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AUSTRALASIAN SOCIETY FOR GENERAL RELATIVITY AND GRAVITATION

Electronic Newsletter -- #24, Summer 2019-20

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The ASGRG has a home web page at http://www.asgrg.org

Items for this newsletter should be emailed to the editor:

asgrg@hotmail.com

The deadline for the next issue is 30 November, 2020.

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CONTENTS:

* REPORT ON 9TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG9): Gingin, Western Australia, 27-30 November, 2017

* 10TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG10): Victoria University of Wellington, New Zealand, 10-13 December, 2019

* 11TH BIENNIAL GENERAL MEETING OF THE ASGRG, 10 December 2019

* MEMBERSHIP DETAILS ONLINE at

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

* FORTHCOMING MEETINGS

* BOOK NOTICE: =A1=B0General Relativity: An Introduction to Black Holes, Gravitational Waves, and Cosmology=A1=B1 by Michael J W Hall

* MEMBERS' ABSTRACTS at gr-qc, July 2017 - November 2019

* ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc, July 2017 - November 2019

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REPORT ON 9TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG9)

Gingin, Western Australia, 27-30 November, 2017

ACGRG9 was the ninth in the series of biennial conferences run by the ASGRG. The venue was the Gravity Discovery Centre in Gingin, about 67 km north of the centre of Perth.

The conference was opened by the Chancellor of the University of Western Australia, Rob French. Keynote talks were given by Tsvi Piran, George Smoot, Jun Luo and Matthew Bailes. In addition, there were two full sessions on Gravitational Wave (GW) Astronomy, two on General Relativity (GR) Theory, one on Space and Cosmology, one on GW Technology, one on Quantum Measurement and GR experiments, one on Multi-Messenger Astronomer, and joint sessions combining GW Astronomy and GW Technology, and GW Astronomy and GR Theory.

For the first time also, ASGRG hosted an Education and Outreach session for science teachers on 30 November, chaired by Eric Thrane. In conjunction with ASGRG9, Tsvi Piran gave a public lecture titled =A1=B0Neutron Stars, Gravitational Waves and the Origin of Gold=A1=B1 at the Gravity Discovery Centre on the evening of 28 November, and George Smoot gave a public lecture titled =A1=B0Gravitational Waves, Merging Black Holes and Merging Binary Neutron Stars=A1=B1 at the UWA University Club Auditorium on the afternoon of 30 November.

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10TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG10)

Victoria University of Wellington, New Zealand, 10-13 December, 2019

ACGRG10 is the tenth in a series of biennial conferences run by the ASGRG with the aim of bringing together researchers from around the world to discuss all aspects of General Relativity, Cosmology and Relativistic Astrophysics including theory and experiment. The programme will include both experimental and theoretical plenary sessions, with invited speakers Joerg Hennig (Otago), Volker Schlue (Melbourne), Maria Eugenia Gabach Clement (Cordoba), Karl Wette (ANU), Krzysztof Bolejko (Tasmania) and Robert Ward (ANU).

ACGRG10 will be hosted by the Victoria University of Wellington from 10 to 13 December, 2019, and is open to anyone with an interest in general relativity.

Local Organising Committee: Matt Visser, David Wiltshire, Joerg Frauendiener, Sebastian Schuster

Scientific Organising Committee: Joerg Frauendiener (Otago), Matt Visser (VUW), David Wiltshire (Canterbury), Susan Scott (ANU), Leo Brewin (Monash), Todd Oliynyk (Monash), Malcolm Anderson (Brunei), Eric Thrane (Monash), Karl Wette (ANU)

For further details, visit the conference website at https://sms.wgtn.ac.nz/Events/AotearoaACGRG10/

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11TH BIENNIAL GENERAL MEETING OF THE ASGRG

The 2019 Biennial General Meeting of the ASGRG will be held in conjunction with ACGRG10, at 5 pm on Tuesday 10 December 2019.

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

President =A8C Joerg Frauendiener

Treasurer - Todd Oliynyk

Secretary - Malcolm Anderson

Officer - Susan Scott

Officer - David Wiltshire

Co-Opted committee members: Eric Thrane, Karl Wette

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MEMBERSHIP DETAILS ONLINE:

ASGRG members are invited to renew their subscriptions by visiting the Membership web page at:

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

Membership is open to anyone interested in General Relativity. Post-graduate students and early career researchers are particularly encouraged to apply.

The annual subscription is A$40 (A$20 for students and retirees). Life membership is available for a one-off payment of A$250.

Members of the Australian Institute of Physics (AIP) are entitled to a 10% discount on all memberships.

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FORTHCOMING MEETINGS

December 10-13, 2019:
9^{th} International Conference on Gravitation and Cosmology (ICGC)

Indian Institute of Science Education and Research (IISER)

Mohali, India

http://14.139.227.202/web/icgc2019/index.html

December
15-20, 2019: 30^{th} Texas Symposium on Relativistic Astrophysics

University of Portsmouth

Portsmouth, United Kingdom

December 19-20, 2019: XII Black Holes Workshop

Guimaraes, Portugal

http://w3.math.uminho.pt/bhw9/12/bhw2019?name=3Dhome

January 13-24, 2020: Nordita Advanced Winter School on Theoretical Cosmology

Nordita

Stockholm, Sweden

https://agenda.albanova.se/conferenceDisplay.py?confId=3D6662

January 13-16, 2020: Gravitational Waves, Black Holes and Fundamental Physics

IFPU Miramare campus

Trieste, Italy

https://indico.sissa.it/event/37/

February 3-7, 2020: SIGRAV International School 2020

=A1=B0Gravity: General Relativity and Beyond.

Astrophysics, Cosmology and Gravitational Waves.=A1=B1

Vietri sul Mare, Italy

https://agenda.infn.it/event/19488/

February 10-14, 2020:
8^{th} Tux Workshop on Quantum Gravity

Tux Center

Tux, Austria

https://www.gravity.physik.fau.de/events/tux8//

February 20-22, 2020:
10^{th} Central European Relativity Seminar

Albert Einstein Institute

Potsdam-Golm, Germany

https://www.univie.ac.at/cers/cers10/

February 24-28, 2020:
56^{th} Karpacz Winter School in Theoretical Physics

=A1=B0Superfluidity and Transport for Multimessenger Physics of Compact Stars=A1=B1

Karpacz, Poland

https://indico.cern.ch/event/823088/overview

April 14-17, 2020: Research School on Random Geometry and Quantum Gravity

Centre International de Rencontres Mathematiques (CIRM)

Marseille, France

https://conferences.cirm-math.fr/2178.html

April 20-24, 2020:
ICRANET: The 4^{th} Zeldovich Meeting

Minsk, Belarus

http://www.icranet.org/index.php?option=3Dcom_content&task=3Dview&id=3D1254

April 27-28, 2020: BritGrav20

School of Mathematics and Statistics, University College Dublin

Dublin, Ireland

https://sites.google.com/view/britgrav20/home

May 11-14, 2020:
6^{th} International Conference on the Nature and Ontology of Spacetime

Hotel Laguna Garden

Albena, Bulgaria

http://www.minkowskiinstitute.org/conferences/2020/

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BOOK NOTICE

Michael J W Hall, who is an ASGRG member and emeritus professor in theoretical physics at the Australian National University, published in March 2018 a book titled =A1=B0General Relativity: An Introduction to Black Holes, Gravitational Waves, and Cosmology=A1=B1.

Publishers: Morgan and Claypool

138 pages, 9 chapters covering: Concepts in special relativity; Tensors in relativity; The equivalence principle and local inertial frames; The motion of freely falling particles in general relativity; The Schwarzschild metric and black holes; Tensors and geometry; Einstein=A1=AFs field equations; Solving the field equations: vacuum solutions; Solving the field equations: cosmological solutions. Plus 3 appendices.

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MEMBERS' ABSTRACTS at gr-qc, July 2017 - November 2019

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 214 papers listed here and in the LIGO section represent 1.79% of the 11987 papers posted or cross-linked to gr-qc between July 2017 and November 2019.

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arXiv:1712.09719 astro-ph.IM gr-qc

**Systematic calibration error requirements for gravitational-wave detectors via the Cram=A8=A6r-Rao bound**

** **

**Authors:** Evan
D. Hall, Craig
Cahillane, Kiwamu
Izumi, Rory
J. E. Smith, Rana
X Adhikari

Abstract: Gravitational-wave (GW) laser interferometers such as Advanced LIGO transduce spacetime strain into optical power fluctuation. Converting this optical power fluctuations back into an estimated spacetime strain requires a calibration process that accounts for both the interferometer's optomechanical response and the feedback control loop used to control the interferometer test masses. Systematic errors in the calibration parameters lead to systematic errors in the GW strain estimate, and hence to systematic errors in the astrophysical parameter estimates in a particular GW signal. In this work we examine this effect for a GW signal similar to GW150914, both for a low-power detector operation similar to the first and second Advanced LIGO observing runs and for a higher-power operation with detuned signal extraction. We set requirements on the accuracy of the calibration such that the astrophysical parameter estimation is limited by errors introduced by random detector noise, rather than calibration systematics. We also examine the impact of systematic calibration errors on the possible detection of a massive graviton.

Journal reference: CQG 36 205006 (2019)

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arXiv:1901.03885 astro-ph.IM gr-qc

**Exploring the sensitivity of gravitational wave detectors to neutron star physics**

** **

**Authors:** Denis
Martynov, Haixing
Miao, Huan
Yang, Francisco
Hernandez Vivanco, Eric
Thrane, Rory
Smith, Paul
Lasky, William
E. East, Rana
Adhikari, Andreas
Bauswein, Aidan
Brooks, Yanbei
Chen, Thomas
Corbitt, Thomas
Corbitt, Hartmut
Grote, Yuri
Levin, Chunnong
Zhao, Alberto
Vecchio

Abstract: The physics of neutron stars can be studied with gravitational waves emitted from coalescing binary systems. Tidal effects become significant during the last few orbits and can be visible in the gravitational-wave spectrum above 500 Hz. After the merger, the neutron star remnant oscillates at frequencies above 1 kHz and can collapse into a black hole. Gravitational-wave detectors with a sensitivity of ~10^{-24} strain/sqHz at 2-4 kHz can observe these oscillations from a source which is ~100 Mpc away . The current observatories, such as LIGO and Virgo, are limited by shot noise at high frequencies and have a sensitivity of > 2 * 10^{-23} strain/sqHz at 3 kHz. In this paper, we propose an optical configuration of gravitational-wave detectors which can be set up in present facilities using the current interferometer topology. This scheme has a potential to reach 7 * 10^{-25} strain/sqHz at 2.5 kHz without compromising the detector sensitivity to black hole binaries. We argue that the proposed instruments have a potential to detect similar amount of post-merger neutron star oscillations as the next generation detectors, such as Cosmic Explorer and Einstein Telescope. We also optimise the arm length of the future detectors for neutron star physics and find that the optimal arm length is ~20 km. These instruments have the potential to observe neutron star post-merger oscillations at a rate of ~30 events per year with a signal-to-noise ratio of 5 or more.

Journal reference: Phys. Rev. D 99, 102004 (2019)

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arXiv:1901.11400 astro-ph.IM gr-qc

**Apparatus to Measure Optical Scatter of Coatings Versus Annealing Temperature**

** **

**Authors:** Joshua
R. Smith, Rana
X Adhikari, Katerin
M. Aleman, Adrian
Avila-Alvarez, Garilynn
Billingsley, Amy
Gleckl, Jazlyn
Guerrero, Ashot
Markosyan, Steven
Penn, Juan
A. Rocha, Dakota
Rose, Robert
Wright

Abstract: Light scattered by amorphous thin-film optical coatings limits the sensitivity of interferometric gravitational-wave detectors. We describe an imaging scatterometer to assess the role that crystal growth during annealing plays in this scatter.

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arXiv:1905.02842 astro-ph.HE gr-qc

**Astrophysical science metrics for next-generation gravitational-wave detectors**

** **

**Authors:** Rana
X Adhikari, P
Ajith, Yanbei
Chen, James
A Clark, Vladimir
Dergachev, Nicolas
V Fotopoulos, Sarah
E. Gossan, Ilya
Mandel, Maria
Okounkova, Vivien
Raymond, Jocelyn
S Read

Abstract: The second generation of gravitational-wave detectors are being built and tuned all over the world. The detection of signals from binary black holes is beginning to fulfill the promise of gravitational-wave astronomy. In this work, we examine several possible configurations for third-generation laser interferometers in existing km-scale facilities. We propose a set of astrophysically motivated metrics to evaluate detector performance. We measure the impact of detector design choices against these metrics, providing a quantitative cost-benefit analyses of the resulting scientific payoffs.

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arXiv:1907.04833 astro-ph.IM gr-qc

**Cosmic Explorer: The U.S. Contribution to Gravitational-Wave Astronomy beyond LIGO**

**Authors:** David
Reitze, Rana
X Adhikari, Stefan
Ballmer, Barry
Barish, Lisa
Barsotti, GariLynn
Billingsley, Duncan
A. Brown, Yanbei
Chen, Dennis
Coyne, Robert
Eisenstein, Matthew
Evans, Peter
Fritschel, Evan
D. Hall, Albert
Lazzarini, Geoffrey
Lovelace, Jocelyn
Read, B.
S. Sathyaprakash, David
Shoemaker, Joshua
Smith, Calum
Torrie, Salvatore
Vitale, Rainer
Weiss, Christopher
Wipf, Michael
Zucker

Abstract: This white paper describes the research and development needed over the next decade to realize "Cosmic Explorer," the U.S. node of a future third-generation detector network that will be capable of observing and characterizing compact gravitational-wave sources to cosmological redshifts.

Journal reference: 2019 BAAS 51(7) 035

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arXiv:1908.06004 gr-qc

**Astrophysics and cosmology with a deci-hertz gravitational-wave detector: TianGO**

** **

**Authors:** Kevin
A. Kuns, Hang
Yu, Yanbei
Chen, Rana
X Adhikari

Abstract: We present the astrophysical science case for a space-based, deci-Hz gravitational-wave (GW) detector. We particularly highlight an ability in inferring a source's sky location, both when combined with a network of ground-based detectors to form a long triangulation baseline, and by itself for the early warning of merger events. Such an accurate location measurement is the key for using GW signals as standard sirens for constraining the Hubble constant. This kind of detector also opens up the possibility of testing type Ia supernovae progenitor hypotheses by constraining the merger rates of white dwarf binaries with both super- and sub-Chandrasekhar masses separately. We will discuss other scientific outcomes that can be delivered, including the precise determination of black hole spins, the constraint of structure formation in the early Universe, and the search for intermediate-mass black holes.

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arXiv:1802.02459 gr-qc

**Vortex Solution of the Gravitational Field Equation of a Twisted Skyrme Strings**

** **

**Authors:** Malcolm
Anderson, Miftachul
Hadi, Andri
Husein

Abstract: We construct non-linear sigma model plus Skyrme term (Skyrme model) with a twist in the gravitational field. To simplify the solution, first we examine non-linear sigma model without Skyrme term, in particular with a twist, which comprises a vortex solution with an added dependence on a twist term mkz, where z is the vertical coordinate. We find that vortex solution for non-linear sigma model with a twist is similar with vortex solution without a twist. The work is still progress.

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arXiv:1802.05129 gr-qc

**Non-Twisting and Twisting Solutions of the Einstein Field Equations of a Skyrmionic String**

** **

**Authors:** Malcolm
Anderson, Miftachul
Hadi, Andri
Husein

Abstract: We construct non-linear sigma
model plus Skyrme term (Skyrme model) with a twist in the gravitational field. We try to solve the Einstein field equations for small and large values of r,
with and without twist. We prove that no non-twisting or twisting solutions extending from r=3D0 to r=3D=A1=DE exist.
At last, we try to solve non-twisting and twisting solutions of the Einstein field equations with a finite radius. We find that there are no solutions with a finite radius that can satisfy the junction conditions at the boundary radius r=3Dr_{b},
where rb is a finite radius.

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arXiv:1907.12645 astro-ph.HE gr-qc

**GROWTH on S190425z: Searching thousands of square degrees to identify an optical or infrared counterpart to a binary neutron star merger with the Zwicky Transient Facility
and Palomar Gattini IR**

** **

**Authors:** Michael
W. Coughlin, Tom=A8=A2s
Ahumada, Shreya
Anand, Kishalay
De, Matthew
J. Hankins, Mansi
M. Kasliwal, Leo
P. Singer, Eric
C. Bellm, Igor
Andreoni, S.
Bradley Cenko, Jeff
Cooke, Christopher
M. Copperwheat, Alison
M. Dugas, Jacob
E. Jencson, Daniel
A. Perley, Po-Chieh
Yu, Varun
Bhalerao, Harsh
Kumar, Joshua
S. Bloom, G.
C. Anupama, Michael
C. B. Ashley, Ashot
Bagdasaryan, Rahul
Biswas, David
A. H. Buckley, Kevin
B. Burdge , et al. (54 additional authors not shown)

Abstract: The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg2 of the initial skymap over the next two nights, corresponding to 46\% integrated probability, ZTF system achieved a depth of =A1=D6\,21 mAB in g- and r-bands. Palomar Gattini-IR covered 2200 square degrees in J-band to a depth of 15.5\,mag, including 32\% integrated probability based on the initial sky map. The revised skymap issued the following day reduced these numbers to 21\% for the Zwicky Transient Facility and 19\% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient "alerts" over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical lightcurves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were ruled-out as supernovae. Palomar Gattini-IR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with single-detector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible.

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arXiv:1712.05192 gr-qc

**Epicyclic oscillations of charged particles in stationary solutions immersed in a magnetic field with application to the Kerr--Newman black hole**

** **

**Authors:** Mustapha
Azreg-Aïnou

Abstract: We consider a stationary metric immersed in a uniform magnetic field and determine general expressions for the epicyclic frequencies of charged particles. Applications to the Kerr--Newman black hole is reach of physical consequences and reveals some new effects among which the existence of radially and vertically stable circular orbits in the region enclosed by the event horizon and the so-called {\textquotedblleft innermost\textquotedblright} stable circular orbit in the plane of symmetry

Journal reference: Int. J. Mod. Phys. D28 (2019) 1950013

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arXiv:1807.01600 gr-qc

**Rotating normal and phantom Einstein--Maxwell--dilaton black holes: Geodesic analysis**

** **

**Authors:** Mustapha
Azreg-Aïnou, Sumarna
Haroon, Mubasher
Jamil, Muhammad
Rizwan

Abstract: Depending on five parameters, rotating counterparts of Einstein--Maxwell--dilaton black holes are derived. We discuss their physical and geometric properties and investigate their null and time-like geodesics including circular orbits. The Lense--Thirring effect is considered.

Journal reference: Int. J. Mod. Phys. D 28 (2019) 1950063

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arXiv:1807.01600 gr-qc

**Rotating normal and phantom Einstein--Maxwell--dilaton black holes: Geodesic analysis**

** **

**Authors:** Mustapha
Azreg-Aïnou, Sumarna
Haroon, Mubasher
Jamil, Muhammad
Rizwan

Abstract: Depending on five parameters, rotating counterparts of Einstein--Maxwell--dilaton black holes are derived. We discuss their physical and geometric properties and investigate their null and time-like geodesics including circular orbits. The Lense--Thirring effect is considered.

Journal reference: Class. Quantum Grav. 35 (2018) 235001

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arXiv:1907.01394 gr-qc

**Gyroscope precession frequency analysis of a five dimensional charged rotating Kaluza-Klein black hole**

** **

**Authors:** Mustapha
Azreg-Aïnou, Mubasher
Jamil, Kai
Lin

Abstract: In this paper, we study the spin precession frequency of a test gyroscope attached to a stationary observer in the five dimensional rotating Kaluza-Klein black hole (RKKBH). We derive the conditions under which the test gyroscope moves along a timelike trajectory in this geometry and the regions where the spin precession frequency diverges. The magnitude of the gyroscope precession frequency around KK black hole diverges at two spatial locations outside the event horizon. However in the static case, the behavior of the Lense Thirring frequency of a gyroscope around KK black hole is much like an ordinary Schwarzschild black hole. Since a rotating Kaluza-Klein black hole is a generalization of Kerr-Newman black hole, we present two mass-independent schemes to distinguish these two spacetimes.

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arXiv:1908.04995 gr-qc astro-ph.HE math-ph

**Cylindrically symmetric n-dimensional (un)charged
de Sitter and anti-de Sitter black holes in generic f(T) gravity**

** **

**Authors:** Mustapha
Azreg-Aïnou

Abstract: Given a generic function f(T) we construct in almost closed forms cylindrically symmetric n-dimensional uncharged and charged de Sitter and anti-de Sitter solutions (including black holes, wormholes and possibly other regular solutions) in f(T) gravity. Applications to some known models are considered.

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arXiv:1811.04495 gr-qc

**Energy-momentum tensor and metric near the Schwarzschild
sphere**

**Authors:** Valentina
Baccetti, Robert
B. Mann, Sebastian
Murk, Daniel
R. Terno

Abstract: Regularity of the horizon radius r_{g} of
a collapsing body constrains a limiting form of a spherically-symmetric energy-momentum tensor near it. Its non-zero limit belongs to one of four classes that are distinguished only by two signs. As a result, close to r_{g} the
geometry can always be described by either an ingoing or outgoing Vaidya metric with increasing or decreasing mass. If according to a distant outside observer the trapped regions form in finite time, then the Einstein equations imply violation of the null
energy condition. In this case the horizon radius and its rate of change determine the metric in its vicinity, and the hypersurface r=3Dr_{g}(t) is
timelike during both the expansion and contraction of the trapped region. We present the implications of these results for the firewall paradox and discuss arguments that the required violation of the null energy condition is incompatible with the standard
analysis of black hole evaporation.

Journal reference: Phys. Rev. D 99, 124014 (2019)

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arXiv:1812.07727 gr-qc

**Black hole evaporation and semiclassical thin shell collapse**

** **

**Authors:** Valentina
Baccetti, Sebastian
Murk, Daniel
R. Terno

Abstract: In case of spherical symmetry, the assumptions of finite-time formation of a trapped region and regularity of its boundary --- the apparent horizon --- are sufficient to identify the form of the metric and energy-momentum tensor in its vicinity. By comparison with the known results for quasistatic evaporation of black holes, we complete the identification of their parameters. Consistency of the Einstein equations allows only two possible types of higher-order terms in the energy-momentum tensor. By using its local conservation, we provide a method of calculation of the higher-order terms, explicitly determining the leading-order regular corrections. Contraction of a spherically symmetric thin dust shell is the simplest model of gravitational collapse. Nevertheless, the inclusion of a collapse-triggered radiation in different extensions of this model leads to apparent contradictions. Using our results, we resolve these contradictions and show how gravitational collapse may be completed in finite time according to a distant observer.

