******************************************************************************

AUSTRALASIAN SOCIETY FOR GENERAL RELATIVITY AND GRAVITATION

Electronic Newsletter -- #21, Winter 2015

******************************************************************************

The ASGRG has a home web page at http://www2.phys.canterbury.ac.nz/ASGRG/

Items for this newsletter should be emailed to the editor:

asgrg@hotmail.com

The deadline for the next issue is 31 October, 2016.

******************************************************************************

CONTENTS:

* REPORT ON ACGRG7, Hamilton Island, 9-11 December, 2013

* MINUTES OF THE 8TH BIENNIAL GENERAL MEETING OF THE ASGRG, 10 December 2013

* ACGRG8, Monash University, Melbourne, 2-4 December 2015

* MEMBERSHIP DETAILS ONLINE at

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

* SUBSCRIPTIONS

* FORTHCOMING MEETINGS

* MEMBERS' ABSTRACTS at gr-qc, December 2013 - June 2015

* ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc, December 2013 - June 2015

******************************************************************************

REPORT ON ACGRG7, Reef View Hotel, Hamilton Island, December 9-11 2013

The 7th Australasian Conference on General Relativity and Gravitation (ACGRG7) was held at the Reef View Hotel on Hamilton Island, Queensland from 9 to 11 December 2013. ACGRG7 was organised by the Australian National University and was the seventh in a series of approximately biennial conferences on general relativity and gravitation organised by the ASGRG.

A total of 34 presentations were given over the three days of ACGRG7, including plenary talks from David Parkinson (“Watching galaxies fall: structure formation in the universe as a probe of gravity”), Rana Adhikari (“Wideband, next-generation, gravitational-wave antenna”), Sheila Rown (“Status and plans for advanced gravitational wave detectors”), Edward Teo (“The Plebanski-Demianski solution: space-time protoType D”), Manuel Tiglio (“Reduced order modelling in general relativity”) and Ryan Shannon (“Characterising the supermassive black hole population with gravitational waves and pulsar timing arrays”).

And, for the first time, the ACGRG programme included a category of shorter “invited talks”, which were given by Kazuaki Kuroda (“KAGRA Status”), Krzysztof Bolejko (“Curvature of the Universe”), Stefan Danilishin (“Need for speed: shall the 3rd generation gravitational wave interferometer be a speedmeter and if so, which species of it?”), David Ottaway (“New cavity technologies for beating thermal noise in third generation GW detectors”), and Matt Visser (“Classical and quantum flux energy conditions”).

The more specialised talks at ACGRG7 covered topics as diverse as the GRACE experiment, distance measurements with WiggleZ, cosmology with the velocity subsample of the 6dF Galaxy Survey, gravitational waves from low mass X-ray binaries, squeezed states in gravitational wave detection, multilayer optical coatings for GW detection, the Gingin high optical power test facility, dark energy and neutrino masses, the nuclear equation of state, sterile neutrinos and gravity, the Newtonian limit of cosmological space times, timescape cosmology, quasilocal energy in cosmology, vacuum space times with constant Weyl scalars, linearised gravitational waves at space-like and null infinity, spikes and matter overdensities, rotating hairy black holes, Clausius entropy of null surfaces, smooth lattice relativity, and the TensorPack software for tensor algebra.

The final event on the closing day, December 11, was the third award of the Kerr Prize, to mark the best student presentation at an ACGRG. As happened at the previous two conferences (in 2009 and 2011), the judges decided to split the prize between two students: Valentina Baccetti of Victoria University of Wellington who spoke about “Clausius entropy for arbitrary bifurcate null surfaces”, and Andrew Wade of the Australian National University, whose talk was entitled “A path length modulation technique for frequency shifting scatter induced noise in squeezing measurements”. The two winners shared the A$200 prize.

The conference banquet was held at the Outrigger Restaurant on the evening of December 10, and a fun time was had by all. Particular thanks should go to the local organisers (Daniel Shaddock, Robert Ward, David Bowman, Susan Scott) for making ACGRG7 such a great success.

******************************************************************************

MINUTES OF THE 8TH BIENNIAL GENERAL MEETING OF THE ASGRG

held at the Reef View Hotel, Hamilton Island, Tuesday 10 December, 2013

The meeting opened at 5.15 p.m.

Apologies: John Schutz

1. The minutes of the 7th Biennial General Meeting, held at Rydges Lakeside Resort, Queenstown, Thursday 9 February 2012, were presented to the meeting. Susan Scott moved that the minutes be accepted, and Matt Visser seconded. The motion was approved.

2. President's Report: David Wiltshire reported that the two major activities the Society had been involved in over the last two years were the current ACGRG and the ASGRG sessions at the 2012 AIP Congress in Sydney. The next AIP Congress was due to be held in Canberra in December 2014.

The last 4 years had been frustrating for the relativity community in Australia, as the proposed gravitational wave observatory (ACIGA) had not been funded, and job prospects in the area were limited. The situation was better in New Zealand, where recent appointments of relativists included Joerg Frauendiener at the University of Otago, and Woe Chet Lim at Waikato.

22 people from Australia and New Zealand attended the Marcel Grossmann meeting in Stockholm (MG13) in 2012, and 27 people from Australia and New Zealand were at the GR20/Amaldi10 conference in Warsaw in 2013.

David also mentioned that the IUPAP Young Scientist Prize is awarded every year, and that the deadline for applications for the 2014 prizes was 1 February.

3. Treasurer's Report: John Steele reported that the Society now had a total of 56 financial members, of which 47 were life members (an increase of 5 since the last ACGRG), 6 were ordinary members and 3 were student members.

The Society's current account contained $14,426, excluding the expenses and income of the current conference. The Society’s main income is derived from life membership fees.

John reported that, as always, the main expense of the Society was bank fees. He had not yet looked into the possibility of setting up a term deposit for the Society (as was mooted at ACGRG6), but he will. There was a need to add a PayPal button to the Society’s website so that members could pay fees online. David Wiltshire said he would ask David Bowman to do this. The Commonwealth Bank should also be informed that the Society no longer wants a charge account.

4. Auditor's Report: The Auditor, John Schutz, certified in an email dated 8 December 2013 that he was satisfied with the Society's accounts.

5. Appointment of Auditor for the next session: John Schutz was nominated to remain the Auditor of the Society's accounts.

6. Election of officers: The following people were elected officers of the ASGRG Committee by acclamation:

President: Leo Brewin

Treasurer: John Steele

Secretary: Malcolm Anderson

Officer: Susan Scott

Officer: David Wiltshire

Paul Lasky and Bram Slagmolen were also co-opted as Committee members.

Leo Brewin thanked the outgoing President, David Wiltshire, and the other members of the old ASGRG Committee for their services over the last two years.

7. Proposed constitutional amendment: The Society's Treasurer, John Steele, submitted a resolution aiming to allow the collection and disbursement of the Society's monies through PayPal as well as by cheque or cash. As required by the Constitution, the proposed amendment was communicated to all members by email on 17 November 2013 (at least 21 days before the BGM).

The resolution proposed that the financial section of the Constitution, which previously read:

“Finances

23.

i) All funds received for on behalf of the Society must be deposited in a bank account.

ii) All payments made by the Society in excess of five-hundred dollars must be made by cheque. All cheques must be signed by the Treasurer and one of two other members of the committee appointed by the Committee for that purpose.

iii) The Treasurer is responsible for the keeping of the financial books and records of the Society, and must present to each meeting of the Committee a report of the financial transactions of the Society since the last committee meeting.

iv) The Treasurer must prepare and have audited an annual balance sheet, and present these to the General Meeting or, in years where there is no General Meeting, to the Committee.”

should be changed to now read:

“Finances

23

i) The Treasurer is responsible for the keeping of the financial books and records of the Society, and must present to each meeting of the Committee a report of the financial transactions of the Society since the last committee meeting.

ii) In any financial year where the Society’s turnover (excluding bank fees) exceeds one thousand dollars, the Treasurer must prepare and have audited a balance sheet, and present these to the General Meeting or, in years where there is no General Meeting, to the Committee.

iii) All funds received by or on behalf of the Society must be deposited in an Australian bank account in the name of the Society, unless they are collected through an account held in the name of the Society with PayPal Australia Pty Ltd (hereinafter ‘PayPal’) under the terms of subclause (v) below. Notwithstanding that, the Treasurer may, with the express approval of the Committee, use cash payments under five-hundred dollars to the Society to pay prizes or small bills under two-hundred dollars at conferences or workshops. Such transactions shall always be noted in the Society’s accounts and receipted.

iv) The Treasurer shall be listed as the Primary Account holder for the Society's PayPal account, be responsible for compliance with PayPal’s terms and conditions and provide separate accounts for it to the committee and at General Meetings. The PayPal account must have the Society's main bank account as Nominated Bank Account, with all the required conditions imposed by PayPal.

v) The Society’s PayPal account may only be used for the collection of funds for specific purposes, such as conferences, and membership fees. Funds in the Society's PayPal account must, within one hundred and twenty days of receipt, be transferred to the Society’s main bank account, or paid out to cover the specific purpose for which they were collected as authorised by the committee, excepting an amount not exceeding five-hundred dollars to cover any disputed payments. If any reserve placed on the Society’s account by PayPal exceeds this amount, the committee must either authorise this reserve or direct the Treasurer to close the PayPal account in line with PayPal’s procedures. If the reserve is authorised, this decision must then be ratified at the Society's next General Meeting.

vi) All payments made by the Society in excess of five-hundred dollars must be made by cheque or by a direct electronic transfer expressly authorised by the Committee. All cheques must be signed by the Treasurer and one of any other members of the Committee appointed by the Committee for that purpose. Notwithstanding this, PayPal’s standard Direct Debit Agreement is permitted over the Society's account.”

The motion to change the constitution was put by John Steele, and seconded by Matt Visser. The meeting voted to accept it.

