The Australian Academy of Science has recognised two scientists from The Australian National University (ANU) for their work on advancing science. Professor Susan Scott from the ANU Centre for Gravitational Physics, and Professor Simon Foote, Director of The John Curtin School of Medical Research, have been elected as new Fellows of the Academy in recognition of their significant contributions to science.

ANU Vice-Chancellor Professor Brian Schmidt AC congratulated the new Fellows and said their work highlights the quality of research being done at ANU. “I congratulate both Susan and Simon, not only on their own career achievements, but also their pioneering research and the excellent example they have set for the next generation of scientists,” Professor Schmidt said.

The Academy’s total fellowship now includes 511 scientists. Professor Scott was recognised for her ground-breaking discoveries in the fields of general relativity and gravitational wave science, while Professor Foote was recognised for his work to fight malaria.

Professor Scott’s theoretical work includes advancing our understanding of both singularities and the global structure of space-time. She has also been a pioneer in the analysis of astrophysical signatures in gravitational wave experiments. “I am delighted to have been elected a Fellow of the Academy. This award is a wonderful recognition of my research work to probe the structure of space-time and the nature of gravitational waves,” she said. “As the first gravitational wave scientist to enter the Academy, I really hope that this will highlight our new field of gravitational wave astronomy.

“The detection of gravitational waves is a discovery that most of us could only dream of happening in our working lifetime in research. To have that happen at this point in my career is amazing. I’ve had 20 years working on this project and now I have the rest of my career to see where this discovery leads.”

Academy of Science President Professor Andrew Holmes congratulated all of the new Fellows and said they had made significant and lasting impacts in their scientific disciplines. “The breadth of scientific talent recognised in this year’s election is truly awe-inspiring,” Professor Holmes said. “From breakthroughs in pure science to spurring scientific innovations, these new Fellows have made an impact on everything from the way we treat disease to how we grow our food to advancing our fundamental knowledge about the world in which we live.”

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MEMBERSHIP DETAILS ONLINE:

Due to requests from members, David Wiltshire has written some HTML scripts which generate membership details online from our records. If you click on

http://www.physics.adelaide.edu.au/ASGRG/members/

you will find a members' list. Clicking on individual members gives their current contact details. By following a further link private details of the subscription status of any member will be sent to their registered email.

This feature should enable us to update our records more frequently in response to members' input, and to allow members to keep track of their subscriptions.

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SUBSCRIPTIONS:

The membership script programs are intended to be run automatically once a year, at the end of July, to give members other than life members details of their current subscription status.

The new version of the subscription form, at

http://www.physics.adelaide.edu.au/ASGRG/subsform.html

has been simplified so that it does not need to be updated each year. Given that our annual fee is modest, members are encouraged to pay for multiple years, and to fill in the years they are paying for. E.g., when

the July 2016 - June 2017 subscriptions are requested, if you wish to pay for July 2017 - June 2018 at the same time, it may simplify matters.

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FORTHCOMING MEETINGS

July 3-9, 2016: “Large Scale Structure and Galaxy Flows”

12th Rencontres du Vietnam

Quy Nhon, Vietnam

http://www.cpt.univ-mrs.fr/~cosmo/CosFlo16/index.php

July 4-7, 2016: “Noncommutative geometry, quantum symmetries and quantum gravity II”

37th Max Born Symposium

Wroclaw University

Wroclaw, Poland

http://ift.uni.wroc.pl/~mborn37/index.html

July 6-8, 2016: 16th Canadian Conference on General Relativity and Relativistic Astrophysics

Vancouver, Canada

http://www.sfu.ca/physics/cosmology/CCGRRA-16/

July 10-15, 2016: 21st International Conference on General Relativity and Gravitation (GR21)

Columbia University, New York

http://www.gr21.org/

July 18-29, 2016: IT from Qubit Summer School

Perimeter Institute for Theoretical Physics

Waterloo, Canada

http://www.perimeterinstitute.ca/conferences/it-qubit-summer-school

August 1-12, 2016: “Astrophysics from LIGO’s First Black Holes

Kavli Institute for Theoretical Physics

University of California, Santa Barbara

https://www.kitp.ucsb.edu/activities/gwaves-m16

August 15-20, 2016: 3rd International Summer School on Quantum Gravity

Beijing Normal University

Beijing, China

http://123.206.13.226/summerschool/en/

August 29 - September 2, 2016: GRavitational-wave Astronomy Meeting in PAris (GRAMPA)

Institut d’Astrophysique de Paris

Paris, France

http://www.iap.fr/vie_scientifique/ateliers/GravitationalWave/2016/index.html

September 5-9, 2016: 11th International LISA Symposium

University of Zurich

Zurich, Switzerland

http://www.physik.uzh.ch/events/lisa2016/

September 8-10, 2016: Workshop on Numerical and Mathematical Relativity

Friedrich-Schiller-Universitaet Jena

Oppburg, Germany

https://www.tpi.uni-jena.de/tiki-index.php?page=NRMR16

September 12-15, 2016: “One Hundred Years of the Schwarzschild Solution”

Spanish-Portuguese Relativity Meeting

Lisbon, Portugal

https://erep2016.tecnico.ulisboa.pt/

September 12-17, 2016: Varying Constants and Fundamental Cosmology (VARCOSMOFUN ’16)

Szczecin, Poland

https://indico.cern.ch/event/462870/

September 19-22, 2016: Recent Developments in Gravity (NEB-17)

Mykonos, Greece

http://www.hsrgc.gr/neb17/

September 19-30, 2016: “Experimental Search for Quantum Gravity”

Frankfurt Institute for Advanced Studies

Frankfurt, Germany

https://indico.fias.uni-frankfurt.de/event/2/

September 25-29, 2016: 3rd Conference of the Polish Society on Relativity

“A Century of General Relativity”

Jagiellonian University

Krakow, Poland

http://th.if.uj.edu.pl/~grawitacja/potor2016/

September 25-30, 2016: 3rd Jose Plinio Baptista School of Cosmology

“The Dark Sector of the Universe”

Universidade Federal do Espirito Santo

Espirito Santo, Brazil

http://www.cosmo-ufes.org/jpbcosmo3.html

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MEMBERS' ABSTRACTS at gr-qc, July 2015 - June 2016

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 78 papers listed here and in the LIGO section represent 1.68% of the 4632 papers posted or cross-linked to gr-qc between July 2015 and June 2016.

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arXiv:1508.05196 [gr-qc]

Static Axisymmetric Einstein Equations in Vacuum: Symmetry, New Solutions and Ricci Solitons

Authors: M. M. Akbar, M. A. H. MacCallum

(Submitted on 21 Aug 2015)

An explicit one-parameter Lie point symmetry of the four-dimensional vacuum Einstein equations with two commuting hypersurface-orthogonal Killing vector fields is presented. The parameter takes values over all of the real line and the action of the group can be effected algebraically on any solution of the system. This enables one to construct particular one-parameter extended families of axisymmetric static solutions and cylindrical gravitational wave solutions from old ones, in a simpler way than most solution-generation techniques, including the prescription given by Ernst for this system. As examples, we obtain the families that generalize the Schwarzschild solution and the C-metric. These in effect superpose a Levi-Civita cylindrical solution on the seeds. Exploiting a correspondence between static solutions of Einstein's equations and Ricci solitons (self-similar solutions of the Ricci flow), this also enables us to construct new steady Ricci solitons.

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arXiv:1508.04602 [gr-qc]

Generalizations of the Abstract Boundary singularity theorem

Authors: Ben E. Whale, Mike J. S. L. Ashley, Susan M. Scott

(Submitted on 19 Aug 2015)

Abstract: The Abstract Boundary singularity theorem was first proven by Ashley and Scott. It links the existence of incomplete causal geodesics in strongly causal, maximally extended spacetimes to the existence of Abstract Boundary essential singularities, i.e., non-removable singular boundary points. We give two generalizations of this theorem: the first to continuous causal curves and the distinguishing condition, the second to locally Lipschitz curves in manifolds such that no inextendible locally Lipschitz curve is totally imprisoned. To do this we extend generalized affine parameters from C1 curves to locally Lipschitz curves.

Journal reference: Class. Quantum Grav. 32 135001 (2015)

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arXiv:1509.00234 [gr-qc]

Wormhole solutions sourced by fluids, II: Three-fluid two-charged sources

Authors: Mustapha Azreg-Aïnou

(Submitted on 1 Sep 2015 (v1), last revised 11 Dec 2015 (this version, v2))

Abstract: Lack of a consistent metric for generating rotating wormholes motivates us to present a new one endowed with interesting physical and geometrical properties. When combined with the generalized method of superposition of fields, which consists in attaching a form of matter to each moving frame, it generates massive and charged (charge without charge) two-fluid-sourced, massive and two-charged three-fluid-sourced, rotating as well as new static wormholes which, otherwise, can hardly be derived by integration. If the lapse function of the static wormhole is bounded from above, no closed timelike curves occur in the rotating counterpart. For positive energy densities dying out faster than 1/r, the angular velocity includes in its expansion a correction term, to the leading one that corresponds to ordinary stars, proportional to lnr/r^4. Such a term is not present in the corresponding expansion for the Kerr-Newman black hole. Based on this observation and our previous work, the dragging effects of falling neutral objects may constitute a substitute for other known techniques used for testing the nature of the rotating black hole candidates that are harbored in the center of galaxies. We discuss the possibility of generating (n+1)-fluid-sourced, n-charged, rotating as well as static wormholes.

Journal reference: Eur. Phys. J. C (2016) 76:7

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arXiv:1512.02065 [gr-qc]

Cyclic and heteroclinic flows near general static spherically symmetric black holes

Authors: Ayyesha K. Ahmed, Mustapha Azreg-Aïnou (C. Author), Mir Faizal, Mubasher Jamil

(Submitted on 7 Dec 2015 (v1), last revised 29 Apr 2016 (this version, v3))

Abstract: We investigate the Michel-type accretion onto a static spherically symmetric black hole. Using a Hamiltonian dynamical approach, we show that the standard method employed for tackling the accretion problem has masked some properties of the fluid flow. We determine new analytical solutions that are neither transonic nor supersonic as the fluid approaches the horizon(s); rather, they remain subsonic for all values of the radial coordinate. Moreover, the three velocity vanishes and the pressure diverges on the horizon(s), resulting in a flowout of the fluid under the effect of its own pressure. This is in favor of an earlier prediction that pressure-dominant regions form near the horizon. This result does not depend on the form of the metric and it applies to a neighborhood of any horizon where the time coordinate is timelike. For anti-de Sitter-like f(R) black holes we discuss the stability of the critical flow and determine separatrix heteroclinic orbits. For de Sitter-like f(R) black holes, we construct polytropic cyclic, non-homoclinic, physical flows connecting the two horizons. These flows become non-relativistic for Hamiltonian values higher than the critical value allowing for a good estimate of the proper period of the flow.

Journal reference: Eur. Phys. J. C (2016) 76:280

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arXiv:1602.03523 [gr-qc]

Astrophysical flows near f(T) gravity black holes

Authors: Ayyesha K. Ahmed, Mustapha Azreg-Aïnou (C. Author), Sebastian Bahamonde, Salvatore Capozziello, Mubasher Jamil

(Submitted on 10 Feb 2016 (v1), last revised 29 Apr 2016 (this version, v2))

Abstract: In this paper, we study the accretion process for fluids flowing near a black hole in the context of f(T) teleparallel gravity. Specifically, by performing a dynamical analysis by a Hamiltonian system, we are able to find the sonic points. After that, we consider different isothermal test fluids in order to study the accretion process when they are falling onto the black hole. We found that these flows can be classified according to the equation of state and the black hole features. Results are compared in f(T) and f(R) gravity.