Journal reference: Phys. Rev. D 100, 064054 (2019)

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arXiv:1904.00506 gr-qc

**Trapped surfaces, energy conditions, and horizon avoidance in spherically-symmetric collapse**

** **

**Authors:** Valentina
Baccetti, Robert
B. Mann, Daniel
R. Terno

Abstract: We consider spherically-symmetric black holes in semiclassical gravity. For a collapsing radiating thin shell we derive a sufficient condition on the exterior geometry that ensures that a black hole is not formed. This is also a sufficient condition for an infalling test particle to avoid the apparent horizon of an existing black hole and approach it only within a certain minimal distance. Taking the presence of a trapped region and its outer boundary --- the apparent horizon--- as the defining feature of black holes, we explore the consequences of their finite time of formation according to a distant observer. Assuming regularity of the apparent horizon we obtain the limiting form of the metric and the energy-momentum tensor in its vicinity that violates the null energy condition (NEC). The metric does not satisfy the sufficient condition for horizon avoidance: a thin shell collapses to form a black hole and test particles (unless too slow) cross into it in finite time. However, there may be difficulty in maintaining the expected range of the NEC violation, and stability against perturbations is not assured.

arXiv:1808.09202 gr-qc

**Dynamical analysis of a first order theory of bulk viscosity**

** **

**Authors:** Giovanni
Acquaviva, Aroonkumar
Beesham

Abstract: We perform a global analysis of curved Friedmann-Robertson-Walker cosmologies in the presence of a viscous fluid. The fluid's bulk viscosity is governed by a first order theory recently proposed in [M. M. Disconzi, T. W. Kephart, and R. J. Scherrer, Phys. Rev. D 91, 043532 (2015)], and the analysis is carried out in a compactified parameter space with dimensionless coordinates. We provide stability properties, cosmological interpretation and thermodynamic features of the critical points.

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arXiv:1902.04800 gr-qc

**Emergence and expansion of cosmic space in an accelerating BIon**

** **

**Authors:** Aroonkumar
Beesham, Alireza
Sepehri

Abstract: We generalize the Padmanabhan [arXiv:hep-th/1206.4916] mechanism to an accelerating BIon and show that the difference between the number of degrees of freedom on the boundary surface and the number of degrees of freedom in a bulk region causes the accelerated expansion of a BIon. We also consider the evolution of a universe which emerges on this BIon, and obtain its Hubble parameter and energy density.

Journal reference: EPJC 78 (2018) 968

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arXiv:1902.07968 gr-qc

**Expansion and contraction of accretion disks of a rotating thermal BIon in a Rindler space-time**

** **

**Authors:** Aroonkumar
Beesham, Alireza
Sepehri

Abstract: In this paper, we consider the evolution of accretion disks of a rotating BIon in a Rindler space-time. This space-time emerges because of the acceleration of the disks in a BIon. A BIon is constructed from a pair of accretion disks that are connected by a wormhole. We will show that in a rotating BIon, by increasing the rotation velocity, the area of one accretion disk grows, while the area of the other shrinks. Also, we consider four types of accretion disks which are produced in a Rindler space-time.

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arXiv:1902.10732 hep-ph gr-qc

**Tachyonic =A6=C4-Tsallis entropy of a thermal tachyonic
BIon**

** **

**Authors:** Aroonkumar
Beesham, Alireza
Sepehri

Abstract: When a brane and an anti-brane come close to each other, the tachyonic potential between them increases and a tachyon wormhole is formed. This configuration, which consists of two branes and a tachyonic wormhole, is called a thermal tachyonic BIon. By considering the thermodynamic behaviour of this system, one finds that its entropy has the same form as that of the Tsallis one. By decreasing the separation between the branes, the tachyonic potential increases, and the entropy grows.

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arXiv:1903.08562 gr-qc

**Cosmological aspects of a hyperbolic solution in f(R,T) gravity**

** **

**Authors:** Ritika
Nagpal, J.
K. Singh, A.
Beesham, Hamid
Shabani

Abstract: This article deals with a cosmological scenario in f(R,T) gravity for a flat FLRW model of the universe. We consider the f(R,T) function as f(R)+f(T) which starts with a quadratic correction of the geometric term f(R) having structure f(R)=3DR+=A6=C1R2, and a linear matter term f(T)=3D2=A6=CBT. To achieve the solution of the gravitational field equations in the f(R,T) formalism, we take the form of a geometrical parameter, i.e. scale factor a(t)=3Dsinh1n(=A6=C2t) \cite{cha}, where =A6=C2 and n are model parameters. An eternal acceleration can be predicted by the model for 0<n<1, while the cosmic transition from the early decelerated phase to the present accelerated epoch can be anticipated for n=A1=DD1. The obtained model facilitate the formation of structure in the Universe according to the Jeans instability condition as our model transits from radiation dominated era to matter dominated era. We study the varying role of the equation of state parameter =A6=D8. We analyze our model by studying the behavior of the scalar field and discuss the energy conditions on our achieved solution. We examine the validity of our model via Jerk parameter, Om diagnostic, Velocity of sound and Statefinder diagnostic tools. We investigate the constraints on the model parameter n and H0 (Hubble constant) using some observational datasets: SNeIa dataset, H(z) (Hubble parameter) dataset, BAO (Baryon Acoustic Oscillation data) and their combinations as joint observational datasets H(z) + SNeIa and H(z) + SNeIa + BAO. It is testified that the present study is well consistent with these observations. We also perform some cosmological tests and a detailed discussion of the model.

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arXiv:1905.10801 gr-qc

**A Dark Energy Quintessence Model of the Universe**

** **

**Authors:** Gopi
Kant Goswami, Anirudh
Pradhan, Aroonkumar
Beesham

Abstract: In this paper, we have presented a model of the FLRW universe filled with matter and dark energy fluids, by assuming an ansatz that deceleration parameter is a linear function of the Hubble constant. This results in a time-dependent DP having decelerating-accelerating transition phase of the universe. This is a quintessence model =A6=D8(de)=A1=DD−1. The quintessence phase remains for the period (0=A1=DCz=A1=DC0.5806). The model is shown to satisfy current observational constraints. Various cosmological parameters relating to the history of the universe have been investigated.

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arXiv:1906.00450 gr-qc

**FLRW accelerating universe with interactive dark energy**

** **

**Authors:** G.
K. Goswami, Anirudh
Pradhan, A.
Beesham

Abstract: We have developed an accelerating cosmological model for the present universe which is phantom for the period (0=A1=DCz=A1=DC1.99) and quintessence phase for (1.99=A1=DCz=A1=DC2.0315). The universe is assumed to be filled with barotropic and dark energy(DE) perfect fluid in which DE interact with matter. For a deceleration parameter(DP) having decelerating-accelerating transition phase of universe, we assume hybrid expansion law for scale factor. The transition red shift for the model is obtained as zt=3D0.956. The model satisfies current observational constraints.

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arXiv:1907.02965 gr-qc

**FRW dark energy cosmological model with hybrid expansion law**

** **

**Authors:** G.
K. Goswami, Anirudh
Pradhan, Meena
Mishra, A.
Beesham

Abstract: In this work, we study a cosmological model of spatially homogeneous and isotropic accelerating universe which exhibits a transition from deceleration to acceleration. For this, Friedmann Robertson Walker(FRW) metric is taken and Hybrid expansion law a(t)=3Dt=A6=C1exp(=A6=C2t) is proposed and derived. We consider the universe to be filled with two types of fluids barotropic and dark energy which have variable equations of state. The evolution of dark energy, Hubble, and deceleration parameters etc., have been described in the form of tables and figures. We consider 581 data's of observed values of distance modulus of various SNe Ia type supernovae from union 2.1 compilation to compare our theoretical results with observations and found that model satisfies current observational constraints. We have also calculated the time and redshift at which acceleration in the Universe had commenced.

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arXiv:1707.01800 astro-ph.CO gr-qc

**Emergence of spatial curvature**

** **

**Authors:** Krzysztof
Bolejko

Abstract: This paper investigates the phenomenon of emergence of spatial curvature. This phenomenon is absent in the Standard Cosmological Model, which has a flat and fixed spatial curvature (small perturbations are considered in the Standard Cosmological Model but their global average vanishes, leading to spatial flatness at all times). This paper shows that with the nonlinear growth of cosmic structures the global average deviates from zero. The analysis is based on the {\em silent universes} (a wide class of inhomogeneous cosmological solutions of the Einstein equations). The initial conditions are set in the early universe as perturbations around the =A6=ABCDM model with =A6=B8m=3D0.31, =A6=B8=A6=AB=3D0.69, and H0=3D67.8 km s−1 Mpc−1. As the growth of structures becomes nonlinear, the model deviates from the =A6=ABCDM model, and at the present instant if averaged over a domain D with volume V=3D(2150Mpc)3 (at these scales the cosmic variance is negligibly small) gives: =A6=B8Dm=3D0.22, =A6=B8D=A6=AB=3D0.61, =A6=B8DR=3D0.15 (in the FLRW limit =A6=B8DR=A1=FA=A6=B8k), and ⟨H⟩D=3D72.2 km s−1 Mpc−1. Given the fact that low-redshift observations favor higher values of the Hubble constant and lower values of matter density, compared to the CMB constraints, the emergence of the spatial curvature in the low-redshift universe could be a possible solution to these discrepancies.

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arXiv:1708.09143 astro-ph.CO gr-qc

**Relativistic numerical cosmology with Silent Universes**

** **

**Authors:** Krzysztof
Bolejko

Abstract: Relativistic numerical cosmology is most often based either on the exact solutions of the Einstein equations, or perturbation theory, or weak-field limit, or the BSSN formalism. The Silent Universe provides an alternative approach to investigate relativistic evolution of cosmological systems. The silent universe is based on the solution of the Einstein equations in 1+3 comoving coordinates with additional constraints imposed. These constraints include: the gravitational field is sourced by dust and cosmological constant only, both rotation and magnetic part of the Weyl tensor vanish, and the shear is diagnosable. This paper describes the code simsilun (free software distributed under the terms of the reposi General Public License), which implements the equations of the Silent Universe. The paper also discusses applications of the Silent Universe and it uses the Millennium simulation to set up the initial conditions for the code simsilun. The simulation obtained this way consists of 16,777,216 worldlines, which are evolved from z=3D80 to z=3D0. Initially, the mean evolution (averaged over the whole domain) follows the evolution of the background =A6=ABCDM model. However, once the evolution of cosmic structures becomes nonlinear, the spatial curvature evolves from =A6=B8K=3D0 to =A6=B8K=A1=D60.1 at the present day. The emergence of the spatial curvature is associated with =A6=B8M and =A6=B8=A6=AB being smaller by approximately 0.05 compared to the =A6=ABCDM.

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arXiv:1712.02967 astro-ph.CO gr-qc

**Emerging spatial curvature can resolve the tension between high-redshift CMB and low-redshift distance ladder measurements of the Hubble constant**

** **

**Authors:** Krzysztof
Bolejko

Abstract: The measurements of the Hubble constant reveal a tension between high-redshift (CMB) and low-redshift (distance ladder) constraints. So far neither observational systematics nor new physics has been successfully implemented to explain this tension away. This paper present a new solution to the Hubble constant problem. The solution is based on the Simsilun simulation (relativistic simulation of the large scale structure of the Universe) with the ray-tracing algorithm implemented. The initial conditions for the Simsilun simulation were set up as perturbations around the =A6=ABCDM model. However, unlike in the Standard Cosmological Model (i.e. =A6=ABCDM model + perturbations), within the Simsilun simulation relativistic and nonlinear evolution of cosmic structures leads to the phenomenon of emerging spatial curvature, where the mean spatial curvature evolves from spatial flatness of the early universe towards slightly curved present-day universe. Consqeuently, the present-day expansion rate is slightly faster compared to the spatially flat =A6=ABCDM model. The results of the ray-tracing analysis show that the universe which starts with initial conditions consistent with the Planck constraints should have the Hubble constant H0=3D72.5=A1=C02.1 km s−1 Mpc−1. When the Simsilun simulation was re-run with no inhomogeneities imposed, the Hubble constant inferred within such a homogeneous simulation was H0=3D68.1=A1=C02.0 km s−1 Mpc−1. Thus, the inclusion of nonlinear relativistic evolution that leads to the emergence of the spatial curvature can explain why the low-redshift measurements favour higher values compared to high-redshift constraints and alleviate the tension between the CMB and distance ladder measurements of the Hubble constant.

Journal reference: Phys. Rev. D 97, 103529 (2018)

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**arXiv:1712.04041**** ****gr-qc**** astro-ph.CO**

**Gravitational entropy and the cosmological no-hair conjecture**

** **

**Authors:** Krzysztof
Bolejko

Abstract: The gravitational entropy and no-hair conjectures seems to predict contradictory future states of our Universe. The growth of the gravitational entropy is associated with the growth of inhomogeneity, while the no-hair conjecture argues that a universe dominated by dark energy should asymptotically approach a homogeneous and isotropic de Sitter state. The aim of this paper is to study these two conjectures. The investigation is based on the Simsilun simulation, which simulates the universe using the approximation of the Silent Universe. The Silent Universe is a solution to the Einstein equations that assumes irrotational, non-viscous, and insulated dust, with vanishing magnetic part of the Weyl curvature. The initial conditions for the Simsilun simulation are sourced from the Millennium simulation, which results with a realistically appearing but relativistic at origin simulation of a universe. The Simsilun simulation is evolved from the early universe (t =3D 25 Myr) till far future (t =3D 1000 Gyr). The results of this investigation show that both conjectures are correct. On global scales, a universe with a positive cosmological constant and non-positive spatial curvature does indeed approach the de Sitter state. At the same time it keeps generating the gravitational entropy.

Journal reference: Phys. Rev. D 97, 083515 (2018)

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arXiv:1907.04495 astro-ph.CO gr-qc

**Direct detection of the cosmic expansion: the redshift drift and the flux drift**

** **

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

Abstract: The redshift drift, the change of cosmological redshift with time, is a direct consequence of the expansion of the Universe. Thus the measurement of the cosmological redshift drift will offer a direct test of our models of cosmology. The magnitude of the effect is very small, i.e. the spectral shift is of order of 10−10−10−9 over the period of a decade, but the next generation facilities such as ELT and SKA will be able to directly detect the expansion of our Universe by the year 2040. In this paper we focus on detectebility of this effect, including strategies of overcoming the kinematic contamination of the cosmological signal. We also show that the redshift drift directly impacts the change of flux. Thus apart from the redshift drift, measurements of the flux drift will provide an additional tool of detecting the expansion of the universe, including its acceleration. We discuss the strategies of detecting the flux drift and show that by including the flux drift signal to the redshift drift signal we boost the chances of a direct detection of the expansion of the Universe. We show that if only the stability of flux is at the level of =A6=A4F/F∼10−6 then the SKA1-mid Array should be able to detect these effects, before the ETL and the full SKA. Thus, by including the flux drift into the SKA1-mid Array's analysis pipeline, we could be able to provide by mid-2030s a direct evidence of the expansion of the universe including its accelerating phase.

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arXiv:1908.01953 astro-ph.CO gr-qc

**Probing the independence within the dark sector in the fluid approximation**

** **

**Authors:** Lawrence
Dam, Krzysztof
Bolejko, Geraint
F. Lewis

Abstract: The standard model of cosmology is based on two unknown dark components that are uncoupled from each other. In this paper we investigate whether there is evidence for an interaction between these components of cold dark matter (CDM) and dark energy (DE). In particular, we focus on a minimal extension and reconstruct the interaction history at low-redshifts non-parametrically using a variation of the commonly used principal component analysis. Although we focus on the interaction in the dark sector, any significant deviation from the standard model that changes the expansion history of the Universe, should leave imprints detectable by our analysis. Thus, detecting signatures of interaction could also be indicative of other non-standard phenomena even if they are not the results of the interaction. It is thus interesting to note that the results presented in this paper do not provide support for the interaction in the dark sector, although the uncertainty is still quite large. In so far as interaction is present but undetectable using current data, we show from a Fisher forecast that forthcoming LSST and DESI surveys will be able to constrain a DM-DE coupling at 20% precision --- enough to falsify the non-interacting scenario, assuming the presence of a modest amount of interaction.

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arXiv:1707.06101 gr-qc

**Bounding the speed of gravity with gravitational wave observations**

** **

**Authors:** Neil
Cornish, Diego
Blas, Germano
Nardini

Abstract: The time delay between gravitational wave signals arriving at widely separated detectors can be used to place upper and lower bounds on the speed of gravitational wave propagation. Using a Bayesian approach that combines the first three gravitational wave detections reported by the LIGO collaboration we constrain the gravitational waves propagation speed c_gw to the 90% credible interval 0.55 c < c_gw < 1.42 c, where c is the speed of light in vacuum. These bounds will improve as more detections are made and as more detectors join the worldwide network. Of order twenty detections by the two LIGO detectors will constrain the speed of gravity to within 20% of the speed of light, while just five detections by the LIGO-Virgo-Kagra network will constrain the speed of gravity to within 1% of the speed of light.

Journal reference: Phys. Rev. Lett. 119, 161102 (2017)

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arXiv:1711.00040 gr-qc

**Inferring the post-merger gravitational wave emission from binary neutron star coalescences**

** **

**Authors:** Katerina
Chatziioannou, James
Alexander Clark, Andreas
Bauswein, Margaret
Millhouse, Tyson
B. Littenberg, Neil
Cornish

Abstract: We present a robust method to characterize the gravitational wave emission from the remnant of a neutron star coalescence. Our approach makes only minimal assumptions about the morphology of the signal and provides a full posterior probability distribution of the underlying waveform. We apply our method on simulated data from a network of advanced ground-based detectors and demonstrate the gravitational wave signal reconstruction. We study the reconstruction quality for different binary configurations and equations of state for the colliding neutron stars. We show how our method can be used to constrain the yet-uncertain equation of state of neutron star matter. The constraints on the equation of state we derive are complimentary to measurements of the tidal deformation of the colliding neutron stars during the late inspiral phase. In the case of a non-detection of a post-merger signal following a binary neutron star inspiral we show that we can place upper limits on the energy emitted.

Journal reference: Phys. Rev. D 96, 124035 (2017)

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arXiv:1712.07132 gr-qc

**Constraining alternative theories of gravity using pulsar timing arrays**

** **

**Authors:** Neil
J. Cornish, Logan
O'Beirne, Stephen
R. Taylor, Nicolas
Yunes

Abstract: The opening of the gravitational wave window by ground-based laser interferometers has made possible many new tests of gravity, including the first constraints on polarization. It is hoped that within the next decade pulsar timing will extend the window by making the first detections in the nano-Hertz frequency regime. Pulsar timing offers several advantages over ground-based interferometers for constraining the polarization of gravitational waves due to the many projections of the polarization pattern provided by the different lines of sight to the pulsars, and the enhanced response to longitudinal polarizations. Here we show that existing results from pulsar timing arrays can be used to place stringent limits on the energy density of longitudinal stochastic gravitational waves. Paradoxically however, we find that longitudinal modes will be very difficult to detect due to the large variance in the pulsar-pulsar correlation patterns for these modes. Existing upper limits on the power spectrum of pulsar timing residuals imply that the amplitude of vector longitudinal and scalar longitudinal modes at frequencies of 1/year are constrained: AVL<4.1=A1=C110−16 and ASL<3.7=A1=C110−17, while the bounds on the energy density for a scale invariant cosmological background are: =A6=B8VLh2<3.5=A1=C110−11 and =A6=B8SLh2<3.2=A1=C110−13.

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arXiv:1801.02617 astro-ph.HE gr-qc

**The NANOGrav 11-year Data Set: Pulsar-timing Constraints On The Stochastic Gravitational-wave Background**

** **

**Authors:** Z.
Arzoumanian, P.
T. Baker, A.
Brazier, S.
Burke-Spolaor, S.
J. Chamberlin, S.
Chatterjee, B.
Christy, J.
M. Cordes, N.
J. Cornish, F.
Crawford, H.
Thankful Cromartie, K.
Crowter, M.
DeCesar, P.
B. Demorest, T.
Dolch, J.
A. Ellis, R.
D. Ferdman, E.
Ferrara, W.
M. Folkner, E.
Fonseca, N.
Garver-Daniels, P.
A. Gentile, R.
Haas, J.
S. Hazboun, E.
A. Huerta , et al. (35 additional authors not shown)

Abstract: We search for an isotropic stochastic gravitational-wave background (GWB) in the newly released 11-year dataset from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). While we find no significant evidence for a GWB, we place constraints on a GWB from a population of supermassive black-hole binaries, cosmic strings, and a primordial GWB. For the first time, we find that the GWB upper limits and detection statistics are sensitive to the Solar System ephemeris (SSE) model used, and that SSE errors can mimic a GWB signal. We developed an approach that bridges systematic SSE differences, producing the first PTA constraints that are robust against SSE uncertainties. We thus place a 95% upper limit on the GW strain amplitude of AGWB<1.45=A1=C110−15 at a frequency of f=3D1 yr−1 for a fiducial f−2/3 power-law spectrum, and with inter-pulsar correlations modeled. This is a factor of ∼2 improvement over the NANOGrav 9-year limit, calculated using the same procedure. Previous PTA upper limits on the GWB will need revision in light of SSE systematic uncertainties. We use our constraints to characterize the combined influence on the GWB of the stellar mass-density in galactic cores, the eccentricity of SMBH binaries, and SMBH--galactic-bulge scaling relationships. We constrain cosmic-string tension using recent simulations, yielding an SSE-marginalized 95% upper limit on the cosmic string tension of G=A6=CC<5.3=A1=C110−11---a factor of ∼2 better than the published NANOGrav 9-year constraints. Our SSE-marginalized 95% upper limit on the energy density of a primordial GWB (for a radiation-dominated post-inflation Universe) is =A6=B8GWB(f)h2<3.4=A1=C110−10.

