of GR. Frame-dragging and the Geodetic effect have also been confirmed. The strong equivalence principle SEP has been confirmed to h = 4.4 x 10^-4, to GR h = 0. Gravitational slip has been constrained to EG = 0.48 +/- 0.10 at z = 0.32, against the GR value, EG = 0.30 +/- 0.07. The first gravitational wave detection from GW150914, in 2015, has confirmed a long-awaited phenomenon that has taken GR testing to higher precision. Gravitational lensing has also confirmed GR to better than 1%. GR continues to be tested, eliminating competing gravity theories.
Comments: 65 pages, 23 Figures, B.Sc. [hons] dissertation
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arXiv:1701.02239 [gr-qc]
Strong Gravitational Lensing by a Charged Kiselev Black Hole
Authors: Mustapha Azreg-Aïnou, Sebastian Bahamonde, Mubasher Jamil
(Submitted on 5 Jan 2017 )
Abstract: We study the gravitational lensing scenario where the lens is a spherically symmetric charged black hole (BH) surrounded by quintessence matter. The null geodesic equations in the curved background of the black hole are derived. The resulting trajectory equation is solved analytically via perturbation and series methods for special choice of parameters and the distance of the closest approach to black hole is calculated. We also derive the lens equation giving the bending angle of light in the curved background. In the strong field approximation, the solution of the lens equation is also obtained for all values of the quintessence parameter $w_q$. For all $w_q$, we show that there are no stable closed null orbits and that corrections to the deflection angle for the Reissner-Nordstr\"om black hole when the observer and the source are at large, but finite, distances from the lens do not depend on the charge up to the inverse of the distances squared. A part of the present work, analyzed however with a different approach, is the extension of {\it Phys. Rev. D \textbf{92}, 084042 (2015)} where the uncharged case has been treated.
Journal reference: Eur. Phys. J. C (2017) 77: 414
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Author: Mustapha Azreg-Aïnou
(Submitted on 20 Jan 2017)
Abstract: We consider a general stationary solution and derive the general laws for accretion of rotating perfect fluids. For non-degenerate and degenerate Fermi and Bose fluids we derive new effects that mimic the center-of-mass-energy effect of two colliding particle in the vicinity of horizons. Non-degenerate fluids see their chemical potential grow arbitrarily and ultra-relativistic Fermi fluids see their specific enthalpy and Fermi momentum grow arbitrarily too while the latter vanishes gradually for non-relativistic Fermi fluids. For degenerate Bose fluids two scenarios remain possible as the fluid approaches a horizon: a) The Bose-Einstein condensation ceases or b) the temperature drops gradually down to zero.
Journal reference: Phys. Rev. D 95, 083002 (2017)
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Author: Mustapha Azreg-Aïnou
(Submitted on 19 Apr 2017)
Abstract: We derive and investigate the physical properties of asymptotically flat noncommutative regular charged black holes with a Gaussian mass density distribution and a Weibull electric charge density distribution. The solutions have a de Sitter behavior in the vicinity of the origin provided the electric charge density is Weibull of the form $r^{n/2}\e^{-r^2/(4\theta^2)}$ with $n\geq 1$. The electric field and temperature are finite for all values of the radial coordinate, mass, and charge. The charge is bounded from above for stability reasons and stable charged quantum particles have mass and charge bounded from below and from above within the simplified semi-classical model we present in this work.\
Comments: 7 pages, 6 figures
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Author: Mustapha Azreg-Aïnou
(Submitted on 14 Jun 2017)
Abstract: We discuss the generic properties of any general mass distribution $\mathcal{D}(r,\theta)$ depending on one parameter $\theta$ and endowed with spherical symmetry. We show (a) that the de Sitter behavior of spacetime at the origin is generic and depends only on $\mathcal{D}(0,\theta)$, (b) that, due to the character of the cumulative distribution of $\mathcal{D}(r,\theta)$, the geometry may posses up to two horizons depending solely on the value of the total mass $M$, and (c) that no scalar invariant nor a thermodynamic entity diverges. We define new two-parameter mathematical distributions mimicking Gaussian and step-like functions and reduce to the Dirac distribution in the limit of vanishing parameter $\theta$. We use these distributions to derive in closed forms asymptotically flat, spherically symmetric, solutions that describe and model a variety of physical and geometric entities ranging from noncommutative black holes, quantum-corrected black holes to stars and dark matter halos for various scaling values of $\theta$. We show that the linear mass density $\pi c^2/G$ is an upper limit for regular-black-hole formation.
Comments: 10 two-column pages, 3 figures, 2 tables
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Authors: Sunil D. Maharaj, Aroonkumar Beesham, Sergey V. Chervon, Aleksandr S. Kubasov
(Submitted on 19 Apr 2017)
Abstract: We consider a chiral cosmological model in the framework of Einstein-Gauss-Bonnet cosmology. Using a decomposition of the latter equations in such a way that the first chiral field is responsible for the Einstein part of the model, while the second field together with the kinetic interaction is connected with the Gauss--Bonnet part of the theory, we find new exact solutions for the 2-component chiral cosmological model with and without the kinetic interaction between fields.
Comments: 9 pages, the article is a reflection of the talk given at the Ulyanovsk International School-Seminar "Problems of theoretical and observation cosmology - UISS 2016", September 19-30, Ulyanovsk, Russia
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Authors: Alireza Sepehri, Anirudh Pradhan, A. Beesham
(Submitted on 14 Jun 2017)
Abstract: In this paper, we demonstrate that compactification in M-theory can lead to a deformation of field theory consistent with the generalized uncertainty principle (GUP).We observe that the matter fields in the M3-brane action contain higher derivative terms. We demonstrate that such terms can also be constructed from a reformulation of the field theory by the GUP. In fact, we will construct the Heisenberg algebra consistent with this deformation, and explicitly demonstrate it to be the Heisenberg algebra obtained from the GUP. Thus, we use compactification in M-theory to motivate for the existence of the GUP.
Journal reference: Mod. Phys. Lett. A, Vol. 32, No. 24 (2017) 1750123
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Authors: Jue Zhang, Chunnong Zhao, Li Ju, David Blair
(Submitted on 6 Sep 2016)
Abstract: Parametric instability is an intrinsic risk in high power laser interferometer gravitational wave detectors, in which the optical cavity modes interact with the acoustic modes of the mirrors leading to exponential growth of the acoustic vibration. In this paper, we investigate the potential parametric instability for a proposed next generation gravitational wave detector based on cooled silicon test masses. It is shown that there would be about 2 unstable modes per test mass, with the highest parametric gain of ~76. The importance of developing suitable instability suppression schemes is emphasized.
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Author: Leo Brewin
(Submitted on 28 Feb 2017)
Abstract: Numerical results, based on a lattice method for computational general relativity, will be presented for Cauchy evolution of initial data for the Brill, Teukolsky and polarised Gowdy space-times. The simple objective of this paper is to demonstrate that the lattice method can, at least for these space-times, match results obtained from contemporary methods. Some of the issues addressed in this paper include the handling of axisymmetric instabilities (in the Brill space-time) and an implementation of a Sommerfeld radiation condition for the Brill and Teukolsky space-times. It will be shown that the lattice method performs particularly well in regard to the passage of the waves through the outer boundary. Questions concerning multiple black-holes, mesh refinement and long term stability will not be discussed here but may form the basis of future work.
Comments: 67 pages, 18 figures
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Authors: Katerina Chatziioannou, Antoine Klein, Neil Cornish, Nicolas Yunes
(Submitted on 9 Jun 2016)
Abstract: Binary systems of two compact objects circularize and spiral toward each other via the emission of gravitational waves. The coupling of the spins of each object with the orbital angular momentum causes the orbital plane to precess, which leads to modulation of the gravitational wave signal. Until now, generating frequency-domain waveforms for fully precessing systems for use in gravitational wave data analysis meant numerically integrating the equations of motion, then Fourier transforming the result, which is very computationally intensive for systems that complete hundreds or thousands of cycles in the sensitive band of a detector. Previously, analytic solutions were only available for certain special cases or for simplified models. Here we describe the construction of closed-form, frequency-domain waveforms for fully-precessing, quasi-circular binary inspirals.