Journal reference: Eur. Phys. J. C (2016) 76:269

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arXiv:1603.07894 [gr-qc]

Vacuum and nonvacuum black holes in a uniform magnetic field

Authors: Mustapha Azreg-Aïnou

(Submitted on 25 Mar 2016)

Abstract: We generalize the known solution for the electromagnetic field when a vacuum, stationary, axisymmetric black hole is immersed in a uniform magnetic field to the case of nonvacuum black holes and determine all linear terms of the vector potential in powers of the magnetic field and the rotation parameter.

Comments: 6 pages

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arXiv:1605.06063 [gr-qc]

Cyclic and semi-cyclic flows of polytropic fluids in f(R) gravity

Authors: Mustapha Azreg-Aïnou

(Submitted on 19 May 2016)

Abstract: We present new accretion solutions of a polytropic perfect fluid onto an f(R)-gravity de Sitter-like black hole. We consider two f(R)-gravity models and obtain finite-period cyclic flows oscillating between the event and cosmological horizons as well as semi-cyclic critical flows executing a two-way motion from and back to the same horizon.

Comments: 6 pages, 3 figures

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arXiv:1605.02590 [gr-qc]

Teleparallel loop quantum cosmology in a system of intersecting branes

Authors: Alireza Sepehri, Anirudh Pradhan, A. Beesham, Jaume de Haro

(Submitted on 9 May 2016 (v1), last revised 7 Jun 2016 (this version, v2))

Abstract: Recently, some authors have removed the big bang singularity in teleparallel Loop Quantum Cosmology (LQC) and have shown that the universe may undergo a number of oscillations. We investigate the origin of this type of teleparallel theory in a system of intersecting branes in M-theory in which the angle between them changes with time. This system is constructed by two intersecting anti-D8-branes, one compacted D4-brane and the other a D3-brane. These branes are built by joining M0-branes which develop in decaying fundamental strings. The compacted D4-brane is located between two intersecting anti-D8 branes and glues to one of them. Our universe is located on the D3 brane which wraps the D4 brane from one end and sticks to one of the anti-D8 branes from another one. In this system, there are three types of ?elds, corresponding to compacted D4 branes, intersecting branes and D3-branes. These ?elds interact with each other and make the angle between branes oscillate. By decreasing this angle and approaching the intersecting anti-D8 branes towards each other, the D4 brane rolls, the D3 brane wraps around the D4 brane, and t he universe contracts. By separating the intersecting branes and increasing the angle, the D4 brane rolls in the opposite direction, the D3 brane separates from it and the expansion branch begins. Also, the interaction between branes in this system gives us the exact form of the relevant Lagrangian for teleparallel LQC.

Comments: 11 pages

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arXiv:1512.07364 [astro-ph.CO]

Differential cosmic expansion and the Hubble flow anisotropy

Authors: Krzysztof Bolejko, M. Ahsan Nazer, David L. Wiltshire

(Submitted on 23 Dec 2015 (v1), last revised 6 Jun 2016 (this version, v3))

Abstract: The Universe on scales 10−100 h^−1 Mpc is dominated by a cosmic web of voids, filaments, sheets and knots of galaxy clusters. These structures participate differently in the global expansion of the Universe: from non-expanding clusters to the above average expansion rate of voids. In this paper we characterize Hubble expansion anisotropies in the COMPOSITE sample of 4534 galaxies and clusters. We concentrate on the dipole and quadrupole in the rest frame of the Local Group. These both have statistically significant amplitudes. These anisotropies, and their redshift dependence, cannot be explained solely by a boost of the Local Group in the Friedmann-Lema\^itre-Robertson-Walker (FLRW) model which expands isotropically in the rest frame of the cosmic microwave background (CMB) radiation. We simulate the local expansion of the Universe with inhomogeneous Szekeres models, which match the standard FLRW model on >100 h^−1 Mpc scales but exhibit nonkinematic relativistic differential expansion on small scales. We restrict models to be consistent with observed CMB temperature anisotropies, while simultaneously fitting the redshift variation of the Hubble expansion dipole. We include features to account for both the Local Void and the "Great Attractor". While this naturally accounts for the Hubble expansion and CMB dipoles, the simulated quadrupoles are smaller than observed. Further refinement to incorporate additional structures may improve this. This would enable a test of the hypothesis that some large angle CMB anomalies result from failing to treat the relativistic differential expansion of the background geometry, a natural feature of solutions to Einstein's equations not included in the current standard model of cosmology.

Journal reference: JCAP06(2016)035

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arXiv:1512.03457 [math.DG]

Two dimensional axisymmetric smooth lattice Ricci flow

Authors: Leo Brewin

(Submitted on 10 Dec 2015)

Abstract: A lattice based method will be presented for numerical investigations of Ricci flow. The method will be applied to the particular case of 2-dimensional axially symmetric initial data on manifolds with S^2 topology. Results will be presented that show that the method works well and agrees with results obtained using contemporary finite difference methods.

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arXiv:1508.02062 [gr-qc]

Probing the Internal Composition of Neutron Stars with Gravitational Waves

Katerina Chatziioannou, Kent Yagi, Antoine Klein, Neil Cornish, Nicolas Yunes

(Submitted on 9 Aug 2015 (v1), last revised 4 Nov 2015 (this version, v2))

Gravitational waves from neutron star binary inspirals contain information about the equation of state of supranuclear matter. In the absence of definitive experimental evidence that determines the correct equation of state, a number of diverse models that give the pressure in a neutron star as function of its density have been proposed. These models differ not only in the approximations and techniques they use to solve the many-body Schr\"odinger equation, but also in the neutron star composition they assume. We study whether gravitational wave observations of neutron star binaries in quasicircular inspirals will allow us to distinguish between equations of state of differing internal composition, thereby providing important information about the properties of extremely high density matter. We carry out a Bayesian model selection analysis, and find that second generation gravitational wave detectors can heavily constrain equations of state that contain only quark matter, but hybrid stars containing both normal and quark matter are harder to distinguish from normal matter stars. A gravitational wave detection with a signal-to-noise ratio of 30 and masses around 1.4M_⊙ could either detect or rule out strange quark stars with a 20 to 1 confidence. The presence of kaon condensates or hyperons in neutron star inner cores cannot be easily confirmed. For example, for the equations of state studied in this paper, even a gravitational wave signal with a signal-to-noise ratio as high as 60 would not allow us to claim a detection of kaon condensates or hyperons with confidence greater than 5 to 1. On the other hand, if kaon condensates and hyperons do not form in neutron stars, a gravitational wave signal with similar signal-to-noise ratio would be able to constrain their existence with an 11 to 1 confidence for high-mass systems.

Comments: 24 pages, 15 figures, final published version

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arXiv:1511.08752 [gr-qc]

Enabling high confidence detections of gravitational-wave bursts

Authors: Tyson B. Littenberg, Jonah B. Kanner, Neil J. Cornish, Margaret Millhouse

(Submitted on 27 Nov 2015)

Abstract: With the advanced LIGO and Virgo detectors taking observations the detection of gravitational waves is expected within the next few years. Extracting astrophysical information from gravitational wave detections is a well-posed problem and thoroughly studied when detailed models for the waveforms are available. However, one motivation for the field of gravitational wave astronomy is the potential for new discoveries. Recognizing and characterizing unanticipated signals requires data analysis techniques which do not depend on theoretical predictions for the gravitational waveform. Past searches for short-duration un-modeled gravitational wave signals have been hampered by transient noise artifacts, or "glitches," in the detectors. In some cases, even high signal-to-noise simulated astrophysical signals have proven difficult to distinguish from glitches, so that essentially any plausible signal could be detected with at most 2-3 σ level confidence. We have put forth the BayesWave algorithm to differentiate between generic gravitational wave transients and glitches, and to provide robust waveform reconstruction and characterization of the astrophysical signals. Here we study BayesWave's capabilities for rejecting glitches while assigning high confidence to detection candidates through analytic approximations to the Bayesian evidence. Analytic results are tested with numerical experiments by adding simulated gravitational wave transient signals to LIGO data collected between 2009 and 2010 and found to be in good agreement.

Comments: 15 pages, 6 figures, submitted to PRD

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arXiv:1512.06829 [gr-qc]

Towards Robust Gravitational Wave Detection with Pulsar Timing Arrays

Authors: Neil J. Cornish, Laura M. Sampson

(Submitted on 21 Dec 2015)

Abstract: Precision timing of highly stable milli-second pulsars is a promising technique for the detection of very low frequency sources of gravitational waves. In any single pulsar, a stochastic gravitational wave signal appears as an additional source of timing noise that can be absorbed by the noise model, and so it is only by considering the coherent response across a network of pulsars that the signal can be distinguished from other sources of noise. In the limit where there are many gravitational wave sources in the sky, or many pulsars in the array, the signals produce a unique tensor correlation pattern that depends only on the angular separation between each pulsar pair. It is this distinct fingerprint that is used to search for gravitational waves using pulsar timing arrays. Here we consider how the prospects for detection are diminished when the statistical isotropy of the timing array or the gravitational wave signal is broken by having a finite number of pulsars and a finite number of sources. We find the standard tensor-correlation analysis to be remarkably robust, with a mild impact on detectability compared to the isotropic limit. Only when there are very few sources and very few pulsars does the standard analysis begin to fail. Having established that the tensor correlations are a robust signature for detection, we study the use of "sky-scrambles" to break the correlations as a way to increase confidence in a detection. This approach is analogous to the use of "time-slides" in the analysis of data from ground based interferometric detectors.

Comments: 11 pages, 12 figures

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arXiv:1606.00953 [gr-qc]

Rapid and reliable sky localization of gravitational wave sources

Authors: Neil J. Cornish

(Submitted on 3 Jun 2016)

Abstract: The first detection of gravitational waves by LIGO from the merger of two compact objects has sparked new interest in detecting electromagnetic counterparts to these violent events. For mergers involving neutron stars, it is thought that prompt high-energy emission in gamma rays and x-rays will be followed days to weeks later by an afterglow in visible light, infrared and radio. Rapid sky localization using the data from a network of gravitational wave detectors is essential to maximize the chances of making a joint detection. Here I describe a new technique that is able to produce accurate, fully Bayesian sky maps in seconds or less. The technique can be applied to spin-precessing compact binaries, and can take into account detector calibration and spectral estimation uncertainties.

Comments: 5 pages, 4 figures

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arXiv:1606.03117 [gr-qc]

Analytic gravitational waveforms for generic precessing compact binaries

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.

Comments: 5 pages, 3 figures

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arXiv:1512.04597 [astro-ph.CO]

Ultracompact minihalos as probes of inflationary cosmology

Authors: Grigor Aslanyan, Layne C. Price, Jenni Adams, Torsten Bringmann, Hamish A. Clark, Richard Easther, Geraint F. Lewis, Pat Scott

(Submitted on 14 Dec 2015)

Abstract: Cosmological inflation generates primordial density perturbations on all scales, including those far too small to contribute to the cosmic microwave background. At these scales, isolated ultracompact minihalos of dark matter can form, well before standard structure formation, if the small-scale perturbations have a large enough amplitude. Such minihalos affect pulsar timing observations and are potentially bright sources of gamma rays. The resulting constraints significantly extend the observable window of inflation and dark matter, simultaneously probing two of the greatest puzzles in modern cosmology.

Comments: 6 pages, 2 figures, 2 tables

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arXiv:1605.09050 [gr-qc]

Generalized Second Law of Thermodynamics with Corrected Entropy in Tachyon Cosmology

Authors: H. Farajollahi, A. Ravanpak, H. Shojaie, M. Abolghasemi

(Submitted on 29 May 2016)

Abstract: This work is to study the generalized second law (GSL) of thermodynamics in tachyon cosmology where the boundary of the universe is assumed to be enclosed by a dynamical apparent horizon. The model is constrained with the observational data. The two logarithmic and power law corrected entropy is also discussed and conditions to validate the GSL and corrected entropies are obtained.