Journal reference: The Astrophysical Journal, Volume 859, Number 1, 2018

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arXiv:1801.09009 gr-qc

**Nature Abhors a Circle**

** **

**Authors:** Nicholas
Loutrel, Samuel
Liebersbach, Nicolas
Yunes, Neil
Cornish

Abstract: The loss of orbital energy and angular momentum to gravitational waves produced in a binary inspiral forces the orbital eccentricity to adiabatically evolve and oscillate. For comparable-mass binaries, the osculating eccentricity is thought to decrease monotonically in the inspiral. Contrary to this, we here show that, once the osculating eccentricity is small enough, radiation reaction forces it to grow secularly before the binary reaches the last stable orbit. We explore this behavior, its physical interpretation and consequences, and its potential impact on future gravitational wave observations.

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arXiv:1803.01944 astro-ph.HE gr-qc

**The construction and use of LISA sensitivity curves**

** **

**Authors:** Travis
Robson, Neil
Cornish, Chang
Liu

Abstract: The Laser Interferometer Space Antenna (LISA) will open the mHz band of the gravitational wave spectrum for exploration. Sensitivity curves are a useful tool for surveying the types of sources that can be detected by the LISA mission. Here we describe how the sensitivity curve is constructed, and how it can be used to compute the signal-to-noise ratio for a wide range of binary systems. We adopt the 2018 LISA Phase-0 reference design parameters. We consider both sky-averaged sensitivities, and the sensitivity to sources at particular sky locations. The calculations are included in a publicly available {\em Python} notebook.

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arXiv:1804.03146 gr-qc

**Constraining the Polarization Content of Gravitational Waves with Astrometry**

** **

**Authors:** Logan
O'Beirne, Neil
J. Cornish

Abstract: Gravitational waves perturb the paths of photons, impacting both the time-of-flight and the arrival direction of light from stars. Pulsar timing arrays can detect gravitational waves by measuring the variations in the time of flight of radio pulses, while astrometry missions such as Gaia can detect gravitational waves from the time-varying changes in the apparent position of a field of stars. Just as gravitational waves impart a characteristic correlation pattern in the arrival times of pulses from pulsars at different sky locations, the deflection of starlight is similarly correlated across the sky. Here we compute the astrometric correlation patterns for the full range of polarization states found in alternative theories of gravity, and decompose the sky-averaged correlation patterns into vector spherical harmonics. We find that the tensor and vector polarization states produce equal power in the electric- and magnetic-type vector spherical harmonics, while the scalar modes produce only electric-type correlations. Any difference in the measured electric and magnetic-type correlations would represent a clear violation of Einstein gravity. The angular correlations functions for the vector and scalar longitudinal modes show the same enhanced response at small angular separations that is familiar from pulsar timing.

Journal reference: Phys. Rev. D 98, 024020 (2018)

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arXiv:1804.03239 gr-qc

**Bayesian reconstruction of gravitational wave bursts using chirplets**

** **

**Authors:** Margaret
Millhouse, Neil
J. Cornish, Tyson
Littenberg

Abstract: The LIGO-Virgo collaboration uses a variety of techniques to detect and characterize gravitational waves. One approach is to use templates - models for the signals derived from Einstein's equations. Another approach is to extract the signals directly from the coherent response of the detectors in LIGO-Virgo network. Both approaches played an important role in the first gravitational wave detections. Here we extend the BayesWave analysis algorithm, which reconstructs gravitational wave signals using a collection of continuous wavelets, to use a generalized wavelet family, known as chirplets, that have time-evolving frequency content. Since generic gravitational wave signals have frequency content that evolves in time, a collection of chirplets provides a more compact representation of the signal, resulting in more accurate waveform reconstructions, especially for low signal-to-noise events, and events that occupy a large time-frequency volume.

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arXiv:1806.00500 gr-qc

**Detecting hierarchical stellar systems with LISA**

** **

**Authors:** Travis
Robson, Neil
J. Cornish, Nicola
Tamanini, Silvia
Toonen

Abstract: A significant fraction of stars are members of gravitationally bound hierarchies containing three or more components. Almost all low mass stars in binaries with periods shorter three days are part of a hierarchical system. We therefore anticipate that a large fraction of compact galactic binaries detected by the Laser Interferometer Space Antenna (LISA) will be members of hierarchical triple or quadruple system. The acceleration imparted by the hierarchical companions can be detected in the gravitational wave signal for outer periods as large as 100 years. For systems with periods that are shorter than, or comparable to, the mission lifetime, it will be possible to measure the period and eccentricity of the outer orbit. LISA observations of hierarchical stellar systems will provide insight into stellar evolution, including the role that Kozai-Lidov oscillations play in driving systems towards merger.

Journal reference: Phys. Rev. D 98, 064012 (2018)

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arXiv:1808.03619 gr-qc

**Mitigation of the instrumental noise transient in gravitational-wave data surrounding GW170817**

** **

**Authors:** Chris
Pankow, Katerina
Chatziioannou, Eve
A. Chase, Tyson
B. Littenberg, Matthew
Evans, Jessica
McIver, Neil
J. Cornish, Carl-Johan
Haster, Jonah
Kanner, Vivien
Raymond, Salvatore
Vitale, Aaron
Zimmerman

Abstract: In the coming years gravitational-wave detectors will undergo a series of improvements, with an increase in their detection rate by about an order of magnitude. Routine detections of gravitational-wave signals promote novel astrophysical and fundamental theory studies, while simultaneously leading to an increase in the number of detections temporally overlapping with instrumentally- or environmentally-induced transients in the detectors (glitches), often of unknown origin. Indeed, this was the case for the very first detection by the LIGO and Virgo detectors of a gravitational-wave signal consistent with a binary neutron star coalescence, GW170817. A loud glitch in the LIGO-Livingston detector, about one second before the merger, hampered coincident detection (which was initially achieved solely with LIGO-Hanford data). Moreover, accurate source characterization depends on specific assumptions about the behavior of the detector noise that are rendered invalid by the presence of glitches. In this paper, we present the various techniques employed for the initial mitigation of the glitch to perform source characterization of GW170817 and study advantages and disadvantages of each mitigation method. We show that, despite the presence of instrumental noise transients louder than the one affecting GW170817, we are still able to produce unbiased measurements of the intrinsic parameters from simulated injections with properties similar to GW170817.

Journal reference: Phys. Rev. D 98, 084016 (2018)

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arXiv:1810.03521 gr-qc

**The Eccentric Behavior of Inspiraling Compact Binaries**

** **

**Authors:** Nicholas
Loutrel, Samuel
Liebersbach, Nicolas
Yunes, Neil
Cornish

Abstract: Eccentricity of binary systems is not a gauge invariant quantity, but has an important impact on the observed gravitational wave signal of such systems, generating power in all possible harmonics of the orbital period. We here clarify the possible discrepancies between different eccentricity parameters used to describe the orbital dynamics of binary systems across different approximations, specifically the post-Newtonian approximation, the self-force approximation, and numerical relativity. To this end, we highlight disparities between the typically used orbit averaged method of evolving binary systems under radiation reaction, and more direct techniques of solving the two-body problem in post-Newtonian theory. We show, both numerically and analytically, that the orbit averaged method breaks down in the late inspiral, failing to capture a strong secular growth in the Keplerian eccentricity parameter and producing a orbital de-phasing relative to direct integration of the two-body equations of motion. We show that the secular growth and de-phasing affect the observed gravitational wave signal, which could bias how accurately we may recover parameters for systems with signal-to-noise ratios ≳100. We further develop a frequency domain post-adiabatic waveform model to capture these effects, and study the precision to which we may estimate parameters with this model through a Fisher information matrix analysis.

Journal reference: Classical & Quantum Gravity, Volume 36, Number 2, 2018

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arXiv:1811.04490 gr-qc

**Detecting Gravitational Wave Bursts with LISA in the presence of Instrumental Glitches**

**Authors:** Travis
Robson, Neil
J. Cornish

Abstract: The Laser Interferometer Space Antenna (LISA) will open a rich discovery space in the milli-Hertz gravitational wave band. In addition to the anticipated signals from many millions of binary systems, this band may contain new and previously un-imagined sources for which we currently have no models. To detect unmodeled and unexpected signals we need to be able to separate them from instrumental noise artifacts, or glitches. Glitches are a regular feature in the data from ground based laser interferometers, and they were also seen in data from the LISA Pathfinder mission. In contrast to the situation on ground, we will not have the luxury of having multiple independent detectors to help separate unmodeled signals from glitches, and new techniques have to be developed. Here we show that unmodeled gravitational wave bursts can be detected with LISA by leveraging the different way in which instrument glitches and gravitational wave bursts imprint themselves in the time-delay interferometery data channels. We show that for signals with periods longer than the light travel time between the spacecraft, the "breathing mode" or Sagnac data combination is key to detection. Conversely, for short period signals it is the time of arrival at each spacecraft that aids separation. We investigate the conditions under which we can distinguish the origin of signals and glitches consisting of a single sine-Gaussian wavelet and determine how well we can characterize the signal. We find that gravitational waves bursts can be unambiguously detected and characterized with just a single data channel (four functioning laser links), though the signal separation and parameter estimation improve significantly when all six laser links are operational.

Journal reference: Phys. Rev. D 99, 024019 (2019)

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arXiv:1812.11585 astro-ph.GA gr-qc

**The NANOGrav 11-Year Data Set: Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries**

** **

**Authors:** K.
Aggarwal, Z.
Arzoumanian, P.
T. Baker, A.
Brazier, M.
R. Brinson, P.
R. Brook, S.
Burke-Spolaor, S.
Chatterjee, J.
M. Cordes, N.
J. Cornish, F.
Crawford, K.
Crowter, H.
T. Cromartie, M.
DeCesar, P.
B. Demorest, T.
Dolch, J.
A. Ellis, R.
D. Ferdman, E.
Ferrara, E.
Fonseca, N.
Garver-Daniels, P.
Gentile, J.
S. Hazboun, A.
M. Holgado, E.
A. Huerta , et al. (38 additional authors not shown)

Abstract: Observations indicate that nearly all galaxies contain supermassive black holes (SMBHs) at their centers. When galaxies merge, their component black holes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves (GWs) that can be detected by pulsar timing arrays (PTAs). We have searched the recently-released North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11-year data set for GWs from individual SMBHBs in circular orbits. As we did not find strong evidence for GWs in our data, we placed 95\% upper limits on the strength of GWs from such sources as a function of GW frequency and sky location. We placed a sky-averaged upper limit on the GW strain of h0<7.3(3)=A1=C110−15 at fgw=3D8 nHz. We also developed a technique to determine the significance of a particular signal in each pulsar using ``dropout' parameters as a way of identifying spurious signals in measurements from individual pulsars. We used our upper limits on the GW strain to place lower limits on the distances to individual SMBHBs. At the most-sensitive sky location, we ruled out SMBHBs emitting GWs with fgw=3D8 nHz within 120 Mpc for M=3D109M=A1=D1, and within 5.5 Gpc for M=3D1010M=A1=D1. We also determined that there are no SMBHBs with M>1.6=A1=C1109M=A1=D1 emitting GWs in the Virgo Cluster. Finally, we estimated the number of potentially detectable sources given our current strain upper limits based on galaxies in Two Micron All-Sky Survey (2MASS) and merger rates from the Illustris cosmological simulation project. Only 34 out of 75,000 realizations of the local Universe contained a detectable source, from which we concluded it was unsurprising that we did not detect any individual sources given our current sensitivity to GWs.

Journal reference: Astrophys. J. 880, 2 (2019)

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arXiv:1904.01438 astro-ph.HE gr-qc

**The Discovery Potential of Space-Based Gravitational Wave Astronomy**

** **

**Authors:** Neil
J. Cornish, Emanuele
Berti, Kelly
Holley-Bockelmann, Shane
Larson, Sean
McWilliams, Guido
Mueller, Priya
Natarajan, Michele
Vallisneri

Abstract: A space-based interferometer operating in the previously unexplored mHz gravitational band has tremendous discovery potential. If history is any guide, the opening of a new spectral band will lead to the discovery of entirely new sources and phenomena. The mHz band is ideally suited to exploring beyond standard model processes in the early universe, and with the sensitivities that can be reached with current technologies, the discovery space for exotic astrophysical systems is vast.

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arXiv:1904.02744 gr-qc

**Constraining alternative polarization states of gravitational waves from individual black hole binaries using pulsar timing arrays**

** **

**Authors:** Logan
O'Beirne, Neil
J. Cornish, Sarah
J. Vigeland, Stephen
R. Taylor

Abstract: Pulsar timing arrays are sensitive to gravitational wave perturbations produced by individual supermassive black hole binaries during their early inspiral phase. Modified gravity theories allow for the emission of gravitational dipole radiation, which is enhanced relative to the quadrupole contribution for low orbital velocities, making the early inspiral an ideal regime to test for the presence of modified gravity effects. Using a theory-agnostic description of modified gravity theories based on the parametrized post-Einsteinian framework, we explore the possibility of detecting deviations from General Relativity using simulated pulsar timing array data, and provide forecasts for the constraints that can be achieved. We generalize the {\tt enterprise} pulsar timing software to account for possible additional polarization states and modifications to the phase evolution, and study how accurately the parameters of simulated signals can be recovered. We find that while a pure dipole model can partially recover a pure quadrupole signal, there is little possibility for confusion when the full model with all polarization states is used. With no signal present, and using noise levels comparable to those seen in contemporary arrays, we produce forecasts for the upper limits that can be placed on the amplitudes of alternative polarization modes as a function of the sky location of the source.

Journal reference: Phys. Rev. D 99, 124039 (2019)

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arXiv:1907.06482 astro-ph.IM**
**gr-qc

**The Laser Interferometer Space Antenna: Unveiling the Millihertz Gravitational Wave Sky**

** **

**Authors:** John
Baker, Jillian
Bellovary, Peter
L. Bender, Emanuele
Berti, Robert
Caldwell, Jordan
Camp, John
W. Conklin, Neil
Cornish, Curt
Cutler, Ryan
DeRosa, Michael
Eracleous, Elizabeth
C. Ferrara, Samuel
Francis, Martin
Hewitson, Kelly
Holley-Bockelmann, Ann
Hornschemeier, Craig
Hogan, Brittany
Kamai, Bernard
J. Kelly, Joey
Shapiro Key, Shane
L. Larson, Jeff
Livas, Sridhar
Manthripragada, Kirk
McKenzie, Sean
T. McWilliams , et al. (17 additional authors not shown)

Abstract: The first terrestrial gravitational wave interferometers have dramatically underscored the scientific value of observing the Universe through an entirely different window, and of folding this new channel of information with traditional astronomical data for a multimessenger view. The Laser Interferometer Space Antenna (LISA) will broaden the reach of gravitational wave astronomy by conducting the first survey of the millihertz gravitational wave sky, detecting tens of thousands of individual astrophysical sources ranging from white-dwarf binaries in our own galaxy to mergers of massive black holes at redshifts extending beyond the epoch of reionization. These observations will inform - and transform - our understanding of the end state of stellar evolution, massive black hole birth, and the co-evolution of galaxies and black holes through cosmic time. LISA also has the potential to detect gravitational wave emission from elusive astrophysical sources such as intermediate-mass black holes as well as exotic cosmological sources such as inflationary fields and cosmic string cusps.

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arXiv:1907.06540 gr-qc

**Noise spectral estimation methods and their impact on gravitational wave measurement of compact binary mergers**

** **

**Authors:** Katerina
Chatziioannou, Carl-Johan
Haster, Tyson
B. Littenberg, Will
M. Farr, Sudarshan
Ghonge, Margaret
Millhouse, James
A. Clark, Neil
Cornish

Abstract: Estimating the parameters of gravitational wave signals detected by ground-based detectors requires an understanding of the properties of the detectors' noise. In particular, the most commonly used likelihood function for gravitational wave data analysis assumes that the noise is Gaussian, stationary, and of known frequency-dependent variance. The variance of the colored Gaussian noise is used as a whitening filter on the data before computation of the likelihood function. In practice the noise variance is not known and it evolves over timescales of dozens of seconds to minutes. We study two methods for estimating this whitening filter for ground-based gravitational wave detectors with the goal of performing parameter estimation studies. The first method uses large amounts of data separated from the specific segment we wish to analyze and computes the power spectral density of the noise through the mean-median Welch method. The second method uses the same data segment as the parameter estimation analysis, which potentially includes a gravitational wave signal, and obtains the whitening filter through a fit of the power spectrum of the data in terms of a sum of splines and Lorentzians. We compare these two methods and argue that the latter is more reliable for gravitational wave parameter estimation.

Journal reference: Phys. Rev. D 100, 104004 (2019)

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arXiv:1909.08644 astro-ph.HE gr-qc

**The NANOGrav 11-Year Data Set: Evolution of Gravitational Wave Background Statistics**

** **

**Authors:** J.
S. Hazboun, J.
Simon, S.
R. Taylor, M.
T. Lam, S.
J. Vigeland, K.
Islo, J.
S. Key, Z.
Arzoumanian, P.
T. Baker, A.
Brazier, P.
R. Brook, S.
Burke-Spolaor, S.
Chatterjee, J.
M. Cordes, N.
J. Cornish, F.
Crawford, K.
Crowter, H.
T. Cromartie, M.
DeCesar, P.
B. Demorest, T.
Dolch, J.
A. Ellis, R.
D. Ferdman, E.
Ferrara, E.
Fonseca , et al. (38 additional authors not shown)

Abstract: An ensemble of inspiraling supermassive black hole binaries should produce a stochastic background of very low frequency gravitational waves. This stochastic background is predicted to be a power law, with a spectral index of -2/3, and it should be detectable by a network of precisely timed millisecond pulsars, widely distributed on the sky. This paper reports a new "time slicing" analysis of the 11-year data release from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) using 34 millisecond pulsars. Methods to flag potential "false positive" signatures are developed, including techniques to identify responsible pulsars. Mitigation strategies are then presented. We demonstrate how an incorrect noise model can lead to spurious signals, and show how independently modeling noise across 30 Fourier components, spanning NANOGrav's frequency range, effectively diagnoses and absorbs the excess power in gravitational-wave searches. This results in a nominal, and expected, progression of our gravitational-wave statistics. Additionally we show that the first interstellar medium event in PSR J1713+0747 pollutes the common red noise process with low-spectral index noise, and use a tailored noise model to remove these effects.

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arXiv:1911.08488 astro-ph.HE gr-qc

**The NANOGrav 11-Year Data Set: Limits on Gravitational Wave Memory**

** **

**Authors:** K.
Aggarwal, Z.
Arzoumanian, P.
T. Baker, A.
Brazier, P.
R. Brook, S.
Burke-Spolaor, S.
Chatterjee, J.
M. Cordes, N.
J. Cornish, F.
Crawford, H.
T. Cromartie, K.
Crowter, M.
Decesar, P.
B. Demorest, T.
Dolch, J.
A. Ellis, R.
D. Ferdman, E.
Ferrara, P.
Gentile, D.
Good, J.
S. Hazboun, A.
M. Holgado, E.
A. Huerta, K.
Islo, R.
Jennings , et al. (34 additional authors not shown)

Abstract: The mergers of supermassive black hole binaries (SMBHB) promise to be incredible sources of gravitational waves (GW). While the oscillatory part of the merger gravitational waveform will be outside the frequency sensitivity range of pulsar timing arrays (PTA), the non-oscillatory GW memory effect is detectable. Further, any burst of gravitational waves will produce GW memory, making memory a useful probe of unmodeled exotic sources and new physics. We searched the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 11-year data set for GW memory. Finding no evidence for GWs, we placed limits on the strain amplitude of GW memory events during the observation period. We then used the strain upper limits to place limits on the rate of GW memory causing events. At a strain of 2.5=A1=C110−14, corresponding to the median upper limit as a function of source sky position, we set a limit on the rate of GW memory events at <0.4 yr−1. That strain corresponds to a SMBHB merger with reduced mass of =A6=C7M∼2=A1=C11010M=A1=D1 at a distance of 1 Gpc. As a test of our analysis, we analyzed the NANOGrav 9-year data set as well. This analysis found an anomolous signal, which does not appear in the 11-year data set. This signal is not a GW, and its origin remains unknown.

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arXiv:1903.07123 gr-qc

**Reflections on the Energy of Black Holes**

** **

**Authors:** Tevian
Dray, Carlo
Rovelli

Abstract: Inside a black hole, there is no local way to say which side of a sphere is the inside, and which is the outside. One can easily be gulled by this fact into mixing up the sign of the energy. We lead the reader astray with a naïve treatment of the energy of a null shell in black hole spacetimes. We then resolve the confusion, showing that global, rather than local, considerations offer good guidance.

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arXiv:1903.04700 astro-ph.CO gr-qc

**Probing the origin of our Universe through cosmic microwave background constraints on gravitational waves**

** **

**Authors:** Sarah
Shandera, Peter
Adshead, Mustafa
Amin, Emanuela
Dimastrogiovanni, Cora
Dvorkin, Richard
Easther, Matteo
Fasiello, Raphael
Flauger, John
T. Giblin Jr, Shaul
Hanany, Lloyd
Knox, Eugene
Lim, Liam
McAllister, Joel
Meyers, Marco
Peloso, Graca
Rocha, Maresuke
Shiraishi, Lorenzo
Sorbo, Scott
Watson

Abstract: The next generation of instruments designed to measure the polarization of the cosmic microwave background (CMB) will provide a historic opportunity to open the gravitational wave window to the primordial Universe. Through high sensitivity searches for primordial gravitational waves, and tighter limits on the energy released in processes like phase transitions, the CMB polarization data of the next decade has the potential to transform our understanding of the laws of physics underlying the formation of the Universe.

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arXiv:1804.03818 gr-qc

**Polytropic Inspired Inflation on the Brane**

** **

**Authors:** Mohammad
Reza Setare, Arvin
Ravanpak, Hossein
Farajollahi

Abstract: The brane inflationary model inspired by a polytropic inflationary idea is studied. In the slow-roll approximation and high energy limit, for a chaotic potential, the model is developed, and its characteristics are discussed. We obtain explicit expressions for the scalar power spectrum, the tensor-scalar ratio, the scalar spectral index and its running in terms of the polytropic parameters. We find a new constraint on the energy scale of the inflation and the brane tension using the WMAP9 data.