Journal reference: Phys Rev Lett.118.051101
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Authors: Joseph D. Romano, Neil J. Cornish
(Submitted on 24 Aug 2016)
Abstract: We review detection methods that are currently in use or have been proposed to search for a stochastic background of gravitational radiation. We consider both Bayesian and frequentist searches using ground-based and space-based laser interferometers, spacecraft Doppler tracking, and pulsar timing arrays; and we allow for anisotropy, non-Gaussianity, and non-standard polarization states. Our focus is on relevant data analysis issues, and not on the particular astrophysical or early Universe sources that might give rise to such backgrounds. We provide a unified treatment of these searches at the level of detector response functions, detection sensitivity curves, and, more generally, at the level of the likelihood function, since the choice of signal and noise models and prior probability distributions are actually what define the search. Pedagogical examples are given whenever possible to compare and contrast different approaches. We have tried to make the article as self-contained and comprehensive as possible, targeting graduate students and new researchers looking to enter this field.
Journal reference: Living Rev Relativ (2017) 20:2
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Authors: Katerina Chatziioannou, Antoine Klein, Nicolas Yunes, Neil Cornish
(Submitted on 11 Mar 2017)
Abstract: The coalescence of compact objects is one of the most promising sources of gravitational waves for ground-based interferometric detectors, such as advanced LIGO and Virgo. Generically, com- pact objects in binaries are expected to be spinning with spin angular momenta misaligned with the orbital angular momentum, causing the orbital plane to precess. This precession adds rich structure to the gravitational waves, introducing such complexity that an analytic closed-form description has been unavailable until now. We here construct the first closed-form frequency- domain gravitational waveforms that are valid for generic spin-precessing quasicircular compact binary inspirals. We first construct time-domain gravitational waves by solving the post-Newtonian precession equations of motion with radiation reaction through multiple scale analysis. We then Fourier transform these time-domain waveforms with the method of shifted uniform asymptotics to obtain closed-form expressions for frequency-domain waveforms. We study the accuracy of these analytic, frequency-domain waveforms relative to waveforms obtained by numerically evolving the post-Newtonian equations of motion and find that they are suitable for unbiased parameter estimation for 99.2%(94.6%) of the binary configurations we studied at a signal-to-noise ratio of 10(25). These new frequency-domain waveforms could be used for detection and parameter estimation studies due to their accuracy and low computational cost.
Journal reference: Phys Rev D.95.104004
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Authors: Neil Cornish, Travis Robson
(Submitted on 29 Mar 2017)
Abstract: Building on the great success of the LISA Pathfinder mission, the outlines of a new LISA mission design were laid out at the $11^{\rm th}$ International LISA Symposium in Zurich. The revised design calls for three identical spacecraft forming an equilateral triangle with 2.5 million kilometer sides, and two laser links per side delivering full polarization sensitivity. With the demonstrated Pathfinder performance for the disturbance reduction system, and a well studied design for the laser metrology, it is anticipated that the new mission will have a sensitivity very close to the original LISA design. This implies that the mid-band performance, between 0.5 mHz and 3 mHz, will be limited by unresolved signals from compact binaries in our galaxy. Here we use the new LISA design to compute updated estimates for the galactic confusion noise, the number of resolvable galactic binaries, and the accuracy to which key parameters of these systems can be measured.
Comments: 4 pages, 5 figures. Prepared for the proceedings of the 11th International LISA Symposium
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Authors: Travis Robson, Neil Cornish
(Submitted on 26 May 2017)
Abstract: The Laser Interferometer Space Antenna (LISA) will explore the source-rich milli-Hertz band of the gravitational wave spectrum. In contrast to ground based detectors, where typical signals are short-lived and discrete, LISA signals are typically long-lived and over-lapping, thus requiring a global data analysis solution that is very different to the source-by-source analysis that has been developed for ground based gravitational wave astronomy. Across the LISA band, gravitational waves are both signals {\em and} noise. The dominant contribution to this so-called confusion noise (better termed unresolved signal noise) is expected to come from short period galactic white dwarf binaries, but all sources, including massive black hole binaries and extreme mass ratio captures will also contribute. Previous estimates for the galactic confusion noise have assumed perfect signal subtraction. Here we provide analytic estimates for the signal subtraction residuals and the impact they have on parameter estimation while for the first time incorporating the effects of noise modeling. The analytic estimates are found using a maximum likelihood approximation to the full global Bayesian analysis. We find that while the confusion noise is {\em lowered} in the global analysis, the waveform errors for individual sources are {\em increased} relative to estimates for isolated signals. We provide estimates for how parameter estimation errors are inflated from various parts of a global analysis.
Comments: 24 pages, 10 figures
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Author: Tevian Dray
(Submitted on 11 Jan 2017)
Abstract: We review the treatment of conservation laws in spacetimes that are glued together in various ways, thus adding a boundary term to the usual conservation laws. Several examples of such spacetimes will be described, including the joining of Schwarzschild spacetimes of different masses, and the possibility of joining regions of different signatures. The opportunity will also be taken to explore some of the less obvious properties of Lorentzian vector calculus.
Comments: To appear in Gravity and the Quantum, Springer 2017
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arXiv:1610.09614 [gr-qc]
Authors: A. Ravanpak, H. Farajollahi, G. F. Fadakar
(Submitted on 30 Oct 2016)
Abstract: In this article we study cosmic dynamics in the context of normal branch of DGP braneworld model. Using the current Planck data, we best fit the model and cosmological parameters in non-flat $\Lambda$DGP. With the transition redshift as a basic variable and statefinder parameters, our result shows that the Universe starts its accelerated expansion phase, slightly earlier than expected in $\Lambda$CDM cosmology. The result also alleviates the coincidence problem of the $\Lambda$CDM model.
Journal reference: Res. Astron. Astrophys., 16 (2016) no.9, 137
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Authors: Arvin Ravanpak, Hossein Farajollahi, Golnaz Farpoor Fadakar
(Submitted on 28 Mar 2017)
Abstract: The varying speed of light (VSL) has been used in cosmological models in which the physical constants vary over time. On the other hand, the Dvali, Gabadadze and Porrati (DGP) brane world model, especially its normal branch has been extensively discussed to justify the current cosmic acceleration. In this article we show that the normal branch of DGP in VSL cosmology leads to a self-accelerating behavior and therefore can interpret cosmic acceleration. Applying statefinder diagnostics demonstrate that our result slightly deviates {\Lambda}CDM model.
Journal reference: Res. Astron. Astrophys., 17 (2017) 26
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Authors: Georgios Doulis, Jörg Frauendiener
(Submitted on 12 Sep 2016)
Abstract: In this work, we study linearised gravitational fields on the entire Minkowski space-time including space-like infinity. The generalised conformal field equations linearised about a Minkowski background are utilised for this purpose. In principle, this conformal representation of Einstein's equations can be used to carry out global simulations of Minkowski space-time. We investigate thoroughly this possibility.
Comments: 25 pages, 8 figures
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Authors: Jörg Frauendiener, Jörg Hennig
(Submitted on 22 Sep 2016)
Abstract: It has recently been demonstrated (Class. Quantum Grav. 31, 085010, 2014) that the conformally invariant wave equation on a Minkowski background can be solved with a fully pseudospectral numerical method. In particular, it is possible to include spacelike infinity into the numerical domain, which is appropriately represented as a cylinder, and highly accurate numerical solutions can be obtained with a moderate number of gridpoints. In this paper, we generalise these considerations to the spherically-symmetric wave equation on a Schwarzschild background. In the Minkowski case, a logarithmic singularity at the future boundary is present at leading order, which can easily be removed to obtain completely regular solutions. An important new feature of the Schwarzschild background is that the corresponding solutions develop logarithmic singularities at infinitely many orders. This behaviour seems to be characteristic for massive space-times. In this sense this work is indicative of properties of the solutions of the Einstein equations near spatial infinity. The use of fully pseudospectral methods allows us to still obtain very accurate numerical solutions, and the convergence properties of the spectral approximations reveal details about the singular nature of the solutions on spacelike and null infinity. These results seem to be impossible to achieve with other current numerical methods. Moreover, we describe how to impose conditions on the asymptotic behaviour of initial data so that the leading-order logarithmic terms are avoided, which further improves the numerical accuracy.