8. Date and venue for ACGRG8: Leo Brewin and Todd Oliynyk proposed that Monash University would host ACGRG8 in December 2015, and the meeting agreed.

9. Other business

2013 Kerr Prize: Leo reminded the meeting that the Kerr Prize for the best student presentation at ACGRG7 would be awarded at the end of the final session on December 11.

Suggested changes to future ACGRG programmes: David McClelland raised concerns about what he saw as a disconnect between the theory and experiment talks at ACGRG7. He pointed out that the Amaldi conferences have tutorials or primers on the topics being discussed at the beginning of each day, and asked if ACGRG could do something similar. He also felt that an event like ACGRG should be devoted to student talks, with the established experts discussing their current research in poster sessions rather than in a formal setting.

GR centenary year: The meeting ended with a reminder that 2015 will mark the centenary of Einstein’s elaboration of the full theory of general relativity. The Society should therefore attempt to engage a high-profile public speaker for ACGRG8 in December of that year.

The meeting closed at 5.55 pm.

******************************************************************************

8TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG8)

Monash University, Melbourne, 2-4 December 2015

ACGRG8 is the eighth in a series of biennial conferences run by the ASGRG with the aim of bringing together researchers from around the world to discuss new findings in mathematical, theoretical, numerical and experimental gravitation, to make contacts and consolidate ideas.

This year’s conference will run from Wednesday 2 December to Friday 4 December, and will be held on the Clayton campus of Monash University, Melbourne.

The confirmed plenary speakers are:

Brian Schmidt (Australian National University)

Bob Wald (University of Chicago)

Matt Choptuik (University of British Columbia)

David McClelland (Australian National University)

Joerg Frauendiener (University of Otago)

Further details about the programme and registration and submission deadlines will be sent out once they are finalised.

******************************************************************************************

MEMBERSHIP DETAILS ONLINE:

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

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

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

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

******************************************************************************

SUBSCRIPTIONS:

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

The new version of the subscription form, at

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

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

the July 2014 - June 2015 subscriptions are requested, if you wish to pay for July 2015 - June 2016 at the same time, it may simplify matters.

******************************************************************************

FORTHCOMING MEETINGS

July 12-18, 2015: 14th Marcel Grossmann Meeting on General Relativity

Rome, Italy

http://www.icra.it/mg/mg14/

July 20-31, 2015: “Quantum Fields and IR Issues in de Sitter Space”

International Institute of Physics,

Federal University of Rio Grande do Norte, Brazil

http://www.iip.ufrn.br/eventsdetail?inf===wTEFVP

July 22-24, 2015: 19th Paris Cosmology Colloquium

“Latest News from the Universe: Lambda Warm Dark Matter Cosmology (ΛWDM), CMB, Dark Matter, Dark Energy, and Sterile Neutrinos”

Ecole International Daniel Chalonge,

Observatoire de Paris, France

http://chalonge.obspm.fr/colloque2015.html

July 27-31, 2015: GR 100 in Rio

“Gravitational Waves Astrophysics”

Centro Brasileiro Pesquisas de Fisicas

August 9-15, 2015: “Hot Topics in General Relativity and Gravitation”

International Centre for Interdisciplinary Science Education,

Quy Nhon, Vietnam

http://www.cpt.univ-mrs.fr/~cosmo/HTGRG-2/index.php

August 20-22, 2015: Extreme Gravity Workshop

Montana State University,

Bozeman, Montana, USA

http://www.physics.montana.edu/gravity/workshop/workshop2015.htm

September 7-11, 2015: Spanish Relativity Meeting (ERE2015)

University of the Balearic Islands

Palma Mallorca, Spain

http://grg.uib.es/ERE2015

September 14 - December 18, 2015: “Mathematical general relativity, compressible fluids, and more”

Centre Emile Borel,

Institut Henri Poincare, Paris

https://philippelefloch.wordpress.com/

September 28 - October 1, 2015: “Geometric Aspects of General Relativity”

University of Montpellier, France

https://indico.math.cnrs.fr/event/663/

September 30 - October 3, 2015: “The Modern Physics of Compact Stars and Relativistic Gravity”

Yerevan State University, Armenia

https://indico.cern.ch/event/360350/

October 5-7, 2015: “100 years of curved space time”

Austrian Academy of Sciences, Vienna

http://www.oeaw.ac.at/gravitation2015/scientific-workshop/program-outline/

November 25-28, 2015: International Conference on General Relativity (ICGR-2015)

Sant Gadge Baba Amravati University,

Amravati, India

http://www.icgr2015.in/

November 30 - December 5, 2015: Einstein Conference 2015

“A Century of General Relativity”

Max Planck Institute for Gravitational Physics,

Harnack House, Berlin

http://www.einsteinconference2015.org/

December 2-4, 2015: 8th Australian Conference on General Relativity and Gravitation (ACGRG8)

Monash University, Melbourne

http://asgrg.org/ACGRG8.html

December 12-14, 2015: International Conference on Relativity and Geometry

in Memory of Andre Lichnerowicz

Institut Henri Poincare, Paris

http://monge.u-bourgogne.fr/gdito/lichnerowicz2015/en/index.php

******************************************************************************

MEMBERS' ABSTRACTS at gr-qc, December 2013 - June 2015

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 114 papers listed here and in the LIGO section represent 1.64% of the 6960 papers posted or cross-linked to gr-qc between December 2013 and June 2015.

----------------------------------------------------------------

arXiv:1405.0970 [gr-qc]

Twisted Skyrmion String

Miftachul Hadi, Malcolm Anderson, Andri Husein

(Submitted on 30 Apr 2014 (v1), last revised 7 May 2014 (this version, v2))

We study nonlinear sigma model, especially Skyrme model without twist and Skyrme model with twist: twisted Skyrmion string. Twist term, mkz, is indicated in vortex solution. Necessary condition for stability of vortex solution has consequence that energy of vortex is minimum and scale-free (vortex solution is neutrally stable to changes in scale). We find numerically that the value of vortex minimum energy per unit length for twisted Skyrmion string is 20.37×1060 eV/m.

Comments: 12 pages, 2 figures. Presented at CTPNP 2014 and submitted to Journal of Physics: Conference Series.

----------------------------------------------------------------

Mathematical modeling of the gravitational field of a twisted Skyrmion string

Miftachul Hadi, Malcolm Anderson, Andri Husein

(Submitted on 22 May 2015)

In this paper we study the gravitational field of a straight string generated from a class of nonlinear sigma models, specifically the Skyrme model without a twist and the Skyrme model with a twist (the twisted Skyrmion string). The twist term, mkz, is included to stabilize the vortex solution. To model the effects of gravity, we replace the Minkowski tensor, ημν, in the standard Skyrme Lagrangian density with a space-time metric tensor, gμν, assumed to be static and cylindrically symmetric. The Einstein equations for the metric and field components are then derived. This work is still in progress.

Comments: 10 pages, prepared for submission to Physical Review D

----------------------------------------------------------------

arXiv:1506.01101 [gr-qc]

The Gravitational Field of a Twisted Skyrmion

Miftachul Hadi, Malcolm Anderson, Andri Husein

(Submitted on 3 Jun 2015 (v1), last revised 24 Jun 2015 (this version, v2))

In this paper we study the gravitational field of a straight string generated from a class of nonlinear sigma models, specifically the Skyrme model with a twist (the twisted Skyrmion). The twist term, mkz, is included to stabilize the vortex solution. To model the effects of gravity, we replace the Minkowski tensor, ημν, in the standard Skyrme Lagrangian density with a space-time metric tensor, gμν, assumed to be static and cylindrically symmetric. The Einstein equations for the metric and field components are then derived. This work is still in progress.

Comments: 5 pages, no figure, presented at ICMNS Bandung, Indonesia, 2 Nov 2014 and submitted to AIP Conference Proceedings

----------------------------------------------------------------

arXiv:1401.0787 [gr-qc]

Regular and conformal regular cores for static and rotating solutions

Mustapha Azreg-Aïnou

(Submitted on 4 Jan 2014 (v1), last revised 10 Jan 2014 (this version, v3))

Using a new metric for generating rotating solutions, we derive in a general fashion the solution of an imperfect fluid and that of its conformal homolog. We discuss the conditions that the stress-energy tensors and invariant scalars be regular. On classical physical grounds, it is stressed that, conformal fluids used as cores for static or rotating solutions, are exempt from any malicious behavior in that they are finite and defined everywhere.

Journal reference: Phys. Lett. B 730 (2014) 95

----------------------------------------------------------------

----------------------------------------------------------------

Generating rotating regular black hole solutions without complexification

Mustapha Azreg-Aïnou

(Submitted on 11 May 2014 (v1), last revised 24 Sep 2014 (this version, v3))

We drop the complexification procedure from the Newman-Janis algorithm and introduce more physical arguments and symmetry properties, and we show how one can generate regular and singular rotating black hole and non-black-hole solutions in Boyer-Lindquist coordinates. We focus on generic rotating regular black holes and show that they are regular on the Kerr-like ring but physical entities are undefined there. We show that rotating regular black holes have much smaller electric charges, and, with increasing charge, they turn into regular non-black-hole solutions well before their Kerr-Newman counterparts become naked singularities. No causality violations occur in the region inside a rotating regular black hole. The separability of the Hamilton-Jacobi equation for neutral particles is also carried out in the generic case, and the innermost boundaries of circular orbits for particles are briefly discussed. Other, but special, properties pertaining to the rotating regular counterpart of the Ay\'on-Beato--Garc\'{\i}a regular static black hole are also investigated.