Journal reference: Astrophys. Space Sci., 350, 325-331, (2014)

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arXiv:1605.09054 [gr-qc]

A 5D Holographic Dark Energy in DGP-Brane Cosmology

Authors: H. Farajollahi, A. Ravanpak

(Submitted on 29 May 2016 (v1), last revised 5 Jun 2016 (this version, v2))

Abstract: This paper is aimed at investigating a 5D holographic dark energy in DGP-BRANE cosmology by employing a combination of Sne Ia, BAO and CMB observational data to fit the cosmological parameters in the model. We describe the dynamic of a FRW for the normal branch (ϵ = +1) of solutions of the induced gravity brane-world model. We take the matter in 5D bulk as holographic dark energy that its holographic nature is reproduced effectively in 4D. The cosmic evolution reveals that the effective 4D holographic dark energy behaves as quintessence while taking into account the 4D cold dark matter results in matter dominated universe followed by late time acceleration.

Journal reference: Astrophys. Space Sci., 349, 961-966, (2014)

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arXiv:1606.00845 [gr-qc]

Cosmological constraints on Agegraphic dark energy in DGP braneworld gravity

Authors: H. Farajollahi, A. Ravanpak, G. F. Fadakar

(Submitted on 2 Jun 2016 (v1), last revised 7 Jun 2016 (this version, v2))

Abstract: A proposal to study the original and new agegraphic dark energy in DGP braneworld cosmology is presented in this work. To verify our model with the observational data, the model is constrained by a variety of independent measurements such as Hubble parameter, cosmic microwave background anisotropies, and baryon acoustic oscillation peaks. The best fitting procedure shows the effectiveness of agegraphic parameter n in distinguishing between the original and new agegraphic dark energy scenarios and subsequent cosmological findings. In particular, the result shows that in both scenarios, our universe enters an agegraphic dark energy dominated phase.

Journal reference: Astrophys. Space Sci., 348, 253-259 (2013)

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arXiv:1606.00852 [gr-qc]

Thermodynamics properties of tachyon cosmology with non-minimal coupling to matter

Authors: H. Farajollahi, A. Ravanpak, M. Abolghasemi

(Submitted on 2 Jun 2016)

Abstract: Recently, we have investigated the dynamics of the universe in tachyon cosmology with non-minimal coupling to matter \cite{faraj}-\cite{faraj3}. In particular, for the interacting holographic dark energy (IHDE), the model is studied in \cite{Ravanpak}. In the current work, a significant observational program has been conducted to unveil the model's thermodynamic properties. Our result shows that the IHDE version of our model better fits the observational data than Λ CDM model. The first and generalized second thermodynamics laws for the universe enveloped by cosmological apparent and event horizon are revisited. From the results, both first and generalized second laws, constrained by the observational data, are satisfied on cosmological apparent horizon.In addition, the total entropy is verified with the observation only if the horizon of the universe is taken as apparent horizon. Then, due to validity of generalized second law, the current cosmic acceleration is also predicted.

Journal reference: Gen. Relativ. Gravit., Volume 45, Issue 2, pp 465-476 (2013)

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arXiv:1510.03136 [astro-ph.SR]

Pinned vortex hopping in a neutron star crust

Authors: Brynmor Haskell, Andrew Melatos

(Submitted on 12 Oct 2015)

Abstract: The motion of superfluid vortices in a neutron star crust is at the heart of most theories of pulsar glitches. Pinning of vortices to ions can decouple the superfluid from the crust and create a reservoir of angular momentum. Sudden large scale unpinning can lead to an observable glitch. In this paper we investigate the scattering of a free vortex off a pinning potential and calculate its mean free path, in order to assess whether unpinned vortices can skip multiple pinning sites and come close enough to their neighbours to trigger avalanches, or whether they simply hop from one pinning site to another giving rise to a more gradual creep. We find that there is a significant range of parameter space in which avalanches can be triggered, thus supporting the hypothesis that they may lie at the origin of pulsar glitches. For realistic values of the pinning force and superfluid drag parameters we find that avalanches are more likely in the higher density regions of the crust where pinning is stronger. Physical differences in stellar parameters, such as mass and temperature, may lead to a switch between creep-like motion and avalanches, explaining the different characteristics of glitching pulsars.

Comments: Submitted to MNRAS

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arXiv:1512.07903 [astro-ph.HE]

Hydrodynamic simulations of pulsar glitch recovery

Authors: G. Howitt, B. Haskell, A. Melatos

(Submitted on 9 Dec 2015 (v1), last revised 4 May 2016 (this version, v2))

Abstract: Glitches are sudden jumps in the spin frequency of pulsars believed to originate in the superfluid interior of neutron stars. Superfluid flow in a model neutron star is simulated by solving the equations of motion of a two-component superfluid consisting of a viscous proton-electron plasma and an inviscid neutron condensate in a spherical Couette geometry. We examine the response of our model neutron star to glitches induced in three different ways: by instantaneous changes of the spin frequency of the inner and outer boundaries, and by instantaneous recoupling of the fluid components in the bulk. All simulations are performed with strong and weak mutual friction. It is found that the maximum size of a glitch that originates in the bulk decreases as the mutual friction strengthens. It is also found that mutual friction determines the fraction of the frequency jump which is later recovered, a quantity known as the 'healing parameter'. These behaviours may explain some of the diversity in observed glitch recoveries.

Comments: Accepted for publication in MNRAS. 15 pages, 9 figures

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arXiv:1603.04304 [astro-ph.SR]

The effect of superfluid hydrodynamics on pulsar glitch sizes and waiting times

Authors: Brynmor Haskell

(Submitted on 14 Mar 2016)

Abstract: Pulsar glitches, sudden jumps in frequency observed in many radio pulsars, may be the macroscopic manifestation of superfluid vortex avalanches on the microscopic scale. Small scale quantum mechanical simulations of vortex motion in a decelerating container have shown that such events are possible and predict power-law distributions for the size of the events, and exponential distributions for the waiting time. Despite a paucity of data, this prediction is consistent with the size and waiting time distributions of most glitching pulsars. Nevertheless a few object appear to glitch quasi-periodically, and exhibit many large glitches, while a recent study of the Crab pulsar has suggested a cut-off deviations from a power-law distribution for smaller glitches. In this paper we incorporate the results of quantum mechanical simulations in a macroscopic scale superfluid hydrodynamics simulation. We show that the effect of vortex coupling to the neutron and proton fluids in the neutron star naturally leads to deviations from power-law distributions for sizes and from exponential distributions for waiting times. In particular we predict a cut-off in the size distribution for small glitches.

Comments: Submitted to MNRAS

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arXiv:1508.06643 [astro-ph.HE]

Gravitational Waves from Neutron Stars: A Review

Authors: Paul D. Lasky

(Submitted on 26 Aug 2015)

Abstract: Neutron stars are excellent emitters of gravitational waves. Squeezing matter beyond nuclear densities invites exotic physical processes, many of which violently transfer large amounts of mass at relativistic velocities, disrupting spacetime and generating copious quantities of gravitational radiation. I review mechanisms for generating gravitational waves with neutron stars. This includes gravitational waves from radio and millisecond pulsars, magnetars, accreting systems and newly born neutron stars, with mechanisms including magnetic and thermoelastic deformations, various stellar oscillation modes and core superfluid turbulence. I also focus on what physics can be learnt from a gravitational wave detection, and where additional research is required to fully understand the dominant physical processes at play.

Journal reference: PASA (2015) Vol 32, pp. 34

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arXiv:1509.07320 [astro-ph.CO]

Gravitational waves from binary supermassive black holes missing in pulsar observations

Authors: R. M. Shannon, V. Ravi, L. T. Lentati, P. D. Lasky, G. Hobbs, M. Kerr, R. N. Manchester, W. A. Coles, Y. Levin, M. Bailes, N. D. R. Bhat, S. Burke-Spolaor, S. Dai, M. J. Keith, S. Osłowski, D. J. Reardon, W. van Straten, L. Toomey, J.-B. Wang, L. Wen, J. S. B. Wyithe, X.-J. Zhu

(Submitted on 24 Sep 2015)

Abstract: Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems will modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrain the characteristic amplitude of this background, A_c,yr, to be < 1.0×10^−15 with 95% confidence. This limit excludes predicted ranges for A_c,yr from current models with 91-99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments, and that higher-cadence and shorter-wavelength observations would result in an increased sensitivity to gravitational waves.

Comments: Published in Science 25 September 2015

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arXiv:1511.02959 [astro-ph.HE]

Hunting Gravitational Waves with Multi-Messenger Counterparts: Australia's Role

Authors: E. J. Howell, A. Rowlinson, D. M. Coward, P. D. Lasky, D. L. Kaplan, E. Thrane, G. Rowell, D. K. Galloway, Fang Yuan, R. Dodson, T. Murphy, G. C. Hill, I. Andreoni, L. Spitler, A. Horton

(Submitted on 10 Nov 2015)

Abstract: The first observations by a worldwide network of advanced interferometric gravitational wave detectors offer a unique opportunity for the astronomical community. At design sensitivity, these facilities will be able to detect coalescing binary neutron stars to distances approaching 400 Mpc, and neutron star-black hole systems to 1 Gpc. Both of these sources are associated with gamma ray bursts which are known to emit across the entire electromagnetic spectrum. Gravitational wave detections provide the opportunity for "multi-messenger" observations, combining gravitational wave with electromagnetic, cosmic ray or neutrino observations. This review provides an overview of how Australian astronomical facilities and collaborations with the gravitational wave community can contribute to this new era of discovery, via contemporaneous follow-up observations from the radio to the optical and high energy. We discuss some of the frontier discoveries that will be made possible when this new window to the Universe is opened.

Comments: This paper has been accepted for publication in PASA as an invited review for a special issue on Gravitational Wave Astronomy

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arXiv:1511.05994 [astro-ph.CO]

Gravitational-wave cosmology across 29 decades in frequency

Paul D. Lasky, Chiara M. F. Mingarelli, Tristan L. Smith, John T. Giblin Jr., Eric Thrane, Daniel J. Reardon, Robert Caldwell, Matthew Bailes, N. D. Ramesh Bhat, Sarah Burke-Spolaor, Shi Dai, James Dempsey, George Hobbs, Matthew Kerr, Yuri Levin, Richard N. Manchester, Stefan Osłowski, Vikram Ravi, Pablo A. Rosado, Ryan M. Shannon, Renée Spiewak, Willem van Straten, Lawrence Toomey, Jingbo Wang, Linqing Wen, Xiaopeng You, Xingjiang Zhu

(Submitted on 18 Nov 2015 (v1), last revised 28 Feb 2016 (this version, v2))

Quantum fluctuations of the gravitational field in the early Universe, amplified by inflation, produce a primordial gravitational-wave background across a broad frequency band. We derive constraints on the spectrum of this gravitational radiation, and hence on theories of the early Universe, by combining experiments that cover 29 orders of magnitude in frequency. These include Planck observations of cosmic microwave background temperature and polarization power spectra and lensing, together with baryon acoustic oscillations and big bang nucleosynthesis measurements, as well as new pulsar timing array and ground-based interferometer limits. While individual experiments constrain the gravitational-wave energy density in specific frequency bands, the combination of experiments allows us to constrain cosmological parameters, including the inflationary spectral index, n_t, and the tensor-to-scalar ratio, r. Results from individual experiments include the most stringent nanohertz limit of the primordial background to date from the Parkes Pulsar Timing Array, \Omega_{\rm gw}(f)<2.3\times10^{-10}. Observations of the cosmic microwave background alone limit the gravitational-wave spectral index at 95\% confidence to n_t\lesssim5 for a tensor-to-scalar ratio of r = 0.11. However, the combination of all the above experiments limits n_t<0.36. Future Advanced LIGO observations are expected to further constrain n_t<0.34 by 2020. When cosmic microwave background experiments detect a non-zero r, our results will imply even more stringent constraints on n_t and hence theories of the early Universe.