Journal reference: Class. Quantum Grav. 34, 045005 (2017)
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Authors: Florian Beyer, Leon Escobar, Jörg Frauendiener
(Submitted on 9 Feb 2017)
Abstract: In this paper, we construct and study solutions of Einstein's equations in vacuum with a positive cosmological constant which can be considered as inhomogeneous generalizations of the Nariai cosmological model. Similar to this Nariai spacetime, our solutions are at the borderline between gravitational collapse and de-Sitter-like exponential expansion. Our studies focus in particular on the intriguing oscillatory dynamics which we discover. Our investigations are carried out both analytically (using heuristic mode analysis arguments) and numerically (using the numerical infrastructure recently introduced by us).
Journal reference: Phys. Rev. D 95, 084030 (2017)
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Authors: Florian Beyer, Jörg Frauendiener, Chris Stevens, Ben Whale
(Submitted on 5 Jun 2017)
Abstract: In this paper we study a numerical implementation for the initial boundary value formulation for the generalized conformal field equations. We propose a formulation which is well suited for the study of the long-time behaviour of perturbed exact solutions such as a Schwarzschild or even a Kerr black hole. We describe the derivation of the implemented equations which we give in terms of the space-spinor formalism. We discuss the conformal Gauss gauge, and a slight generalization thereof which seems to be particularly useful in the presence of boundaries. We discuss the structure of the equations at the boundary and propose a method for imposing boundary conditions which allow the correct number of degrees of freedom to be freely specified while still preserving the constraints. We show that this implementation yields a numerically well-posed system by testing it on a simple case of gravitational perturbations of Minkowski space-time and subsequently with gravitational perturbations of Schwarzschild space-time.
Comments: 27 pages, 7 figures
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Authors: Florian Beyer, Leon Escobar, Jörg Frauendiener
(Submitted on 20 Jun 2017)
Abstract: In this paper we consider the hyperbolic formulation of the constraints introduced by R\'acz. Using the numerical framework recently developed by us we construct initial data sets which can be interpreted as nonlinear perturbations of Schwarzschild data in Kerr-Schild coordinates and investigate their asymptotics. Our results suggest that, unless one finds a way to exploit the freedom to pick the free part of the initial data in some suitable way, generic initial data sets obtained by this method may violate fundamental asymptotic conditions.
Comments: 26 pages, 10 figures
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Authors: L. Sun, A. Melatos, P. D. Lasky, C. T. Y. Chung, N. S. Darman
(Submitted on 30 Sep 2016)
Abstract: We present the results of a cross-correlation search for gravitational waves from SNR 1987A using the second year of LIGO Science Run 5 data. The frequency band 75--450\,Hz is searched. No evidence of gravitational waves is found. A 90\% confidence upper limit of $h_0 \leq 3.8\times10^{-25}$ is placed on the gravitational wave strain at the most sensitive frequency near 150\,Hz. This corresponds to an ellipticity of $\epsilon \leq 8.2\times10^{-4}$ and improves on previously published strain upper limits by a factor $\approx4$. We perform a comprehensive suite of validations of the search algorithm and identify several computational savings which marginally sacrifice sensitivity in order to streamline the parameter space being searched. We estimate detection thresholds and sensitivities through Monte-Carlo simulations.
Journal reference: Phys. Rev. D 94, 082004 (2016)
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Authors: Hayley J. Macpherson, Paul D. Lasky, Daniel J. Price
(Submitted on 16 Nov 2016)
Abstract: We perform three-dimensional numerical relativity simulations of homogeneous and inhomogeneous expanding spacetimes, with a view towards quantifying non-linear effects from cosmological inhomogeneities. We demonstrate fourth-order convergence with errors less than one part in 10^6 in evolving a flat, dust Friedmann-Lemaitre-Roberston-Walker (FLRW) spacetime using the Einstein Toolkit within the Cactus framework. We also demonstrate agreement to within one part in 10^3 between the numerical relativity solution and the linear solution for density, velocity and metric perturbations in the Hubble flow over a factor of ~350 change in scale factor (redshift). We simulate the growth of linear perturbations into the non-linear regime, where effects such as gravitational slip and tensor perturbations appear. We therefore show that numerical relativity is a viable tool for investigating nonlinear effects in cosmology.
Comments: 12 pages, 10 figures, accepted for publication in Phys. Rev. D
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Authors: Lucy O. McNeill, Eric Thrane, Paul D. Lasky
(Submitted on 6 Feb 2017)
Abstract: Gravitational-wave memory manifests as a permanent distortion of an idealized gravitational-wave detector and arises generically from energetic astrophysical events. For example, binary black hole mergers are expected to emit memory bursts a little more than an order of magnitude smaller in strain than the oscillatory parent waves. We introduce the concept of "orphan memory": gravitational-wave memory for which there is no detectable parent signal. In particular, high-frequency gravitational-wave bursts ($\gtrsim$ kHz) produce orphan memory in the LIGO/Virgo band. We show that Advanced LIGO measurements can place stringent limits on the existence of high-frequency gravitational waves, effectively increasing the LIGO bandwidth by orders of magnitude. We investigate the prospects for and implications of future searches for orphan memory.
Comments: 5 pages, 4 figures
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Authors: Eric Thrane, Paul Lasky, Yuri Levin
(Submitted on 16 Jun 2017)
Abstract: General relativity's no-hair theorem states that isolated astrophysical black holes are described by only two numbers: mass and spin. As a consequence, there are strict relationships between the frequency and damping time of the different modes of a perturbed Kerr black hole. Testing the no-hair theorem has been a longstanding goal of gravitational-wave astronomy. The recent detection of gravitational waves from black hole mergers would seem to make such tests imminent. We investigate how constraints on black hole ringdown parameters scale with the loudness of the ringdown signal--subject to the constraint that the post-merger remnant must be allowed to settle into a perturbative, Kerr-like state. In particular, we require that--for a given detector--the gravitational waveform predicted by numerical relativity is indistinguishable from an exponentially damped sine after time tcut. The investigation yields a surprising result: by requiring the post-merger remnant to settle into a perturbative state, constraints on ringdown parameters do not shrink monotonically with louder signals. Since the no-hair theorem describes black holes in the asymptotic t -> infinity limit, more sensitive measurements probe later times without necessarily providing tighter constraints on ringdown frequencies and damping times. As a result, there is no clear threshold beyond which gravitational-wave detectors can be said to have unambiguously validated the no-hair theorem, only a series of constraints probing later and later times following the merger. Preliminary investigations are unable to explain this result in terms of a numerical relativity artifact.
Comments: 5 pages, 2 figures
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Authors: Daniele Gregoris, Woei Chet Lim, Alan Coley
(Submitted on 8 May 2017)
Abstract: In this paper, we expand our previous work [1], using the entire family of Bianchi type V solutions as seed solutions of the Stephani transformation. Among the generated solutions, we observe a number of interesting phenomena. The most interesting phenomenon is the intersecting spikes. Other interesting phenomena are saddle states and close-to-FL epoch.