Journal reference: Phys. Rev. D 90, 064041 (2014)

----------------------------------------------------------------

----------------------------------------------------------------

----------------------------------------------------------------

Black hole thermodynamics: No inconsistency via the inclusion of the missing P-V terms

Mustapha Azreg-Aïnou

(Submitted on 10 Nov 2014 (v1), last revised 23 Mar 2015 (this version, v2))

The early literature on black hole thermodynamics ignored the P-V term associated with the existence of a fundamental physical constant in the black hole solution. The inclusion of this constant in the first law becomes inconsistent with the Smarr relation. Once the missing P-V term introduced is, it becomes customary to introduce it only in problems where there is a negative cosmological constant. This practice is inherited from cosmological approaches which consider the quantity −Λ/8π as the constant pressure due to a cosmological fluid. However, the notions of pressure and thermodynamic volume in black hole thermodynamics are very different from their counterparts in classical thermodynamics. From this point of view, there is \textit{a priori} no compelling reason to not extend this notion of pressure and associate a partial pressure with each external" density 8πTtt. In this work, we associate a partial pressure with a variable mass parameter as well as with each tt component of the effective stress-energy tensor Teffμν but not with the linear component of the electromagnetic field. Using the field equations Gμν=8πTeffμν, we derive universal expressions for the enthalpy, internal energy, free energies, thermodynamic volume, equation of state, law of corresponding states, criticality, and critical exponents of static (nonrotating) charged black holes, with possibly a variable mass parameter, whether they are solutions to the Einstein field equations or not. We extend the derivation to the case where the black hole is immersed in the field of a quintessence force and to the multiforce case. Many applications and extensions are considered, including applications to regular black holes derived in previous and present work. No inconsistency has been noticed in their thermodynamics.

Journal reference: Phys. Rev. D 91, 064049 (2015)

----------------------------------------------------------------

arXiv:1412.8282 [gr-qc]

Confined-exotic-matter wormholes with no gluing effects -- Imaging supermassive type (1) wormholes and black holes

Mustapha Azreg-Aïnou

(Submitted on 29 Dec 2014)

We classify wormholes endowed with redshift effects and finite mass into three types. Type (1) wormholes have their radial pressure dying out faster, as one moves away from the throat, than any other component of the stress-energy and thus violate the least the local energy conditions. We introduce a novel and generalizable method for deriving, with no cutoff in the stress-energy or gluing, the three types of wormholes. We focus on type (1) wormholes and construct different asymptotically flat solutions with finite, upper- and lower-bounded, mass M. It is observed that the radial pressure is negative, and the null energy condition is violated, only inside a narrow shell, adjacent to the throat, of relative spacial extent ϵ. Reducing the relative size of the shell, without harming the condition of traversability, yields an inverse square law of ϵ versus M for supermassive wormholes. We show that the diameter of the shadow of this type (1) supermassive wormhole overlaps with that of the black hole candidate at the center of the Milky Way and that the recent derivation, using the up-to-date millimeter-wavelength very long baseline interferometry made in Astrophys. J. \textbf{795} 134 (2014) [arXiv:1409.4690], remains inconclusive.

We show that redshift-free wormholes, with positive energy density, have one of their barotropic equations of state in the phantom regime, have their stress energy tensor traceless, and are anisotropic. They are all type (3) wormholes having their variable equations of state approaching 1 and −1 at spatial infinity. We also introduce a new approach for deriving new redshift-free wormholes.

----------------------------------------------------------------

----------------------------------------------------------------

----------------------------------------------------------------

arXiv:1408.4201 [gr-qc]

The Attached Point Topology of the Abstract Boundary For Space-Time

Richard A. Barry, Susan M. Scott

(Submitted on 19 Aug 2014)

Singularities play an important role in General Relativity and have been shown to be an inherent feature of most physically reasonable space-times. Despite this, there are many aspects of singularities that are not qualitatively or quantitatively understood. The abstract boundary construction of Scott and Szekeres has proven to be a flexible tool with which to study the singular points of a manifold. The abstract boundary construction provides a 'boundary' for any n-dimensional, paracompact, connected, Hausdorff, smooth manifold. Singularities may then be defined as entities in this boundary - the abstract boundary. In this paper a topology is defined, for the first time, for a manifold together with its abstract boundary. This topology, referred to as the attached point topology, thereby provides us with a description of how the abstract boundary is related to the underlying manifold. A number of interesting properties of the topology are considered, and in particular, it is demonstrated that the attached point topology is Hausdorff.

Journal reference: Class. Quantum Grav. 28(16) 165003 2011

----------------------------------------------------------------

arXiv:1408.4203 [gr-qc]

The Strongly Attached Point Topology of the Abstract Boundary For Space-Time

Richard A. Barry, Susan M. Scott

(Submitted on 19 Aug 2014)

The abstract boundary construction of Scott and Szekeres provides a ‘boundary’ for any n-dimensional, paracompact, connected, Hausdorff, smooth manifold. Singularities may then be defined as objects within this boundary. In a previous paper by the authors, a topology referred to as the attached point topology was defined for a manifold and its abstract boundary, thereby providing us with a description of how the abstract boundary is related to the underlying manifold. In this paper, a second topology, referred to as the strongly attached point topology, is presented for the abstract boundary construction. Whereas the abstract boundary was effectively disconnected from the manifold in the attached point topology, it is very much connected in the strongly attached point topology. A number of other interesting properties of the strongly attached point topology are considered, each of which support the idea that it is a very natural and appropriate topology for a manifold and its abstract boundary.

Journal reference: Class. Quantum Grav. 31(12) 125004 2014

----------------------------------------------------------------

arXiv:1312.0795 [gr-qc]

Exact Global Phantonical Solutions in the Emergent Universe

A. Beesham, S. V. Chervon, S. D. Maharaj, A. S. Kubasov

(Submitted on 3 Dec 2013 (v1), last revised 5 Dec 2013 (this version, v2))

We present new classes of exact solutions for an Emergent Universe supported by phantom and canonical scalar fields in the framework of a two-component chiral cosmological model. We outline in detail the method of deriving exact solutions, discuss the potential and kinetic interaction for the model and calculate key cosmological parameters. We suggest that this this model be called a {\it phantonical Emergent Universe} because of the necessity to have phantom and canonical chiral fields. The solutions obtained are valid for all time.

----------------------------------------------------------------

arXiv:1312.1770 [gr-qc]

An Emergent Universe with Dark Sector Fields in a Chiral Cosmological Model

A. Beesham, S. V. Chervon, S. D. Maharaj, A. S. Kubasov

(Submitted on 6 Dec 2013)

We consider the emergent universe scenario supported by a chiral cosmological model with two interacting dark sector fields: phantom and canonical. We investigate the general properties of the evolution of the kinetic and potential energies as well as the development of the equation of state with time. We present three models based on asymptotic solutions and investigate the phantom part of the potential and chiral metric components. The exact solution corresponding to a global emergent universe scenario, starting from the infinite past and evolving to the infinite future, has been obtained for the first time for a chiral cosmological model. The behavior of the chiral metric components responsible for the kinetic interaction between the phantom and canonical scalar fields has been analyzed as well.

Journal reference: Quantum Matter 2: 388-395 (2013)

----------------------------------------------------------------

arXiv:1405.3459 [gr-qc]

Nonlinear bulk viscosity and the stability of accelerated expansion in FRW spacetime

G. Acquaviva, A. Beesham

(Submitted on 14 May 2014)

In the context of dark energy solutions, we consider a Friedmann-Robertson-Walker spacetime filled with a non-interacting mixture of dust and a viscous fluid, whose bulk viscosity is governed by the nonlinear model proposed in [15]. Through a phase space analysis of the equivalent dynamical system, existence and stability of critical solutions are established and the respective scale factors are computed. The results point towards the possibility of describing the current accelerated expansion of the Universe by means of the abovementioned nonlinear model for viscosity.

----------------------------------------------------------------

arXiv:1405.7219 [gr-qc]

An Emergent Universe supported by chiral cosmological fields in Einstein--Gauss--Bonnet gravity

Sergey V. Chervon, Sunil D. Maharaj, Aroonkumar Beesham and Aleksandr Kubasov

(Submitted on 28 May 2014)

We propose the application of the chiral cosmological model (CCM) for the Einstein--Gauss--Bonnet (EGB) theory of gravitation with the aim of finding new models of the Emergent Universe (EmU) scenario. We analysed the EmU supported by two chiral cosmological fields for a spatially flat universe, while we have used three chiral fields when we investigated open and closed universes. To prove the validity of the EmU scenario we fixed the scale factor and found the exact solution by decomposition of EGB equations and solving the chiral field dynamics equation. To this end, we suggested the decomposition of the EGB equations in such a way that the first chiral field is responsible for the Einstein part of the model, while the second field, together with kinetic interaction term, is connected with the Gauss--Bonnet part of the theory. We proved that both fields are phantom ones under this decomposition, and that the model has a solution if the kinetic interaction between the fields equals a constant. We have presented the exact solution in terms of cosmic time. This was done for a spatially flat universe. In the case of open and closed universes we introduced the third chiral field (canonical for closed and phantom for open universe) which is responsible for the EGB and curvature parts. The solution of the third field equation is obtained in quadratures. Thus we have proved that the CCM is able to support EmU scenario in EGB gravity for spatially flat, open and closed universes.

Report number: LGCA/5-2014

----------------------------------------------------------------

arXiv:1505.01965 [gr-qc]

A phase space analysis for nonlinear bulk viscous cosmology

Giovanni Acquaviva, Aroonkumar Beesham

(Submitted on 8 May 2015)

We consider a Friedmann-Robertson-Walker spacetime filled with both viscous radiation and nonviscous dust. The former has a bulk viscosity which is proportional to an arbitrary power of the energy density, i.e. \zeta \propto \rho_v^{\nu}, and viscous pressure satisfying a nonlinear evolution equation. The analysis is carried out in the context of dynamical systems and the properties of solutions corresponding to the fixed points are discussed. For some ranges of the relevant parameter \nu we find that the trajectories in the phase space evolve from a FRW singularity towards an asymptotic de Sitter attractor, confirming and extending previous analysis in the literature. For other values of the parameter, instead, the behaviour differs from previous works.