Journal reference: Gravitation and Cosmology, volume 24, issue 1, pp 52-56 (2018)

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arXiv:1709.09792 gr-qc

**Fully pseudospectral solution of the conformally invariant wave equation near the cylinder at spacelike infinity. III: Nonspherical Schwarzschild waves and singularities
at null infinity**

** **

**Authors:** Jörg
Frauendiener, Jörg
Hennig

Abstract: We extend earlier numerical and analytical considerations of the conformally invariant wave equation on a Schwarzschild background from the case of spherically symmetric solutions, discussed in Class. Quantum Grav. 34, 045005 (2017), to the case of general, nonsymmetric solutions. A key element of our approach is the modern standard representation of spacelike infinity as a cylinder. With a decomposition into spherical harmonics, we reduce the four-dimensional wave equation to a family of two-dimensional equations. These equations can be used to study the behaviour at the cylinder, where the solutions turn out to have logarithmic singularities at infinitely many orders. We derive regularity conditions that may be imposed on the initial data, in order to avoid the first singular terms. We then demonstrate that the fully pseudospectral time evolution scheme can be applied to this problem leading to a highly accurate numerical reconstruction of the nonsymmetric solutions. We are particularly interested in the behaviour of the solutions at future null infinity, and we numerically show that the singularities spread from the cylinder to null infinity. The observed numerical behaviour is consistent with similar logarithmic singularities found analytically on the cylinder. Finally, we demonstrate that even solutions with singularities at low orders can be obtained with high accuracy by virtue of a coordinate transformation that converts functions with logarithmic singularities into smooth solutions.

Journal reference: Class. Quantum Grav. 35, 065015 (2018)

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arXiv:1808.07914 gr-qc

**Notes on the Sagnac effect in General Relativity**

** **

**Authors:** Joerg
Frauendiener

Abstract: The Sagnac effect can be described as the difference in travel time between two photons traveling along the same path in opposite directions. In this paper we explore the consequences of this characterisation in the context of General Relativity. We derive a general expression for this time difference in an arbitrary space-time for arbitrary paths. In general, this formula is not very useful since it involves solving a differential equation along the path. However, we also present special cases where a closed form expression for the time difference can be given. The main part of the paper deals with the discussion of the effect in a small neighbourhood of an arbitrarily moving observer in their arbitrarily rotating reference frame. We also discuss the special case of stationary space-times and point out the relationship between the Sagnac effect and Fizeau's "aether-drag" experiment.

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arXiv:1808.08653 gr-qc

**Gravitational waves and the Sagnac effect**

** **

**Authors:** Jörg
Frauendiener

Abstract: Light propagating in opposite directions around the same loop in general shows a relative phase shift when recombined. This phenomenon is known as the Sagnac effect after Georges Sagnac who, in 1913, demonstrated with an interferometer on a rotating table that the phase shift depended on the angular velocity of the table. In previous work we have given a very general formula for the Sagnac effect, valid in full general relativity. The relativistic effect not only contains the `classical' contribution from the rotation of the laboratory but also contributions due its acceleration and due to incoming gravitational waves. Here, we point out a major consequence of this gravitational effect which may have implications for third generation gravitational wave detectors. We describe an `antenna' design which picks out specific components of the Weyl tensor describing the incident gravitational waves.

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arXiv:1902.05670 gr-qc

**Numerical initial data deformation exploiting a gluing construction: I. Exterior asymptotic Schwarzschild**

** **

**Authors:** Boris
Daszuta, Jörg
Frauendiener

Abstract: In this work a new numerical technique to prepare Cauchy data for the initial value problem (IVP) formulation of Einstein's field equations is presented. Directly inspired by the exterior asymptotic gluing (EAG) result of Corvino (2000) our (pseudo)-spectral scheme is demonstrated under the assumption of axisymmetry so as to fashion composite Hamiltonian constraint satisfying initial data featuring internal binary black holes (BBH) as glued to exterior Schwarzschild initial data in isotropic form. The generality of the method is illustrated in a comparison of the ADM mass of EAG initial data sets featuring internal BBHs as modelled by Brill-Lindquist and Misner data.In contrast to the recent work of Doulis and Rinne (2016), and Pook-Kolb and Giulini (2018) we do not make use of the York-Lichnerowicz conformal framework to reformulate the constraints.

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arXiv:1903.06329 gr-qc

**Numerical construction of initial data sets of binary black hole type using a parabolic-hyperbolic formulation of the vacuum constraint equations**

** **

**Authors:** Florian
Beyer, Leon
Escobar, Jörg
Frauendiener, Joshua
Ritchie

Abstract: In this paper we investigate the parabolic-hyperbolic formulation of the vacuum constraint equations introduced by R{=A8=A2}cz with a view to constructing multiple black hole initial data sets without spin. In order to respect the natural properties of this configuration, we foliate the spatial domain with 2-spheres. It is then a consequence of these equations that they must be solved as an initial value problem evolving outwards towards spacelike infinity. Choosing the free data and the "strong field boundary conditions" for these equations in a way which mimics asymptotically flat and asymptotically spherical binary black hole initial data sets, our focus in this paper is on the analysis of the asymptotics of the solutions. In agreement with our earlier results, our combination of analytical and numerical tools reveals that these solutions are in general not asymptotically flat, but have a cone geometry instead. In order to remedy this and approximate asymptotically Euclidean data sets, we then propose and test an iterative numerical scheme.

Journal reference: Class. Quantum Grav. 36 175005 (2019)

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arXiv:1903.12482 cs.MS gr-qc

**COFFEE -- An MPI-parallelized Python package for the numerical evolution of differential equations**

**Authors:** Georgios
Doulis, Jörg
Frauendiener, Chris
Stevens, Ben
Whale

Abstract: COFFEE (ConFormal Field Equation Evolver) is a Python package primarily developed to numerically evolve systems of partial differential equations over time using the method of lines. It includes a variety of time integrators and finite differencing stencils with the summation-by-parts property, as well as pseudo-spectral functionality for angular derivatives of spin-weighted functions. Some additional capabilities include being MPI-parallelisable on a variety of different geometries, HDF data output and post processing scripts to visualize data, and an actions class that allows users to create code for analysis after each timestep.

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arXiv:1707.07974 quant-ph gr-qc

**On two recent proposals for witnessing nonclassical gravity**

** **

**Authors:** Michael
J. W. Hall, Marcel
Reginatto

Abstract: Two very similar proposals have been made recently for witnessing nonclassical features of gravity, by Bose et al. and by Marletto and Vedral. However, while these proposals are asserted to be very general, they are in fact based on a very strong claim: that quantum systems cannot become entangled via a classical intermediary. We point out that the support provided for this claim is only applicable to a very limited class of quantum-classical interaction models, corresponding to Koopman-type dynamics. We show that the claim is also valid for mean-field models, but that it is contradicted by explicit counterexamples based on the configuration-ensemble model. Thus, neither proposal provides a definitive test of nonclassical gravity.

Journal reference: J Phys A 51, 085303 (2018)

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arXiv:1801.10206 math.AP gr-qc

**Ergoregions between two ergospheres**

** **

**Authors:** Gregory
Eskin, Michael
Hall

Abstract: For a stationary spacetime metric, black holes are spatial regions which disturbances may not propagate out of. In our previous work an existence and regularity theorem was proven for black holes in two space dimensions, in the case where the boundary of the ergoregion is a simple closed curve surrounding a singularity. In this paper we study the case of an annular ergoregion, whose boundary has two components.

Journal reference: Methods of Funct. Anal. Topology, 24(2018), No. 2, 98-106, 83c57

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arXiv:1809.04989 gr-qc

**Entanglement of quantum fields via classical gravity**

** **

**Authors:** Marcel
Reginatto, Michael
J. W. Hall

Abstract: We consider the coupling of quantum fields to classical gravity in the formalism of ensembles on configuration space, a model that allows a consistent formulation of interacting classical and quantum systems. Explicit calculations show that there are solutions for which two quantum fields are in an entangled state, even though their interaction occurs solely via a common classical gravitational field, and that such entangled solutions can evolve from initially unentangled ones. These results support the observation of a previous paper that an observed generation of entanglement would not provide a definitive test of the nonclassical nature of gravity.

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arXiv:1709.10340 astro-ph.HE gr-qc

**Superfluidity and Superconductivity in Neutron Stars**

** **

**Authors:** Brynmor
Haskell, Armen
Sedrakian

Abstract: This review focuses on applications of the ideas of superfluidity and superconductivity in neutron stars in a broader context, ranging from the microphysics of pairing in nucleonic superfluids to macroscopic manifestations of superfluidity in pulsars. The exposition of the basics of pairing, vorticity and mutual friction can serve as an introduction to the subject. We also review some topics of recent interest, including the various types of pinning of vortices, glitches, and oscillations in neutron stars containing superfluid phases of baryonic matter.

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arXiv:1711.05550 astro-ph.HE gr-qc

**The enigmatic spin evolution of PSR J0537-6910: r-modes, gravitational waves and the case for continued timing**

**Authors:** N.
Andersson, D.
Antonopoulou, C.
M. Espinoza, B.
Haskell, W.
C. G. Ho

Abstract: We discuss the unique spin evolution of the young X-ray pulsar PSR J0537-6910, a system in which the regular spin down is interrupted by glitches every few months. Drawing on the complete timing data from the Rossi X-ray Timing Explorer (RXTE, from 1999-2011), we argue that a trend in the inter-glitch behaviour points to an effective braking index close to n=3D7, much larger than expected. This value is interesting because it would accord with the neutron star spinning down due to gravitational waves from an unstable r-mode. We discuss to what extent this, admittedly speculative, scenario may be consistent and if the associated gravitational-wave signal would be within reach of ground based detectors. Our estimates suggest that one may, indeed, be able to use future observations to test the idea. Further precision timing would help enhance the achievable sensitivity and we advocate a joint observing campaign between the Neutron Star Interior Composition ExploreR (NICER) and the LIGO-Virgo network.

Journal reference: Astrophys. J. 864, 137 (2018)

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arXiv:1712.01547 astro-ph.HE gr-qc

**Probing neutron star interiors with pulsar glitches**

** **

**Authors:** Brynmor
Haskell

Abstract: Pulsar glitches are thought to be probes of the superfluid interior of neutron stars. These sudden jumps in frequency observed in many pulsars are generally assumed to be the macroscopic manifestation of superfluid vortex motion on a microscopic scale. Resolving and modelling such phenomena on the scale of a neutron star is, however, a challenging problem which still remains open, fifty years after the discovery of pulsars. In this article I will review recent theoretical progress, both on the microscopic level and on the macroscopic level, and discuss which constraints on the models can be provided by observations.

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arXiv:1801.01413 astro-ph.HE gr-qc

**Modelling pulsar glitches: the hydrodynamics of superfluid vortex avalanches in neutron stars**

** **

**Authors:** Vadym
Khomenko, Brynmor
Haskell

Abstract: The dynamics of quantised vorticity in neutron star interiors is at the heart of most pulsar glitch models. However, the large number of vortices (up to =A1=D61013) involved in a glitch and the huge disparity in scales between the femtometer scale of vortex cores and the kilometre scale of the star makes quantum dynamical simulations of the problem computationally intractable. In this paper we take a first step towards developing a mean field prescription to include the dynamics of vortices in large scale hydrodynamical simulations of superfluid neutron stars. We consider a one dimensional setup and show that vortex accumulation and differential rotation in the neutron superfluid lead to propagating waves, or `avalanches', as solutions for the equations of motion for the superfluid velocities. We introduce an additional variable, the fraction of free vortices, and test different prescriptions for its advection with the superfluid flow. We find that the new terms lead to solutions with a linear component in the rise of a glitch, and that, in specific setups, they can give rise to glitch precursors and even to decreases in frequency, or `anti-glitches'.

Journal reference: Publications of the Astronomical Society of Australia, Volume 35, id.e020 (2018)

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arXiv:1805.11277 astro-ph.HE gr-qc

**Fundamental physics and the absence of sub-millisecond pulsars**

** **

**Authors:** B.
Haskell, J.
L. Zdunik, M.
Fortin, M.
Bejger, R.
Wijnands, A.
Patruno

Abstract: Observations of the spin distribution of rapidly rotating neutron stars show evidence for a lack of stars spinning at frequencies larger than f=A1=D6700 Hz, well below the predictions of theoretical equations of state. This has generally been taken as evidence of an additional spin-down torque operating in these systems and it has been suggested that gravitational wave torques may be operating and be linked to a potentially observable signal. In this paper we aim to determine whether additional spin-down torques are necessary, or whether the observed limit of f=A1=D6700 Hz could correspond to the mass-shedding frequency for the observed systems and is simply a consequence of the, currently unknown, state of matter at high densities. Given our ignorance with regard to the true equation of state of matter above nuclear saturation densities, we make minimal physical assumption and only demand causality in the core. We then connect our causally-limited equation of state to a realistic microphysical crustal equation of state for densities below nuclear saturation density. This produces a limiting model that will give the lowest possible maximum frequency, which we compare to observational constraints on neutron star masses and frequencies. We also compare our findings with the constraints on the tidal deformability obtained in the observations of the GW170817 event. We find that the lack of pulsars spinning faster than f=A1=D6700 Hz is not compatible with our causal limited `minimal' equation of state, for which the breakup frequency cannot be lower than fmax=A1=D61200 Hz. A low frequency cutoff, around f=A1=D6800 Hz could only be possible if we assume that these systems do not contain neutron stars with masses above M=A1=D62M=A1=D1. This would have to be due either to selection effects, or possibly to a phase transition in the interior of the neutron star.

Journal reference: A&A 620, A69 (2018) A&A 620, A69 (2018)

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arXiv:1806.02822 astro-ph.HE gr-qc

**Evidence for a minimum ellipticity in millisecond pulsars**

** **

**Authors:** G.
Woan, M.
D. Pitkin, B.
Haskell, D.
I. Jones, P.
D. Lasky

Abstract: Neutron stars spin down over time due to a number of energy-loss processes. We provide tantalizing population-based evidence that millisecond pulsars (MSPs) have a minimum ellipticity of =A6=C5=A1=D610−9 around their spin axis and that, consequently, some spin down mostly through gravitational-wave emission. We discuss the implications of such a minimum ellipticity in terms of the internal magnetic field strengths and nuclear matter composition of neutron stars and show it would result in the Advanced LIGO and Virgo gravitational-wave detectors, or their upgrades, detecting gravitational waves from some known MSPs in the near future.

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arXiv:1811.09419 gr-qc

**The dynamics of neutron star crusts: Lagrangian perturbation theory for a relativistic superfluid-elastic system**

** **

**Authors:** N.
Andersson, B.
Haskell, G.
L. Comer, L.
Samuelsson

Abstract: The inner crust of a mature neutron star is composed of an elastic lattice of neutron-rich nuclei penetrated by free neutrons. These neutrons can flow relative to the crust once the star cools below the superfluid transition temperature. In order to model the dynamics of this system, which is relevant for a range of problems from pulsar glitches to magnetar seismology and continuous gravitational-wave emission from rotating deformed neutron stars, we need to understand general relativistic Lagrangian perturbation theory for elastic matter coupled to a superfluid component. This paper develops the relevant formalism to the level required for astrophysical applications.

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arXiv:1907.04946 gr-qc

**How to search for gravitational waves from r-modes
of known pulsars**

** **

**Authors:** Santiago
Caride, Ra
Inta, Benjamin
J. Owen, Binod
Rajbhandari

Abstract: Searches for continuous gravitational waves from known pulsars so far have been targeted at or near the spin frequency or double the spin frequency of each pulsar, appropriate for mass quadrupole emission. But some neutron stars might radiate strongly through current quadrupoles via r-modes, which oscillate at about four thirds the spin frequency. We show for the first time how to construct searches over appropriate ranges of frequencies and spin-down parameters to target r-modes from known pulsars. We estimate computational costs and sensitivities of realistic r-mode searches using the coherent F-statistic, and find that feasible searches for known pulsars can beat spin-down limits on gravitational wave emission even with existing LIGO and Virgo data.

Journal reference: Phys. Rev. D 100, 064013 (2019)

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arXiv:1711.09226 astro-ph.HE gr-qc

**Inferring the population properties of binary neutron stars with gravitational-wave measurements of spin**

** **

**Authors:** Xing-Jiang
Zhu, Eric
Thrane, Stefan
Osłowski, Yuri
Levin, Paul
D. Lasky

Abstract: The recent LIGO-Virgo detection of gravitational waves from a binary neutron star inspiral event GW170817 and the discovery of its accompanying electromagnetic signals mark a new era for multimessenger astronomy. In the coming years, advanced gravitational-wave detectors are likely to detect tens to hundreds of similar events. Neutron stars in binaries can possess significant spin, which is imprinted on the gravitational waveform via the effective spin parameter =A6=D6eff. We explore the astrophysical inferences made possible by gravitational-wave measurements of =A6=D6eff. First, using a fiducial model informed by radio observations, we estimate that =A1=D615−30% of binary neutron stars should have spins measurable at ≳90% confidence level by advanced detectors assuming the spin axis of the recycled neutron star aligns with the total orbital angular momentum of the binary. Second, using Bayesian inference, we show that it is possible to tell whether or not the spin axis of the recycled neutron star tends to be aligned with the binary orbit using ≳30 detections. Finally, interesting constraints can be placed on neutron star magnetic field decay after ≳300 detections, if the spin periods and magnetic field strengths of Galactic binary neutron stars are representative of the merging population.

Journal reference: Phys. Rev. D 98, 043002 (2018)

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arXiv:1805.01481 astro-ph.HE gr-qc

**X-ray guided gravitational-wave search for binary neutron star merger remnants**

** **

**Authors:** Nikhil
Sarin, Paul
D. Lasky, Letizia
Sammut, Greg
Ashton

Abstract: X-ray observations of some short gamma-ray bursts indicate that a long-lived neutron star can form as a remnant of a binary neutron star merger. We develop a gravitational-wave detection pipeline for a long-lived binary neutron star merger remnant guided by these counterpart electromagnetic observations. We determine the distance out to which a gravitational-wave signal can be detected with Advanced LIGO at design sensitivity and the Einstein Telescope using this method, guided by X-ray data from GRB140903A as an example. Such gravitational waves can in principle be detected out to ∼ 20 Mpc for Advanced LIGO and ∼ 450 Mpc for the Einstein Telescope assuming a fiducial ellipticity of 10−2. However, in practice we can rule out such high values of the ellipticity as the total energy emitted in gravitational waves would be greater than the total rotational energy budget of the system. We show how these observations can be used to place upper limits on the ellipticity using these energy considerations. For GRB140903A, the upper limit on the ellipticity is 10−3, which lowers the detectable distance to ∼ 2 Mpc and ∼ 45 Mpc for Advanced LIGO and the Einstein Telescope, respectively.

Journal reference: Phys. Rev. D 98 043011 (2018)

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arXiv:1807.00990 astro-ph.HE gr-qc

**Gravitational-wave memory: waveforms and phenomenology**

** **

**Authors:** Colm
Talbot, Eric
Thrane, Paul
D. Lasky, Fuhui
Lin

Abstract: The non-linear gravitational-wave memory effect is a prediction of general relativity in which test masses are permanently displaced by gravitational radiation. We implement a method for calculating the expected memory waveform from an oscillatory gravitational-wave time series. We use this method to explore the phenomenology of gravitational-wave memory using a numerical relativity surrogate model. Previous methods of calculating the memory have considered only the dominant oscillatory (ℓ=3D2, m=3D|2|) mode in the spherical harmonic decomposition or the post-Newtonian expansion. We explore the contribution of higher-order modes and reveal a richer phenomenology than is apparent with ℓ=3D|m|=3D2 modes alone. We also consider the `memory of the memory' in which the memory is, itself, a source of memory, which leads to a small, O(10−4), correction to the memory waveform. The method is implemented in the python package {\tt\sc GWMemory}, which is made publicly available.

Journal reference: Phys. Rev. D 98, 064031 (2018)

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arXiv:1807.01711 astro-ph.CO gr-qc

**Einstein's Universe: Cosmological structure formation in numerical relativity**

** **

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

Abstract: We perform large-scale cosmological simulations that solve Einstein's equations directly via numerical relativity. Starting with initial conditions sampled from the cosmic microwave background, we track the emergence of a cosmic web without the need for a background cosmology. We measure the backreaction of large-scale structure on the evolution of averaged quantities in a matter-dominated universe. Although our results are preliminary, we find the global backreaction energy density is of order 10−8 compared to the energy density of matter in our simulations, and is thus unlikely to explain accelerating expansion under our assumptions. Sampling scales above the homogeneity scale of the Universe (100−180h−1Mpc), in our chosen gauge, we find 2−3% variations in local spatial curvature.

Journal reference: Phys. Rev. D 99, 063522 (2019)

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arXiv:1807.01714 astro-ph.CO gr-qc

**The trouble with Hubble: Local versus global expansion rates in inhomogeneous cosmological simulations with numerical relativity**

** **

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

Abstract: In a fully inhomogeneous, anisotropic cosmological simulation performed by solving Einstein's equations with numerical relativity, we find a local measurement of the effective Hubble parameter differs by less than 1\% compared to the global value. This variance is consistent with predictions from Newtonian gravity. We analyse the averaged local expansion rate on scales comparable to Type 1a supernova surveys, and find that local variance cannot resolve the tension between the \citet{riess2018b} and \citet{planck2018a} measurements.