Comments: accepted for publication in Physical Review X

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arXiv:1512.04950 [astro-ph.CO]

Detectability of Gravitational Waves from High-Redshift Binaries

Authors: Pablo A. Rosado, Paul D. Lasky, Eric Thrane, Xingjiang Zhu, Ilya Mandel, Alberto Sesana

(Submitted on 15 Dec 2015 (v1), last revised 11 Mar 2016 (this version, v2))

Abstract: Recent non-detection of gravitational-wave backgrounds from pulsar timing arrays casts further uncertainty on the evolution of supermassive black hole binaries. We study the capabilities of current gravitational-wave observatories to detect individual binaries and demonstrate that, contrary to conventional wisdom, some are in principle detectable throughout the Universe. In particular, a binary with rest-frame mass ≳10^10M_⊙ can be detected by current timing arrays at arbitrarily high redshifts. The same claim will apply for less massive binaries with more sensitive future arrays. As a consequence, future searches for nanohertz gravitational waves could be expanded to target evolving high-redshift binaries. We calculate the maximum distance at which binaries can be observed with pulsar timing arrays and other detectors, properly accounting for redshift and using realistic binary waveforms.

Journal reference: PRL 116, 101102 (2016)
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arXiv:1512.05368 [astro-ph.HE]

Observationally constraining gravitational wave emission from short gamma-ray burst remnants

Authors: Paul D Lasky, Kostas Glampedakis

(Submitted on 16 Dec 2015 (v1), last revised 22 Feb 2016 (this version, v2))

Abstract: Observations of short gamma-ray bursts indicate ongoing energy injection following the prompt emission, with the most likely candidate being the birth of a rapidly rotating, highly magnetised neutron star. We utilise X-ray observations of the burst remnant to constrain properties of the nascent neutron star, including its magnetic field-induced ellipticity and the saturation amplitude of various oscillation modes. Moreover, we derive strict upper limits on the gravitational wave emission from these objects by looking only at the X-ray light curve, showing the burst remnants are unlikely to be detected in the near future using ground-based gravitational wave interferometers such as Advanced LIGO.

Comments: Accepted for publication in MNRAS

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arXiv:1602.05570 [astro-ph.IM]

From Spin Noise to Systematics: Stochastic Processes in the First International Pulsar Timing Array Data Release

L. Lentati, R. M. Shannon, W. A. Coles, J. P. W. Verbiest, R. van Haasteren, J. A. Ellis, R. N. Caballero, R. N. Manchester, Z. Arzoumanian, S. Babak, C. G. Bassa, N. D. R. Bhat, P. Brem, M. Burgay, S. Burke-Spolaor, D. Champion, S. Chatterjee, I. Cognard, J. M. Cordes, S. Dai, P. Demorest, G. Desvignes, T. Dolch, R. D. Ferdman, E. Fonseca, J. R. Gair, M. E. Gonzalez, E. Graikou, L. Guillemot, J. W. T. Hessels, G. Hobbs, G. H. Janssen, G. Jones, R. Karuppusamy, M. Keith, M. Kerr, M. Kramer, M. T. Lam, P. D. Lasky, A. Lassus, P. Lazarus, T. J. W. Lazio, K. J. Lee, L. Levin, K. Liu, R. S. Lynch, D. R. Madison, J. McKee, M. McLaughlin, S. T. McWilliams, C. M. F. Mingarelli, D. J. Nice, S. Osłowski, T. T. Pennucci, B. B. P. Perera, D. Perrodin, A. Petiteau, A. Possenti, S. M. Ransom, D. Reardon, et al. (23 additional authors not shown)

(Submitted on 16 Feb 2016)

We analyse the stochastic properties of the 49 pulsars that comprise the first International Pulsar Timing Array (IPTA) data release. We use Bayesian methodology, performing model selection to determine the optimal description of the stochastic signals present in each pulsar. In addition to spin-noise and dispersion-measure (DM) variations, these models can include timing noise unique to a single observing system, or frequency band. We show the improved radio-frequency coverage and presence of overlapping data from different observing systems in the IPTA data set enables us to separate both system and band-dependent effects with much greater efficacy than in the individual PTA data sets. For example, we show that PSR J1643−1224 has, in addition to DM variations, significant band-dependent noise that is coherent between PTAs which we interpret as coming from time-variable scattering or refraction in the ionised interstellar medium. Failing to model these different contributions appropriately can dramatically alter the astrophysical interpretation of the stochastic signals observed in the residuals. In some cases, the spectral exponent of the spin noise signal can vary from 1.6 to 4 depending upon the model, which has direct implications for the long-term sensitivity of the pulsar to a stochastic gravitational-wave (GW) background. By using a more appropriate model, however, we can greatly improve a pulsar's sensitivity to GWs. For example, including system and band-dependent signals in the PSR J0437−4715 data set improves the upper limit on a fiducial GW background by ∼60% compared to a model that includes DM variations and spin-noise only.

Comments: 29 pages. 16 figures. Accepted for publication in MNRAS

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arXiv:1605.01415 [astro-ph.HE]

Detecting gravitational-wave memory with LIGO: implications of GW150914

Authors: Paul D. Lasky, Eric Thrane, Yuri Levin, Jonathan Blackman, Yanbei Chen

(Submitted on 4 May 2016)

Abstract: It may soon be possible for Advanced LIGO to detect hundreds of binary black hole mergers per year. We show how the accumulation of many such measurements will allow for the detection of gravitational-wave memory: a permanent displacement of spacetime that comes from strong-field, general relativistic effects. We estimate that Advanced LIGO operating at design sensitivity may be able to make a signal-to-noise ratio 3(5) detection of memory with ∼35(90) events with masses and distance similar to GW150914. Given current merger rate estimates (of one such event per ∼16 days), this could happen in as few as ∼1.5(4) years of coincident data collection. We highlight the importance of incorporating higher-order gravitational-wave modes for parameter estimation of binary black hole mergers, and describe how our methods can also be used to detect higher-order modes themselves before Advanced LIGO reaches design sensitivity.

Comments: Submitted for publication.

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arXiv:1507.02754 [gr-qc]

Non-orthogonally transitive G_2 spike solution

Authors: Woei Chet Lim

(Submitted on 10 Jul 2015)

Abstract: We generalize the orthogonally transitive (OT) G_2 spike solution to the non-OT G_2 case. This is achieved by applying Geroch's transformation on a Kasner seed. The new solution contains two more parameters than the OT G_2 spike solution. Unlike the OT G_2 spike solution, the new solution always resolves its spike.

Journal reference: Class. Quantum Grav. 32 (2015) 162001
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arXiv:1511.07095 [gr-qc]

Spikes and matter inhomogeneities in massless scalar field models

Authors: Alan Coley, Woei Chet Lim

(Submitted on 23 Nov 2015)

Abstract: We shall discuss the general relativistic generation of spikes in a massless scalar field or stiff perfect fluid model. We first investigate orthogonally transitive (OT) G_2 stiff fluid spike models both heuristically and numerically, and give a new exact OT G_2 stiff fluid spike solution. We then present a new two-parameter family of non-OT G_2 stiff fluid spike solutions, obtained by the generalization of non-OT G_2 vacuum spike solutions to the stiff fluid case by applying Geroch's transformation on a Jacobs seed. The dynamics of these new stiff fluid spike solutions is qualitatively different from that of the vacuum spike solutions, in that the matter (stiff fluid) feels the spike directly and the stiff fluid spike solution can end up with a permanent spike. We then derive the evolution equations of non-OT G_2 stiff fluid models, including a second perfect fluid, in full generality, and briefly discuss some of their qualitative properties and their potential numerical analysis. Finally, we discuss how a fluid, and especially a stiff fluid or massless scalar field, affects the physics of the generation of spikes.

Comments: 24 pages, 3 figures.

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arXiv:1606.07177 [gr-qc]

On the first G_1 stiff fluid spike solution in General Relativity

Authors: Alan Coley, Daniele Gregoris, Woei Chet Lim

(Submitted on 23 Jun 2016 (v1), last revised 1 Jul 2016 (this version, v2))

Abstract: Using the Geroch transformation we obtain the first example of an exact stiff fluid spike solution to the Einstein field equations in a closed form exhibiting a spacelike G_1 group of symmetries (i.e., with a single isometry). This new solution is of Petrov type I and exhibits a spike crossing which persists to the past, which allows us to better understand spike crossings in the context of structure formation.

Comments: 13 pages, 1 figure. Minor changes. New references added

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arXiv:1509.06159 [gr-qc]

Accelerating Horndeski cosmologies screening the vacuum energy

Authors: Prado Martin-Moruno, Nelson J. Nunes, Francisco S. N. Lobo

(Submitted on 21 Sep 2015)

Abstract: In the context of Horndeski cosmologies, we consider a dynamical adjustment mechanism able to screen any value of the vacuum energy of the matter fields leading to a fixed de Sitter geometry. Thus, we present the most general scalar-tensor cosmological models without higher than second order derivatives in the field equation that have a fixed spatially flat de Sitter critical point for any kind of material content or vacuum energy. These models allow us to understand the current accelerated expansion of the universe as the result of the evolution towards the critical point when it is an attractor.

Comments: 6 pages; contribution to the proceedings of "The Fourtheenth Marcel Grossmann Meeting on General Relativity", University of Rome "La Sapienza", Rome, July 12-18, 2015, based on a talk delivered at the AT1 parallel session

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arXiv:1509.07398 [gr-qc]

The quantum realm of the "Little Sibling" of the Big Rip singularity

Authors: Imanol Albarran, Mariam Bouhmadi-López, Francisco Cabral, Prado Martín-Moruno

(Submitted on 24 Sep 2015)

Abstract: We analyse the quantum behaviour of the "Little Sibling" of the Big Rip singularity (LSBR) [1]. The quantisation is carried within the geometrodynamical approach given by the Wheeler--DeWitt (WDW) equation. The classical model is based on a Friedmann--Lema\^{i}tre--Robertson--Walker Universe filled by a perfect fluid that can be mapped to a scalar field with phantom character. We analyse the WDW equation in two setups. In the first step, we consider the scale factor as the single degree of freedom, which from a classical perspective parametrises both the geometry and the matter content given by the perfect fluid. We then solve the WDW equation within a WKB approximation, for two factor ordering choices. On the second approach, we consider the WDW equation with two degrees of freedom: the scale factor and a scalar field. We solve the WDW equation, with the Laplace--Beltrami factor-ordering, using a Born--Oppenheimer approximation. In both approaches, we impose the DeWitt (DW) condition as a potential criterion for singularity avoidance. We conclude that in all the cases analysed the DW condition can be verified, which might be an indication that the LSBR can be avoided or smoothed in the quantum approach.

Journal reference: JCAP 11 (2015) 044

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arXiv:1510.00158 [gr-qc]

Semi-classical and nonlinear energy conditions

Authors: Prado Martin-Moruno, Matt Visser

(Submitted on 1 Oct 2015)

Abstract: We consider the characteristics of nonlinear energy conditions and of quantum extensions of these and the usual energy conditions. We show that they are satisfied by some quantum vacuum states that violate the usual energy conditions.

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arXiv: 1511.00655 [gr-qc]

Cosmology of the de Sitter Horndeski models

Authors: Nelson J. Nunes, Prado Martin-Moruno, Francisco S. N. Lobo

(Submitted on 29 Oct 2015)

Abstract: Horndeski models with a de Sitter critical point for any kind of material content may provide a mechanism to alleviate the cosmological constant problem. We study the cosmological evolution of two classes of families - the linear models and the non-linear models with shift symmetry. We conclude that the latter models can deliver a background dynamics compatible with the latest observational data.