Comments: 23 pages. 11 figures
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Authors: Patrick Kwee, John Miller, Tomoki Isogai, Lisa Barsotti, Matthew Evans
(Submitted on 11 Apr 2017)
Abstract: Squeezed states of light have been successfully employed in interferometric gravitational-wave detectors to reduce quantum noise, thus becoming one of the most promising options for extending the astrophysical reach of the generation of detectors currently under construction worldwide. In these advanced instruments, quantum noise will limit sensitivity over the entire detection band. Therefore, to obtain the greatest benefit from squeezing, the injected squeezed state must be filtered using a long-storage-time optical resonator, or "filter cavity", so as to realise a frequency dependent rotation of the squeezed quadrature. Whilst the ultimate performance of a filter cavity is determined by its storage time, several practical decoherence and degradation mechanisms limit the experimentally achievable quantum noise reduction. In this paper we develop an analytical model to explore these mechanisms in detail. As an example, we apply our results to the 16 m filter cavity design currently under consideration for the Advanced LIGO interferometers.
Journal reference: Phys Rev D.90.062006
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Authors: John Miller, Silvie Ngo, Adam J. Mullavey, Bram J. J. Slagmolen, Daniel A. Shaddock, David E. McClelland
(Submitted on 11 Apr 2017)
Abstract: We present the first demonstration of real-time closed-loop control and deterministic tuning of an independently suspended Fabry-Perot optical cavity using digitally-enhanced heterodyne interferometry, realising a peak sensitivity of $\sim$10 pm$/\sqrt{\mathrm{Hz}}$ over the 10-1000 Hz frequency band. The methods presented are readily extensible to multiple coupled cavities. As such, we anticipate that refinements of this technique may find application in future interferometric gravitational-wave detectors.
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Authors: Chao Liu, Todd A. Oliynyk
(Submitted on 14 Jan 2017)
Abstract: We establish the existence of $1$-parameter families of $\epsilon$-dependent solutions to the Einstein-Euler equations with a positive cosmological constant $\Lambda >0$ and a linear equation of state $p=\epsilon^2 K \rho$, $0<K\leq 1/3$, for the parameter values $0<\epsilon < \epsilon_0$. These solutions exist globally to the future, converge as $\epsilon \searrow 0$ to solutions of the cosmological Poison-Euler equations of Newtonian gravity, and are inhomogeneous non-linear perturbations of FLRW fluid solutions.
Comments: 57 pages
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Authors: Arthur George Suvorov, Andrew Melatos
(Submitted on 10 Aug 2016)
Abstract: The Ernst formulation of the Einstein equations is generalised to accommodate $f(R)$ theories of gravity. It is shown that, as in general relativity, the axisymmetric $f(R)$ field equations for a vacuum spacetime that is either stationary or cylindrically symmetric reduce to a single, non-linear differential equation for a complex-valued scalar function. As a worked example, we apply the generalised Ernst equations to derive a $f(R)$ generalisation of the Zipoy-Voorhees metric, which may be used to describe the gravitational field outside of an ellipsoidal neutron star. We also apply the theory to investigate the phase speed of large-amplitude gravitational waves in $f(R)$ gravity in the context of soliton-like solutions that display shock-wave behaviour across the causal boundary.
Comments: 12 pages, zero figures. Accepted for publication in PRD
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Thomas Callister, A. Sylvia Biscoveanu, Nelson Christensen, Maximiliano Isi, Andrew Matas, Olivier Minazzoli, Tania Regimbau, Mairi Sakellariadou, Jay Tasson, Eric Thrane
(Submitted on 26 Apr 2017)
Abstract: The direct observation of gravitational waves with Advanced LIGO offers novel opportunities to test general relativity in strong-field, highly dynamical regimes. One such opportunity is the measurement of gravitational-wave polarizations. While general relativity predicts only two tensor gravitational-wave polarizations, general metric theories of gravity allow for up to four additional vector and scalar modes. The detection of these alternative polarizations would represent a clear violation of general relativity. However, the current generation of ground-based detectors is unable to sensitively determine the polarization content of transient gravitational-wave signals. Observation of the stochastic gravitational-wave background, in contrast, offers a means of directly measuring gravitational-wave polarizations. The stochastic background, arising from the superposition of many individually unresolvable gravitational-wave signals, may be detectable by Advanced LIGO at design-sensitivity. In this paper, we present a Bayesian method with which to detect and characterize the polarization of the stochastic background. We explore prospects for estimating parameters of the background, and quantify the limits that Advanced LIGO can place on vector and scalar polarizations in the absence of a detection. Finally, we investigate how the addition of future terrestrial detectors like Advanced Virgo will aid our ability to detect or constrain alternative polarizations in the stochastic background. We find that, although the addition of Advanced Virgo does not notably improve detection prospects, it may dramatically improve our ability to estimate the parameters of backgrounds of mixed polarization.
Comments: 24 pages, 20 figures; to be submitted to PRX
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Authors: Cheng-Gang Shao, Yu-Jie Tan, Wen-Hai Tan, Shan-Qing Yang, Jun Luo, Michael Edmund Tobar, Quentin G. Bailey, J.C. Long, E. Weisman, Rui Xu, Alan Kostelecky
(Submitted on 20 Jul 2016)
Abstract: Short-range experiments testing the gravitational inverse-square law at the submillimeter scale offer uniquely sensitive probes of Lorentz invariance. A combined analysis of results from the short-range gravity experiments HUST-2015, HUST-2011, IU-2012, and IU-2002 permits the first independent measurements of the 14 nonrelativistic coefficients for Lorentz violation in the pure-gravity sector at the level of $10^{-9}$ m$^2$, improving by an order of magnitude the sensitivity to numerous types of Lorentz violation involving quadratic curvature derivatives and curvature couplings.
Journal reference: Phys.Rev.Lett.117:071102,2016
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Authors: Cheng-Gang Shao, Ya-Fen Chen, Yu-Jie Tan, Jun Luo, Shan-Qing Yang, Michael Edmund Tobar
(Submitted on 6 Nov 2016)
Abstract: Recently, first limits on putative Lorentz invariance violation coefficients in the pure gravity sector were determined by the reanalysis of short-range gravity experiments. Such experiments search for new physics at sidereal frequencies. They are not, however, designed to optimize the signal strength of a Lorentz invariance violation force; in fact the Lorentz violating signal is suppressed in the planar test mass geometry employed in those experiments. We describe a short-range torsion pendulum experiment with enhanced sensitivity to possible Lorentz violating signals. A periodic, striped test mass geometry is used to augment the signal. Careful arrangement of the phases of the striped patterns on opposite ends of the pendulum further enhances the signal while simultaneously suppressing the Newtonian background.
Comments: Accepted Phys. Rev. D
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Authors: Maxim Goryachev, Ben T. McAllister, Michael E. Tobar
(Submitted on 21 Mar 2017)
Abstract: Using eigenmode analysis and full 3D FEM modelling, we demonstrate that a closed cavity built of an array of elementary harmonic oscillators with negative mutual couplings exhibits a dispersion curve with lower order modes corresponding to higher frequencies. Such cavity arrays may help to achieve infinitely large mode volumes for boosting sensitivity of the axion searches, where the mode volume for the composed array scales proportional to the number of elements, but the frequency remains constant. The negatively coupled cavity array is demonstrated with magnetically coupling coils, where the sign of next-neighbour coupling (controlled with their chirality) sets the dispersion curve properties of the resonator array medium. Furthermore, we show that similar effects can be achieved using only positively coupled cavities of different frequencies assembled in periodic cells. This principle is demonstrated for the multi-post re-entrant system, which can be realised with an array of straight metallic rods organised in chiral structures.