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arXiv:1810.03227 astro-ph.CO gr-qc

**Parkes Pulsar Timing Array constraints on ultralight scalar-field dark matter**

** **

**Authors:** Nataliya
K. Porayko, Xingjiang
Zhu, Yuri
Levin, Lam
Hui, George
Hobbs, Aleksandra
Grudskaya, Konstantin
Postnov, Matthew
Bailes, N.
D. Ramesh Bhat, William
Coles, Shi
Dai, James
Dempsey, Michael
J. Keith, Matthew
Kerr, Michael
Kramer, Paul
D. Lasky, Richard
N. Manchester, Stefan
Osłowski, Aditya
Parthasarathy, Vikram
Ravi, Daniel
J. Reardon, Pablo
A. Rosado, Christopher
J. Russell, Ryan
M. Shannon, Ren=A8=A6e
Spiewak , et al. (5 additional authors not shown)

Abstract: It is widely accepted that dark matter contributes about a quarter of the critical mass-energy density in our Universe. The nature of dark matter is currently unknown, with the mass of possible constituents spanning nearly one hundred orders of magnitude. The ultralight scalar field dark matter, consisting of extremely light bosons with m∼10−22 eV and often called "fuzzy" dark matter, provides intriguing solutions to some challenges at sub-Galactic scales for the standard cold dark matter model. As shown by Khmelnitsky and Rubakov, such a scalar field in the Galaxy would produce an oscillating gravitational potential with nanohertz frequencies, resulting in periodic variations in the times of arrival of radio pulses from pulsars. The Parkes Pulsar Timing Array (PPTA) has been monitoring 20 millisecond pulsars at two to three weeks intervals for more than a decade. In addition to the detection of nanohertz gravitational waves, PPTA offers the opportunity for direct searches for fuzzy dark matter in an astrophysically feasible range of masses. We analyze the latest PPTA data set which includes timing observations for 26 pulsars made between 2004 and 2016. We perform a search in this data set for evidence of ultralight dark matter in the Galaxy using Bayesian and Frequentist methods. No statistically significant detection has been made. We therefore place upper limits on the local dark matter density. Our limits, improving on previous searches by a factor of two to five, constrain the dark matter density of ultralight bosons with m=A1=DC10−23 eV to be below 6GeVcm−3 with 95\% confidence in the Earth neighborhood. Finally, we discuss the prospect of probing the astrophysically favored mass range m≳10−22 eV with next-generation pulsar timing facilities.

Journal reference: Phys. Rev. D 98, 102002 (2018)

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arXiv:1811.02042 astro-ph.IM astro-ph.HE gr-qc

**Bilby: A user-friendly Bayesian inference library for gravitational-wave astronomy**

** **

**Authors:** Gregory
Ashton, Moritz
Huebner, Paul
D. Lasky, Colm
Talbot, Kendall
Ackley, Sylvia
Biscoveanu, Qi
Chu, Atul
Divarkala, Paul
J. Easter, Boris
Goncharov, Francisco
Hernandez Vivanco, Jan
Harms, Marcus
E. Lower, Grant
D. Meadors, Denyz
Melchor, Ethan
Payne, Matthew
D. Pitkin, Jade
Powell, Nikhil
Sarin, Rory
J. E. Smith, Eric
Thrane

Abstract: Bayesian parameter estimation is fast becoming the language of gravitational-wave astronomy. It is the method by which gravitational-wave data is used to infer the sources' astrophysical properties. We introduce a user-friendly Bayesian inference library for gravitational-wave astronomy, Bilby. This python code provides expert-level parameter estimation infrastructure with straightforward syntax and tools that facilitate use by beginners. It allows users to perform accurate and reliable gravitational-wave parameter estimation on both real, freely-available data from LIGO/Virgo, and simulated data. We provide a suite of examples for the analysis of compact binary mergers and other types of signal model including supernovae and the remnants of binary neutron star mergers. These examples illustrate how to change the signal model, how to implement new likelihood functions, and how to add new detectors. Bilby has additional functionality to do population studies using hierarchical Bayesian modelling. We provide an example in which we infer the shape of the black hole mass distribution from an ensemble of observations of binary black hole mergers.

Journal reference: The Astrophysical Journal Supplement Series (2019) 241, 27

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arXiv:1811.11183 gr-qc

**Computing Fast and Reliable Gravitational Waveforms of Binary Neutron Star Merger Remnants**

** **

**Authors:** Paul
J. Easter, Paul
D. Lasky, Andrew
R. Casey, Luciano
Rezzolla, Kentaro
Takami

Abstract: Gravitational waves have been detected from the inspiral of a binary neutron-star, GW170817, which allowed constraints to be placed on the neutron star equation of state. The equation of state can be further constrained if gravitational waves from a post-merger remnant are detected. Post-merger waveforms are currently generated by numerical-relativity simulations, which are computationally expensive. Here we introduce a hierarchical model trained on numerical-relativity simulations, which can generate reliable post-merger spectra in a fraction of a second. Our spectra have mean fitting factors of 0.95, which compares to fitting factors of 0.76 and 0.85 between different numerical-relativity codes that simulate the same physical system. This method is the first step towards generating large template banks of spectra for use in post-merger detection and parameter estimation.

Journal reference: Phys. Rev. D 100, 043005 (2019)

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arXiv:1901.03885 astro-ph.IM gr-qc

**Exploring the sensitivity of gravitational wave detectors to neutron star physics**

** **

**Authors:** Denis
Martynov, Haixing
Miao, Huan
Yang, Francisco
Hernandez Vivanco, Eric
Thrane, Rory
Smith, Paul
Lasky, William
E. East, Rana
Adhikari, Andreas
Bauswein, Aidan
Brooks, Yanbei
Chen, Thomas
Corbitt, Thomas
Corbitt, Hartmut
Grote, Yuri
Levin, Chunnong
Zhao, Alberto
Vecchio

Abstract: The physics of neutron stars can be studied with gravitational waves emitted from coalescing binary systems. Tidal effects become significant during the last few orbits and can be visible in the gravitational-wave spectrum above 500 Hz. After the merger, the neutron star remnant oscillates at frequencies above 1 kHz and can collapse into a black hole. Gravitational-wave detectors with a sensitivity of ~10^{-24} strain/sqHz at 2-4 kHz can observe these oscillations from a source which is ~100 Mpc away. The current observatories, such as LIGO and Virgo, are limited by shot noise at high frequencies and have a sensitivity of > 2 * 10^{-23} strain/sqHz at 3 kHz. In this paper, we propose an optical configuration of gravitational-wave detectors which can be set up in present facilities using the current interferometer topology. This scheme has a potential to reach 7 * 10^{-25} strain/sqHz at 2.5 kHz without compromising the detector sensitivity to black hole binaries. We argue that the proposed instruments have a potential to detect similar amount of post-merger neutron star oscillations as the next generation detectors, such as Cosmic Explorer and Einstein Telescope. We also optimise the arm length of the future detectors for neutron star physics and find that the optimal arm length is ~20 km. These instruments have the potential to observe neutron star post-merger oscillations at a rate of ~30 events per year with a signal-to-noise ratio of 5 or more.

Journal reference: Phys. Rev. D 99, 102004 (2019)

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arXiv:1903.05778 gr-qc

**Accelerated detection of the binary neutron star gravitational-wave background**

** **

**Authors:** Francisco
Hernandez Vivanco, Rory
Smith, Eric
Thrane, Paul
D. Lasky

Abstract: Most gravitational-wave signals from binary neutron star coalescences are too weak to be individually resolved with current detectors. We demonstrate how to extract a population of sub-threshold binary neutron star signals using Bayesian parameter estimation. Assuming a merger rate of one signal every two hours, we find that this gravitational-wave background can be detected after approximately three months of observation with Advanced LIGO and Virgo at design sensitivity, versus several years using the standard cross-correlation algorithm. We show that the algorithm can distinguish different neutron star equations of state using roughly seven months of Advanced LIGO and Virgo design-sensitivity data. This is in contrast to the standard cross-correlation method, which cannot.

Journal reference: Phys. Rev. D 100, 043023 (2019)

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arXiv:1905.01387 astro-ph.HE gr-qc

**Neutron Star Merger Remnants: Braking Indices, Gravitational Waves, and the Equation Of State**

** **

**Authors:** Paul
D. Lasky, Nikhil
Sarin, Greg
Ashton

Abstract: The binary neutron star merger GW170817/GRB170817A confirmed that at least some neutron star mergers are the progenitors of short gamma-ray bursts. Many short gamma-ray bursts have long-term x-ray afterglows that have been interpreted in terms of post-merger millisecond magnetars---rapidly rotating, highly magnetised, massive neutron stars. We review our current understanding of millisecond magnetars born in short gamma-ray bursts, focusing particularly three main topics. First, whether millisecond magnetars really do provide the most plausible explain for the x-ray plateau. Second, determining and observing the gravitational-wave emission from these remnants. Third, determining the equation of state of nuclear matter from current and future x-ray and gravitational-wave measurements.

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arXiv:1907.01124 astro-ph.HE gr-qc

**Rotational evolution of the Vela pulsar during the 2016 glitch**

** **

**Authors:** Gregory
Ashton, Paul
D. Lasky, Vanessa
Graber, Jim
Palfreyman

Abstract: The 2016 Vela glitch observed by the Mt Pleasant radio telescope provides the first opportunity to study pulse-to-pulse dynamics of a pulsar glitch, opening up new possibilities to study the neutron star's interior. We fit models of the star's rotation frequency to the pulsar data, and present three new results. First, we constrain the glitch rise time to less than 12.6s with 90% confidence, almost three times shorter than the previous best constraint. Second, we find definitive evidence for a rotational-frequency overshoot and fast relaxation following the glitch. Third, we find evidence for a slow-down of the star's rotation immediately prior to the glitch. The overshoot is predicted theoretically by some models; we discuss implications of the glitch rise and overshoot decay times on internal neutron-star physics. The slow down preceding the glitch is unexpected; we propose the slow-down may trigger the glitch by causing a critical lag between crustal superfluid and the crust.

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arXiv:1909.02698 gr-qc

**Measuring the neutron star equation of state with gravitational waves: the first forty binary neutron star mergers**

** **

**Authors:** Francisco
Hernandez Vivanco, Rory
Smith, Eric
Thrane, Paul
D. Lasky, Colm
Talbot, Vivien
Raymond

Abstract: Gravitational waves from binary neutron star coalescences contain rich information about matter at supranuclear densities encoded by the neutron star equation of state. We can measure the equation of state by analyzing the tidal interactions between neutron stars, which is quantified by the tidal deformability. Multiple merger events are required to probe the equation of state over a range of neutron star masses. The more events included in the analysis, the stronger the constraints on the equation of state. In this paper, we build on previous work to explore the constraints that LIGO and Virgo are likely to place on the neutron star equation of state by combining the first forty binary neutron star detections, a milestone we project to be reached during the first year of accumulated design-sensitivity data. We carry out Bayesian inference on a realistic mock dataset of binaries to obtain posterior distributions for neutron star tidal parameters. In order to combine posterior samples from multiple observations, we employ a random forest regressor, which allows us to efficiently interpolate the likelihood distribution. Assuming a merger rate of 1540 Gpc−3 yr−1 and a LIGO-Virgo detector network operating for one year at the sensitivity of the third-observation run, plus an additional eight months of design sensitivity, we find that the radius of a 1.4 M=A1=D1 neutron star can be constrained to ∼10% at 90% confidence. At the same time, the pressure at twice the nuclear saturation density can be constrained to ∼45 % at 90% confidence.

Journal reference: Phys. Rev. D 100, 103009 (2019)

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arXiv:1910.12330 astro-ph.HE gr-qc

**Ultra-relativistic astrophysics using multi-messenger observations of double neutron stars with LISA and the SKA**

** **

**Authors:** Eric
Thrane, Stefan
Osłowski, Paul
Lasky

Abstract: Recent work highlights that tens of Galactic double neutron stars are likely to be detectable in the millihertz band of the space-based gravitational-wave observatory, LISA. Kyutoku and Nishino point out that some of these binaries might be detectable as radio pulsars using the Square Kilometer Array (SKA). We point out that the joint LISA+SKA detection of a fgw≳1 mHz binary, corresponding to a binary period of ≲400 s, would enable precision measurements of ultra-relativistic phenomena. We show that, given plausible assumptions, multi-messenger observations of ultra-relativistic binaries can be used to constrain the neutron star equation of state with remarkable fidelity. It may be possible to measure the mass-radius relation with a precision of =A1=D60.2% after 10 yr of observations with the SKA. Such a measurement would be roughly an order of magnitude more precise than possible with other proposed observations. We summarize some of the other remarkable science made possible with multi-messenger observations of millihertz binaries, and discuss the prospects for the detection of such objects.

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arXiv:1911.07998 gr-qc

**Memory Effect or Cosmic String? Classifying Gravitational-Wave Bursts with Bayesian Inference**

** **

**Authors:** Atul
K. Divakarla, Eric
Thrane, Paul
D. Lasky, Bernard
F. Whiting

Abstract: A variety of gravitational-wave transient sources can be modeled in the Fourier domain using a power law. This simple power-law model provides a reasonable approximation for gravitational-wave bursts from cosmic string cusps, cosmic string kinks, and the memory effect. Each of these sources is described using a different spectral index. In this work, we simulate interferometer strain data with injections of power-law and memory bursts to demonstrate parameter estimation, signal detection, and model selection. We show how Bayesian inference can be used to measure the power-law spectral index, thereby distinguishing between different astrophysical scenarios.

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arXiv:1911.12496 astro-ph.HE gr-qc

**Thanks for the memory: measuring gravitational-wave memory in the first LIGO/Virgo gravitational-wave transient catalog**

** **

**Authors:** Moritz
H=A8=B9bner, Colm
Talbot, Paul
D. Lasky, Eric
Thrane

Abstract: Gravitational-wave memory, a strong-field effect of general relativity, manifests itself as a permanent displacement in spacetime. We develop a Bayesian framework to detect gravitational-wave memory with the Advanced LIGO/Virgo detector network. We apply this algorithm on the ten binary black hole mergers in LIGO/Virgo's first transient gravitational-wave catalog. We find no evidence of memory, which is consistent with expectations. In order to estimate when memory will be detected, we use the best current population estimates to construct a realistic sample of binary black hole observations for LIGO/Virgo at design sensitivity. We show that an ensemble of O(2000) binary black hole observations can be used to find definitive evidence for gravitational-wave memory. We conclude that memory is likely to be detected in the early A+/Virgo+ era.

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arXiv:1911.05415 gr-qc

**Expanding space, redshifts, and rigidity: Conceptual issues in cosmology**

** **

**Authors:** Colin
MacLaurin

Abstract: I examine the interpretation of photon redshifts in curved spacetime, as being gravitational or Doppler in origin. In Friedmann-Lemaître-Robertson-Walker spacetime, redshifts between comoving observers are often attributed to "expanding space", whereas in Schwarzschild spacetime, redshifts between static observers are attributed to "gravitational" causes. Yet various authors have suggested a freely falling observer congruence would interpret any redshift as Doppler, whereas a rigid congruence must interpret it as gravitational since there is no relative motion. I realise this proposal by explicitly constructing coordinate systems for rigid motion in the above spacetimes. This includes an extensive analysis of observer-dependent distance measurement in curved spacetime. I also introduce Rindler acceleration, the Milne model, and Newtonian cosmology.

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arXiv:1911.05988 gr-qc

**Schwarzschild spacetime under generalised Gullstrand-Painlev=A8=A6 slicing**

** **

**Authors:** Colin
MacLaurin

Abstract: We investigate a foliation of Schwarzschild spacetime determined by observers freely falling in the radial direction. This is described using a generalisation of Gullstrand-Painlev=A8=A6 coordinates which allows for any possible radial velocity. This foliation provides a contrast with the usual static foliation implied by Schwarzschild coordinates. The 3-dimensional spaces are distinct for the static and falling observers, so the embedding diagrams, spatial measurement, simultaneity, and time at infinity are also distinct, though the 4-dimensional spacetime is unchanged. Our motivation is conceptual understanding, to counter Newton-like viewpoints. In future work, this alternate foliation may shed light on open questions regarding quantum fields, analogue gravity, entropy, energy, and other quantities. This article is aimed at experienced relativists, whereas a forthcoming series is intended for a general audience of physicists, mathematicians, and philosophers.

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arXiv:1911.07500 gr-qc

**Clarifying spatial distance measurement**

** **

**Authors:** Colin
MacLaurin

Abstract: We examine length measurement in curved spacetime, based on the 1+3-splitting of a local observer frame. This situates extended objects within spacetime, in terms of a given coordinate which serves as an external reference. The radar metric is shown to coincide with the spatial projector, but these only give meaningful results on the observer's 3-space, where they reduce to the metric. Examples from Schwarzschild spacetime are given.

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arXiv:1911.08726 gr-qc

**Cosmic cable**

** **

**Authors:** Colin
MacLaurin

Abstract: I investigate the relativistic mechanics of an extended "cable" in an arbitrary static, spherically symmetric spacetime. Such hypothetical bodies have been proposed as tests of energy and thermodynamics: by lowering objects toward a black hole, scooping up Hawking radiation, or mining energy from the expansion of the universe. I review existing work on stationary cables, which demonstrates an interesting "redshift" of tension, and extend to a case of rigid motion. By using a partly restrained cable to turn a turbine, the energy harvested is up to the equivalent of the cable's rest mass, concurring with the quasistatic case. Still, the total Killing energy of the system is conserved.

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arXiv:1707.03741 gr-qc

**Tracking our Universe to de Sitter by a Horndeski scalar**

** **

**Authors:** Cristiano
Germani, Prado
Martin-Moruno

Abstract: Assuming both that our Universe is evolving into a de Sitter space and a vanishing cosmological constant, leaves only the option that the observed acceleration is provided by a "kinetic" energy of a scalar field. From an effective field theory point of view, the absence of Ostrogradsky instabilities restricts the choice to shift-symmetric Horndeski theories. Within these theories, we find the conditions for the existence of a de Sitter critical point in a universe filled by matter, radiation and a Horndeski scalar. Moreover, we show that this point is a universal attractor and we provide the tracking trajectory. Therefore, if a de Sitter fixed point exists within these models, our Universe will eventually evolve into a de Sitter space. As an example, we have discussed the case of the combined Galileon-Slotheon system, in which the Galileon is kinetically non-minimal coupled to the Einstein tensor. Interestingly, we have also found that the tracker trajectory of this system does not follow previous literature assumptions.

Journal reference: Physics of the Dark Universe 18C (2017) pp. 1-5

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arXiv:1707.04172 gr-qc

**Generalized Rainich conditions, generalized stress-energy conditions, and the Hawking-Ellis classification**

** **

**Authors:** Prado
Mart=A8=AAn-Moruno, Matt
Visser

Abstract: The (generalized) Rainich conditions are algebraic conditions which are polynomial in the (mixed-component) stress-energy tensor. As such they are logically distinct from the usual classical energy conditions (NEC, WEC, SEC, DEC), and logically distinct from the usual Hawking-Ellis (Segr=A8=A6-Pleba=A8=BDski) classification of stress-energy tensors (type I, type II, type III, type IV). There will of course be significant inter-connections between these classification schemes, which we explore in the current article. Overall, we shall argue that it is best to view the (generalized) Rainich conditions as a refinement of the classical energy conditions and the usual Hawking-Ellis classification.

Journal reference: Classical and Quantum Gravity 34 (2017) 225014

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arXiv:1802.00865 gr-qc hep-th

**Essential core of the Hawking--Ellis types**

** **

**Authors:** Prado
Martin-Moruno, Matt
Visser

Abstract: The Hawking-Ellis (Segre-Plebanski) classification of possible stress-energy tensors is an essential tool in analyzing the implications of the Einstein field equations in a more-or-less model-independent manner. In the current article the basic idea is to simplify the Hawking-Ellis type I, II, III, and IV classification by isolating the "essential core" of the type II, type III, and type IV stress-energy tensors; this being done by subtracting (special cases of) type I to simplify the (Lorentz invariant) eigenvalue structure as much as possible without disturbing the eigenvector structure. We will denote these "simplified cores" type II0, type III0, and type IV0. These "simplified cores" have very nice and simple algebraic properties. Furthermore, types I and II0 have very simple classical interpretations, while type IV0 is known to arise semi-classically (in renormalized expectation values of standard stress-energy tensors). In contrast type III0 stands out in that it has neither a simple classical interpretation, nor even a simple semi-classical interpretation. We will also consider the robustness of this classification considering the stability of the different Hawking-Ellis types under perturbations. We argue that types II and III are definitively unstable, whereas types I and IV are stable. .

Journal reference: Classical and Quantum Gravity 35 (2018) 125003

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arXiv:1802.03290 gr-qc

**f****(****R****)**** quantum
cosmology: avoiding the Big Rip**

** **

**Authors:** Ana
Alonso-Serrano, Mariam
Bouhmadi-Lopez, Prado
Martin-Moruno

Abstract: Extended theories of gravity have gathered a lot of attention over the last years, for they not only provide an excellent framework to describe the inflationary era but also yields an alternative to the elusive and mysterious dark energy. Among the different extended theories of gravity, on this work we focus on metric f(R) theories. In addition, it is well known that if the late-time acceleration of the universe is stronger than the one induced by a cosmological constant then some future cosmic singularities might arise, being the Big Rip the most virulent one. Following this reasoning, on this work, we analyse the Big Rip singularity in the framework of f(R) quantum geometrodynamics. Invoking the DeWitt criterium, i. e. that the wave function vanishes at the classical singularity, we proof that a class of solutions to the Wheeler-DeWitt equation fulfilling this condition can be found. Therefore, this result hints towards the avoidance of the Big Rip in metric f(R) theories of gravity.

Journal reference: Phys. Rev. D 98, 104004 (2018)

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arXiv:1805.09629 gr-qc

**Modified gravity as a diagravitational medium**

** **

**Authors:** Jose
A. R. Cembranos, Mario
Coma Diaz, Prado
Martin-Moruno

Abstract: In this letter we reflect on the propagation of gravitational waves in alternative theories of gravity, which are typically formulated using extra gravitational degrees of freedom in comparison to General Relativity. We propose to understand that additional structure as forming a diagravitational medium for gravitational waves characterized by a refractive index. Furthermore, we shall argue that the most general diagravitational medium has associated an anisotropic dispersion relation. In some situations a refractive index tensor, which takes into account both the deflection of gravitational waves due to the curvature of a non-flat spacetime and the modifications of the general relativistic predictions, can be defined. The most general media, however, entail the consideration of at least two independent tensors.

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arXiv:1806.02094 gr-qc

**Hawking-Ellis type III spacetime geometry**

** **

**Authors:** Prado
Martin-Moruno, Matt
Visser

Abstract: The type III (and the "essential core" type III0) stress-energy tensors in the Hawking-Ellis (Segre-Plebanski) classification stand out in that there is to date no known source (either classical or semi-classical) leading to type III stress-energy. (In contrast the Hawking-Ells types I and II occur classically, and type IV is known to occur semi-classically). We instead start by asking the obverse question: What sort of spacetime (assuming the Einstein equations) needs a type III stress-energy to support it? One key observation is that type III is incompatible with either planar or spherical symmetry, so one should be looking at spacetimes of low symmetry (or no symmetry). Finding such a type III spacetime is a matter of somehow finding an appropriate ansatz for the metric, calculating the Einstein tensor, and analyzing the pattern of (Lorentz invariant) eigenvalues and eigenvectors. Herein we report some (partial) success along these lines - we explicitly exhibit several (somewhat unnatural) spacetime geometries with a type III Einstein tensor. We then build an explicit but somewhat odd Lagrangian model leading (in Minkowski space) to type III stress-energy. While we still have no fully acceptable general physical model for type III stress-energy, we can at least say something about what such a stress-energy tensor would entail.