----------------------------------------------------------------

arXiv:1409.3284 [gr-qc]

Time evolution of parametric instability in large-scale gravitational-wave interferometers

Stefan L. Danilishin, Sergey P. Vyatchanin, David G. Blair, Ju Li, Chunnong Zhao

(Submitted on 11 Sep 2014 (v1), last revised 6 Dec 2014 (this version, v3))

We present a study of three-mode parametric instability in large-scale gravitational-wave detectors. Previous work used a linearised model to study the onset of instability. This paper presents a non-linear study of this phenomenon, which shows that the initial stage of exponential rise of the amplitudes of a higher order optical mode and the mechanical internal mode of the mirror is followed by a saturation phase, in which all three participating modes reach a new equilibrium state with constant oscillation amplitudes. Results suggest that stable operation of interferometers may be possible in the presence of such instabilities, thereby simplifying the task of suppression.

Journal reference: Phys. Rev. D 90, 122008 (2014)

----------------------------------------------------------------

arXiv:1501.01542 [physics.optics]

Parametric Instability in Long Optical Cavities and Suppression by Dynamic Transverse Mode Frequency Modulation

Chunnong Zhao, Li Ju, Qi Fang, Carl Blair, Jiayi Qin, David Blair, Jerome Degallaix, Hiroaki Yamamoto

(Submitted on 7 Jan 2015 (v1), last revised 13 Jan 2015 (this version, v2))

Three mode parametric instability has been predicted in Advanced gravitational wave detectors. Here we present the first observation of this phenomenon in a large scale suspended optical cavity designed to be comparable to those of advanced gravitational wave detectors. Our results show that previous modelling assumptions that transverse optical modes are stable in frequency except for frequency drifts on a thermal deformation time scale is unlikely to be valid for suspended mass optical cavities. We demonstrate that mirror figure errors cause a dependence of transverse mode offset frequency on spot position. Combined with low frequency residual motion of suspended mirrors, this leads to transverse mode frequency modulation which suppresses the effective parametric gain. We show that this gain suppression mechanism can be enhanced by laser spot dithering or fast thermal modulation. Using Advanced LIGO test mass data and thermal modelling we show that gain suppression factors of 10-20 could be achieved for individual modes, sufficient to greatly ameliorate the parametric instability problem.

----------------------------------------------------------------

arXiv:1403.1718 [astro-ph.CO]

Nonlinear Chaplygin Gas Cosmologies

P. P. Avelino, K. Bolejko, G. F. Lewis

(Submitted on 7 Mar 2014 (v1), last revised 20 May 2014 (this version, v2))

We study the nonlinear regime of Unified Dark Energy models, using Generalized Chaplygin Gas cosmologies as a representative example, and introduce a new parameter characterizing the level of small scale clustering in these scenarios. We show that viable Generalized Chaplygin Gas cosmologies, consistent with the most recent observational constraints, may be constructed for any value of the Generalized Chaplygin Gas parameter by considering models with a sufficiently high level of nonlinear clustering.

Journal reference: Phys.Rev.D89:103004,2014

----------------------------------------------------------------

arXiv:1412.4976 [astro-ph.CO]

A new test of the FLRW metric using distance sum rule

Syksy Rasanen, Krzysztof Bolejko, Alexis Finoguenov

(Submitted on 16 Dec 2014 (v1), last revised 30 Dec 2014 (this version, v2))

We present a new test of the validity of the Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) metric, based on comparing the distance from redshift 0 to z1 and from z1 to z2 to the distance from 0 to z2. If the universe is described by the FLRW metric, the comparison provides a model-independent measurement of spatial curvature. The test is kinematic and relies on geometrical optics, it is independent of the matter content of the universe and the applicability of the Einstein equation on cosmological scales. We apply the test to observations, using the Union2.1 compilation of supernova distances and Sloan Lens ACS Survey galaxy strong lensing data. The FLRW metric is consistent with the data, and the spatial curvature parameter is constrained to be −1.22<ΩK0<0.63, or −0.08<ΩK0<0.97 with a prior from the cosmic microwave background and the local Hubble constant, though modelling of the lenses causes significant systematic uncertainty.

Report number: HIP-2014-37/TH

----------------------------------------------------------------

arXiv:1505.00067 [gr-qc]

A numerical study of the Regge Calculus and Smooth Lattice methods on a Kasner cosmology

Leo Brewin

(Submitted on 1 May 2015)

Two lattice based methods for numerical relativity, the Regge Calculus and the Smooth Lattice Relativity, will be compared with respect to accuracy and computational speed in a full 3+1 evolution of initial data representing a standard Kasner cosmology. It will be shown that both methods provide convergent approximations to the exact Kasner cosmology. It will also be shown that the Regge Calculus is of the order of 110 times slower than the Smooth Lattice method.

----------------------------------------------------------------

arXiv:1402.3581 [gr-qc]

Spin-Precession: Breaking the Black Hole--Neutron Star Degeneracy

Katerina Chatziioannou, Neil Cornish, Antoine Klein, Nicolas Yunes

(Submitted on 14 Feb 2014 (v1), last revised 18 Dec 2014 (this version, v2))

Mergers of compact stellar remnant are prime targets for the LIGO/Virgo gravitational wave detectors. One hopes that the gravitational wave signals from these merger events can be used to study the mass and spin distribution of stellar remnants, and provide information about black hole horizons and the material properties of neutron stars. However, it has been suggested that degeneracies in the way that the star's mass and spin are imprinted in the waveforms may make it impossible to distinguish between black holes and neutron stars. Here we show that the precession of the orbital plane due to spin-orbit coupling breaks the mass-spin degeneracy, and allows us to distinguish between standard neutron stars and alternative possibilities, such as black holes or exotic neutron stars with large masses and spins.

Journal reference: ApJ, 798, L17 (2015)

----------------------------------------------------------------

arXiv:1404.3180 [gr-qc]

Detection and Parameter Estimation of Gravitational Waves from Compact Binary Inspirals with Analytical Double-Precessing Templates

Katerina Chatziioannou, Neil Cornish, Antoine Klein, Nicolas Yunes

(Submitted on 11 Apr 2014 (v1), last revised 14 May 2014 (this version, v2))

We study the performance of various analytical frequency-domain templates for detection and parameter estimation of gravitational waves from spin-precessing, quasi-circular, compact binary inspirals. We begin by assessing the extent to which non-spinning, spin-aligned, and the new (analytical, frequency-domain, small-spin) double-precessing frequency-domain templates can be used to detect signals from such systems. For effective, dimensionless spin values above 0.2, the use of non-spinning or spin-aligned templates for detection purposes will result in a loss of up to 30 of all events, while in the case of the double-precessing model, this never exceeds 6. Moreover, even for signals from systems with small spins, non-spinning and spin-aligned templates introduce large biases in the extracted masses and spins. The use of a model that encodes spin-induced precession effects, such as the double-precessing model, improves the mass and spin extraction by up to an order of magnitude. The additional information encoded in the spin-orbit interaction is invaluable if one wishes to extract the maximum amount of information from gravitational wave signals.

DOI: 10.1103/PhysRevD.89.104023

----------------------------------------------------------------

----------------------------------------------------------------

arXiv:1407.7038 [gr-qc]

Projected Constraints on Scalarization with Gravitational Waves from Neutron Star Binaries

Laura Sampson, Nicolas Yunes, Neil Cornish, Marcelo Ponce, Enrico Barausse, Antoine Klein, Carlos Palenzuela, Luis Lehner

(Submitted on 25 Jul 2014 (v1), last revised 28 Nov 2014 (this version, v2))

Certain scalar-tensor theories have the property of endowing stars with scalar hair, sourced either by the star's own compactness (spontaneous scalarization) or, for binary systems, by the companion's scalar hair (induced scalarization) or by the orbital binding energy (dynamical scalarization). Scalarized stars in binaries present different conservative dynamics than in General Relativity, and can also excite a scalar mode in the metric perturbation that carries away dipolar radiation. As a result, the binary orbit shrinks faster than predicted in General Relativity, modifying the rate of decay of the orbital period. In spite of this, scalar-tensor theories can pass existing binary pulsar tests, because observed pulsars may not be compact enough or sufficiently orbitally bound to activate scalarization. Gravitational waves emitted during the last stages of compact binary inspirals are thus ideal probes of scalarization effects. For the standard projected sensitivity of advanced LIGO, we here show that, if neutron stars are sufficiently compact to enter the detector's sensitivity band already scalarized, then gravitational waves could place constraints at least comparable to binary pulsars. If the stars dynamically scalarize while inspiraling in band, then constraints are still possible provided the scalarization occurs sufficiently early in the inspiral, roughly below an orbital frequency of 50Hz. In performing these studies, we derive an easy-to-calculate data analysis measure, an integrated phase difference between a General Relativistic and a modified signal, that maps directly to the Bayes factor so as to determine whether a modified gravity effect is detectable. Finally, we find that custom-made templates are equally effective as model-independent, parameterized post-Einsteinian waveforms at detecting such modified gravity effects at realistic signal-to-noise ratios.

Comments: 26 pages, 15 figures, 2 tables. Version resubmitted to PRD after referee report.