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Authors: Heling Deng, Jaume Garriga, Alexander Vilenkin
(Submitted on 12 Dec 2016)
Abstract: In theories with a broken discrete symmetry, Hubble sized spherical domain walls may spontaneously nucleate during inflation. These objects are subsequently stretched by the inflationary expansion, resulting in a broad distribution of sizes. The fate of the walls after inflation depends on their radius. Walls smaller than a critical radius fall within the cosmological horizon early on and collapse due to their own tension, forming ordinary black holes. But if a wall is large enough, its repulsive gravitational field becomes dominant much before the wall can fall within the cosmological horizon. In this "supercritical" case, a wormhole throat develops, connecting the ambient exterior FRW universe with an interior baby universe, where the exponential growth of the wall radius takes place. The wormhole pinches off in a time-scale comparable to its light-crossing time, and black holes are formed at its two mouths. As discussed in previous work, the resulting black hole population has a wide distribution of masses and can have significant astrophysical effects. The mechanism of black hole formation has been previously studied for a dust-dominated universe. Here we investigate the case of a radiation-dominated universe, which is more relevant cosmologically, by using numerical simulations in order to find the initial mass of a black hole as a function of the wall size at the end of inflation. For large supercritical domain walls, this mass nearly saturates the upper bound according to which the black hole cannot be larger than the cosmological horizon. We also find that the subsequent accretion of radiation satisfies a scaling relation, resulting in a mass increase by about a factor of 2.
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Authors: Ali Masoumi, Alexander Vilenkin, Masaki Yamada
(Submitted on 12 Dec 2016)
Abstract: We develop analytic and numerical techniques for studying the statistics of slow-roll inflation in random Gaussian landscapes. As an illustration of these techniques, we analyze small-field inflation in a one-dimensional landscape. We calculate the probability distributions for the maximal number of e-folds and for the spectral index of density fluctuations $n_s$ and its running $\alpha_s$. These distributions have a universal form, insensitive to the correlation function of the Gaussian ensemble. We outline possible extensions of our methods to a large number of fields and to models of large-field inflation. These methods do not suffer from potential inconsistencies inherent in the Brownian motion technique, which has been used in most of the earlier treatments.
Comments: 25 pages, 6 figures
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Authors: Ali Masoumi, Alexander Vilenkin, Masaki Yamada
(Submitted on 23 Apr 2017)
Abstract: In the landscape perspective, our Universe begins with a quantum tunneling from an eternally-inflating parent vacuum, followed by a period of slow-roll inflation. We investigate the tunneling process and calculate the probability distribution for the initial conditions and for the number of e-folds of slow-roll inflation, modeling the landscape by a small-field one-dimensional random Gaussian potential. We find that such a landscape is fully consistent with observations, but the probability for future detection of spatial curvature is rather low, $P \sim 10^{-3}$.
Comments: 24 pages, 13 figures; (v2) minor changes, an appendix explaining new features of tunneling added
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Authors: Finnian Gray, Jessica Santiago, Sebastian Schuster, Matt Visser
(Submitted on 19 Oct 2016)
Abstract: So-called "twisted" black holes have recently been proposed by Zhang (1609.09721 [gr-qc]), and further considered by Chen and Jing (1610.00886 [gr-qc]), and more recently by Ong (1610.05757 [gr-qc]). While these spacetimes are certainly Ricci-flat, and so mathematically satisfy the vacuum Einstein equations, they are also merely minor variants on Taub--NUT spacetimes. Consequently they exhibit several unphysical features that make them quite unreasonable as realistic astrophysical objects. Specifically, these "twisted" black holes are not (globally) asymptotically flat. Furthermore, they contain closed timelike curves that are not hidden behind any event horizon --- the most obvious of these closed timelike curves are small azimuthal circles around the rotation axis, but the effect is more general. The entire region outside the horizon is infested with closed timelike curves.
Journal reference: Mod. Phys. Lett. A, Vol. 32, No. 18 (2017) 1771001
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Author: Matt Visser (Victoria University of Wellington)
(Submitted on 24 Oct 2016)
Abstract: Some 65 years ago (1951) Wolfgang Pauli noted that the net zero-point energy density could be set to zero by a carefully fine-tuned cancellation between bosons and fermions. In the current article I will argue in a slightly different direction: The zero-point energy density is only one component of the zero-point stress energy tensor, and it is this tensor quantity that is in many ways the more fundamental object of interest. I shall demonstrate that Lorentz invariance of the zero-point stress energy tensor implies finiteness of the zero-point stress energy tensor, and vice versa. Under certain circumstances, (in particular, but not limited to, the finite QFTs), Pauli's cancellation mechanism will survive the introduction of particle interactions. I shall then relate the discussion to BSM physics, to the cosmological constant, and to Sakharov-style induced gravity.
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Author: Matt Visser (Victoria University of Wellington)
(Submitted on 18 Feb 2017)
Abstract: It is an article of folklore that the collection of ideas identified as Euclidean quantum gravity may be derived from ordinary Lorentzian signature gravity by the procedure of Wick rotation. This note will attempt to shed some light on this relatively ill-understood procedure. I argue that it proves inappropriate and unhelpful to regard Wick rotation in terms of a complex deformation of the time coordinate. Rather, Wick rotation can more usefully be viewed as a complex deformation of the spacetime metric. This simple reformulation of the Wick rotation procedure, while it leaves flat space physics unaffected, has profound implications for quantum gravity.
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Authors: Prado Martin-Moruno (Universidad Complutense de Madrid), Matt Visser (Victoria University of Wellington)
(Submitted on 20 Feb 2017)
Abstract: The standard energy conditions of classical general relativity are (mostly) linear in the stress-energy tensor, and have clear physical interpretations in terms of geodesic focussing, but suffer the significant drawback that they are often violated by semi-classical quantum effects. In contrast, it is possible to develop non-standard energy conditions that are intrinsically non-linear in the stress-energy tensor, and which exhibit much better well-controlled behaviour when semi-classical quantum effects are introduced, at the cost of a less direct applicability to geodesic focussing. In this article we will first review the standard energy conditions and their various limitations. (Including the connection to the Hawking--Ellis type I, II, III, and IV classification of stress-energy tensors). We shall then turn to the averaged, nonlinear, and semi-classical energy conditions, and see how much can be done once semi-classical quantum effects are included.
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Authors: Ana Alonso-Serrano (Charles University of Prague), Matt Visser (Victoria University of Wellington)
(Submitted on 1 Apr 2017)
Abstract: The nature of coarse graining is intuitively "obvious", but it is rather difficult to find explicit and calculable models of the coarse graining process (and the resulting entropy flow) discussed in the literature. What we would like to have at hand is some explicit and calculable process that takes an arbitrary system, with specified initial entropy S, and monotonically and controllably drives the entropy to its maximum value. This does not have to be a physical process, in fact for some purposes it is better to deal with a gedanken-process, since then it is more obvious how the "hidden information" is hiding in the fine-grain correlations that one is simply agreeing not to look at. We shall present several simple mathematically well-defined and easy to work with conceptual models for coarse graining. We shall consider both the classical Shannon and quantum von Neumann entropies, including models based on quantum decoherence, and analyze the entropy flow in some detail. When coarse-graining the quantum von Neumann entropy, we find it extremely useful to introduce an adaptation of Hawking's super-scattering matrix. These explicit models that we shall construct allow us to quantify and keep clear track of the entropy that appears when coarse-graining the system, and the information that can be hidden in unobserved correlations. (While not the focus of the current article, in the longer run these considerations are of interest when addressing black hole information puzzle.)
Journal reference: Entropy 19:5 (2017) 207. (Special Issue: Black Hole Thermodynamics II.)
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Author: Matt Visser (Victoria University of Wellington)
(Submitted on 16 May 2017)
Abstract: Classical point particles in Newtonian gravity obey, as they do in general relativity, the universality of free fall. However classical structured particles, (for instance with a mass quadrupole moment), need not obey the universality of free fall. Quantum mechanically, an elementary "point" particle can be described by a localized wave-packet, for which we can define a probability quadrupole moment. This probability quadrupole can, under plausible hypotheses, affect the universality of free fall. This raises an important issue of principle, as possible quantum violations of the universality of free fall would fundamentally impact on our ideas of what "quantum gravity" might look like. I will present an estimate of the size of the effect, and discuss where if at all it might be measured.