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arXiv:1806.11020 gr-qc

**Graviton-photon oscillation in alternative theories of gravity**

** **

**Authors:** Jose
A. R. Cembranos, Mario
Coma Diaz, Prado
Martin-Moruno

Abstract: In this paper we investigate graviton-photon oscillation in the presence of an external magnetic field in alternative theories of gravity. Whereas the effect of an effective refractive index for the electromagnetic radiation was already considered in the literature, we develop the first approach to take into account the effect of the modification of the predictions for gravitational waves in alternative theories of gravity in the phenomenon of graviton-photon mixing.

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arXiv:1902.04232 gr-qc

**Vaidya spacetimes, black-bounces, and traversable wormholes**

** **

**Authors:** Alex
Simpson, Prado
Martin-Moruno, Matt
Visser

Abstract: We consider a non-static evolving version of the regular "black-bounce"/traversable wormhole geometry recently introduced in JCAP02(2019)042 [arXiv:1812.07114 [gr-qc]]. We first re-write the static metric using Eddington-Finkelstein coordinates, and then allow the mass parameter m to depend on the null time coordinate (a la Vaidya). The spacetime metric is

ds^{2
}=3D −(1−2m(w)/Ö(r^{2}+a^{2}))dw^{2
}− (=A1=C02dwdr)
+ (r^{2}+a^{2})(d=A6=C8^{2}+sin^{2}=A6=C8d=A6=D5^{2}).

Here w=3D{u,v} denotes the {outgoing,ingoing} null time coordinate; representing {retarded,advanced} time. This spacetime is still simple enough to be tractable, and neatly interpolates between Vaidya spacetime, a black-bounce, and a traversable wormhole. We show how this metric can be used to describe several physical situations of particular interest, including a growing black-bounce, a wormhole to black-bounce transition, and the opposite black-bounce to wormhole transition.

Journal reference: Classical and Quantum Gravity 36 # 14 (2019) 145007

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arXiv:1904.01836 gr-qc

**Phantom singularities and their quantum fate: general relativity and beyond**

** **

**Authors:** Mariam
Bouhmadi-L=A8=AEpez, Claus
Kiefer, Prado
Mart=A8=AAn-Moruno

Abstract: Cosmological observations allow the possibility that dark energy is caused by phantom fields. These fields typically lead to the occurrence of singularities in the late Universe. We review here the status of phantom singularities and their possible avoidance in a quantum theory of gravity. We first introduce phantom energy and discuss its behavior in cosmology. We then list the various types of singularities that can occur from its presence. We also discuss the possibility that phantom behavior is mimicked by an alternative theory of gravity. We finally address the quantum cosmology of these models and discuss in which sense the phantom singularities can be avoided.

Journal reference: Gen Relativ Gravit (2019) 51: 135

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arXiv:1907.01269 gr-qc

**The type III stress-energy tensor: Ugly Duckling of the Hawking-Ellis classification**

** **

**Authors:** Prado
Mart=A8=AAn-Moruno, Matt
Visser

Abstract: We present some advances in the understanding of type III stress-energy tensors as per the Hawking-Ellis classification. Type I and type II naturally appear in classical situations, and can also describe semiclassical effects. Type IV often shows up in semiclassical gravity. Type III is much more subtle. We focus our attention on type III0 stress-energy tensors, which capture the essence ("essential core") of type III. Reflecting on known purely phenomenological examples, ("gyratons"), we are able to generalize the geometry generated by those type III0 stress-energy tensors. Moreover, we also succeed in extending work by Griffiths based on massless Weyl spinors by finding a fundamental classical bosonic Lagrangian description of these type III0 stress-energy tensors. To the best of our knowledge this is the first time in the literature that a consistent classical bosonic Lagrangian formulation for type III0 stress-energy has been found.

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arXiv:1907.13081 gr-qc

**The classical and quantum fate of the Little Sibling of the Big Rip in f(R) cosmology**

** **

**Authors:** Teodor
Borislavov Vasilev, Mariam
Bouhmadi-L=A8=AEpez, Prado
Mart=A8=AAn-Moruno

Abstract: The Little Sibling of the Big Rip is a cosmological abrupt event predicted by some phantom cosmological models that could describe our Universe. When this event is approached the observable Universe and its expansion rate grow infinitely, but its cosmic derivative remains finite. In this work we have obtained the group of metric f(R) theories of gravity that reproduce this classical cosmological background evolution. Furthermore, we have considered the quantization of some of the resulting models in the framework of quantum geometrodynamics, showing that the DeWitt criterion can be satisfied. Therefore, as it also happens in General Relativity, this event may be avoided in f(R) quantum cosmology.

Journal reference: Phys. Rev. D 100, 084016 (2019)

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arXiv:1709.01628 gr-qc

**Causal properties of nonlinear gravitational waves in modified gravity**

** **

**Authors:** Arthur
George Suvorov, Andrew
Melatos

Abstract: Some exact, nonlinear, vacuum gravitational wave solutions are derived for certain polynomial f(R) gravities. We show that the boundaries of the gravitational domain of dependence, associated with events in polynomial f(R) gravity, are not null as they are in general relativity. The implication is that electromagnetic and gravitational causality separate into distinct notions in modified gravity, which may have observable astrophysical consequences. The linear theory predicts that tachyonic instabilities occur, when the quadratic coefficient a2 of the Taylor expansion of f(R) is negative, while the exact, nonlinear, cylindrical wave solutions presented here can be superluminal for all values of a2. Anisotropic solutions are found, whose wave-fronts trace out time- or space-like hypersurfaces with complicated geometric properties. We show that the solutions exist in f(R) theories that are consistent with Solar System and pulsar timing experiments.

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arXiv:1810.03812 gr-qc

**Directed searches for gravitational waves from ultralight bosons**

** **

**Authors:** Maximiliano
Isi, Ling
Sun, Richard
Brito, Andrew
Melatos

Abstract: Gravitational-wave detectors can search for yet-undiscovered ultralight bosons, including those conjectured to solve problems in particle physics, high-energy theory and cosmology. Ground-based instruments could probe boson masses between 10−15 eV to 10−11 eV, which are largely inaccessible to other experiments. In this paper, we explore the prospect of searching for the continuous gravitational waves generated by boson clouds around known black holes. We carefully study the predicted waveforms and use the latest-available numerical results to model signals for different black-hole and boson parameters. We then demonstrate the suitability of a specific method (hidden Markov model tracking) to efficiently search for such signals, even when the source parameters are not perfectly known and allowing for some uncertainty in theoretical predictions. We empirically study this method's sensitivity and computational cost in the context of boson signals, finding that it will be possible to target remnants from compact-binary mergers localized with at least three instruments. For signals from scalar clouds, we also compute detection horizons for future detectors (Advanced LIGO, LIGO Voyager, Cosmic Explorer and the Einstein Telescope). Among other results, we find that, after one year of observation, an Advanced LIGO detector at design sensitivity could detect these sources up to over 100 Mpc, while Cosmic Explorer could reach over 104 Mpc. These projections offer a more complete picture than previous estimates based on analytic approximations to the signal power or idealized search strategies. Finally, we discuss specific implications for the followup of compact-binary coalescences and black holes in x-ray binaries. Along the way, we review the basic physics of bosons around black holes, in the hope of providing a bridge between the theory and data-analysis literatures.

Journal reference: Phys. Rev. D 99, 084042 (2019)

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arXiv:1711.10896 gr-qc

**Cosmological Newtonian limits on large spacetime scales**

** **

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

Abstract: We establish the existence of 1-parameter families of =A6=C5-dependent solutions to the Einstein-Euler equations with a positive cosmological constant =A6=AB>0 and a linear equation of state p=3D=A6=C52K=A6=D1, 0<K=A1=DC1/3, for the parameter values 0<=A6=C5<=A6=C50. These solutions exist globally on the manifold M=3D(0,1]=A1=C1R3, are future complete, and converge as =A6=C5=A8K0 to solutions of the cosmological Poisson-Euler equations. They represent inhomogeneous, nonlinear perturbations of a FLRW fluid solution where the inhomogeneities are driven by localized matter fluctuations that evolve to good approximation according to Newtonian gravity.

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arXiv:1907.04071 math.AP gr-qc

**The Fuchsian approach to global existence for hyperbolic equations**

** **

**Authors:** Florian
Beyer, Todd
A. Oliynyk, J.
Arturo Olvera-Santamar=A8=AAa

Abstract: We analyze the Cauchy problem for symmetric hyperbolic equations with a time singularity of Fuchsian type and establish a global existence theory along with decay estimates for evolutions towards the singular time under a small initial data assumption. We then apply this theory to semilinear wave equations near spatial infinity on Minkowski and Schwarzschild spacetimes, and to the relativistic Euler equations with Gowdy symmetry on Kasner spacetimes.

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arXiv:1907.08192 math.AP gr-qc

**Dynamical relativistic liquid bodies**

** **

**Authors:** Todd
A. Oliynyk

Abstract: We establish the local-in-time existence of solutions to the relativistic Euler equations representing dynamical liquid bodies in vacuum.

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arXiv:1712.09997 gr-qc

**Uncertainty principle for momentum, torsional regularization, and bare charge**

** **

**Authors:** Nikodem
Popławski

Abstract: We show that in the presence of the torsion tensor Skij,
whose existence is required by the consistency of the conservation law for the total angular momentum of a Dirac particle in curved spacetime with relativistic quantum mechanics, the quantum commutation relation for the four-momentum is given by [p_{i},p_{j}]=3D2iℏS^{k}_{ij}p_{k}.
We propose that this relation replaces the integration in the momentum space in Feynman diagrams with the summation over the discrete momentum eigenvalues. We derive a prescription for this summation that agrees with convergent integrals:

=A1=D2d^{4}p/(p^{2}+=A6=A4)^{s
}=A1=FA
4=A6=D0U^{s}^{−}^{2
}=A1=C6_{l}_{=3D}_{1}^{=A1=DE
}=A1=D2_{0}^{=A6=D0}^{/}^{2}d=A6=D5
sin^{4}=A6=D5
n^{s}^{−}^{3}/[sin=A6=D5+U=A6=A4n]^{s},

where n=3D Ö[l(l+1)] and 1/ÖU is a constant on the order of the Planck mass, determined by the Einstein-Cartan theory of gravity. We show that this prescription regularizes ultraviolet-divergent integrals in loop diagrams. We extend this prescription to tensor integrals and apply it to vacuum polarization. We derive a finite, gauge-invariant vacuum polarization tensor and a finite running coupling that agrees with the low-energy limit of the standard quantum electrodynamics. Including loops from all charged fermions, we find a finite value for the bare electric charge of an electron: =A1=D6 −1.22e. Torsional regularization, originating from the noncommutativity of the momentum and spin-torsion coupling, therefore provides a realistic, physical mechanism for eliminating infinities in quantum field theory: quantum electrodynamics with torsion is ultraviolet complete.

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arXiv:1801.08076 physics.pop-ph gr-qc

**The simplest origin of the big bounce and inflation**

** **

**Authors:** Nikodem
Popławski

Abstract: Torsion is a geometrical object, required by quantum mechanics in curved spacetime, which may naturally solve fundamental problems of general theory of relativity and cosmology. The black-hole cosmology, resulting from torsion, could be a scenario uniting the ideas of the big bounce and inflation, which were the subject of a recent debate of renowned cosmologists.

Journal reference: Int. J. Mod. Phys. D 27, 1847020 (2018)

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arXiv:1807.07068 hep-th gr-qc

**Torsional regularization of vertex function**

** **

**Authors:** Nikodem
Popławski

Abstract: The noncommutativity of the momentum components, arising from spacetime torsion coupled to spin, replaces the integration over the momentum in loop Feynman diagrams with the summation over the momentum eigenvalues. This prescription regularizes logarithmically divergent integrals by turning them into convergent sums. We apply torsional regularization to the vertex function in quantum electrodynamics in the one-loop correction. The magnetic form factor is consistent with Schwinger's value for the anomalous magnetic dipole moment of a fermion. We show that it depends on the mass of a fermion, which may explain the observed deviation of the magnetic moment of a muon from the predicted theoretical value. We also show that the electric form factor is ultraviolet convergent.

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arXiv:1808.08327 gr-qc

**Big bounce and closed universe from spin and torsion**

** **

**Authors:** Gabriel
Unger, Nikodem
Popławski

Abstract: We analyze the dynamics of a homogeneous and isotropic universe in the Einstein−−Cartan theory of gravity. The coupling between the spin and torsion prevents gravitational singularities and replaces the Big Bang with a nonsingular big bounce, at which the universe transitions from contraction to expansion. We show that a closed universe exists only when the product of the scale factor and temperature is higher than a particular threshold, contrary to a flat universe and an open universe, which are not restricted. During inflation, this product must increase to another threshold, so that the universe can reach dark-energy acceleration.

Journal reference: Astrophys. J. 870, 78 (2019)

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arXiv:1906.11824 gr-qc

**Analysis of big bounce in Einstein--Cartan cosmology**

** **

**Authors:** Jordan
L. Cubero, Nikodem
J. Popławski

Abstract: We analyze the dynamics of a homogeneous and isotropic universe in the Einstein--Cartan theory of gravity. The spin of fermions produces spacetime torsion that prevents gravitational singularities and replaces the big bang with a nonsingular big bounce. We show that a closed universe exists only when a particular function of its scale factor and temperature is higher than some threshold value, whereas an open and a flat universes do not have such a restriction. We also show that a bounce of the scale factor is double: as the temperature increases and then decreases, the scale factor decreases, increases, decreases, and then increases.

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arXiv:1910.10819 physics.pop-ph gr-qc

**Black Hole Genesis and origin of inertia**

** **

**Authors:** Nikodem
Popławski

Abstract: I propose that if the universe was born as a baby universe on the other side of the event horizon of a black hole existing in a parent universe, then the corresponding white hole provides the absolute inertial frame of reference in the universe. The principle of relativity then allows to construct an infinity of other inertial frames. Consequently, this scenario could give the origin of inertia and complete Einstein's general theory of relativity by making it consistent with Mach's principle.

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arXiv:1903.02143 math.MG gr-qc

**The global properties of the finiteness and continuity of the Lorentzian distance**

** **

**Authors:** Adam
Rennie, Ben
Whale

Abstract: It is well-known that global hyperbolicity implies that the Lorentzian distance is finite and continuous. By carefully analysing the causes of discontinuity of the Lorentzian distance, we show that in most other respects the finiteness and continuity of the Lorentzian distance is independent of the causal structure. The proof of these results relies on the properties of a class of generalised time functions introduced by the authors in \cite{RennieWhale2016}.

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arXiv:1807.07427 astro-ph.IM gr-qc

**Implications of dedicated seismometer measurements on Newtonian-noise cancellation for Advanced LIGO**

** **

**Authors:** M.
W. Coughlin, J.
Harms, J.
Driggers, D.
J. McManus, N.
Mukund, M.
P. Ross, B.
J. J. Slagmolen, K.
Venkateswara

Abstract: Newtonian gravitational noise from seismic fields will become a limiting noise source at low frequency for second-generation, gravitational-wave detectors. It is planned to use seismic sensors surrounding the detectors' test masses to coherently subtract Newtonian noise using Wiener filters derived from the correlations between the sensors and detector data. In this work, we use data from a seismometer array deployed at the corner station of the LIGO Hanford detector combined with a tiltmeter for a detailed characterization of the seismic field and to predict achievable Newtonian-noise subtraction levels. As was shown previously, cancellation of the tiltmeter signal using seismometer data serves as the best available proxy of Newtonian-noise cancellation. According to our results, a relatively small number of seismometers is likely sufficient to perform the noise cancellation due to an almost ideal two-point spatial correlation of seismic surface displacement at the corner station, or alternatively, a tiltmeter deployed under each of the two test masses of the corner station at Hanford will be able to efficiently cancel Newtonian noise. Furthermore, we show that the ground tilt to differential arm-length coupling observed during LIGO's second science run is consistent with gravitational coupling.

Journal reference: Phys. Rev. Lett. 121, 221104 (2018)

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arXiv:1804.09413 astro-ph.HE gr-qc

**Astroseismology of neutron stars from gravitational waves in the limit of perfect measurement**

** **

**Authors:** Arthur
G Suvorov

Abstract: The oscillation spectrum of a perturbed neutron star is intimately related to the physical properties of the star, such as the equation of state. Observing pulsating neutron stars therefore allows one to place constraints on these physical properties. However, it is not obvious exactly how much can be learnt from such measurements. If we observe for long enough, and precisely enough, is it possible to learn everything about the star? A classical result in the theory of spectral geometry states that one cannot uniquely `hear the shape of a drum'. More formally, it is known that an eigenfrequency spectrum may not uniquely correspond to a particular geometry; some `drums' may be indistinguishable from a normal-mode perspective. In contrast, we show that the drum result does not extend to perturbations of simple neutron stars within general relativity -- in the case of axial (toroidal) perturbations of static, perfect fluid stars, a quasi-normal mode spectrum uniquely corresponds to a stellar profile. We show in this paper that it is not possible for two neutron stars, with distinct fluid profiles, to oscillate in an identical manner. This result has the information-theoretic consequence that gravitational waves completely encode the properties of any given oscillating star: unique identifications are possible in the limit of perfect measurement.

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arXiv:1810.02975 astro-ph.HE gr-qc

**Monopolar and quadrupolar gravitational radiation from magnetically deformed neutron stars in modified gravity**

** **

**Authors:** Arthur
G Suvorov

Abstract: Some modified theories of gravity are known to predict monopolar, in addition to the usual quadrupolar and beyond, gravitational radiation in the form of `breathing' modes. For the same reason that octupole and higher-multipole terms often contribute negligibly to the overall wave strain, monopole terms tend to dominate. We investigate both monopolar and quadrupolar continuous gravitational radiation from neutron stars deformed through internal magnetic stresses. We adopt the Parameterised-Post-Newtonian formalism to write down equations describing the leading-order stellar properties in a theory-independent way, and derive some exact solutions for stars with mixed poloidal-toroidal magnetic fields. We then turn to the specific case of scalar-tensor theories to demonstrate how observational upper limits on the gravitational-wave luminosity of certain neutron stars may be used to place constraints on modified gravity parameters, most notably the Eddington parameter =A6=C3. For conservative, purely poloidal models with characteristic field strength given by the spindown minimum, upper limits for the Vela pulsar yield 1−=A6=C3≲4.2=A1=C110−3. For models containing a strong toroidal field housing ∼99% of the internal magnetic energy, we obtain the bound 1−=A6=C3≲8.0=A1=C110−7. This latter bound is an order of magnitude tighter than those obtained from current Solar system experiments, though applies to the strong-field regime.

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arXiv:1905.02021 gr-qc

**Gravitational perturbations of a Kerr black hole in f(R) gravity**

** **

**Authors:** Arthur
G Suvorov

Abstract: Modified theories of gravity are often built such that they contain general relativity as a limiting case. This inclusion property implies that the Kerr metric is common to many families of theories. For example, all analytic f(R) theories with vanishing constant term admit the Kerr solution. In any given theory, however, the response of the gravitational field to astrophysical disturbances is tied to the structure of the field equations. As such, even if black holes are Kerr, the underlying theory can, in principle, be probed through gravitational distortions. In this paper, we study linear perturbations of a Kerr black hole in f(R) gravity using the Newman-Penrose formalism. We show that, as in general relativity, the equations governing the perturbed metric, which depend on the quadratic term of the function f, completely decouple.

Journal reference: Phys. Rev. D 99, 124026 (2019)

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arXiv:1908.11609 gr-qc

**A metric on the space of neutron star models in general relativity and modified gravity**

** **

**Authors:** Arthur
G Suvorov

Abstract: Intuitively, some pairs of neutron star models can be thought of as being `closer' together than others, in the sense that more precise observations might be required to distinguish between them than would be necessary for other pairs. In this paper, borrowing ideas from the study of geometrodynamics, we introduce a mathematical formalism to define a geometric distance between stellar models, to provide a quantitative meaning for this notion of `closeness'. In particular, it is known that the set of all Riemannian metrics on a manifold itself admits the structure of a Riemannian manifold (`configuration manifold'), which comes equipped with a canonical metric. By thinking of a stationary star as being a particular 3+1 metric, the structure of which is determined through the Tolman-Oppenheimer-Volkoff relations and their generalisations, points on a suitably restricted configuration manifold can be thought of as representing different stars, and distances between these points can be computed. We develop the necessary mathematical machinery to build the configuration manifold of neutron star models, and provide some worked examples to illustrate how this space might be used in future studies of neutron star structure.

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arXiv:1712.00688 gr-qc

**The optimal search for an astrophysical gravitational-wave background**

** **

**Authors:** Rory
Smith, Eric
Thrane

Abstract: Roughly every 2-10 minutes, a pair of stellar mass black holes merge somewhere in the Universe. A small fraction of these mergers are detected as individually resolvable gravitational-wave events by advanced detectors such as LIGO and Virgo. The rest contribute to a stochastic background. We derive the statistically optimal search strategy for a background of unresolved binaries. Our method applies Bayesian parameter estimation to all available data. Using Monte Carlo simulations, we demonstrate that the search is both "safe" and effective: it is not fooled by instrumental artefacts such as glitches, and it recovers simulated stochastic signals without bias. Given realistic assumptions, we estimate that the search can detect the binary black hole background with about one day of design sensitivity data versus =A1=D640 months using the traditional cross-correlation search. This framework independently constrains the merger rate and black hole mass distribution, breaking a degeneracy present in the cross-correlation approach. The search provides a unified framework for population studies of compact binaries, which is cast in terms of hyper-parameter estimation. We discuss a number of extensions and generalizations including: application to other sources (such as binary neutron stars and continuous-wave sources), simultaneous estimation of a continuous Gaussian background, and applications to pulsar timing.