----------------------------------------------------------------

arXiv:1408.5158 [gr-qc]

Fast Frequency-domain Waveforms for Spin-Precessing Binary Inspirals

Antoine Klein, Neil Cornish, Nicolás Yunes

(Submitted on 21 Aug 2014 (v1), last revised 9 Dec 2014 (this version, v2))

The detection and characterization of gravitational wave signals from compact binary coalescence events relies on accurate waveform templates in the frequency domain. The stationary phase approximation (SPA) can be used to compute closed-form frequency-domain waveforms for non-precessing, quasi-circular binary inspirals. However, until now, no fast frequency-domain waveforms have existed for generic, spin-precessing quasi-circular compact binary inspirals. Templates for these systems have had to be computed via a discrete Fourier transform of finely sampled time-domain waveforms, which is far more computationally expensive than those constructed directly in the frequency-domain, especially for those systems that are dominated by the inspiral part. There are two obstacles to deriving frequency-domain waveforms for precessing systems: (i) the spin-precession equations do not admit closed-form solutions for generic systems; (ii) the SPA fails catastrophically. Presently there is no general solution to the first problem, so we must resort to numerical integration of the spin precession equations. This is not a significant obstacle, as numerical integration on the slow precession timescale adds very little to the computational cost of generating the waveforms. Our main result is to solve the second problem, by providing an alternative to the SPA that we call the method of Shifted Uniform Asymptotics, or SUA, that cures the divergences in the SPA. The construction of frequency-domain templates using the SUA can be orders of magnitude more efficient than the time-domain ones obtained through a discrete Fourier transform. Moreover, this method is very faithful to the discrete Fourier transform, with mismatches on the order of 10−5.

Journal reference: Phys. Rev. D 90, 124029 (2014)

----------------------------------------------------------------

arXiv:1410.3835 [gr-qc]

BayesWave: Bayesian Inference for Gravitational Wave Bursts and Instrument Glitches

Neil J. Cornish, Tyson B. Littenberg

(Submitted on 14 Oct 2014 (v1), last revised 7 May 2015 (this version, v3))

A central challenge in Gravitational Wave Astronomy is identifying weak signals in the presence of non-stationary and non-Gaussian noise. The separation of gravitational wave signals from noise requires good models for both. When accurate signal models are available, such as for binary Neutron star systems, it is possible to make robust detection statements even when the noise is poorly understood. In contrast, searches for "un-modeled" transient signals are strongly impacted by the methods used to characterize the noise. Here we take a Bayesian approach and introduce a multi-component, variable dimension, parameterized noise model that explicitly accounts for non-stationarity and non-Gaussianity in data from interferometric gravitational wave detectors. Instrumental transients (glitches) and burst sources of gravitational waves are modeled using a Morlet-Gabor continuous wavelet frame. The number and placement of the wavelets is determined by a trans-dimensional Reversible Jump Markov Chain Monte Carlo algorithm. The Gaussian component of the noise and sharp line features in the noise spectrum are modeled using the BayesLine algorithm, which operates in concert with the wavelet model.

Comments: 36 pages, 15 figures, Version accepted by Class. Quant. Grav

----------------------------------------------------------------

arXiv:1410.3852 [gr-qc]

BayesLine: Bayesian Inference for Spectral Estimation of Gravitational Wave Detector Noise

Tyson B. Littenberg, Neil J. Cornish

(Submitted on 14 Oct 2014)

Gravitational wave data from ground-based detectors is dominated by instrument noise. Signals will be comparatively weak, and our understanding of the noise will influence detection confidence and signal characterization. Mis-modeled noise can produce large systematic biases in both model selection and parameter estimation. Here we introduce a multi-component, variable dimension, parameterized model to describe the Gaussian-noise power spectrum for data from ground-based gravitational wave interferometers. Called BayesLine, the algorithm models the noise power spectral density using cubic splines for smoothly varying broad-band noise and Lorentzians for narrow-band line features in the spectrum. We describe the algorithm and demonstrate its performance on data from the fifth and sixth LIGO science runs. Once fully integrated into LIGO/Virgo data analysis software, BayesLine will produce accurate spectral estimation and provide a means for marginalizing inferences drawn from the data over all plausible noise spectra.

----------------------------------------------------------------

arXiv:1412.6479 [gr-qc]

Multivariate Classification with Random Forests for Gravitational Wave Searches of Black Hole Binary Coalescence

Paul T. Baker, Sarah Caudill, Kari A. Hodge, Dipongkar Talukder, Collin Capano, Neil J. Cornish

(Submitted on 19 Dec 2014)

Searches for gravitational waves produced by coalescing black hole binaries with total masses ≳25M⊙ use matched filtering with templates of short duration. Non-Gaussian noise bursts in gravitational wave detector data can mimic short signals and limit the sensitivity of these searches. Previous searches have relied on empirically designed statistics incorporating signal-to-noise ratio and signal-based vetoes to separate gravitational wave candidates from noise candidates. We report on sensitivity improvements achieved using a multivariate candidate ranking statistic derived from a supervised machine learning algorithm. We apply the random forest of bagged decision trees technique to two separate searches in the high mass (≳25M⊙) parameter space. For a search which is sensitive to gravitational waves from the inspiral, merger, and ringdown (IMR) of binary black holes with total mass between 25M⊙ and 100M⊙, we find sensitive volume improvements as high as 70±13−109±11\% when compared to the previously used ranking statistic. For a ringdown-only search which is sensitive to gravitational waves from the resultant perturbed intermediate mass black hole with mass roughly between 10M⊙ and 600M⊙, we find sensitive volume improvements as high as 61±4−241±12\% when compared to the previously used ranking statistic. We also report how sensitivity improvements can differ depending on mass regime, mass ratio, and available data quality information. Finally, we describe the techniques used to tune and train the random forest classifier that can be generalized to its use in other searches for gravitational waves.

Report number: LIGO Document P1400231

----------------------------------------------------------------

arXiv:1501.05343 [astro-ph.GA]

NANOGrav Constraints on Gravitational Wave Bursts with Memory

Z. Arzoumanian, A. Brazier, S. Burke-Spolaor, S. J. Chamberlin, S. Chatterjee, B. Christy, J. M. Cordes, N. J. Cornish, P. B. Demorest, X. Deng, T. Dolch, J. A. Ellis, R. D. Ferdman, E. Fonseca, N. Garver-Daniels, F. Jenet, G. Jones, V. M. Kaspi, M. Koop, M. T. Lam, T. J. W. Lazio, L. Levin, A. N. Lommen, D. R. Lorimer, J. Luo, R. S. Lynch, D. R. Madison, M. A. McLaughlin, S. T. McWilliams, D. J. Nice, N. Palliyaguru, T. T. Pennucci, S. M. Ransom, X. Siemens, I. H. Stairs, D. R. Stinebring, K. Stovall, J. Swiggum, M. Vallisneri, R. van Haasteren, Y. Wang, W. W. Zhu

(Submitted on 21 Jan 2015)

Among efforts to detect gravitational radiation, pulsar timing arrays are uniquely poised to detect "memory" signatures, permanent perturbations in spacetime from highly energetic astrophysical events such as mergers of supermassive black hole binaries. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) observes dozens of the most stable millisecond pulsars using the Arecibo and Green Bank radio telescopes in an effort to study, among other things, gravitational wave memory. We herein present the results of a search for gravitational wave bursts with memory (BWMs) using the first five years of NANOGrav observations. We develop original methods for dramatically speeding up searches for BWM signals. In the directions of the sky where our sensitivity to BWMs is best, we would detect mergers of binaries with reduced masses of 109 M⊙ out to distances of 30 Mpc; such massive mergers in the Virgo cluster would be marginally detectable. We find no evidence for BWMs. However, with our non-detection, we set upper limits on the rate at which BWMs of various amplitudes could have occurred during the time spanned by our data--e.g., BWMs with amplitudes greater than 10−13 must occur at a rate less than 1.5 yr−1.

Comments: 12 pages, 7 figures. We will submit this article to ApJ two weeks from now.

----------------------------------------------------------------

arXiv:1502.05735 [gr-qc]

Fisher vs. Bayes : A comparison of parameter estimation techniques for massive black hole binaries to high redshifts with eLISA

Edward K. Porter, Neil J. Cornish

(Submitted on 19 Feb 2015)

Massive black hole binaries are the primary source of gravitational waves (GW) for the future eLISA observatory. The detection and parameter estimation of these sources to high redshift would provide invaluable information on the formation mechanisms of seed black holes, and on the evolution of massive black holes and their host galaxies through cosmic time. The Fisher information matrix has been the standard tool for GW parameter estimation in the last two decades. However, recent studies have questioned the validity of using the Fisher matrix approach. For example, the Fisher matrix approach sometimes predicts errors of ≥100% in the estimation of parameters such as the luminosity distance and sky position. With advances in computing power, Bayesian inference is beginning to replace the Fisher matrix approximation in parameter estimation studies. In this work, we conduct a Bayesian inference analysis for 120 sources situated at redshifts of between 0.1≤z≤13.2, and compare the results with those from a Fisher matrix analysis. The Fisher matrix results suggest that for this particular selection of sources, eLISA would be unable to localize sources at redshifts of z≲6. In contrast, Bayesian inference provides finite error estimations for all sources in the study, and shows that we can establish minimum closest distances for all sources. The study further predicts that we should be capable with eLISA, out to a redshift of at least z≤13, of predicting a maximum error in the chirp mass of ≲1%, the reduced mass of ≲20%, the time to coalescence of 2 hours, and to a redshift of z∼5, the inclination of the source with a maximum error of ∼60 degrees.

----------------------------------------------------------------

arXiv:1503.02662 [gr-qc]

Constraining the Solution to the Last Parsec Problem with Pulsar Timing

Laura Sampson, Neil J. Cornish, Sean T. McWilliams

(Submitted on 9 Mar 2015)

The detection of a stochastic gravitational-wave signal from the superposition of many inspiraling supermassive black holes with pulsar timing arrays (PTAs) is likely to occur within the next decade. With this detection will come the opportunity to learn about the processes that drive black-hole-binary systems toward merger through their effects on the gravitational-wave spectrum. We use Bayesian methods to investigate the extent to which effects other than gravitational-wave emission can be distinguished using PTA observations. We show that, even in the absence of a detection, it is possible to place interesting constraints on these dynamical effects for conservative predictions of the population of tightly bound supermassive black-hole binaries. For instance, if we assume a relatively weak signal consistent with a low number of bound binaries and a low black-hole-mass to galaxy-mass correlation, we still find that a non-detection by a simulated array, with a sensitivity that should be reached in practice within a few years, disfavors gravitational-wave-dominated evolution with an odds ratio of ∼30:1. Such a finding would suggest either that all existing astrophysical models for the population of tightly bound binaries are overly optimistic, or else that some dynamical effect other than gravitational-wave emission is actually dominating binary evolution even at the relatively high frequencies/small orbital separations probed by PTAs.