Comments: 6 pages, 2 figures. Contribution to the proceedings of "The Fourtheenth Marcel Grossmann Meeting on General Relativity", University of Rome "La Sapienza", Rome, July 12-18, 2015, based on a talk delivered at the DE1 parallel session

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arXiv:1512.07659 [gr-qc]

Novel stability approach of thin-shell gravastars

Authors: Francisco S. N. Lobo, Prado Martín-Moruno, Nadiezhda Montelongo-García, Matt Visser

(Submitted on 23 Dec 2015)

Abstract: We develop an extremely general and robust framework that can be adapted to wide classes of generic spherically symmetric thin-shell gravastars. The thin shell (transition layer) will be permitted to move freely in the bulk spacetimes, permitting a fully dynamic analysis. This will then allow us to perform a general stability analysis, where it is explicitly shown that stability of the gravastar is related to the properties of the matter residing in the thin-shell transition layer.

Comments: 6 pages; contribution to the proceedings of the "The Fourteenth Marcel Grossmann Meeting on General Relativity", University of Rome "La Sapienza", Rome, July 12-18, 2015, based on an invited talk delivered at the BS1-"Black Hole foils, Boson stars" parallel session

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arXiv:1512.08474 [gr-qc]

A novel approach to thin-shell wormholes and applications

Authors: Francisco S. N. Lobo, Mariam Bouhmadi-López, Prado Martín-Moruno, Nadiezhda Montelongo-García, Matt Visser

(Submitted on 28 Dec 2015)

Abstract: A novel framework is presented that can be adapted to a wide class of generic spherically symmetric thin-shell wormholes. By using the Darmois--Israel formalism, we analyze the stability of arbitrary spherically symmetric thin-shell wormholes to linearized perturbations around static solutions. We demonstrate in full generality that the stability of the wormhole is equivalent to choosing suitable properties for the exotic material residing on the wormhole throat. As an application, we consider the thin-shell variant of the Ellis wormhole for the cases of a vanishing momentum flux and non-zero external fluxes.

Comments: 6 pages; contribution to the proceedings of the "The Fourteenth Marcel Grossmann Meeting on General Relativity", University of Rome "La Sapienza", Rome, July 12-18, 2015, based on an invited talk delivered at the BH4-"Gravitational fields with sources: From compact objects to black holes" parallel session

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arXiv:1512.02291 [gr-qc]

Testing modified gravity and no-hair relations for the Kerr-Newman metric through quasi-periodic oscillations of galactic microquasars

Authors: Arthur George Suvorov, Andrew Melatos

(Submitted on 8 Dec 2015)

Abstract: We construct multipole moments for stationary, asymptotically flat, spacetime solutions to higher-order curvature theories of gravity. The moments are defined using 3+1 techniques involving timelike Killing vector constructions as in the classic papers by Geroch and Hansen. Using the fact that the Kerr-Newman metric is a vacuum solution to a particular class of f(R) theories of gravity, we compute all its moments, and find that they admit recurrence relations similar to those for the Kerr solution in general relativity. It has been proposed previously that modelling the measured frequencies of quasi-periodic oscillations from galactic microquasars enables experimental tests of the no-hair theorem. We explore the possibility that, even if the no-hair relation is found to break down in the context of general relativity, there may be an f(R) counterpart that is preserved. We apply the results to the microquasars GRS 1915 +105 and GRO J1655 -40 using the diskoseismology and kinematic resonance models, and constrain the spins and `charges' [which are not really electric charges in the f(R) context] of their black holes.

Comments: 14 pages, 5 figures; Accepted for publication in PRD

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arXiv:1606.02412 [astro-ph.IM]

Hidden Markov model tracking of continuous gravitational waves from a neutron star with wandering spin

Authors: S. Suvorova, L. Sun, A. Melatos, W. Moran, R. J. Evans

(Submitted on 8 Jun 2016)

Abstract: Gravitational wave searches for continuous-wave signals from neutron stars are especially challenging when the star's spin frequency is unknown a priori from electromagnetic observations and wanders stochastically under the action of internal (e.g. superfluid or magnetospheric) or external (e.g. accretion) torques. It is shown that frequency tracking by hidden Markov model (HMM) methods can be combined with existing maximum likelihood coherent matched filters like the F-statistic to surmount some of the challenges raised by spin wandering. Specifically it is found that, for an isolated, biaxial rotor whose spin frequency walks randomly, HMM tracking of the F-statistic output from coherent segments with duration T_drift = 10d over a total observation time of T_obs = 1yr can detect signals with wave strains h0 > 2e-26 at a noise level characteristic of the Advanced Laser Interferometer Gravitational Wave Observatory (Advanced LIGO). For a biaxial rotor with randomly walking spin in a binary orbit, whose orbital period and semi-major axis are known approximately from electromagnetic observations, HMM tracking of the Bessel-weighted F-statistic output can detect signals with h0 > 8e-26. An efficient, recursive, HMM solver based on the Viterbi algorithm is demonstrated, which requires ~10^3 CPU-hours for a typical, broadband (0.5-kHz) search for the low-mass X-ray binary Scorpius X-1, including generation of the relevant F-statistic input. In a "realistic" observational scenario, Viterbi tracking successfully detects 41 out of 50 synthetic signals without spin wandering in Stage I of the Scorpius X-1 Mock Data Challenge convened by the LIGO Scientific Collaboration down to a wave strain of h0 = 1.1e-25, recovering the frequency with a root-mean-square accuracy of <= 4.3e-3 Hz.

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arXiv:1510.08834 [astro-ph.CO]

Non-parametric reconstruction of an inflaton potential from Einstein-Cartan-Sciama-Kibble gravity with particle production

Authors: Shantanu Desai, Nikodem J. Poplawski

(Submitted on 18 Oct 2015)

Abstract: The coupling between spin and torsion in the Einstein-Cartan-Sciama-Kibble theory of gravity generates gravitational repulsion at very high densities, which prevents a singularity in a black hole and may create there a new universe. We show that quantum particle production in such a universe near the last bounce, which represents the Big Bang gives the dynamics that solves the horizon, flatness, and homogeneity problems in cosmology. For a particular range of the particle production coefficient, we obtain a nearly constant Hubble parameter that gives an exponential expansion of the universe with more than 60 e-folds, which lasts about ∼10^−42 s. This scenario can thus explain cosmic inflation without requiring a fundamental scalar field and reheating. From the obtained time dependence of the scale factor, we follow the prescription of Ellis and Madsen to reconstruct in a non-parametric way a scalar field potential which gives the same dynamics of the early universe. This potential gives the slow-roll parameters of cosmic inflation, from which we calculate the tensor-to-scalar ratio, the scalar spectral index of density perturbations, and its running as functions of the production coefficient. We find that these quantities do not significantly depend on the scale factor at the Big Bounce. Our predictions for these quantities are consistent with the Planck 2015 observations.

Journal reference: Phys. Lett. B 755, 183 (2016)

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arXiv:1512.09365 [gr-qc]

Kinetization of scalar field by torsion

Authors: Nikodem J. Poplawski

(Submitted on 31 Dec 2015)

Abstract: We show that a scalar field without a kinetic term in the Lagrangian density, coupled to a covariant divergence of the torsion vector in the Einstein-Cartan theory of gravity, becomes kinetic in its general-relativistic equivalent formulation. Dynamical scalar fields may therefore be emergent.

Comments: 3 pages

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arXiv:1512.04183 [gr-qc]

Optomechanical tests of a Schrödinger-Newton equation for gravitational quantum mechanics

Authors: C. C. Gan, C. M. Savage, S. Z. Scully

(Submitted on 14 Dec 2015 (v1), last revised 3 May 2016 (this version, v2))

Abstract: We show that optomechanical systems can test the Schr\"{o}dinger-Newton equation of gravitational quantum mechanics due to Yang et al. This equation is motivated by semiclassical gravity, a widely used theory of interacting gravitational and quantum fields. From the many-body Schr\"{o}dinger-Newton equation follows an approximate equation for the center-of-mass dynamics of macroscopic objects. This predicts a distinctive double-peaked signature in the output optical quadrature power spectral density of certain optomechanical systems. Since the Schr\"{o}dinger-Newton equation lacks free parameters, these will allow its experimental confirmation or refutation.

Comments: 8 pages. Revised in response to referees. Corrections and convention changes

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arXiv:1604.04305 [astro-ph.HE]

Gravitational radiation from neutron stars deformed by crustal Hall drift

Authors: Arthur George Suvorov, Alpha Mastrano, Ulrich Geppert

(Submitted on 14 Apr 2016)

Abstract: A precondition for the radio emission of pulsars is the existence of strong, small-scale magnetic field structures (`magnetic spots') in the polar cap region. Their creation can proceed via crustal Hall drift out of two qualitatively and quantitatively different initial magnetic field configurations: a field confined completely to the crust and another which penetrates the whole star. The aim of this study is to explore whether these magnetic structures in the crust can deform the star sufficiently to make it an observable source of gravitational waves. We model the evolution of these field configurations, which can develop, within ∼10^4 - 10^5 yr, magnetic spots with local surface field strengths ∼10^14 G maintained over > ∼ 10^6 yr. Deformations caused by the magnetic forces are calculated. We show that, under favourable initial conditions, a star undergoing crustal Hall drift can have ellipticity ∼ 10^-6, even with sub-magnetar polar field strengths, after ∼ 10^5 yr. A pulsar rotating at ∼10^2 Hz with such  is a promising gravitational-wave source candidate. Since such large deformations can be caused only by a particular magnetic field configuration that penetrates the whole star and whose maximum magnetic energy is concentrated in the outer core region, gravitational wave emission observed from radio pulsars can thus inform us about the internal field structures of young neutron stars.

Comments: 14 pages, 6 figures, 5 tables. Accepted for publication in MNRAS

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arXiv:1507.02416 [gr-qc]

Deformed hyperbolic black holes

Authors: Yu Chen, Yen-Kheng Lim, Edward Teo

(Submitted on 9 Jul 2015 (v1), last revised 12 Aug 2015 (this version, v3))

Abstract: Black holes with planar or hyperbolic horizons are known to exist in AdS space, alongside the usual ones with spherical horizons. In this paper, we consider a one-parameter generalisation of these black holes that is contained in the AdS C-metric. In terms of the domain-structure analysis recently developed for such solutions, these black holes have a domain in the shape of a triangle. It is shown that the horizons of these black holes are deformed hyperbolic spaces, with the new parameter controlling the amount of deformation. The space-times are static and completely regular outside the horizons. We argue that these black holes are hyperbolic analogues of the "slowly accelerating" spherical black holes known to exist in AdS space.

Journal reference: Phys. Rev. D 92, 044058 (2015).

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arXiv:1604.07527 [gr-qc]

Black holes with bottle-shaped horizons

Authors: Yu Chen, Edward Teo

(Submitted on 26 Apr 2016)

Abstract: We present a new class of four-dimensional AdS black holes with non-compact event horizons of finite area. The event horizons are topologically spheres with one puncture, with the puncture pushed to infinity in the form of a cusp. Because of the shape of their event horizons, we call such black holes "black bottles". The solution was obtained as a special case of the Plebanski-Demianski solution, and may describe either static or rotating black bottles. For certain ranges of parameters, an acceleration horizon may also appear in the space-time. We study the full parameter space of the solution, and the various limiting cases that arise. In particular, we show how the rotating black hole recently discovered by Klemm arises as a special limit.