Journal reference: Phys. Rev. X 8, 021019 (2018)

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arXiv:1802.00885 gr-qc

**Measurement and subtraction of Schumann resonances at gravitational-wave interferometers**

** **

**Authors:** Michael
W. Coughlin, Alessio
Cirone, Patrick
Meyers, Sho
Atsuta, Valerio
Boschi, Andrea
Chincarini, Nelson
L. Christensen, Rosario
De Rosa, Anamaria
Effler, Irene
Fiori, Mark
Golkowski, Melissa
Guidry, Jan
Harms, Kazuhiro
Hayama, Yuu
Kataoka, Jerzy
Kubisz, Andrzej
Kulak, Michael
Laxen, Andrew
Matas, Janusz
Mlynarczyk, Tsutomu
Ogawa, Federico
Paoletti, Jacobo
Salvador, Robert
Schofield, Kentaro
Somiya, Eric Thrane

Abstract: Correlated magnetic noise from Schumann resonances threatens to contaminate the observation of a stochastic gravitational-wave background in interferometric detectors. In previous work, we reported on the first effort to eliminate global correlated noise from the Schumann resonances using Wiener filtering, demonstrating as much as a factor of two reduction in the coherence between magnetometers on different continents. In this work, we present results from dedicated magnetometer measurements at the Virgo and KAGRA sites, which are the first results for subtraction using data from gravitational-wave detector sites. We compare these measurements to a growing network of permanent magnetometer stations, including at the LIGO sites. We show how dedicated measurements can reduce coherence to a level consistent with uncorrelated noise. We also show the effect of mutual magnetometer attraction, arguing that magnetometers should be placed at least one meter from one another.

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arXiv:1805.03761 astro-ph.IM gr-qc

**All-sky radiometer for narrowband gravitational waves using folded data**

** **

**Authors:** Boris
Goncharov, Eric
Thrane

Abstract: We demonstrate an all-sky search for persistent, narrowband gravitational waves using mock data. The search employs radiometry to sidereal-folded data in order to uncover persistent sources of gravitational waves with minimal assumptions about the signal model. The method complements continuous-wave searches, which are finely tuned to search for gravitational waves from rotating neutron stars while providing a means of detecting more exotic sources that might be missed by dedicated continuous-wave techniques. We apply the algorithm to simulated Gaussian noise at the level of LIGO design sensitivity. We project the strain amplitude sensitivity for the algorithm for a LIGO network in the first observing run to be h0=A1=D61.2=A1=C110−24 (1% false alarm probability, 10% false dismissal probability). We include treatment of instrumental lines and detector artifacts using time-shifted LIGO data from the first observing run.

Journal reference: Phys. Rev. D 98, 064018 (2018)

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arXiv:1806.02346 astro-ph.GA gr-qc

**The minimum and maximum gravitational-wave background from supermassive binary black holes**

** **

**Authors:** Xing-Jiang
Zhu, Weiguang
Cui, Eric
Thrane

Abstract: The gravitational-wave background from supermassive binary black holes (SMBBHs) has yet to be detected. This has led to speculations as to whether current pulsar timing array limits are in tension with theoretical predictions. In this paper, we use electromagnetic observations to constrain the SMBBH background from above and below. To derive the {\em maximum} amplitude of the background, we suppose that equal-mass SMBBH mergers fully account for the local black hole number density. This yields a maximum characteristic signal amplitude at a period of one year Ayr<2.4=A1=C110−15, which is comparable to the pulsar timing limits. To derive the {\em minimum} amplitude requires an electromagnetic observation of an SMBBH. While a number of candidates have been put forward, there are no universally-accepted electromagnetic detections in the nanohertz band. We show the candidate 3C 66B implies a lower limit, which is inconsistent with limits from pulsar timing, casting doubt on its binary nature. Alternatively, if the parameters of OJ 287 are known accurately, then Ayr>6.1=A1=C110−17 at 95\% confidence level. If one of the current candidates can be established as a bona fide SMBBH, it will immediately imply an astrophysically interesting lower limit.

Journal reference: MNRAS (2019), 482, 2588

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arXiv:1902.03300 astro-ph.HE gr-qc

**The mass distribution of Galactic double neutron stars
**

** **

**Authors:** Nicholas
Farrow, Xing-Jiang
Zhu, Eric
Thrane

Abstract: The conventional wisdom, dating back to 2012, is that the mass distribution of Galactic double neutron stars is well-fit by a Gaussian distribution with a mean of 1.33M=A1=D1 and a width of 0.09M=A1=D1. With the recent discovery of new Galactic double neutron stars and GW170817, the first neutron star merger event to be observed with gravitational waves, it is timely to revisit this model. In order to constrain the mass distribution of double neutron stars, we perform Bayesian inference using a sample of 17 Galactic double neutron stars effectively doubling the sample used in previous studies. We expand the space of models so that the recycled neutron star need not be drawn from the same distribution as the non-recycled companion. Moreover, we consider different functional forms including uniform, single-Gaussian, and two-Gaussian distributions. While there is insufficient data to draw firm conclusions, we find positive support (a Bayes factor of 9) for the hypothesis that recycled and non-recycled neutron stars have distinct mass distributions. The most probable model---preferred with a Bayes factor of 29 over the conventional model---is one in which the recycled neutron star mass is distributed according to a two-Gaussian distribution and the non-recycled neutron star mass is distributed uniformly. We show that precise component mass measurements of =A1=D620 double neutron stars are required in order to determine with high confidence (a Bayes factor of 150) if recycled and non-recycled neutron stars come from a common distribution. Approximately 60 are needed in order to establish the detailed shape of the distributions.

Journal reference: 2019, ApJ, 876, 18

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arXiv:1903.09260 astro-ph.HE gr-qc

**Cosmology and the Early Universe**

**Authors:** B.
S. Sathyaprakash, Enis
Belgacem, Daniele
Bertacca, Chiara
Caprini, Giulia
Cusin, Yves
Dirian, Xilong
Fan, Daniel
Figueroa, Stefano
Foffa, Evan
Hall, Jan
Harms, Michele
Maggiore, Vuk
Mandic, Andrew
Matas, Tania
Regimbau, Mairi
Sakellariadou, Nicola
Tamanini, Eric
Thrane

Abstract: This Astro-2020 White Paper deals with what we might learn from future gravitational wave observations about the early universe phase transitions and their energy scales, primordial black holes, Hubble parameter, dark matter and dark energy, modified theories of gravity and extra dimensions.

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arXiv:1904.02863 astro-ph.IM gr-qc

**Parallelized Inference for Gravitational-Wave Astronomy**

** **

**Authors:** Colm
Talbot, Rory
Smith, Eric
Thrane, Gregory
B. Poole

Abstract: Bayesian inference is the workhorse of gravitational-wave astronomy, for example, determining the mass and spins of merging black holes, revealing the neutron star equation of state, and unveiling the population properties of compact binaries. The science enabled by these inferences comes with a computational cost that can limit the questions we are able to answer. This cost is expected to grow. As detectors improve, the detection rate will go up, allowing less time to analyze each event. Improvement in low-frequency sensitivity will yield longer signals, increasing the number of computations per event. The growing number of entries in the transient catalog will drive up the cost of population studies. While Bayesian inference calculations are not entirely parallelizable, key components are embarrassingly parallel: calculating the gravitational waveform and evaluating the likelihood function. Graphical processor units (GPUs) are adept at such parallel calculations. We report on progress porting gravitational-wave inference calculations to GPUs. Using a single code - which takes advantage of GPU architecture if it is available - we compare computation times using modern GPUs (NVIDIA P100) and CPUs (Intel Gold 6140). We demonstrate speed-ups of ∼50=A1=C1 for compact binary coalescence gravitational waveform generation and likelihood evaluation and more than 100=A1=C1 for population inference within the lifetime of current detectors. Further improvement is likely with continued development. Our python-based code is publicly available and can be used without familiarity with the parallel computing platform, CUDA.

Journal reference: Phys. Rev. D 100, 043030 (2019)

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arXiv:1905.05477 astro-ph.IM gr-qc

**Higher order gravitational-wave modes with likelihood reweighting**

** **

**Authors:** Ethan
Payne, Colm
Talbot, Eric
Thrane

Abstract: The gravitational waveform of a merging stellar-mass binary is described at leading order by a quadrupolar mode. However, the complete waveform includes higher-order modes, which encode valuable information not accessible from the leading-order mode alone. Despite this, the majority of astrophysical inferences so far obtained with observations of gravitational waves employ only the leading order mode because calculations with higher-order modes are often computationally challenging. We show how to efficiently incorporate higher-order modes into astrophysical inference calculations with a two step procedure. First, we carry out Bayesian parameter estimation using a computationally cheap leading-order-mode waveform, which provides an initial estimate of binary parameters. Second, we weight the initial estimate using higher-order mode waveforms in order to fold in the extra information from the full waveform. We use mock data to demonstrate the effectiveness of this method. We apply the method to each binary black hole event in the first gravitational-wave transient catalog GWTC-1 to obtain posterior distributions and Bayesian evidence with higher-order modes. Performing Bayesian model selection on the events in GWTC-1, we find only a weak preference for waveforms with higher order modes. We discuss how this method can be generalized to a variety of other applications.

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arXiv:1909.11872 gr-qc

**Gravitational wave detection without boot straps: a Bayesian approach**

** **

**Authors:** Gregory
Ashton, Eric
Thrane, Rory
J. E. Smith

Abstract: In order to separate astrophysical gravitational-wave signals from instrumental noise, which often contains transient non-Gaussian artifacts, astronomers have traditionally relied on bootstrap methods such as time slides. Bootstrap methods sample with replacement, comparing single-observatory data to construct a background distribution, which is used to assign a false-alarm probability to candidate signals. While bootstrap methods have played an important role establishing the first gravitational-wave detections, there are limitations. First, as the number of detections increases, it makes increasingly less sense to treat single-observatory data as bootstrap-estimated noise, when we know that the data are filled with astrophysical signals, some resolved, some unresolved. Second, it has been known for a decade that background estimation from time-slides eventually breaks down due to saturation effects, yielding incorrect estimates of significance. Third, the false alarm probability cannot be used to weight candidate significance, for example when performing population inference on a set of candidates. Given recent debate about marginally resolved gravitational-wave detection claims, the question of significance has practical consequences. We propose a Bayesian framework for calculating the odds that a signal is of astrophysical origin versus instrumental noise without bootstrap noise estimation. We show how the astrophysical odds can safely accommodate glitches. We argue that it is statistically optimal. We demonstrate the method with simulated noise and provide examples to build intuition about this new approach to significance.

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arXiv:1911.01379 astro-ph.HE gr-qc

**Constraining short gamma-ray burst jet properties with gravitational waves and gamma rays**

** **

**Authors:** Sylvia
Biscoveanu, Eric
Thrane, Salvatore
Vitale

Abstract: Gamma-ray burst (GRB) prompt emission is highly beamed, and understanding the jet geometry and beaming configuration can provide information on the poorly understood central engine and circum-burst environment. Prior to the advent of gravitational-wave astronomy, astronomers relied on observations of jet breaks in the multi-wavelength afterglow to determine the GRB opening angle, since the observer's viewing angle relative to the system cannot be determined from the electromagnetic data alone. Gravitational-wave observations, however, provide an independent measurement of the viewing angle. We describe a Bayesian method for determining the geometry of short GRBs using coincident electromagnetic and gravitational-wave observations. We demonstrate how an ensemble of multi-messenger detections can be used to measure the distributions of the jet energy, opening angle, Lorentz factor, and angular profile of short GRBs; we find that for a population of 100 such observations, we can constrain the mean of the opening angle distribution to within 10 degrees regardless of the angular emission profile. Conversely, the constraint on the energy distribution depends on the shape of the profile, which can be distinguished.

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arXiv:1911.00644 gr-qc

**On ab initio closed-form expressions for gravitational waves**

** **

**Authors:** Manuel
Tiglio, Aar=A8=AEn
Villanueva

Abstract: We introduce an approach for finding ab initio, high accuracy, closed-form expressions for the gravitational waves emitted by binary systems. Our expressions are built from numerical surrogate models based on numerical relativity simulations, which have been shown to be essentially indistinguishable from each other, with the advantage that our expressions can be written explicitly in a few lines. The key new ingredient in this approach is symbolic regression through genetic programming. The minimum overlap obtained in the proof of concept here presented, compared to ground truth solutions, is 99%.

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arXiv:1803.07755 physics.ins-det gr-qc

**Broadband Axion Dark Matter Haloscopes via Electric Sensing**

** **

**Authors:** Ben
T. McAllister, Maxim
Goryachev, Jeremy
Bourhill, Eugene
N. Ivanov, Michael
E. Tobar

Abstract: The mass of axion dark matter
is only weakly bounded by cosmological observations, necessitating a variety of detection techniques over several orders of magnitude of mass ranges. Axions haloscopes based on resonant cavities have become the current standard to search for dark matter axions.
Such structures are inherently narrowband and for low masses the volume of the required cavity becomes prohibitively large. Broadband low-mass detectors have already been proposed using inductive magnetometer sensors and a gapped toroidal solenoid magnet.
In this work we propose an alternative, which uses electric sensors in a conventional solenoidal magnet aligned in the laboratory z-axis, as implemented in standard haloscope experiments. In the presence of the DC magnetic field, the inverse Primakoff effect
causes a time varying permanent electric vacuum polarization in the z-direction to oscillate at the axion Compton frequency, which induces an oscillating electromotive force. We propose non-resonant techniques to detect this oscillating elctromotive force
by implementing a capacitive sensor or an electric dipole antenna coupled to a low noise amplifier. We present the first experimental results and discuss the foundations and potential of this proposal. Preliminary results constrain ga=A6=C3=A6=C3>∼2.35=A1=C110^{−}^{12} GeV−1 in
the mass range of 2.08=A1=C110^{−}^{11} to 2.2=A1=C110^{−}^{11} eV,
and demonstrate potential sensitivity to axion-like dark matter with masses in the range of 10^{−}^{12} to 10^{−}^{8} eV.

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arXiv:1809.01654 hep-ph gr-qc

**Modified Axion Electrodynamics as Impressed Electromagnetic Sources Through Oscillating Background Polarization and Magnetization**

** **

**Authors:** Michael
Edmund Tobar, Ben
T. McAllister, Maxim
Goryachev

Abstract: We present a reformulation of axion modified electrodynamics with all modifications redefined within the constitutive relations between the D,H,B and E fields. This allows the interpretation of the axion induced background bound charge, polarization current and background polarization and magnetization satisfying the charge-current continuity equation. This representation is of similar form to photon sector odd-parity Lorentz invariance violating background fields. We show that when a DC B-field is applied an oscillating background polarization is induced at a frequency equivalent to the axion mass. In contrast, when DC E-field is applied, an oscillating background magnetization is induced at a frequency equivalent to the axion mass. We show that these terms are equivalent to impressed source terms, analogous to the way that voltage and current sources are impressed into Maxwell's equations in circuit and antenna theory. The impressed source terms represent the conversion of external energy into electromagnetic energy, and in the case of axion modified electrodynamics this is due to the inverse Primakoff effect converting energy from axions into photons. The axion induced oscillating polarization under a DC magnetic field is analogous to a permanent polarised electret oscillating at the axion Compton frequency, which sources an electromotive force from an effective impressed magnetic current source. In particular, it is shown that the impressed electrical DC current that drives the solenoidal magnetic DC field of an electromagnet, induces an impressed magnetic current parallel to the DC electrical current, oscillating at the Compton frequency of the axion. We show that the magnetic current drives a voltage source through an electric vector potential and also defines the boundary condition of the oscillating axion induced polarization inside and outside the electromagnet.

Journal reference: Physics of the Dark Universe 26 (2019) 100339

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arXiv:1812.11123 gr-qc

**Combined Search for a Lorentz-Violating Force in Short-Range Gravity Varying as the Inverse Sixth Power of Distance**

** **

**Authors:** Cheng-Gang
Shao, Ya-Fen
Chen, Yu-Jie
Tan, Shan-Qing
Yang, Jun
Luo, Michael
Edmund Tobar, J.
C. Long, E.
Weisman, Alan
Kostelecky

Abstract: Precision measurements of the
inverse-square law via experiments on short-range gravity provide sensitive tests of Lorentz symmetry. A combined analysis of data from experiments at the Huazhong University of Science and Technology and Indiana University sets simultaneous limits on all
22 coefficients for Lorentz violation correcting the Newton force law as the inverse sixth power of distance. Results are consistent with no effect at the level of 10^{−}^{12} m^{4}.

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arXiv:1903.03346 quant-ph gr-qc

**Testing of Generalized Uncertainty Principle With Macroscopic Mechanical Oscillators and Pendulums**

** **

**Authors:** P.
A. Bushev, J.
Bourhill, M.
Goryachev, N.
Kukharchyk, E.
Ivanov, S.
Galliou, M.
E. Tobar, S.
Danilishin

Abstract: Recent progress in observing
and manipulating mechanical oscillators at quantum regime provides new opportunities of studying fundamental physics, for example, to search for low energy signatures of quantum gravity. For example, it was recently proposed that such devices can be used to
test quantum gravity effects, by detecting the change in the [x,p] commutation relation that could result from quantum gravity corrections. We show that such a correction results in a dependence of a resonant frequency of a mechanical oscillator on its amplitude,
which is known as amplitude-frequency effect. By implementing this new method we measure amplitude-frequency effect for 0.3 kg ultra high-Q sapphire split-bar mechanical resonator and for 10 mg quartz bulk acoustic wave resonator. Our experiments with sapphire
resonator have established the upper limit on quantum gravity correction constant for =A6=C2_{0}<5=A1=C110^{6} which
is a factor of 6 better than previously detected. The reasonable estimates of =A6=C20 from
experiments with quartz resonators yield an even more stringent limit of 4=A1=C110^{4}.
The data sets of 1936 measurement of physical pendulum period by Atkinson results in significantly stronger limitations on =A6=C2_{0}≪1.
Yet, due to the lack of proper pendulum frequency stability measurement in these experiments, the exact upper bound on =A6=C2_{0} can
not be reliably established. Moreover, pendulum based systems only allow testing a specific form of the modified commutator that depends on the mean value of momentum. The electro-mechanical oscillators to the contrary enable testing of any form of generalized
uncertainty principle directly due to much higher stability and a higher degree of control. .

Journal reference: Phys. Rev. D 100, 066020 (2019)

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arXiv:1811.10917 gr-qc

**Reduced phase space optics for general relativity: Symplectic ray bundle transfer**

** **

**Authors:** Nezihe
Uzun

Abstract: In the paraxial regime of Newtonian optics, propagation of an ensemble of rays is represented by a symplectic ABCD transfer matrix defined on a reduced phase space. Here, we present its analogue for general relativity. Starting from general relativistic Fermat's principle, we obtain a geodesic deviation action up to quadratic order following a pre-existing method constructed via Synge's world function. We find the corresponding Hamiltonian function and the reduced phase space coordinates that are composed of the components of the Jacobi fields projected on an observational screen. Our ray bundle transfer matrix is then obtained through the matrix representation of the Lie operator associated with this quadratic Hamiltonian. Moreover, Etherington's distance reciprocity between any two points is shown to be equivalent to the symplecticity conditions of our ray bundle transfer matrix. We further interpret the bundle propagation as a free canonical transformation with a generating function that is equal to the geodesic deviation action. We present it in the form of matrix inner products. A phase space distribution function and the associated Liouville equation is also provided. Finally, we briefly sketch the potential applications of our construction. Those include reduced phase space and null bundle averaging; factorization of light propagation in any spacetime uniquely into its thin lens, pure magnifier and fractional Fourier transformer components; wavization of the ray bundle; reduced polarization optics and autonomization of the bundle propagation on the phase space to find its invariants and obtain the stability analysis.

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arXiv:1707.07457 gr-qc

**Entropy budget for Hawking evaporation**

** **

**Authors:** Ana
Alonso-Serrano, Matt
Visser

Abstract: Blackbody radiation, emitted from a furnace and described by a Planck spectrum, contains (on average) an entropy of 3.9=A1=C02.5 bits per photon. Since normal physical burning is a unitary process, this amount of entropy is compensated by the same amount of "hidden information" in correlations between the photons. The importance of this result lies in the posterior extension of this argument to the Hawking radiation from black holes, demonstrating that the assumption of unitarity leads to a perfectly reasonable entropy/information budget for the evaporation process. In order to carry out this calculation we adopt a variant of the "average subsystem" approach, but consider a tripartite pure system that includes the influence of the rest of the universe, and which allows "young" black holes to still have a non-zero entropy; which we identify with the standard Bekenstein entropy.

Journal reference: Universe 2017, 3(3), 58

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arXiv:1707.09755 quant-ph gr-qc

**Multi-partite analysis of average-subsystem entropies**

** **

**Authors:** Ana
Alonso-Serrano, Matt
Visser

Abstract: So-called average subsystem entropies are defined by first taking partial traces over some pure state to define density matrices, then calculating the subsystem entropies, and finally averaging over the pure states to define the average subsystem entropies. These quantities are standard tools in quantum information theory, most typically applied in bipartite systems. We shall first present some extensions to the usual bipartite analysis, (including a calculation of the average tangle, and a bound on the average concurrence), follow this with some useful results for tripartite systems, and finally extend the discussion to arbitrary multi-partite systems. A particularly nice feature of tri-partite and multi-partite analyses is that this framework allows one to introduce an "environment" for small subsystems to couple to.

Journal reference: Phys. Rev. A 96, 052302 (2017)

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**arXiv:1710.06139**** **gr-qc math-ph

**Near-horizon geodesics for astrophysical and idealised black holes: Coordinate
velocity and coordinate acceleration**

** **

**Authors:** Petarpa
Boonserm, Tritos
Ngampitipan, Matt
Visser

Abstract: Geodesics
(by definition) have an intrinsic 4-acceleration zero. However, when expressed in terms of coordinates, the coordinate acceleration d^{2}x_{i}/dt^{2} can
very easily be non-zero, and the coordinate velocity dxi/dt can behave unexpectedly. The situation becomes extremely delicate in the near-horizon
limit---for both astrophysical and idealised black holes---where an inappropriate choice of coordinates can quite easily lead to significant confusion. We shall carefully explore the relative merits of horizon-penetrating versus horizon-non-penetrating coordinates,
arguing that in the near-horizon limit the coordinate acceleration d^{2}x_{i}/dt^{2} is best interpreted in terms of
horizon-penetrating coordinates.