----------------------------------------------------------------

arXiv:1505.07179 [gr-qc]

Phase-coherent mapping of gravitational-wave backgrounds using ground-based laser interferometers

Joseph D. Romano, Stephen R. Taylor, Neil J. Cornish, Jonathan Gair, Chiara M. F. Mingarelli, Rutger van Haasteren

(Submitted on 27 May 2015)

We extend the formalisms developed in Gair et al. and Cornish and van Haasteren to create maps of gravitational-wave backgrounds using a network of ground-based laser interferometers. We show that in contrast to pulsar timing arrays, which are insensitive to half of the gravitational-wave sky (the curl modes), a network of ground-based interferometers is sensitive to both the gradient and curl components of the background. The spatial separation of a network of interferometers, or of a single interferometer at different times during its rotational and orbital motion around the Sun, allows for recovery of both components. We derive expressions for the response functions of a laser interferometer in the small-antenna limit, and use these expressions to calculate the overlap reduction function for a pair of interferometers. We also construct maximum-likelihood estimates of the + and x-polarization modes of the gravitational-wave sky in terms of the response matrix for a network of ground-based interferometers, evaluated at discrete times during Earth's rotational and orbital motion around the Sun. We demonstrate the feasibility of this approach for some simple simulated backgrounds (a single point source and spatially-extended distributions having only grad or curl components), calculating maximum-likelihood sky maps and uncertainty maps based on the (pseudo)inverse of the response matrix. The distinction between this approach and standard methods for mapping gravitational-wave power is also discussed.

----------------------------------------------------------------

arXiv:1505.08084 [gr-qc]

When is a gravitational-wave signal stochastic?

Neil J. Cornish, Joseph D. Romano

(Submitted on 29 May 2015)

We discuss the detection of gravitational-wave backgrounds in the context of Bayesian inference and suggest a practical definition of what it means for a signal to be considered stochastic---namely, that the Bayesian evidence favors a stochastic signal model over a deterministic signal model. A signal can further be classified as Gaussian-stochastic if a Gaussian signal model is favored. In our analysis we use Bayesian model selection to choose between several signal and noise models for simulated data consisting of uncorrelated Gaussian detector noise plus a superposition of sinusoidal signals from an astrophysical population of gravitational-wave sources. For simplicity, we consider co-located and co-aligned detectors with white detector noise, but the method can be extended to more realistic detector configurations and power spectra. The general trend we observe is that a deterministic model is favored for small source numbers, a non-Gaussian stochastic model is preferred for intermediate source numbers, and a Gaussian stochastic model is preferred for large source numbers. However, there is very large variation between individual signal realizations, leading to fuzzy boundaries between the three regimes. We find that a hybrid, trans-dimensional model comprised of a deterministic signal model for individual bright sources and a Gaussian-stochastic signal model for the remaining confusion background outperforms all other models in most instances.

----------------------------------------------------------------

arXiv:1504.03682 [astro-ph.CO]

Signatures of the Very Early Universe: Inflation, Spatial Curvature and Large Scale Anomalies

Grigor Aslanyan, Richard Easther

(Submitted on 14 Apr 2015 (v1), last revised 24 Jun 2015 (this version, v2))

A short inflationary phase may not erase all traces of the primordial universe. Associated observables include both spatial curvature and "anomalies" in the microwave background or large scale structure. The present curvature ΩK,0 reflects the initial curvature, ΩK,start, and the angular size of anomalies depends on kstart, the comoving horizon size at the onset of inflation. We estimate posteriors for ΩK,start and kstart using current data and simulations, and show that if either quantity is measured to have a non-zero value, both are likely to be observable. Mappings from ΩK,start and kstart to present-day observables depend strongly on the primordial equation of state; ΩK,0 spans ten orders of magnitude for a given ΩK,start while a simple and general relationship connects ΩK,0 and kstart. We show that current bounds on ΩK,0 imply that if kstart is measurable, the curvature was already small when inflation began. Finally, since the energy density changes slowly during inflation, primordial gravitational wave constraints require that a short inflationary phase is preceded by a nontrivial pre-inflationary phase with critical implications for the expected value of ΩK,start.

Journal reference: Phys. Rev. D 91, 123523 (2015)

----------------------------------------------------------------

arXiv:1502.02648 [gr-qc]

Phase plane analysis and statefinder diagnostic of agegraphic dark energy in 5D Brans-Dicke cosmology

Amin Salehi, Hossein Farajollahi, Jafar Sadeghi, M.Pourali

(Submitted on 9 Feb 2015)

We present an autonomous phase-plane describing the evolution of field equations containing an agegraphic dark energy in 5D Brans- Dicke cosmology. To observationally verify the numerical results, we simultaneously solve the equations by constraining the model parameters with SNe Ia data. We find conditions for the existence and stability of the critical points (states of the universe) and numerically examine the cosmological parameters. We also investigate the model by means of statefinder diagnostic.

----------------------------------------------------------------

arXiv:1503.06335 [gr-qc]

Tachyonic Intermediate Inflation in DGP Cosmology; consistency with new observations

Arvin Ravanpak, Hossein Farajollahi, Golnaz Farpoor Fadakar

(Submitted on 21 Mar 2015)

In this article we study an intermediate inflationary model in the context of Dvali-Gabadadze-Porrati (DGP) cosmology caused by a tachyon scalar field. Considering slow-roll inflation we discuss the dynamics of the Universe. Using perturbation theory, we estimate some of the model parameters numerically and compare them with observations, particularly with Planck Temperature data released in 2013 (PT13), nine-years data set of Wilkinson Microwave Anisotropy Probe (WMAP9) and data from second Background Imaging of Cosmic Extragalactic Polarization instrument (BICEP2).

----------------------------------------------------------------

arXiv:1403.1338 [gr-qc]

Numerical evolution of plane gravitational waves in the Friedrich-Nagy gauge

Jörg Frauendiener, Chris Stevens

(Submitted on 6 Mar 2014 (v1), last revised 25 Jul 2014 (this version, v3))

The first proof of well-posedness of an initial boundary value problem for the Einstein equations was given in 1999 by Friedrich and Nagy. They used a frame formalism with a particular gauge for formulating the equations. This `Friedrich-Nagy' (FN) gauge has never been implemented for use in numerical simulations before because it was deemed too complicated. In this paper we present an implementation of the FN gauge for systems with two commuting space-like Killing vectors. We investigate the numerical performance of this formulation for plane wave space-times, reproducing the well-known Khan-Penrose solution for colliding impulsive plane waves and exhibiting a gravitational wave `ping-pong'.

Journal reference: Phys. Rev. D 89, 104026 (2014)

----------------------------------------------------------------

arXiv:1502.07427 [gr-qc]

A spectral method for half-integer spin fields based on spin-weighted spherical harmonics

Florian Beyer, Boris Daszuta, Joerg Frauendiener

(Submitted on 26 Feb 2015)

We present a new spectral scheme for analysing functions of half-integer spin-weight on the 2-sphere and demonstrate the stability and convergence properties of our implementation. The dynamical evolution of the Dirac equation on a manifold with spatial topology of S2 via pseudo-spectral method is also demonstrated.

----------------------------------------------------------------

arXiv:1505.05920 [gr-qc]

Numerical solutions of Einstein's equations for cosmological spacetimes with spatial topology S3 and symmetry group U(1)

Florian Beyer, Leon Escobar, Jörg Frauendiener

(Submitted on 21 May 2015)

We apply a single patch pseudo-spectral scheme based on integer spin-weighted spherical harmonics presented in [1, 2] to Einstein's equations. The particular hyperbolic reduction of Einstein's equations which we use is obtained by a covariant version of the generalized harmonic formalism and Geroch's symmetry reduction. In this paper we focus on spacetimes with a spatial S3-topology and symmetry group U(1). We discuss analytical and numerical issues related to our implementation. As a test, we reproduce numerically exact inhomogeneous cosmological solutions of the vacuum Einstein field equations obtained in [3].

----------------------------------------------------------------

arXiv:1410.5238 [physics.ins-det]

Extending the frequency range of digital noise measurements to the microwave domain

Stephen R. Parker, Eugene N. Ivanov, John G. Hartnett

(Submitted on 20 Oct 2014)

We describe the use of digital phase noise test sets at frequencies well beyond the sampling rate of their analog-to-digital converters. The technique proposed involves the transfer of phase fluctuations from an arbitrary high carrier frequency to within the operating frequency range of the digital instrument. The validity of the proposed technique has been proven via comparison with conventional methods. Digital noise measurements eliminate the need for calibration and improve consistency of experimental results. Mechanisms limiting the resolution of spectral measurements are also discussed.

Journal reference: Parker, S.R.; Ivanov, E.N.; Hartnett, J.G., "Extending the Frequency Range of Digital Noise Measurements to the Microwave Domain," Microwave Theory and Techniques, IEEE Transactions on , vol.62, no.2, pp.368,372, Feb. 2014

----------------------------------------------------------------

Cryogenic resonant microwave cavity searches for hidden sector photons

Stephen R. Parker, John G. Hartnett, Rhys G. Povey, Michael E. Tobar

(Submitted on 20 Oct 2014)

The hidden sector photon is a weakly interacting hypothetical particle with sub-eV mass that kinetically mixes with the photon. We describe a microwave frequency light shining through a wall experiment where a cryogenic resonant microwave cavity is used to try and detect photons that have passed through an impenetrable barrier, a process only possible via mixing with hidden sector photons. For a hidden sector photon mass of 53 μeV we limit the hidden photon kinetic mixing parameter χ<1.7×10−7, which is an order of magnitude lower than previous bounds derived from cavity experiments in the same mass range. In addition, we use the cryogenic detector cavity to place new limits on the kinetic mixing parameter for hidden sector photons as a form of cold dark matter.