Comments: 35 pages, 6 figures

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arXiv:1606.02415 [gr-qc]

Rotating and accelerating black holes with cosmological constant

Authors: Yu Chen, Cheryl Ng, Edward Teo

(Submitted on 8 Jun 2016)

Abstract: We propose a new form of the rotating C-metric with cosmological constant, which generalises the form found by Hong and Teo for the Ricci-flat case. This solution describes the entire class of spherical black holes undergoing rotation and acceleration in dS or AdS space-time. The new form allows us to identify the complete ranges of coordinates and parameters of this solution. We perform a systematic study of its geometrical and physical properties, and of the various limiting cases that arise from it.

Comments: 36 pages, 5 figures

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arXiv:1602.08341 [gr-qc]

Cartographic distortions make dielectric spacetime analog models imperfect mimickers

Authors: Mohsen Fathi, Robert T. Thompson

(Submitted on 25 Feb 2016 (v1), last revised 22 Jun 2016 (this version, v3))

Abstract: It is commonly assumed that if the optical metric of a dielectric medium is identical to the metric of a vacuum space-time then light propagation through the dielectric mimics light propagation in the vacuum. However, just as the curved surface of the Earth cannot be mapped into a flat plane without distortion of some surface features, so too is it impossible to project the behavior of light from the vacuum into a dielectric analog residing in Minkowski space-time without introducing distortions. We study the covariance properties of dielectric analog space-times and the kinematics of a congruence of light in the analog, and show how certain features can be faithfully emulated in the analog depending on the choice of projection, but that not all features can be simultaneously emulated without distortion. These findings indicate conceptual weaknesses in the idea of using analog space-times as a basis for transformation optics, and we show that a certain formulation of transformation optics closely related to analog space-times resolves these issues.

Journal reference: Phys. Rev. D 93, 124026 (2016)

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arXiv:1508.02490 [hep-ph]

Bounds on higher-order Lorentz-violating photon sector coefficients from an asymmetric optical ring resonator experiment

Authors: Stephen R. Parker, Matthew Mewes, Fred N. Baynes, Michael E. Tobar

(Submitted on 11 Aug 2015)

Abstract: Optical resonators provide a powerful tool for testing aspects of Lorentz invariance. Here, we present a reanalysis of an experiment where a path asymmetry was created in an optical ring resonator by introducing a dielectric prism in one arm. The frequency difference of the two fundamental counter-propagating modes was then recorded as the apparatus was orientation-modulated in the laboratory. By assuming that the minimal Standard-Model Extension coefficients vanish we are able to place bounds on higher-order parity-odd Lorentz-violating coefficients of the Standard-Model Extension. The results presented in this work set the first constraints on two previously unbounded linear combinations of d=8 parity-odd nonbirefringent nondispersive coefficients of the photon sector.

Comments: 6 pages, 4 figures, 3 tables, accepted for publication in Physics Letters A

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arXiv:1512.05547 [hep-ph]

Axion Dark Matter Coupling to Resonant Photons via Magnetic Field

Authors: Ben T. McAllister, Stephen R. Parker, Michael E. Tobar

(Submitted on 17 Dec 2015 (v1), last revised 10 Jun 2016 (this version, v4))

Abstract: We show that the magnetic component of the photon field produced by dark matter axions via the two-photon coupling mechanism in a Sikivie Haloscope is an important parameter passed over in previous analysis and experiments. The interaction of the produced photons will be resonantly enhanced as long as they couple to the electric or magnetic mode structure of the Haloscope cavity. For typical Haloscope experiments the electric and magnetic coupling is the same and implicitly assumed in past sensitivity calculations. However, for future planned searches such as those at high frequency, which synchronize multiple cavities, the sensitivity will be altered due to different magnetic and electric couplings. We define the complete electromagnetic form factor and discuss its implications for current and future high and low mass axion searches, including some effects which have been overlooked, due to the assumption that the two couplings are the same.

Journal reference: Phys. Rev. Lett. 116, 161804 (2016)

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arXiv:1602.07861 [gr-qc]

Quasilocal energy exchange and the null cone

Authors: Nezihe Uzun

(Submitted on 25 Feb 2016)

Abstract: Energy is at best defined quasilocally in general relativity. Quasilocal energy definitions depend on the conditions one imposes on the boundary Hamiltonian, i.e., how a finite region of spacetime is "isolated". Here, we propose a method to define and investigate systems in terms of their matter plus gravitational energy content. We adopt a generic construction, that involves embedding of an arbitrary dimensional worldsheet into an arbitrary dimensional spacetime, to a 2 + 2 picture. In our case, the closed 2-dimensional spacelike surface S, that is orthogonal to the 2-dimensional timelike worldsheet T at every point, encloses the system in question. The integrability conditions of T and S correspond to three null tetrad gauge conditions once we transform our notation to the one of the null cone observables. We interpret the Raychaudhuri equation of T as a work-energy relation for systems that are not in equilibrium with their surroundings. We achieve this by identifying the quasilocal charge densities corresponding to rotational and nonrotational degrees of freedom, in addition to a relative work density associated with tidal fields. We define the corresponding quasilocal charges that appear in our work-energy relation and which can potentially be exchanged with the surroundings. These charges and our tetrad conditions are invariant under Type-III Lorentz transformations, i.e., the boosting of the observers in the directions orthogonal to S. We apply our construction to a radiating Vaidya spacetime, a C-metric and the interior of a Lanczos-van Stockum dust metric. The delicate issues related to the axially symmetric stationary spacetimes and possible extensions to the Kerr geometry are also discussed.

Comments: 32 pages, 13 figures

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arXiv:1511.01162 [gr-qc]

On burning a lump of coal

Authors: Ana Alonso-Serrano (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)

(Submitted on 3 Nov 2015)

Abstract: Burning something, (e.g. the proverbial lump of coal, or an encyclopaedia for that matter), in a blackbody furnace leads to an approximately Planck emission spectrum with an average entropy/information transfer of approximately 3.9±2.5 bits per emitted photon. This quantitative and qualitative result depends only on the underlying unitarity of the quantum physics of burning, combined with the statistical mechanics of blackbody radiation. The fact that the utterly standard and unitarity preserving process of burning something (in fact, burning anything) nevertheless *has* an associated entropy/information budget, and the quantitative *size* of that entropy/information budget, is a severely under-appreciated feature of standard quantum statistical physics.

Comments: 11 pages

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arXiv:1512.01890 [gr-qc]

Entropy/information flux in Hawking radiation

Authors: Ana Alonso-Serrano (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)

(Submitted on 7 Dec 2015)

Abstract: Blackbody radiation contains (on average) an entropy of 3.9±2.5 bits per photon. This applies not only to the proverbial case of "burning a lump of coal", but also to the Hawking radiation from both analogue black holes and general relativistic black holes. The flip side of this observation is the information budget: If the emission process is unitary, (as it certainly is for normal physical/chemical burning, and also for the Hawking emission from analogue black holes), then this entropy is exactly compensated by the "hidden information" in the correlations. We shall now extend this argument to the Hawking radiation from general relativistic black holes, (where previous discussion is both heated and inconclusive), demonstrating that the assumption of unitarity leads to a perfectly reasonable entropy/information budget without any hint of a "firewall". The assumption of unitarity instead has a different implication --- the horizon (if present) cannot be an *event* horizon, it must be an *apparent/trapping* horizon, or some variant thereof.
The key technical aspect of our calculation is the "average subsystem" approach, but applied to a tripartite pure system consisting of the (black hole)+(Hawking radiation)+(rest of universe).

Comments: 32 pages, 6 figures

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arXiv:1512.05018 [gr-qc]

Greybody factors for Schwarzschild black holes: Path-ordered exponentials and product integrals

Authors: Finnian Gray (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)

(Submitted on 16 Dec 2015)

Abstract: In recent work concerning the sparsity of the Hawking flux [arXiv:1506.03975v2], we found it necessary to re-examine what is known regarding the greybody factors of black holes, with a view to extending and expanding on some old results from the 1970s. Focussing specifically on Schwarzschild black holes, we re-calculated and re-assessed the greybody factors using a path-ordered-exponential approach, a technique which has the virtue of providing a semi-explicit formula for the relevant Bogoliubov coefficients. These path-ordered-exponentials, (being based on a "transfer matrix" formalism), are closely related to so-called "product integrals", leading to quite straightforward and direct numerical evaluation, while avoiding any need for numerically solving differential equations. Furthermore, while considerable analytic information is already available regarding both the high-frequency and low-frequency asymptotics of these greybody factors, numerical approaches seem better adapted to finding suitable "global models" for these greybody factors in the intermediate frequency regime, where most of the Hawking flux is concentrated. Working in a more general context, these path-ordered-exponential techniques are also likely to be of interest for generic barrier-penetration problems.

Comments: 28 pages; 10 figures

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arXiv:1512.05729 [gr-qc]

Buchert coarse-graining and the classical energy conditions

Authors: Matt Visser (Victoria University of Wellington)

(Submitted on 17 Dec 2015 (v1), last revised 7 Jan 2016 (this version, v2))

Abstract: So-called "Buchert averaging" is actually a coarse-graining procedure, where fine detail is "smeared out" due to limited spatio-temporal resolution. For technical reasons, (to be explained herein), "averaging" is not really an appropriate term, and I shall consistently describe the process as a "coarse-graining". Because Einstein gravity is nonlinear the coarse-grained Einstein tensor is typically not equal to the Einstein tensor of the coarse-grained spacetime geometry. The discrepancy can be viewed as an "effective" stress-energy. To keep otherwise messy technical issues firmly under control, I shall work with conformal-FLRW (CFLRW) cosmologies. These CFLRW-based models are particularly tractable, and are also particularly attractive observationally: the CMB is not distorted. In this CFLRW context one can prove some rigorous theorems regarding the interplay between Buchert coarse-graining, tracelessness of the effective stress-energy, and the classical energy conditions.

Comments: 6 Pages; contribution to the MG14 conference - Rome 2015; V2: Significant changes. One key inequality now proved, (not on the basis of usual averaging arguments), but on the basis of the Cauchy-Schwarz inequality. The trace of the effective stress-energy can be non-zero and of either sign. The effective stress-energy satisfies many of the classical energy conditions

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arXiv:1512.05809 [gr-qc]

Sparsity of the Hawking flux

Authors: Matt Visser (Victoria University of Wellington), Finnian Gray (Victoria University of Wellington), Sebastian Schuster (Victoria University of Wellington), Alexander Van-Brunt (Victoria University of Wellington)

(Submitted on 17 Dec 2015)

Abstract: It is (or should be) well-known that the Hawking flux that reaches spatial infinity is extremely sparse, and extremely thin, with the Hawking quanta, one-by-one, slowly dribbling out of the black hole. The typical time between quanta reaching infinity is much larger than the timescale set by the energy of the quanta. Among other things, this means that the Hawking evaporation of a black hole should be viewed as a sequential cascade of 2-body decays.

Comments: 6 pages; MG14 conference - Rome - 2015

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arXiv:1601.03355 [gr-qc]

Global properties of physically interesting Lorentzian spacetimes

Authors: Deloshan Nawarajan (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)

(Submitted on 13 Jan 2016)

Abstract: Under normal circumstances most members of the general relativity community focus almost exclusively on the local properties of spacetime, such as the locally Euclidean structure of the manifold and the Lorentzian signature of the metric tensor. When combined with the classical Einstein field equations this gives an extremely successful empirical model of classical gravity and classical matter --- at least as long as one does not ask too many awkward questions about global issues, (such as global topology and global causal structure). We feel however that this is a tactical error --- even without invoking full-fledged "quantum gravity" we know that the standard model of particle physics is also an extremely good representation of some parts of empirical reality; and we had better be able to carry over all the good features of the standard model of particle physics --- at least into the realm of semi-classical quantum gravity. Doing so gives us some interesting global features that spacetime should possess: On physical grounds spacetime should be space-orientable, time-orientable, and spacetime-orientable, and it should possess a globally defined tetrad (vierbein, or in general a globally defined vielbein/n-bein). So on physical grounds spacetime should be parallelizable. This strongly suggests that the metric is not the fundamental physical quantity; a very good case can be made for the tetrad being more fundamental than the metric. Furthermore, a globally-defined "almost complex structure" is almost unavoidable. Ideas along these lines have previously been mooted, but much is buried in the pre-arXiv literature and is either forgotten or inaccessible. We shall revisit these ideas taking a perspective very much based on empirical physical observation.