Journal reference: Universe 4 (2018) 68

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arXiv:1711.11500 gr-qc

**Rastall gravity is equivalent to Einstein gravity**

** **

**Authors:** Matt
Visser

Abstract: Rastall gravity, originally developed in 1972, is currently undergoing a significant surge in popularity. Rastall gravity purports to be a modified theory of gravity, with a non-conserved stress-energy tensor, and an unusual non-minimal coupling between matter and geometry, the Rastall stress-energy satisfying nabla_b [T_R]^{ab} =3D {=A6=CB/4} g^{ab} nabla_b R. Unfortunately, a deeper look shows that Rastall gravity is completely equivalent to Einstein gravity --- usual general relativity. The gravity sector is completely standard, based as usual on the Einstein tensor, while in the matter sector Rastall's stress-energy tensor corresponds to an artificially isolated part of the physical conserved stress-energy.

Journal reference: Physics Letters B 782 (2018) 83--86

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arXiv:1801.05549 gr-qc

**Bespoke analogue space-times: Meta-material mimics**

** **

**Authors:** Sebastian
Schuster, Matt
Visser

Abstract: Modern meta-materials allow one to construct electromagnetic media with almost arbitrary bespoke permittivity, permeability, and magneto-electric tensors. If (and only if) the permittivity, permeability, and magneto-electric tensors satisfy certain stringent compatibility conditions, can the meta-material be fully described (at the wave optics level) in terms of an effective Lorentzian metric --- an analogue spacetime. We shall consider some of the standard black-hole spacetimes of primary interest in general relativity, in various coordinate systems, and determine the equivalent meta-material susceptibility tensors in a laboratory setting. In static black hole spacetimes (Schwarzschild and the like) certain eigenvalues of the susceptibility tensors will be seen to diverge on the horizon. In stationary black hole spacetimes (Kerr and the like) certain eigenvalues of the susceptibility tensors will be seen to diverge on the ergo-surface.

Journal reference: General Relativity and Gravitation 50 (2018) 55

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arXiv:1802.00651 hep-ph gr-qc

**Post-Newtonian particle physics in curved spacetime**

** **

**Authors:** Matt
Visser

Abstract: In three very recent papers, (an initial paper by Morishima and Futamase, and two subsequent papers by Morishima, Futamase, and Shimizu), it has been argued that the observed experimental anomaly in the anomalous magnetic moment of the muon might be explained using general relativity. It is my melancholy duty to report that these articles are fundamentally flawed in that they fail to correctly implement the Einstein equivalence principle of general relativity. Insofar as one accepts the underlying logic behind these calculations (and so rejects general relativity) the claimed effect due to the Earth's gravity will be swamped by the effect due to Sun (by a factor of fifteen), and by the effect due to the Galaxy (by a factor of two thousand). In contrast, insofar as one accepts general relativity, then the claimed effect will be suppressed by an extra factor of [(size of laboratory)/(radius of Earth)]^2. Either way, the claimed effect is not compatible with explaining the observed experimental anomaly in the anomalous magnetic moment of the muon.

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arXiv:1802.04785 gr-qc

**Vorticity in analogue spacetimes**

** **

**Authors:** Stefano
Liberati, Sebastian
Schuster, Giovanni
Tricella, Matt
Visser

Abstract: Analogue
spacetimes can be used to probe and study physically interesting spacetime geometries by constructing, either theoretically or experimentally, some notion of an effective Lorentzian metric [g_{eff}(g,V,=A6=AE)]_{ab}.
These effective metrics generically depend on some physical background metric g_{ab}, often flat Minkowski space =A6=C7_{ab},
some "medium" with 4-velocity Va, and possibly some additional background fields =A6=AE.
Electromagnetic analogue models date back to the 1920s, acoustic analogue models to the 1980s, and BEC-based analogues to the 1990s. The acoustic analogue models have perhaps the most rigorous mathematical formulation, and these acoustic analogue models really
work best in the absence of vorticity, if the medium has an irrotational flow. This makes it difficult to model rotating astrophysical spacetimes, spacetimes with non-zero angular momentum, and in the current article we explore the extent to which one might
hope to be able to model astrophysical spacetimes with angular momentum, (thereby implying vorticity in the 4-velocity of the medium).

Journal reference: Phys. Rev. D 99, 044025 (2019)

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arXiv:1802.06550 gr-qc

**Non-perturbative results for the luminosity and area distances**

** **

**Authors:** Dimitar
Ivanov, Stefano
Liberati, Matteo
Viel, Matt
Visser

Abstract: The
notion of luminosity distance is most often defined in purely FLRW (Friedmann-Lemaitre-Robertson-Walker) cosmological spacetimes, or small perturbations thereof. However, the abstract notion of luminosity distance is actually much more robust than this, and
can be defined non-perturbatively in almost arbitrary spacetimes. Some quite general results are already known, in terms of dA_{observer}/d=A6=B8_{source},
the cross-sectional area per unit solid angle of a null geodesic spray emitted from some source and subsequently detected by some observer. We shall reformulate these results in terms of a suitably normalized null geodesic affine parameter and the van Vleck
determinant, =A6=A4_{v}V. The contribution due to the null geodesic affine parameter is effectively the inverse square law for luminosity,
and the van Vleck determinant can be viewed as providing a measure of deviations from the inverse square law. This formulation is closely related to the so-called Jacobi determinant, but the van Vleck determinant has somewhat nicer analytic properties and
wider and deeper theoretical base in the general relativity, quantum physics, and quantum field theory communities. In the current article we shall concentrate on non-perturbative results, leaving near-FLRW perturbative investigation for future work.

Journal reference: JCAP06 (2018) 040

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arXiv:1802.09807 gr-qc

**Boyer-Lindquist space-times and beyond: Meta-material analogues**

** **

**Authors:** Sebastian
Schuster, Matt
Visser

Abstract: Physically reasonable stationary axisymmetric spacetimes can (under very mild technical conditions) be put into Boyer-Lindquist form. Unfortunately a metric presented in Boyer-Lindquist form is not well-adapted to the "quasi-Cartesian" meta-material analysis we developed in our previous article on "bespoke analogue spacetimes" (arXiv:1801.05549 [gr-qc]). In the current article we first focus specifically on spacetime metrics presented in Boyer-Lindquist form, and determine the equivalent meta-material susceptibility tensors in a laboratory setting. We then turn to analyzing generic stationary spacetimes, again determining the equivalent meta-material susceptibility tensors. While the background laboratory metric is always taken to be Riemann-flat, we now allow for arbitrary curvilinear coordinate systems. Finally, we reconsider static spherically symmetric spacetimes, but now in general spherical polar rather than quasi-Cartesian coordinates. The article provides a set of general tools for mimicking various interesting spacetimes by using non-trivial susceptibility tensors in general laboratory settings.

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arXiv:1803.03933 gr-qc

**Towards a Gordon form of the Kerr spacetime**

** **

**Authors:** Stefano
Liberati, Giovanni
Tricella, Matt
Visser

Abstract: It is not currently known how to put the Kerr spacetime metric into the so-called Gordon form, although the closely related Kerr-Schild form of the Kerr metric is well known. A Gordon form for the Kerr geometry, if it could be found, would be particularly useful in developing analogue models for the Kerr spacetime, since the Gordon form is explicitly given in terms of the 4-velocity and "refractive index" of an effective medium. In the current article we report progress toward this goal. First we present the Gordon form for an approximation to Kerr spacetime in the slow-rotation limit, obtained by suitably modifying the well-known Lense-Thirring form of the slow-rotation metric. Second we present the Gordon form for the Kerr spacetime in the near-null limit, (the 4-velocity of the medium being close to null). That these two perturbative approximations to the Kerr spacetime in Gordon form exist gives us some confidence that ultimately one might be able to write the exact Kerr spacetime in this form.

Journal reference: Classical and Quantum Gravity 35 (2018) 155004

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arXiv:1803.04106 gr-qc

**Tolman temperature gradients in a gravitational field**

** **

**Authors:** Jessica
Santiago, Matt
Visser

Abstract: Tolman's relation for the temperature gradient in an equilibrium self-gravitating general relativistic fluid is broadly accepted within the general relativity community. However, the concept of temperature gradients in thermal equilibrium continues to cause confusion in other branches of physics, since it contradicts naive versions of the laws of classical thermodynamics. In this paper we discuss the crucial role of the universality of free fall, and how thermodynamics emphasises the great distinction between gravity and other forces. To do so we will present an argument given by Maxwell and apply it to an electro-thermal system, concluding with an reductio ad absurdum. Among other issues we shall show that Tolman temperature gradients could also (in principle) have been derived circa 1905 - a decade before the development of full general relativity.

Journal ref: European Journal of Physics 40 #2 (2019) 025604

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arXiv:1803.08766 gr-qc

**Perturbative treatment of the luminosity distance**

**Authors:** Dimitar
Ivanov, Stefano
Liberati, Matteo
Viel, Matt
Visser

Abstract: We derive a generalized luminosity distance versus redshift relation for a linearly perturbed FLRW (Friedmann-Lemaitre-Robertson-Walker) metric with two scalar mode excitations. We use two equivalent approaches, based on the Jacobi map and the van Vleck determinant respectively. We apply the resultant formula to two simple models - an exact FLRW universe and an approximate FLRW universe perturbed by a single scalar mode sinusoidally varying with time. For both models we derive a cosmographic expansion for d_L in terms of z. We comment on the interpretation of our results and their possible application to more realistic cosmological models.

Journal reference: Phys. Rev. D 98, 063505 (2018)

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arXiv:1805.02675 gr-qc

**On the viability of regular black holes**

** **

**Authors:** Ra=A8=B2l
Carballo-Rubio, Francesco
Di Filippo, Stefano
Liberati, Costantino
Pacilio, Matt
Visser

Abstract: The evaporation of black holes raises a number of conceptual issues, most of them related to the final stages of evaporation, where the interplay between the central singularity and Hawking radiation cannot be ignored. Regular models of black holes replace the central singularity with a nonsingular spacetime region, in which an effective classical geometric description is available. It has been argued that these models provide an effective, but complete, description of the evaporation of black holes at all times up to their eventual disappearance. However, here we point out that known models fail to be self-consistent: the regular core is exponentially unstable against perturbations with a finite timescale, while the evaporation time is infinite, therefore making the instability impossible to prevent. We also discuss how to overcome these difficulties, highlighting that this can be done only at the price of accepting that these models cannot be fully predictive regarding the final stages of evaporation.

Journal reference: JHEP 2018 (2018) 23

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arXiv:1805.03781 gr-qc

**The exponential metric represents a traversable wormhole**

** **

**Authors:** Petarpa
Boonserm, Tritos
Ngampitipan, Alex
Simpson, Matt
Visser

Abstract: For various reasons a number of authors have mooted an "exponential form" for the spacetime metric:

ds^{2}
=3D −e^{−2m}/r dt^{2} + e^{+2m}/r {dr^{2} + r^{2}(d=A6=C8^{2}+sin^{2}=A6=C8d=A6=D5^{2})}.

While the weak-field behaviour matches nicely with weak-field general relativity, and so also automatically matches nicely with the Newtonian gravity limit, the strong-field behaviour is markedly different. Proponents of these exponential metrics have very much focussed on the absence of horizons --- it is certainly clear that this geometry does not represent a black hole. However, the proponents of these exponential metrics have failed to note that instead one is dealing with a traversable wormhole --- with all of the interesting and potentially problematic features that such an observation raises. If one wishes to replace all the black hole candidates astronomers have identified with traversable wormholes, then certainly a careful phenomenological analysis of this quite radical proposal should be carried out.

Journal reference: Phys. Rev. D 98, 084048 (2018)

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arXiv:1805.05583 gr-qc

**Gravity's universality: The physics underlying Tolman temperature gradients**

** **

**Authors:** Jessica
Santiago, Matt
Visser

Abstract: We provide a simple and clear verification of the physical need for temperature gradients in equilibrium states when gravitational fields are present. Our argument will be built in a completely kinematic manner, in terms of the gravitational red-shift/blue-shift of light, together with a relativistic extension of Maxwell's two column argument. We conclude by showing that it is the universality of the gravitational interaction (the uniqueness of free-fall) that ultimately permits Tolman's equilibrium temperature gradients without any violation of the laws of thermodynamics.

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arXiv:1807.02915 gr-qc

**Tolman-like temperature gradients in stationary spacetimes**

** **

**Authors:** Jessica
Santiago, Matt
Visser

Abstract: It is (or should be) well known that specification of a heat bath requires both a temperature and a 4-velocity, the rest frame of the heat bath. In static spacetimes there is a very natural and unique candidate for the 4-velocity of the heat bath, the normalized timelike Killing vector. However in stationary non-static spacetimes the situation is considerably more subtle, and several different "natural" 4-velocity fields suitable for characterizing the rest frame of a heat bath can be defined - thus Buchdahl's 1949 analysis for the Tolman temperature gradient in a stationary spacetime is only part of the story. In particular, the heat bath most suitable for describing the Hawking radiation from a rotating black hole is best described in terms of a gradient flow normal to the spacelike hypersurfaces, not in terms of Killing vectors.

Journal reference: Phys. Rev. D 98, 064001 (2018)

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arXiv:1808.07987 gr-qc

**Electromagnetic analogue space-times, analytically and algebraically**

** **

**Authors:** Sebastian
Schuster, Matt
Visser

Abstract: While quantum field theory could more aptly be called the "quantum field framework" − as it encompasses a vast variety of varying concepts and theories − in comparison, relativity, both special and general, is more commonly portrayed as less of a "general framework". Viewed from this perspective, the paradigm of "analogue space-times" is to promote the specific theory of general relativity (Einstein gravity) to a framework which covers relativistic phenomena at large. Ultimately, this then also gives rise to new proposals for experiments in the laboratory, as it allows one to move general features of the "relativistic framework" from general relativity to entirely new fields. This allows experiments looking into analogies of currently unobservable phenomena of general relativity proper. The only requirement for this to work is the presence of a notion of an upper limit for propagation speeds in this new field. Systems of such a kind abound in physics, as all hyperbolic wave equations fulfil this requirement. Consequently, models for analogue space-times can be found aplenty. We shall demonstrate this here in two separate analogue space-time models, both taken from electrodynamics in continuous media. First of all, one can distinguish between analytic analogue models (where the analogy is based on some specific hyperbolic differential equation), on the one hand, and algebraic models (where the analogy is fashioned from the more or less explicit appearance of a metric tensor), on the other hand. Yet this distinction is more than just a matter of taste: The analogue space-time model's nature will also determine which physical concepts from general relativity can be taken easily into an experimental context. Examples of this will the main aim of this paper, and the Hawking effect in one of the two models considered the example of most immediate experimental interest.

Journal reference: Classical and Quantum Gravity 36, 134004 (2019)

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arXiv:1809.08238 gr-qc

**Phenomenological aspects of black holes beyond general relativity**

** **

**Authors:** Ra=A8=B2l
Carballo-Rubio, Francesco
Di Filippo, Stefano
Liberati, Matt
Visser

Abstract: While singularities are inevitable in the classical theory of general relativity, it is commonly believed that they will not be present when quantum gravity effects are taken into account in a consistent framework. In particular, the structure of black holes should be modified in frameworks beyond general relativity that aim at regularizing singularities. Being agnostic on the nature of such theory, in this paper we classify the possible alternatives to classical black holes and provide a minimal set of phenomenological parameters that describe their characteristic features. The introduction of these parameters allows us to study, in a largely model-independent manner and taking into account all the relevant physics, the phenomenology associated with these quantum-modified black holes. We perform an extensive analysis of different observational channels and obtain the most accurate characterization of the viable constraints that can be placed using current data. Aside from facilitating a critical revision of previous work, this analysis also allows us to highlight how different channels are capable of probing certain features but are oblivious to others, and pinpoint the theoretical aspects that should be addressed in order to strengthen these tests.

.

Journal reference: Phys. Rev. D 98, 124009 (2018)

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arXiv:1809.10412 gr-qc

**Evading the Trans-Planckian problem with Vaidya spacetimes**

** **

**Authors:** Ivan
Booth, Bradley
Creelman, Jessica
Santiago, Matt
Visser

Abstract: Hawking radiation, when treated in the ray optics limit, exhibits the unfortunate trans-Planckian problem --- a Hawking photon near spatial infinity, if back-tracked to the immediate vicinity of the horizon is hugely blue-shifted and found to have had trans-Planckian energy. (And if back-tracked all the way to the horizon, the photon is formally infinitely blue-shifted, and formally acquires infinite energy.) Unruh has forcefully argued that this implies that the Hawking flux represents a vacuum instability in the presence of a horizon, and that the Hawking photons are actually emitted from some region exterior to the horizon. We seek to make this idea more precise and somewhat explicit by building a purely kinematical model for Hawking evaporation based on two Vaidya spacetimes (outer and inner) joined across a thin time-like boundary layer. The kinematics of this model is already quite rich, and we shall defer consideration of the dynamics for subsequent work. In particular we shall present an explicit calculation of the the 4-acceleration of the shell (including the effects of gravity, motion, and the outgoing null flux) and relate this 4-acceleration to the Unruh temperature.

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arXiv:1812.07114 gr-qc

**Black-bounce to traversable wormhole**

** **

**Authors:** Alex
Simpson, Matt
Visser

Abstract: So-called "regular black holes" are a topic currently of considerable interest in the general relativity and astrophysics communities. Herein we investigate a particularly interesting regular black hole spacetime described by the line element

ds^{2}
=3D −(1−2m/Ö(r^{2}+a^{2}))dt^{2}
+ (1−2m/Ö(r^{2}+a^{2}))^{-}^{1}dr^{2}
+ (r^{2}+a^{2})(d=A6=C8^{2}+sin^{2}=A6=C8d=A6=D5^{2}).

This spacetime neatly interpolates between the standard Schwarzschild black hole and the Morris-Thorne traversable wormhole; at intermediate stages passing through a black-bounce (into a future incarnation of the universe), an extremal null-bounce (into a future incarnation of the universe), and a traversable wormhole. As long as the parameter a is non-zero the geometry is everywhere regular, so one has a somewhat unusual form of "regular black hole", where the "origin" r=3D0 can be either spacelike, null, or timelike. Thus this spacetime generalizes and broadens the class of "regular black holes" beyond those usually considered.

Journal ref: JCAP 1902 (2019) 042

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arXiv:1908.03261 gr-qc

**Opening the Pandora's box at the core of black holes**

** **

**Authors:** Ra=A8=B2l
Carballo-Rubio, Francesco
Di Filippo, Stefano
Liberati, Matt
Visser

Abstract: Unless the reality of spacetime singularities is assumed, astrophysical black holes cannot be identical to their mathematical counterparts obtained as solutions of the Einstein field equations. Mechanisms for singularity regularization would spark deviations with respect to the predictions of general relativity, although these deviations are generally presumed to be negligible for all practical purposes. Nonetheless, the strength and nature of these deviations remain open questions, given the present uncertainties about the dynamics of quantum gravity. We present here a geometric classification of all spherically symmetric spacetimes that could result from singularity regularization, using a kinematic construction that is both exhaustive and oblivious to the dynamics of the fields involved. Due to the minimal geometric assumptions behind it, this classification encompasses virtually all modified gravity theories, and any theory of quantum gravity in which an effective description in terms of an effective metric is available. The first noteworthy conclusion of our analysis is that the number of independent classes of geometries that can be constructed is remarkably limited, with no more than a handful of qualitatively different possibilities. But our most surprising result is that this catalogue of possibilities clearly demonstrates that the degree of internal consistency and the strength of deviations with respect to general relativity are strongly, and positively, correlated. Hence, either quantum fluctuations of spacetime come to the rescue and solve these internal consistency issues, or singularity regularization will percolate to macroscopic (near-horizon) scales, radically changing our understanding of black holes and opening new opportunities to test quantum gravity.

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arXiv:1908.11058 gr-qc

**The Kiselev black hole is neither perfect fluid, nor is it quintessence**

** **

**Authors:** Matt
Visser

Abstract: The Kiselev black hole spacetime,

ds2
=3D −(1−2m/r−K/r^{1+3w})dt^{2} + dr^{2}/(1−2m/r−K/r^{1+3w})+r^{2}d=A6=B8_{2}^{2},

is an extremely popular toy model, with over 200 direct and indirect citations as of 2019. Unfortunately, despite repeated assertions to the contrary, this is not a perfect fluid spacetime. The relative pressure anisotropy and average pressure are easily calculated to satisfy

=A6=A4=3D=A6=A4p/{bar
p} =3D (p_{r}−p_{t})/((p_{r}+2p_{t})/3) =3D −3(1+w)/(2w);
{bar p}/=A6=D1 =3D (p_{r}+2p_{t})/(3=A6=D1) =3D w.

The relative pressure anisotropy =A6=A4 is generally a non-zero constant, (unless w=3D−1, corresponding to Schwarzschild-(anti)-de Sitter spacetime). Kiselev's original paper was very careful to point this out in the calculation, but then in the discussion made a somewhat unfortunate choice of terminology which has (with very limited exceptions) been copied into the subsequent literature. Perhaps worse, Kiselev's use of the word "quintessence" does not match the standard usage in the cosmology community, leading to another level of unfortunate and unnecessary confusion. Very few of the subsequent follow-up papers get these points right, so a brief explicit comment is warranted.

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arXiv:1909.06755 gr-qc

**ISCOs and OSCOs in the presence of positive cosmological constant**

** **

**Authors:** Petarpa
Boonserm, Tritos
Ngampitipan, Alex
Simpson, Matt
Visser

Abstract: Normally one thinks of the observed cosmological constant as being so small that it can be utterly neglected on typical astrophysical scales, only affecting extremely large-scale cosmology at Gigaparsec scales. Indeed, in those situations where the cosmological constant only has a quantitative influence on the physics, a separation of scales argument guarantees the effect is indeed negligible. The exception to this argument arises when the presence of a cosmological constant qualitatively changes the physics. One example of this phenomenon is the existence of outermost stable circular orbits (OSCOs) in the presence of a positive cosmological constant. Remarkably the size of these OSCOs are of a magnitude to be astrophysically interesting. For instance: for galactic masses the OSCOs are of order the inter-galactic spacing, for galaxy cluster masses the OSCOs are of order the size of the cluster.

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arXiv:1910.08008 gr-qc

**Decomposition of total stress-energy for the generalised Kiselev black hole**

** **

**Authors:** Petarpa
Boonserm, Tritos
Ngampitipan, Alex
Simpson, Matt
Visser