Journal reference: S.R. Parker, J.G. Hartnett, R.G. Povey, and M.E. Tobar, "Cryogenic resonant microwave cavity searches for hidden sector photons," Phys. Rev. D 88, 112004, 2013

----------------------------------------------------------------

arXiv:1412.6954 [hep-ph]

Direct Terrestrial Measurement of the Spatial Isotropy of the Speed of Light to 10−18

M. Nagel, S. R. Parker, E. V. Kovalchuk, P. L. Stanwix, J. G. Hartnett, E. N. Ivanov, A. Peters, M. E. Tobar

(Submitted on 22 Dec 2014)

Lorentz symmetry is a foundational property of modern physics, underlying both the standard model of particles and general relativity. It is anticipated that these two theories are merely low energy approximations of a single theory of the four fundamental forces that is unified and consistent at the Planck scale. Many unifying proposals allow for Lorentz symmetry to be broken, with observable effects appearing at Planck-suppressed levels. Thus precision tests of Lorentz Invariance Violation (LIV) are needed to assess and guide theoretical efforts. The most significant consequence of Lorentz symmetry is the isotropic nature of the speed of light, which remains invariant under rotation and boost transformations. In this work we use two ultra-stable oscillator frequency sources to perform a modern Michelson-Morley experiment and make the most precise measurement to date of the spatial isotropy of the speed of light, constraining Δc/c to 9.2±10.7×10−19 (95C.I.). This order of magnitude improvement over the current state-of-the-art allows us to undertake the first terrestrial test of LIV in electrodynamics at the Planck-suppressed electroweak unification scale, finding no significant violation of Lorentz symmetry.

----------------------------------------------------------------

arXiv:1501.04857 [physics.gen-ph]

Deriving time from the geometry of space

James M. Chappell, John G. Hartnett, Nicolangelo Iannella, Derek Abbott

(Submitted on 15 Jan 2015 (v1), last revised 8 Apr 2015 (this version, v2))

The Minkowski formulation of special relativity reveals the essential four-dimensional nature of spacetime, consisting of three space and one time dimension. Recognizing its fundamental importance, a variety of arguments have been proposed over the years attempting to derive the Minkowski spacetime structure from fundamental physical principles. In this paper we illustrate how Minkowski spacetime follows naturally from the geometric properties of three dimensional Clifford space modeled with multivectors. This approach also generalizes spacetime to an eight dimensional space as well as doubling the size of the Lorentz group. This description of spacetime also provides a new geometrical interpretation of the nature of time.

----------------------------------------------------------------

arXiv:1407.8254 [astro-ph.SR]

Gravitational waves from rapidly rotating neutron stars

Brynmor Haskell, Nils Andersson, Caroline D`Angelo, Nathalie Degenaar, Kostas Glampedakis, Wynn C.G. Ho, Paul D. Lasky, Andrew Melatos, Manuel Oppenoorth, Alessandro Patruno, Maxim Priymak

(Submitted on 31 Jul 2014)

Rapidly rotating neutron stars in Low Mass X-ray Binaries have been proposed as an interesting source of gravitational waves. In this chapter we present estimates of the gravitational wave emission for various scenarios, given the (electromagnetically) observed characteristics of these systems. First of all we focus on the r-mode instability and show that a 'minimal' neutron star model (which does not incorporate exotica in the core, dynamically important magnetic fields or superfluid degrees of freedom), is not consistent with observations. We then present estimates of both thermally induced and magnetically sustained mountains in the crust. In general magnetic mountains are likely to be detectable only if the buried magnetic field of the star is of the order of B≈1012 G. In the thermal mountain case we find that gravitational wave emission from persistent systems may be detected by ground based interferometers. Finally we re-asses the idea that gravitational wave emission may be balancing the accretion torque in these systems, and show that in most cases the disc/magnetosphere interaction can account for the observed spin periods.

Comments: To appear in 'Gravitational Waves Astrophysics: 3rd Session of the Sant Cugat Forum on Astrophysics, 2014', Editor: Carlos F. Sopuerta

----------------------------------------------------------------

arXiv:1501.06039 [astro-ph.SR]

Detecting gravitational waves from mountains on neutron stars in the Advanced Detector Era

Brynmor Haskell, Maxim Priymak, Alessandro Patruno, Manuel Oppenoorth, Andrew Melatos, Paul Lasky

(Submitted on 24 Jan 2015)

Rapidly rotating Neutron Stars (NSs) in Low Mass X-ray Binaries (LMXBs) are thought to be interesting sources of Gravitational Waves (GWs) for current and next generation ground based detectors, such as Advanced LIGO and the Einstein Telescope. The main reason is that many of the NS in these systems appear to be spinning well below their Keplerian breakup frequency, and it has been suggested that torques associated with GW emission may be setting the observed spin period. This assumption has been used extensively in the literature to assess the strength of the likely gravitational wave signal. There is now, however, a significant amount of theoretical and observation work that suggests that this may not be the case, and that GW emission is unlikely to be setting the spin equilibrium period in many systems. In this paper we take a different starting point and predict the GW signal strength for two physical mechanisms that are likely to be at work in LMXBs: crustal mountains due to thermal asymmetries and magnetically confined mountains. We find that thermal crustal mountains in transient LMXBs are unlikely to lead to detectable GW emission, while persistent systems are good candidates for detection by Advanced LIGO and by the Einstein Telescope. Detection prospects are pessimistic for the magnetic mountain case, unless the NS has a buried magnetic field of B≈1012 G, well above the typically inferred exterior dipole fields of these objects. Nevertheless, if a system were to be detected by a GW observatory, cyclotron resonant scattering features in the X-ray emission could be used to distinguish between the two different scenarios.

Comments: submitted to MNRAS

----------------------------------------------------------------

arXiv:1502.07062 [astro-ph.SR]

Models of Pulsar Glitches

Brynmor Haskell, Andrew Melatos

(Submitted on 25 Feb 2015)

Radio pulsars provide us with some of the most stable clocks in the universe. Nevertheless several pulsars exhibit sudden spin-up events, known as glitches. More than forty years after their first discovery, the exact origin of these phenomena is still open to debate. It is generally thought that they an observational manifestation of a superfluid component in the stellar interior and provide an insight into the dynamics of matter at extreme densities. In recent years there have been several advances on both the theoretical and observational side, that have provided significant steps forward in our understanding of neutron star interior dynamics and possible glitch mechanisms. In this article we review the main glitch models that have been proposed and discuss our understanding, in the light of current observations.

Comments: Preprint of an article published in IJMPD 24, issue 3, 2015, 530008.

----------------------------------------------------------------

arXiv:1408.4601 [gr-qc]

The Kerr-Schild ansatz revised

Donato Bini, Andrea Geralico, Roy P. Kerr

(Submitted on 20 Aug 2014)

Kerr-Schild metrics have been introduced as a linear superposition of the flat spacetime metric and a squared null vector field, say k, multiplied by some scalar function, say H. The basic assumption which led to Kerr solution was that k be both geodesic and shearfree. This condition is relaxed here and Kerr-Schild ansatz is revised by treating Kerr-Schild metrics as {\it exact linear perturbations} of Minkowski spacetime. The scalar function H is taken as the perturbing function, so that Einstein's field equations are solved order by order in powers of H. It turns out that the congruence must be geodesic and shearfree as a consequence of third and second order equations, leading to an alternative derivation of Kerr solution.

Journal reference: IJGMMP 7, 693 (2010)

----------------------------------------------------------------

arXiv:1503.03298 [astro-ph.HE]

Pulsar timing noise and the minimum observation time to detect gravitational waves with pulsar timing arrays

Paul D. Lasky, Andrew Melatos, Vikram Ravi, George Hobbs

(Submitted on 11 Mar 2015)

The sensitivity of pulsar timing arrays to gravitational waves is, at some level, limited by timing noise. Red timing noise - the stochastic wandering of pulse arrival times with a red spectrum - is prevalent in slow-spinning pulsars and has been identified in many millisecond pulsars. Phenomenological models of timing noise, such as from superfluid turbulence, suggest that the timing noise spectrum plateaus below some critical frequency, fc, potentially aiding the hunt for gravitational waves. We examine this effect for individual pulsars by calculating minimum observation times, Tmin(fc), over which the gravitational wave signal becomes larger than the timing noise plateau. We do this in two ways: 1) in a model-independent manner, and 2) by using the superfluid turbulence model for timing noise as an example to illustrate how neutron star parameters can be constrained. We show that the superfluid turbulence model can reproduce the data qualitatively from a number of pulsars observed as part of the Parkes Pulsar Timing Array. We further show how a value of fc, derived either through observations or theory, can be related to Tmin. This provides a diagnostic whereby the usefulness of timing array pulsars for gravitational-wave detection can be quantified.

Comments: Accepted for publication in MNRAS

----------------------------------------------------------------

arXiv:1504.05889 [gr-qc]

Gravitational waves from Sco X-1: A comparison of search methods and prospects for detection with advanced detectors

C. Messenger, H. J. Bulten, S. G. Crowder, V. Dergachev, D. K. Galloway, E. Goetz, R. J. G. Jonker, P. D. Lasky, G. D. Meadors, A. Melatos, S. Premachandra, K. Riles, L. Sammut, E. H. Thrane, J. T. Whelan, Y. Zhang

(Submitted on 22 Apr 2015)

The low-mass X-ray binary Scorpius X-1 (Sco X-1) is potentially the most luminous source of continuous gravitational-wave radiation for interferometers such as LIGO and Virgo. For low-mass X-ray binaries this radiation would be sustained by active accretion of matter from its binary companion. With the Advanced Detector Era fast approaching, work is underway to develop an array of robust tools for maximizing the science and detection potential of Sco X-1. We describe the plans and progress of a project designed to compare the numerous independent search algorithms currently available. We employ a mock-data challenge in which the search pipelines are tested for their relative proficiencies in parameter estimation, computational efficiency, robust- ness, and most importantly, search sensitivity. The mock-data challenge data contains an ensemble of 50 Scorpius X-1 (Sco X-1) type signals, simulated within a frequency band of 50-1500 Hz. Simulated detector noise was generated assuming the expected best strain sensitivity of Advanced LIGO and Advanced VIRGO (4×10−24 Hz−1/2). A distribution of signal amplitudes was then chosen so as to allow a useful comparison of search methodologies. A factor of 2 in strain separates the quietest detected signal, at 6.8×10−26 strain, from the torque-balance limit at a spin frequency of 300 Hz, although this limit could range from 1.2×10−25 (25 Hz) to 2.2×10−26 (750 Hz) depending on the unknown frequency of Sco X-1. With future improvements to the search algorithms and using advanced detector data, our expectations for probing below the theoretical torque-balance strain limit are optimistic.