Comments: 8 pages; Essay awarded an honourable mention in the 2017 Gravity Research Foundation Essay Competition
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Authors: Sebastian Schuster (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 20 Jun 2017)
Abstract: Using electromagnetism to study analogue space-times is tantamount to having consistency conditions for when a given (meta-)material would provide an analogue space-time model or - vice versa - characterising which given metric could be modelled with a (meta-)material. This is most readily done by keeping the formalisms as close to each other as possible. While partially covariant formulations of the electrodynamics of media have been around for a long while, here we provide a fully unified and fully covariant approach. This enables us even to generalize the consistency conditions to arbitrary background metrics beyond flat space-time electrodynamics. We also show how the familiar matrices for permittivity $\epsilon$, permeability $\mu^{-1}$, and optical activity $\zeta$ can be seen as the three independent pieces of the Bel decomposition for the constitutive tensor $Z^{abcd}$, i.e., the components of an orthogonal decomposition with respect to a given observer with four-velocity $V^a$. At a technical level, we will employ both the Moore-Penrose pseudo-inverse and the closely related pseudo-determinant.
Comments: 24 pages
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Authors: Lars Andersson, Siyuan Ma, Claudio Paganini, Bernard F. Whiting
(Submitted on 10 Jul 2016)
Abstract: A generalization of the mode stability result of Whiting (1989) for the Teukolsky equation is proved for the case of real frequencies. The main result of the paper states that a separated solution of the Teukolsky equation governing massless test fields on the Kerr spacetime, which is purely outgoing at infinity, and purely ingoing at the horizon, must vanish. This has the consequence, that for real frequencies, there are linearly independent fundamental solutions of the radial Teukolsky equation which are purely ingoing at the horizon, and purely outgoing at infinity, respectively. This fact yields a representation formula for solutions of the inhomogenous Teukolsky equation.
Comments: 20 pages, 4 figures.
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Authors: Jonathan E. Thompson, Hector Chen, Bernard F. Whiting
(Submitted on 18 Nov 2016)
Beginning with the pioneering work of Regge and Wheeler (Phys. Rev. 108, 1957), there have been many studies of perturbations away from the Schwarzschild spacetime background. In particular several authors (e.g. Moncrief, Ann. Phys 88, 1974) have investigated gauge invariant quantities of the Regge-Wheeler (RW) gauge. Steven Detweiler also investigated perturbations of Schwarzschild in his own gauge, which he denoted the "easy (EZ) gauge", and which he was in the process of adapting for use in the second-order self-force problem. We present here a compilation of some of his working results, arising from notes for which there seems to have been no manuscript in preparation. In particular, we list the gauge invariant quantities used by Detweiler, as well as explain the process by which he found them.
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Authors: Alan A. Coley, David L. Wiltshire
(Submitted on 29 Dec 2016)
Abstract: General relativity is a set of physical and geometric principles, which lead to a set of (Einstein) field equations that determine the gravitational field, and to the geodesic equations that describe light propagation and the motion of particles on the background. But open questions remain, including: What is the scale on which matter and geometry are dynamically coupled in the Einstein equations? Are the field equations valid on small and large scales? What is the largest scale on which matter can be coarse grained while following a geodesic of a solution to Einstein's equations? We address these questions. If the field equations are causal evolution equations, whose average on cosmological scales is not an exact solution of the Einstein equations, then some simplifying physical principle is required to explain the statistical homogeneity of the late epoch Universe. Such a principle may have its origin in the dynamical coupling between matter and geometry at the quantum level in the early Universe. This possibility is hinted at by diverse approaches to quantum gravity which find a dynamical reduction to two effective dimensions at high energies on one hand, and by cosmological observations which are beginning to strongly restrict the class of viable inflationary phenomenologies on the other. We suggest that the foundational principles of general relativity will play a central role in reformulating the theory of spacetime structure to meet the challenges of cosmology in the 21st century.
Journal reference: Phys. Scripta 92 (2017) 053001
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Authors: Lawrence H. Dam, Asta Heinesen, David L. Wiltshire
(Submitted on 22 Jun 2017)
Abstract: Parameters that quantify the acceleration of cosmic expansion are conventionally determined within the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) model, which fixes spatial curvature to be homogeneous. Generic averages of Einstein's equations in inhomogeneous cosmology lead to models with non-rigidly evolving average spatial curvature, and different parametrizations of apparent cosmic acceleration. The timescape cosmology is a viable example of such a model without dark energy. Using the largest available supernova data set, the JLA catalogue, we find that the timescape model fits the luminosity distance-redshift data with a likelihood that is statistically indistinguishable from the standard spatially flat $\Lambda$ cold dark matter cosmology by Bayesian comparison. In the timescape case cosmic acceleration is non-zero but has a marginal amplitude, with best-fitting apparent deceleration parameter, $q_0=-0.043^{+0.004}_{-0.000}$. Systematic issues regarding standardization of supernova light curves are analysed. Cuts of data at the statistical homogeneity scale affect light curve parameter fits independent of cosmology. A cosmological model dependence of empirical changes to the mean colour parameter is also found. Irrespective of which model ultimately fits better, we argue that as a competitive model with a non-FLRW expansion history, the timescape model may prove a useful diagnostic tool for disentangling selection effects and astrophysical systematics from the underlying expansion history.
Journal reference: Mon.Not.Roy.Astron.Soc. 472 (2017) 835-851
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ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc,
July 2016 - June 2017
The LIGO Scientific Collaboration is a consortium of scientific institutions doing work on the Laser Interferometer Gravitational-Wave Observatory (LIGO), which consists of two laser interferometers 3030 km apart, one at Hanford, Washington State and the other at Livingston, Louisiana. The LIGO Scientific Collaboration includes ASGRG members David Blair, Philip Charlton, Neil Cornish, Ju Li, David McClelland, John Miller, Susan Scott, Bram Slagmolen, Eric Thrane, Peter Veitch and Bernard Whiting.
Listed below are all the abstracts listed on gr-qc from July 2016 to June 2017 from consortia that include at least one ASGRG member as a co-author – these are mostly LIGO abstracts, but there are occasionally some from eLISA and Virgo.
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Authors: B. P. Abbott et al.
(Submitted on 8 Jul 2016)
Abstract: We describe a directed search for continuous gravitational waves in data from the sixth LIGO science run. The target was the nearby globular cluster NGC 6544 at a distance of 2.7 kpc. The search covered a broad band of frequencies along with first and second frequency derivatives for a fixed sky position. The search coherently integrated data from the two LIGO interferometers over a time span of 9.2 days using the matched-filtering F-statistic. We found no gravitational-wave signals and set 95% confidence upper limits as stringent as 6.0 X 10^{-25} on intrinsic strain and 8.5 X 10^{-6} on fiducial ellipticity. These values beat the indirect limits from energy conservation for stars with characteristic spindown ages older than 300 years and are within the range of theoretical predictions for possible neutron-star ellipticities. An important feature of this search was use of a barycentric resampling algorithm which substantially reduced computational cost; this method will be used extensively in searches of Advanced LIGO and Virgo detector data.