Comments: 19 pages

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arXiv:1601.03532 [gr-qc]

Cartesian Kerr-Schild variation on the Newman-Janis ansatz

Authors: Deloshan Nawarajan (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)

(Submitted on 14 Jan 2016)

Abstract: The Newman-Janis ansatz is a procedure (an "ansatz" or "trick") for obtaining the Kerr spacetime from the Schwarzschild spacetime. This 50 year old "trick" continues to generate heated discussion and debate even to this day. Most of the debate focusses on whether the Newman-Janis procedure can be upgraded to the status of an "algorithm", or if it is perhaps merely an inspired "ansatz", or possibly just a random "trick" of no deep physical significance. (That the Newman-Janis procedure very quickly led to the discovery of the Kerr-Newman spacetime is a point very much in its favour.) In the current article we will not answer these deeper questions, we shall instead present a much simpler alternative variation on the theme of the Newman-Janis ansatz that might be easier to work with. We shall present a 2-step version of the Newman-Janis trick that works directly with the Kerr-Schild "Cartesian" metric presentation of the Kerr spacetime. That is, we show how the original 4-step Newman--Janis procedure can, (using the interplay between oblate spheroidal and Cartesian coordinates), be reduced to a considerably cleaner 2-step process.

Comments: 10 pages

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arXiv:1601.06766 [quant-ph]

On the observation of nonclassical excitations in Bose-Einstein condensates

Authors: Andreas Finke, Piyush Jain, Silke Weinfurtner

(Submitted on 25 Jan 2016)

Abstract: In the recent experimental and theoretical literature well-established nonclassicality criteria from the field of quantum optics have been directly applied to the case of excitations in matter-waves. Among these are violations of Cauchy-Schwarz inequalities, Glauber-Sudarshan P-nonclassicality, sub-Poissonian number-difference squeezing (also known as the two-mode variance) and the criterion of nonseparability. We review the strong connection of these criteria and their meaning in quantum optics, and point out differences in the interpretation between light and matter waves. We then calculate observables for a homogenous Bose-Einstein condensate undergoing an arbitrary modulation in the interaction parameter at finite initial temperature, within both the quantum theory as well as a classical reference. We conclude that to date in experiments relevant for analogue gravity, nonclassical effects have not conclusively been observed and conjecture that additional, noncommuting, observables have to be measured to this end.

Comments: 11 pages, 1 figure

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arXiv:1603.02746 [gr-qc]

The imprint of the Hawking effect in subcritical flows

Authors: Antonin Coutant, Silke Weinfurtner

(Submitted on 9 Mar 2016)

Abstract: We study the propagation of low frequency shallow water waves on a one dimensional flow of varying depth. When taking into account dispersive effects, the linear propagation of long wavelength modes on uneven bottoms excites new solutions of the dispersion relation which possess a much shorter wavelength. The peculiarity is that one of these new solutions has a negative energy. When the flow becomes supercritical, this mode has been shown to be responsible for the (classical) analog of the Hawking effect. For subcritical flows, the production of this mode has been observed numerically and experimentally, but the precise physics governing the scattering remained unclear. In this work, we provide an analytic treatment of this effect in subcritical flows. We analyze the scattering of low frequency waves using a new perturbative series, derived from a generalization of the Bremmer series. We show that the production of short wavelength modes is governed by a complex value of the position: a complex turning point. Using this method, we investigate various flow profiles, and derive the main characteristics of the induced spectrum.

Comments: 35 pages, 10 figures

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arXiv:1605.09618 [hep-th]

Extrinsic curvature in 2-dimensional Causal Dynamical Triangulation

Authors: Lisa Glaser, Thomas P. Sotiriou, Silke Weinfurtner

(Submitted on 31 May 2016)

Abstract: Causal Dynamical Triangulations (CDT) is a non-perturbative quantisation of general relativity. Ho\v{r}ava-Lifshitz gravity on the other hand modifies general relativity to allow for perturbative quan- tisation. Past work has given rise to the speculation that Ho\v{r}ava-Lifshitz gravity might correspond to the continuum limit of CDT. In this paper we add another piece to this puzzle by applying the CDT quantisation prescription directly to Ho\v{r}ava-Lifshitz gravity in 2 dimensions. We derive the continuum Hamiltonian and we show that it matches exactly the Hamiltonian one derives from canonically quantising the Ho\v{r}ava-Lifshitz action. Unlike the standard CDT case, here the intro- duction of a foliated lattice does not impose further restriction on the configuration space and, as a result, lattice quantisation does not leave any imprint on continuum physics as expected.

Comments: 9 pages, 3 figures

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arXiv:1507.01720 [gr-qc]

High-order comparisons between post-Newtonian and perturbative self forces

Authors: Luc Blanchet, Guillaume Faye, Bernard F. Whiting

(Submitted on 7 Jul 2015)

Abstract: Recent numerical and analytic computations based on the self-force (SF) formalism in general relativity showed that half-integral post-Newtonian (PN) terms, i.e. terms involving odd powers of 1/c, arise in the redshift factor of small mass-ratio black-hole binaries on exact circular orbits. Although those contributions might seem puzzling at first sight for conservative systems that are invariant under time-reversal, they are in fact associated with the so-called non-linear tail-of-tail effect. We shall describe here how the next-to-next-to-leading order contributions beyond the first half-integral 5.5PN conservative effect (i.e. up to order 7.5PN included) have been obtained by means of the standard PN formalism applied to binary systems of point-like objects. The resulting redshift factor in the small mass-ratio limit fully agrees with that of the SF approach.

Comments: 6 pages; contribution to the proceedings of the 50th Rencontres de Moriond, "Gravitation: 100 years after GR"

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arXiv:1512.03313 [astro-ph.CO]

Observational Challenges for the Standard FLRW Model

Authors: Thomas Buchert, Alan A. Coley, Hagen Kleinert, Boudewijn F. Roukema, David L. Wiltshire

(Submitted on 10 Dec 2015 (v1), last revised 8 Feb 2016 (this version, v2))

Abstract: We summarise some of the main observational challenges for the standard Friedmann-Lemaitre-Robertson-Walker cosmological model and describe how results recently presented in the parallel session `Large--scale Structure and Statistics' (DE3) at the `Fourteenth Marcel Grossman Meeting on General Relativity' are related to these challenges.

Journal reference: Int. J. Mod. Phys. D 25, 1630007 (2016)

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ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc,

July 2015 - June 2016

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, Ra Inta, Ju Li, David McClelland, Andrew Melatos, Jesper Munch, Craig Savage, Susan Scott, Daniel Shaddock, Bram Slagmolen, Peter Veitch, Bernard Whiting and Chunnong Zhao.

Listed below are all the abstracts listed on gr-qc from July 2015 to June 2016 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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arXiv:1510.03474 [gr-qc]

A search of the Orion spur for continuous gravitational waves using a "loosely coherent" algorithm on data from LIGO interferometers

Authors: J. Aasi et al.

(Submitted on 12 Oct 2015 (v1), last revised 14 Oct 2015 (this version, v2))

Abstract: We report results of a wideband search for periodic gravitational waves from isolated neutron stars within the Orion spur towards both the inner and outer regions of our Galaxy. As gravitational waves interact very weakly with matter, the search is unimpeded by dust and concentrations of stars. One search disk (A) is 6.87∘ in diameter and centered on 20h10m54.71s+33∘33′25.29", and the other (B) is 7.45∘ in diameter and centered on 8h35m20.61s−46∘49′25.151". We explored the frequency range of 50-1500 Hz and frequency derivative from 0 to −5×10^−9 Hz/s. A multi-stage, loosely coherent search program allowed probing more deeply than before in these two regions, while increasing coherence length with every stage.
Rigorous followup parameters have winnowed initial coincidence set to only 70 candidates, to be examined manually. None of those 70 candidates proved to be consistent with an isolated gravitational wave emitter, and 95% confidence level upper limits were placed on continuous-wave strain amplitudes. Near 169 Hz we achieve our lowest 95% CL upper limit on worst-case linearly polarized strain amplitude h_0 of 6.3×10^−25, while at the high end of our frequency range we achieve a worst-case upper limit of 3.4×10^−24 for all polarizations and sky locations.

Journal reference: Phys. Rev. D 93, 042006 (2016)

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arXiv:1510.03621 [astro-ph.IM]

First low frequency all-sky search for continuous gravitational wave signals

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 13 Oct 2015)

Abstract: In this paper we present the results of the first low frequency all-sky search of continuous gravitational wave signals conducted on Virgo VSR2 and VSR4 data. The search covered the full sky, a frequency range between 20 Hz and 128 Hz with a range of spin-down between −1.0×10^−10 Hz/s and +1.5×10^−11 Hz/s, and was based on a hierarchical approach. The starting point was a set of short Fast Fourier Transforms (FFT), of length 8192 seconds, built from the calibrated strain data. Aggressive data cleaning, both in the time and frequency domains, has been done in order to remove, as much as possible, the effect of disturbances of instrumental origin. On each dataset a number of candidates has been selected, using the FrequencyHough transform in an incoherent step. Only coincident candidates among VSR2 and VSR4 have been examined in order to strongly reduce the false alarm probability, and the most significant candidates have been selected. The criteria we have used for candidate selection and for the coincidence step greatly reduce the harmful effect of large instrumental artifacts. Selected candidates have been subject to a follow-up by constructing a new set of longer FFTs followed by a further incoherent analysis. No evidence for continuous gravitational wave signals was found, therefore we have set a population-based joint VSR2-VSR4 90% confidence level upper limit on the dimensionless gravitational wave strain in the frequency range between 20 Hz and 128 Hz. This is the first all-sky search for continuous gravitational waves conducted at frequencies below 50 Hz. We set upper limits in the range between about 10^−24 and 2×10^−23 at most frequencies. Our upper limits on signal strain show an improvement of up to a factor of ∼ 2 with respect to the results of previous all-sky searches at frequencies below 80 Hz.

Journal reference: Phys. Rev. D 93, 042007 (2016)

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arXiv:1511.04398 [gr-qc]

All-sky search for long-duration gravitational wave transients with LIGO

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 13 Nov 2015 (v1), last revised 18 Feb 2016 (this version, v2))

Abstract: We present the results of a search for long-duration gravitational wave transients in two sets of data collected by the LIGO Hanford and LIGO Livingston detectors between November 5, 2005 and September 30, 2007, and July 7, 2009 and October 20, 2010, with a total observational time of 283.0 days and 132.9 days, respectively. The search targets gravitational wave transients of duration 10 - 500 s in a frequency band of 40 - 1000 Hz, with minimal assumptions about the signal waveform, polarization, source direction, or time of occurrence. All candidate triggers were consistent with the expected background; as a result we set 90% confidence upper limits on the rate of long-duration gravitational wave transients for different types of gravitational wave signals. For signals from black hole accretion disk instabilities, we set upper limits on the source rate density between 3.4×10^−5- 9.4×10^−4 Mpc^−3 yr^−1 at 90% confidence. These are the first results from an all-sky search for unmodeled long-duration transient gravitational waves.

Journal reference: Phys. Rev. D 93, 042005 (2016)

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arXiv:1602.03839 [gr-qc]

GW150914: First results from the search for binary black hole coalescence with Advanced LIGO

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 11 Feb 2016 (v1), last revised 27 Apr 2016 (this version, v3))

Abstract: On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) simultaneously observed the binary black hole merger GW150914. We report the results of a matched-filter search using relativistic models of compact-object binaries that recovered GW150914 as the most significant event during the coincident observations between the two LIGO detectors from September 12 to October 20, 2015. GW150914 was observed with a matched filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1 {\sigma}.