Report number: LIGO-P1500225
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 25 Jul 2016)
Abstract: We report here the non-detection of gravitational waves from the merger of binary neutron star systems and neutron-star--black-hole systems during the first observing run of Advanced LIGO. In particular we searched for gravitational wave signals from binary neutron star systems with component masses $\in [1,3] M_{\odot}$ and component dimensionless spins $< 0.05$. We also searched for neutron-star--black-hole systems with the same neutron star parameters, black hole mass $\in [2,99] M_{\odot}$ and no restriction on the black hole spin magnitude. We assess the sensitivity of the two LIGO detectors to these systems, and find that they could have detected the merger of binary neutron star systems with component mass distributions of $1.35\pm0.13 M_{\odot}$ at a volume-weighted average distance of $\sim$ 70Mpc, and for neutron-star--black-hole systems with neutron star masses of $1.4M_\odot$ and black hole masses of at least $5M_\odot$, a volume-weighted average distance of at least $\sim$ 110Mpc. From this we constrain with 90% confidence the merger rate to be less than 12,600 Gpc$^{-3}$yr$^{-1}$ for binary-neutron star systems and less than 3,600 Gpc$^{-3}$yr$^{-1}$ for neutron-star--black-hole systems. We find that if no detection of neutron-star binary mergers is made in the next two Advanced LIGO and Advanced Virgo observing runs we would place significant constraints on the merger rates. Finally, assuming a rate of $10^{+20}_{-7}$Gpc$^{-3}$yr$^{-1}$ short gamma ray bursts beamed towards the Earth and assuming that all short gamma-ray bursts have binary-neutron-star (neutron-star--black-hole) progenitors we can use our 90% confidence rate upper limits to constrain the beaming angle of the gamma-ray burst to be greater than ${2.3^{+1.7}_{-1.1}}^{\circ}$ (${4.3^{+3.1}_{-1.9}}^{\circ}$).
Comments: 17 pages, 8 figures
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 5 Aug 2016)
Abstract: The first direct gravitational-wave detection was made by the Advanced Laser Interferometer Gravitational Wave Observatory on September 14, 2015. The GW150914 signal was strong enough to be apparent, without using any waveform model, in the filtered detector strain data. Here, features of the signal visible in the data are analyzed using concepts from Newtonian physics and general relativity, accessible to anyone with a general physics background. The simple analysis presented here is consistent with the fully general-relativistic analyses published elsewhere,in showing that the signal was produced by the inspiral and subsequent merger of two black holes. The black holes were each of approximately 35 Msun, still orbited each other as close as ~350 km apart, and subsequently merged to form a single black hole. Similar reasoning, directly from the data, is used to roughly estimate how far these black holes were from the Earth, and the energy that they radiated in gravitational waves.
Journal reference: Annalen der Physik, Volume 529, Issue 1-2, January 2017, 1600209
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 9 Nov 2016)
Abstract: We present the results from an all-sky search for short-duration gravitational waves in the data of the first run of the Advanced LIGO detectors between September 2015 and January 2016. The search algorithms use minimal assumptions on the signal morphology, so they are sensitive to a wide range of sources emitting gravitational waves. The analyses target transient signals with duration ranging from milliseconds to seconds over the frequency band of 32 to 4096 Hz. The first observed gravitational-wave event, GW150914, has been detected with high confidence in this search; other known gravitational-wave events fall below the search's sensitivity. Besides GW150914, all of the search results are consistent with the expected rate of accidental noise coincidences. Finally, we estimate rate-density limits for a broad range of non-BBH transient gravitational-wave sources as a function of their gravitational radiation emission energy and their characteristic frequency. These rate-density upper-limits are stricter than those previously published by an order-of-magnitude.
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 22 Nov 2016)
Abstract: Parameter estimates of GW150914 were obtained using Bayesian inference, based on three semi-analytic waveform models for binary black hole coalescences. These waveform models differ from each other in their treatment of black hole spins, and all three models make some simplifying assumptions, notably to neglect sub-dominant waveform harmonic modes and orbital eccentricity. Furthermore, while the models are calibrated to agree with waveforms obtained by full numerical solutions of Einstein's equations, any such calibration is accurate only to some non-zero tolerance and is limited by the accuracy of the underlying phenomenology, availability, quality, and parameter-space coverage of numerical simulations. This paper complements the original analyses of GW150914 with an investigation of the effects of possible systematic errors in the waveform models on estimates of its source parameters. To test for systematic errors we repeat the original Bayesian analyses on mock signals from numerical simulations of a series of binary configurations with parameters similar to those found for GW150914. Overall, we find no evidence for a systematic bias relative to the statistical error of the original parameter recovery of GW150914 due to modeling approximations or modeling inaccuracies. However, parameter biases are found to occur for some configurations disfavored by the data of GW150914: for binaries inclined edge-on to the detector over a small range of choices of polarization angles, and also for eccentricities greater than $\sim$0.05. For signals with higher signal-to-noise ratio than GW150914, or in other regions of the binary parameter space (lower masses, larger mass ratios, or higher spins), we expect that systematic errors in current waveform models may impact gravitational-wave measurements, making more accurate models desirable for future observations.
Journal reference: Class. Quantum Grav. 34 (2017) 104002
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Authors: LIGO Scientific Collaboration, Virgo Collaboration, IPN Collaboration
(Submitted on 23 Nov 2016)
Abstract: We present the results of the search for gravitational waves (GWs) associated with $\gamma$-ray bursts detected during the first observing run of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). We find no evidence of a GW signal for any of the 41 $\gamma$-ray bursts for which LIGO data are available with sufficient duration. For all $\gamma$-ray bursts, we place lower bounds on the distance to the source using the optimistic assumption that GWs with an energy of $10^{-2}M_\odot c^2$ were emitted within the $16$-$500\,$Hz band, and we find a median 90% confidence limit of 71$\,$Mpc at 150$\,$Hz. For the subset of 19 short/hard $\gamma$-ray bursts, we place lower bounds on distance with a median 90% confidence limit of 90$\,$Mpc for binary neutron star (BNS) coalescences, and 150 and 139$\,$Mpc for neutron star-black hole coalescences with spins aligned to the orbital angular momentum and in a generic configuration, respectively. These are the highest distance limits ever achieved by GW searches. We also discuss in detail the results of the search for GWs associated with GRB 150906B, an event that was localized by the InterPlanetary Network near the local galaxy NGC 3313, which is at a luminosity distance of 54$\,$Mpc ($z=0.0124$). Assuming the $\gamma$-ray emission is beamed with a jet half-opening angle $\leq 30^{\circ}$, we exclude a BNS and a neutron star-black hole in NGC 3313 as the progenitor of this event with confidence $>99$%. Further, we exclude such progenitors up to a distance of 102$\,$Mpc and 170$\,$Mpc, respectively.
Report number: P1600298
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 6 Dec 2016)
Abstract: A wide variety of astrophysical and cosmological sources are expected to contribute to a stochastic gravitational-wave background. Following the observations of GW150914 and GW151226, the rate and mass of coalescing binary black holes appear to be greater than many previous expectations. As a result, the stochastic background from unresolved compact binary coalescences is expected to be particularly loud. We perform a search for the isotropic stochastic gravitational-wave background using data from Advanced LIGO's first observing run. The data display no evidence of a stochastic gravitational-wave signal. We constrain the dimensionless energy density of gravitational waves to be $\Omega_0<1.7\times 10^{-7}$ with 95% confidence, assuming a flat energy density spectrum in the most sensitive part of the LIGO band (20-86 Hz). This is a factor of ~33 times more sensitive than previous measurements. We also constrain arbitrary power-law spectra. Finally, we investigate the implications of this search for the background of binary black holes using an astrophysical model for the background.
Journal reference: Phys. Rev. Lett. 118, 121101 (2017)
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 6 Dec 2016)
Abstract: We employ gravitational-wave radiometry to map the gravitational waves stochastic background expected from a variety of contributing mechanisms and test the assumption of isotropy using data from Advanced LIGO's first observing run. We also search for persistent gravitational waves from point sources with only minimal assumptions over the 20 - 1726 Hz frequency band. Finding no evidence of gravitational waves from either point sources or a stochastic background, we set limits at 90% confidence. For broadband point sources, we report upper limits on the gravitational wave energy flux per unit frequency in the range $F_{\alpha,\Theta}(f) < (0.1 - 56) \times 10^{-8}$ erg cm$^{-2}$ s$^{-1}$ Hz$^{-1}$ (f/25 Hz)$^{\alpha-1}$ depending on the sky location $\Theta$ and the spectral power index $\alpha$. For extended sources, we report upper limits on the fractional gravitational wave energy density required to close the Universe of $\Omega(f,\Theta) < (0.39-7.6) \times 10^{-8}$ sr$^{-1}$ (f/25 Hz)$^\alpha$ depending on $\Theta$ and $\alpha$. Directed searches for narrowband gravitational waves from astrophysically interesting objects (Scorpius X-1, Supernova 1987 A, and the Galactic Center) yield median frequency-dependent limits on strain amplitude of $h_0 <$ (6.7, 5.5, and 7.0) $\times 10^{-25}$ respectively, at the most sensitive detector frequencies between 130 - 175 Hz. This represents a mean improvement of a factor of 2 across the band compared to previous searches of this kind for these sky locations, considering the different quantities of strain constrained in each case.