Journal reference: Phys. Rev. D 93, 122003 (2016)

Report number: LIGO-P1500269

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arXiv:1602.03840 [gr-qc]

Properties of the Binary Black Hole Merger GW150914

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 11 Feb 2016 (v1), last revised 20 Jun 2016 (this version, v2))

Abstract: On September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected a gravitational-wave transient (GW150914); we characterize the properties of the source and its parameters. The data around the time of the event were analyzed coherently across the LIGO network using a suite of accurate waveform models that describe gravitational waves from a compact binary system in general relativity. GW150914 was produced by a nearly equal mass binary black hole of 36+5−4M_⊙ and 29+4−4M_⊙; for each parameter we report the median value and the range of the 90% credible interval. The dimensionless spin magnitude of the more massive black hole is bound to be <0.7 (at 90% probability). The luminosity distance to the source is 410+160−180 Mpc, corresponding to a redshift 0.09+0.03−0.04 assuming standard cosmology. The source location is constrained to an annulus section of 610 deg^2, primarily in the southern hemisphere. The binary merges into a black hole of 62+4−4M_⊙ and spin 0.67+0.05−0.07. This black hole is significantly more massive than any other inferred from electromagnetic observations in the stellar-mass regime.

Journal reference: Phys. Rev. Lett. 116, 241102 (2016)

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arXiv:1602.03842 [astro-ph.HE]

The Rate of Binary Black Hole Mergers Inferred from Advanced LIGO Observations Surrounding GW150914

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 11 Feb 2016 (v1), last revised 13 Jun 2016 (this version, v2))

Abstract: A transient gravitational-wave signal, GW150914, was identified in the twin Advanced LIGO detectors on September 14, 2015 at 09:50:45 UTC. To assess the implications of this discovery, the detectors remained in operation with unchanged configurations over a period of 39 d around the time of the signal. At the detection statistic threshold corresponding to that observed for GW150914, our search of the 16 days of simultaneous two-detector observational data is estimated to have a false alarm rate (FAR) of <4.9×10^−6 yr^−1, yielding a p-value for GW150914 of <2×10^−7. Parameter estimation followup on this trigger identifies its source as a binary black hole (BBH) merger with component masses (m1,m2)=(36+5−4,29+4−4)M_⊙ at redshift z=0.09+0.03−0.04 (median and 90\% credible range). Here we report on the constraints these observations place on the rate of BBH coalescences. Considering only GW150914, assuming that all BBHs in the Universe have the same masses and spins as this event, imposing a search FAR threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a 90% credible range of merger rates between 2 – 53 Gpc^−3 yr^−1 (comoving frame). Incorporating all search triggers that pass a much lower threshold while accounting for the uncertainty in the astrophysical origin of each trigger, we estimate a higher rate, ranging from 13 – 600 Gpc^−3 yr^−1 depending on assumptions about the BBH mass distribution. All together, our various rate estimates fall in the conservative range 2 – 600 Gpc^−3 yr^−1.

Report number: LIGO-P1500217

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arXiv:1602.08492 [astro-ph.HE]

Localization and broadband follow-up of the gravitational-wave transient GW150914

Authors: B. P. Abbott et al.

(Submitted on 26 Feb 2016 (v1), last revised 3 Jun 2016 (this version, v3))

Abstract: A gravitational-wave (GW) transient was identified in data recorded by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) detectors on 2015 September 14. The event, initially designated G184098 and later given the name GW150914, is described in detail elsewhere. By prior arrangement, preliminary estimates of the time, significance, and sky location of the event were shared with 63 teams of observers covering radio, optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In this Letter we describe the low-latency analysis of the GW data and present the sky localization of the first observed compact binary merger. We summarize the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network circulars, giving an overview of the participating facilities, the GW sky localization coverage, the timeline and depth of the observations. As this event turned out to be a binary black hole merger, there is little expectation of a detectable electromagnetic (EM) signature. Nevertheless, this first broadband campaign to search for a counterpart of an Advanced LIGO source represents a milestone and highlights the broad capabilities of the transient astronomy community and the observing strategies that have been developed to pursue neutron star binary merger events. Detailed investigations of the EM data and results of the EM follow-up campaign are being disseminated in papers by the individual teams.

Report number: LIGO-P1500227-v12

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arXiv:1604.07864 [astro-ph.HE]

Supplement: Localization and broadband follow-up of the gravitational-wave transient GW150914

Authors: B. P. Abbott et al.

(Submitted on 26 Apr 2016 (v1), last revised 17 Jun 2016 (this version, v2))

Abstract: This Supplement provides supporting material for arXiv:1602.08492 . We briefly summarize past electromagnetic (EM) follow-up efforts as well as the organization and policy of the current EM follow-up program. We compare the four probability sky maps produced for the gravitational-wave transient GW150914, and provide additional details of the EM follow-up observations that were performed in the different bands.

Report number: LIGO-P1600137-v2

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arXiv:1605.01785 [gr-qc]

A First Targeted Search for Gravitational-Wave Bursts from Core-Collapse Supernovae in Data of First-Generation Laser Interferometer Detectors

Authors: B. P. Abbott et al.

(Submitted on 5 May 2016 (v1), last revised 19 May 2016 (this version, v2))

Abstract: We present results from a search for gravitational-wave bursts coincident with a set of two core-collapse supernovae observed between 2007 and 2011. We employ data from the Laser Interferometer Gravitational-wave Observatory (LIGO), the Virgo gravitational-wave observatory, and the GEO 600 gravitational-wave observatory. The targeted core-collapse supernovae were selected on the basis of (1) proximity (within approximately 15 Mpc), (2) tightness of observational constraints on the time of core collapse that defines the gravitational-wave search window, and (3) coincident operation of at least two interferometers at the time of core collapse. We find no plausible gravitational-wave candidates. We present the probability of detecting signals from both astrophysically well-motivated and more speculative gravitational-wave emission mechanisms as a function of distance from Earth, and discuss the implications for the detection of gravitational waves from core-collapse supernovae by the upgraded Advanced LIGO and Virgo detectors.

Report number: LIGO-P1400208

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arXiv:1605.03233 [gr-qc]

Comprehensive All-sky Search for Periodic Gravitational Waves in the Sixth Science Run LIGO Data

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 10 May 2016)

Abstract: We report on a comprehensive all-sky search for periodic gravitational waves in the frequency band 100-1500 Hz and with a frequency time derivative in the range of [−1.18,+1.00]×10^−8 Hz/s. Such a signal could be produced by a nearby spinning and slightly non-axisymmetric isolated neutron star in our galaxy. This search uses the data from the Initial LIGO sixth science run and covers a larger parameter space with respect to any past search.
A Loosely Coherent detection pipeline was applied to follow up weak outliers in both Gaussian (95% recovery rate) and non-Gaussian (75% recovery rate) bands. No gravitational wave signals were observed, and upper limits were placed on their strength. Our smallest upper limit on worst-case (linearly polarized) strain amplitude h_0 is 9.7×10^−25 near 169 Hz, while at the high end of our frequency range we achieve a worst-case upper limit of 5.5×10^−24. Both cases refer to all sky locations and entire range of frequency derivative values.

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arXiv:1606.01210 [gr-qc]

An improved analysis of GW150914 using a fully spin-precessing waveform model

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 3 Jun 2016)

Abstract: This paper presents updated estimates of source parameters for GW150914, a binary black-hole coalescence event detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) on September 14, 2015 [1]. Reference presented parameter estimation [2] of the source using a 13-dimensional, phenomenological precessing-spin model (precessing IMRPhenom) and a 11-dimensional nonprecessing effective-one-body (EOB) model calibrated to numerical-relativity simulations, which forces spin alignment (nonprecessing EOBNR). Here we present new results that include a 15-dimensional precessing-spin waveform model (precessing EOBNR) developed within the EOB formalism. We find good agreement with the parameters estimated previously [2], and we quote updated component masses of 35+5−3M_⊙ and 30+3−4M_⊙ (where errors correspond to 90% symmetric credible intervals). We also present slightly tighter constraints on the dimensionless spin magnitudes of the two black holes, with a primary spin estimate 0.65 and a secondary spin estimate 0.75 at 90% probability. Reference [2] estimated the systematic parameter-extraction errors due to waveform-model uncertainty by combining the posterior probability densities of precessing IMRPhenom and nonprecessing EOBNR. Here we find that the two precessing-spin models are in closer agreement, suggesting that these systematic errors are smaller than previously quoted.

Report number: LIGO-P1600048

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arXiv:1606.01262 [gr-qc]

Directly comparing GW150914 with numerical solutions of Einstein's equations for binary black hole coalescence

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 3 Jun 2016)

Abstract: We compare GW150914 directly to simulations of coalescing binary black holes in full general relativity, accounting for all the spin-weighted quadrupolar modes, and separately accounting for all the quadrupolar and octopolar modes. Consistent with the posterior distributions reported in LVC_PE[1] (at 90% confidence), we find the data are compatible with a wide range of nonprecessing and precessing simulations. Followup simulations performed using previously-estimated binary parameters most resemble the data. Comparisons including only the quadrupolar modes constrain the total redshifted mass Mz \in [64 - 82M_\odot], mass ratio q = m2/m1 \in [0.6,1], and effective aligned spin \chi_eff \in [-0.3, 0.2], where \chi_{eff} = (S1/m1 + S2/m2) \cdot\hat{L} /M. Including both quadrupolar and octopolar modes, we find the mass ratio is even more tightly constrained. Simulations with extreme mass ratios and effective spins are highly inconsistent with the data, at any mass. Several nonprecessing and precessing simulations with similar mass ratio and \chi_{eff} are consistent with the data. Though correlated, the components' spins (both in magnitude and directions) are not significantly constrained by the data. For nonprecessing binaries, interpolating between simulations, we reconstruct a posterior distribution consistent with previous results. The final black hole's redshifted mass is consistent with Mf,z between 64.0 - 73.5M_\odot and the final black hole's dimensionless spin parameter is consistent with af = 0.62 - 0.73. As our approach invokes no intermediate approximations to general relativity and can strongly reject binaries whose radiation is inconsistent with the data, our analysis provides a valuable complement to LVC_PE[1].

Report number: LIGO P1500263

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arXiv:1606.04856 [gr-qc]

Binary Black Hole Mergers in the first Advanced LIGO Observing Run

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 15 Jun 2016 (v1), last revised 22 Jun 2016 (this version, v2))

Abstract: The first observational run of the Advanced LIGO detectors, from September 12, 2015 to January 19, 2016, saw the first detections of gravitational waves from binary black hole mergers. In this paper we present full results from a search for binary black hole merger signals with total masses up to 100M_⊙ and detailed implications from our observations of these systems. Our search, based on general-relativistic models of gravitational wave signals from binary black hole systems, unambiguously identified two signals, GW150914 and GW151226, with a significance of greater than 5σ over the observing period. It also identified a third possible signal, LVT151012, with substantially lower significance, which has a 87% probability of being of astrophysical origin. We provide detailed estimates of the parameters of the observed systems. Both GW150914 and GW151226 provide an unprecedented opportunity to study the two-body motion of a compact-object binary in the large velocity, highly nonlinear regime. We do not observe any deviations from general relativity, and place improved empirical bounds on several high-order post-Newtonian coefficients. From our observations we infer stellar-mass binary black hole merger rates lying in the range 9−240 Gpc^−3 yr^−1. These observations are beginning to inform astrophysical predictions of binary black hole formation rates, and indicate that future observing runs of the Advanced detector network will yield many more gravitational wave detections.

Report number: LIGO-P1600088

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