Comments: 14 pages, 4 figures
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Authors: C. Biwer, D. Barker, J. C. Batch, J. Betzwieser, R. P. Fisher, E. Goetz, S. Kandhasamy, S. Karki, J. S. Kissel, A. P. Lundgren, D. M. Macleod, A. Mullavey, K. Riles, J. G. Rollins, K. A. Thorne, E. Thrane, T. D. Abbott, B. Allen, D. A. Brown, P. Charlton, S. G. Crowder, P. Fritschel, J. B. Kanner, M. Landry, C. Lazzaro, M. Millhouse, M. Pitkin, R. L. Savage, P. Shawhan, D. H. Shoemaker, J. R. Smith, L. Sun, J. Veitch, S. Vitale, A. J. Weinstein, N. Cornish, R. C. Essick, M. Fays, E. Katsavounidis, J. Lange, T. B. Littenberg, R. Lynch, P. M. Meyers, F. Pannarale, R. Prix, R. O'Shaughnessy, D. Sigg
(Submitted on 23 Dec 2016)
Abstract: Hardware injections are simulated gravitational-wave signals added to the Laser Interferometer Gravitational-wave Observatory (LIGO). The detectors' test masses are physically displaced by an actuator in order to simulate the effects of a gravitational wave. The simulated signal initiates a control-system response which mimics that of a true gravitational wave. This provides an end-to-end test of LIGO's ability to observe gravitational waves. The gravitational-wave analyses used to detect and characterize signals are exercised with hardware injections. By looking for discrepancies between the injected and recovered signals, we are able to characterize the performance of analyses and the coupling of instrumental subsystems to the detectors' output channels. This paper describes the hardware injection system and the recovery of injected signals representing binary black hole mergers, a stochastic gravitational wave background, spinning neutron stars, and sine-Gaussians.
Report number: LIGO-P1600285
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 26 Jan 2017)
Abstract: We present the result of searches for gravitational waves from 200 pulsars using data from the first observing run of the Advanced LIGO detectors. We find no significant evidence for a gravitational-wave signal from any of these pulsars, but we are able to set the most constraining upper limits yet on their gravitational-wave amplitudes and ellipticities. For eight of these pulsars, our upper limits give bounds that are improvements over the indirect spin-down limit values. For another 32, we are within a factor of 10 of the spin-down limit, and it is likely that some of these will be reachable in future runs of the advanced detector. Taken as a whole, these new results improve on previous limits by more than a factor of two.
Report number: LIGO-P1600159
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 12 Apr 2017)
Abstract: Results are presented from a semi-coherent search for continuous gravitational waves from the brightest low-mass X-ray binary, Scorpius X-1, using data collected during the first Advanced LIGO observing run (O1). The search combines a frequency domain matched filter (Bessel-weighted $\mathcal{F}$-statistic) with a hidden Markov model to track wandering of the neutron star spin frequency. No evidence of gravitational waves is found in the frequency range 60-650 Hz. Frequentist 95% confidence strain upper limits, $h_0^{95\%} = 4.0\times10^{-25}$, $8.3\times10^{-25}$, and $3.0\times10^{-25}$ for electromagnetically restricted source orientation, unknown polarization, and circular polarization, respectively, are reported at 106 Hz. They are $\leq 10$ times higher than the theoretical torque-balance limit at 106 Hz.
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 15 Apr 2017)
Abstract: During their first observational run, the two Advanced LIGO detectors attained an unprecedented sensitivity, resulting in the first direct detections of gravitational-wave signals and GW151226, produced by stellar-mass binary black hole systems. This paper reports on an all-sky search for gravitational waves (GWs) from merging intermediate mass black hole binaries (IMBHBs). The combined results from two independent search techniques were used in this study: the first employs a matched-filter algorithm that uses a bank of filters covering the GW signal parameter space, while the second is a generic search for GW transients (bursts). No GWs from IMBHBs were detected, therefore, we constrain the rate of several classes of IMBHB mergers. The most stringent limit is obtained for black holes of individual mass $100\,M_\odot$, with spins aligned with the binary orbital angular momentum. For such systems, the merger rate is constrained to be less than $0.93~\mathrm{Gpc^{-3}\,yr}^{-1}$ in comoving units at the $90\%$ confidence level, an improvement of nearly 2 orders of magnitude over previous upper limits.
Journal reference: Phys Rev D.96.022001
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 6 Jun 2017)
Abstract: We describe the observation of GW170104, a gravitational-wave signal produced by the coalescence of a pair of stellar-mass black holes. The signal was measured on January 4, 2017 at 10:11:58.6 UTC by the twin advanced detectors of the Laser Interferometer Gravitational-Wave Observatory during their second observing run, with a network signal-to-noise ratio of 13 and a false alarm rate less than 1 in 70,000 years. The inferred component black hole masses are $31.2^{+8.4}_{-6.0}\,M_\odot$ and $19.4^{+5.3}_{-5.9}\,M_\odot$ (at the 90% credible level). The black hole spins are best constrained through measurement of the effective inspiral spin parameter, a mass-weighted combination of the spin components perpendicular to the orbital plane, $\chi_\mathrm{eff} = -0.12^{+0.21}_{-0.30}.$ This result implies that spin configurations with both component spins positively aligned with the orbital angular momentum are disfavored. The source luminosity distance is $880^{+450}_{-390}~\mathrm{Mpc}$ corresponding to a redshift of $z = 0.18^{+0.08}_{-0.07}$. We constrain the magnitude of modifications to the gravitational-wave dispersion relation and perform null tests of general relativity. Assuming that gravitons are dispersed in vacuum like massive particles, we bound the graviton mass to $m_g \le 7.7 \times 10^{-23}~\mathrm{eV}/c^2$. In all cases, we find that GW170104 is consistent with general relativity.
Journal reference: Phys. Rev. Lett., 118(22):221101, 2017
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Authors: The LIGO Scientific Collaboration, the Virgo Collaboration
(Submitted on 9 Jun 2017)
Abstract: We present the results of a semicoherent search for continuous gravitational waves from the low-mass X-ray binary Scorpius X-1, using data from the first Advanced LIGO observing run. The search method uses details of the modelled, parametrized continuous signal to combine coherently data separated by less than a specified coherence time, which can be adjusted to trade off sensitivity against computational cost. A search was conducted over the frequency range from 25 Hz to 2000 Hz, spanning the current observationally-constrained range of the binary orbital parameters. No significant detection candidates were found, and frequency-dependent upper limits were set using a combination of sensitivity estimates and simulated signal injections. The most stringent upper limit was set at 175 Hz, with comparable limits set across the most sensitive frequency range from 100 Hz to 200 Hz. At this frequency, the 95 pct upper limit on signal amplitude h0 is 2.3e-25 marginalized over the unknown inclination angle of the neutron star's spin, and 8.03e-26 assuming the best orientation (which results in circularly polarized gravitational waves). These limits are a factor of 3-4 stronger than those set by other analyses of the same data, and a factor of about 7 stronger than the best upper limits set using initial LIGO data. In the vicinity of 100 Hz, the limits are a factor of between 1.2 and 3.5 above the predictions of the torque balance model, depending on inclination angle, if the most likely inclination angle of 44 degrees is assumed, they are within a factor of 1.7.
Report number: LIGO-P1600297-v24
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