Report number: LIGO-P1400217-v3

----------------------------------------------------------------

arXiv:1405.5252 [gr-qc]

Demonstration of the spike phenomenon using the LTB models

Alan Coley, Woei Chet Lim

(Submitted on 20 May 2014)

We demonstrate the occurrence of permanent spikes using the Lemaitre-Tolman-Bondi models, chosen because the solutions are exact and can be analyzed by qualitative dynamical systems methods. Three examples are given and illustrated numerically. The third example demonstrates that spikes can form directly in the matter density, as opposed to indirectly in previous studies of spikes in the Kasner regime. Spikes provide an alternative general relativistic mechanism for generating exceptionally large structures observed in the Universe.

Journal reference: Class. Quantum Grav. 31 (2014) 115012

----------------------------------------------------------------

Arthur Suvorov, Anthony W.C. Lun

(Submitted on 14 Jan 2014)

We present some results regarding metric perturbations in general relativity and other metric theories of gravity. In particular, using the Newman Penrose variables, we write down and discuss the equations which govern tensor field perturbations of ranks 0,±1 and ±2 (scalar,vector,tensor bosonic perturbations) over certain space-times that admit specific background metrics expressible in isotropic coordinates. Armed with these equations, we are able to develop the Hadamard series which can be associated with the fundamental solution of the equations, wherein we introduce an inhomogeneous singularity at the physical space-time point of the perturbing particle. The Hadamard series is then used to generate closed form solutions by making choices for an appropriate ansatz solution. In particular, we solve for the spin-weighted electrostatic potential for the Reissner-Nordstrom black hole and for the fully dynamical potential for the Friedmann-Robertson-Walker cosmological solution.

----------------------------------------------------------------

arXiv:1407.2446 [gr-qc]

The little sibling of the big rip singularity

Mariam Bouhmadi-Lopez, Ahmed Errahmani, Prado Martin-Moruno, Taoufik Ouali, Yaser Tavakoli

(Submitted on 9 Jul 2014 (v1), last revised 24 Jun 2015 (this version, v2))

We present a new cosmological event, which we named the little sibling of the big rip. This event is much smoother than the big rip singularity. When the little sibling of the big rip is reached, the Hubble rate and the scale factor blow up but the cosmic derivative of the Hubble rate does not. This abrupt event takes place at an infinite cosmic time where the scalar curvature explodes. We show that a doomsday \'a la little sibling of the big rip is compatible with an accelerating universe, indeed at present it would mimic perfectly a LCDM scenario. It turns out that eventhough the event seems to be harmless as it takes place in the infinite future, the bound structures in the universe would be unavoidably destroyed on a finite cosmic time from now. The model can be motivated by considering that the weak energy condition should not be abusibely violated in our Universe, and it could give us some hints about the status of recently formulated nonlinear energy conditions.

Journal reference: Int.J.Mod.Phys.D Vol. 24, No. 10 (2015) 1550078 (20 pages)

----------------------------------------------------------------

arXiv:1407.7758 [gr-qc]

Wormholes minimally violating the null energy condition

Mariam Bouhmadi-Lopez, Francisco S. N. Lobo, Prado Martin-Moruno

(Submitted on 29 Jul 2014 (v1), last revised 22 Oct 2014 (this version, v2))

We consider novel wormhole solutions supported by a matter content that minimally violates the null energy condition. More specifically, we consider an equation of state in which the sum of the energy density and radial pressure is proportional to a constant with a value smaller than that of the inverse area characterising the system, i.e., the area of the wormhole mouth. This approach is motivated by a recently proposed cosmological event, denoted "the little sibling of the big rip", where the Hubble rate and the scale factor blow up but the cosmic derivative of the Hubble rate does not [1]. By using the cut-and-paste approach, we match interior spherically symmetric wormhole solutions to an exterior Schwarzschild geometry, and analyze the stability of the thin-shell to linearized spherically symmetric perturbations around static solutions, by choosing suitable properties for the exotic material residing on the junction interface radius. Furthermore, we also consider an inhomogeneous generalisation of the equation of state considered above and analyse the respective stability regions. In particular, we obtain a specific wormhole solution with an asymptotic behaviour corresponding to a global monopole.

Journal reference: JCAP 1411 (2014) 007

----------------------------------------------------------------

arXiv:1411.4401 [gr-qc]

Semiclassical energy conditions and wormholes

Prado Martin-Moruno

(Submitted on 17 Nov 2014)

We consider the nonlinear energy conditions and their quantum extensions. These new energy conditions behave much better than the usual pointwise energy conditions in the presence of semiclassical quantum effects. Analogous quantum extensions for the linear energy conditions are not always satisfied as one enters the quantum realm, but they can be used to constrain the violation of the classical conditions. Thus, the existence of wormholes supported by a fluid which violates the null energy condition in a controlled way is of particular interest.

Comments: 6 pages; contribution to the proceedings of the Spanish Relativity Meeting 2014 (ERE 2014), Valencia, 1-5 September 2014

----------------------------------------------------------------

arXiv:1502.03236 [gr-qc]

Horndeski theories self-tuning to a de Sitter vacuum

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

(Submitted on 11 Feb 2015 (v1), last revised 27 Mar 2015 (this version, v2))

We consider Horndeski cosmological models able to screen the vacuum energy coming from any field theory assuming that after this screening the space should be in a de Sitter vacuum with a particular value of the cosmological constant specified by the theory of gravity itself. The most general scalar-tensor cosmological models without higher than second order derivatives in the field equations that have a spatially flat de Sitter critical point for any kind of material content or vacuum energy are, therefore, presented. These models could allow us to understand the current accelerated expansion of the universe as the result of a dynamical evolution towards a de Sitter attractor.

Journal reference: Phys.Rev.D91:084029,2015

----------------------------------------------------------------

arXiv:1502.05878 [gr-qc]

Attracted to de Sitter: cosmology of the linear Horndeski models

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

(Submitted on 19 Feb 2015 (v1), last revised 6 May 2015 (this version, v2))

We consider Horndeski cosmological models, with a minisuperspace Lagrangian linear in the field derivative, that are able to screen any vacuum energy and material content leading to a spatially flat de Sitter vacuum fixed by the theory itself. Furthermore, we investigate particular models with a cosmic evolution independent of the material content and use them to understand the general characteristics of this framework. We also consider more realistic models, which we denote the "term-by-term" and "tripod" models, focusing attention on cases in which the critical point is indeed an attractor solution and the cosmological history is of particular interest.

Comments: V1: 25 pages, 14 figures. V2: 5 references added, minor clarifications added. This version accepted for publication in JCAP

----------------------------------------------------------------

arXiv:1502.05878 [gr-qc]

Attracted to de Sitter: cosmology of the linear Horndeski models

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

(Submitted on 19 Feb 2015 (v1), last revised 6 May 2015 (this version, v2))

We consider Horndeski cosmological models, with a minisuperspace Lagrangian linear in the field derivative, that are able to screen any vacuum energy and material content leading to a spatially flat de Sitter vacuum fixed by the theory itself. Furthermore, we investigate particular models with a cosmic evolution independent of the material content and use them to understand the general characteristics of this framework. We also consider more realistic models, which we denote the "term-by-term" and "tripod" models, focusing attention on cases in which the critical point is indeed an attractor solution and the cosmological history is of particular interest.

Journal reference: JCAP 1505 (2015) 033

----------------------------------------------------------------

arXiv:1505.06585 [gr-qc]

Accelerating universe as a result of an adjustment mechanism

Prado Martin-Moruno, Nelson J. Nunes

(Submitted on 25 May 2015)

In this essay we propose that the theory of gravity's vacuum is described by a de Sitter geometry. Under this assumption we consider an adjustment mechanism able to screen any value of the vacuum energy of the matter fields. We discuss the most general scalar-tensor cosmological models with second order equations of motion that have a fixed de Sitter critical point for any kind of material content. These models give rise to interesting cosmological evolutions that we shall discuss.

Comments: 7 pages, 2 figures. Essay awarded honorable mention in the Gravity Research Foundation essay competition 2015.

----------------------------------------------------------------

arXiv:1506.02497 [gr-qc]

Attracted to de Sitter II: cosmology of the shift-symmetric Horndeski models

Prado Martin-Moruno, Nelson J. Nunes

(Submitted on 8 Jun 2015 (v1), last revised 9 Jun 2015 (this version, v2))

Horndeski models with a de Sitter critical point for any kind of material content can provide a mechanism to alleviate the cosmological constant problem. They allow us to understand the current accelerated expansion of the universe as the result of the dynamical approach to the critical point when it is an attractor. We show that this critical point is indeed an attractor for the shift-symmetric subfamily of models with these characteristics. We study the resulting cosmological scenario and conclude that their background dynamics is compatible with the latest observational data.

----------------------------------------------------------------

arXiv:1502.05768 [physics.ins-det]

Thermal noise of gram-scale cantilever flexures

Thanh T-H. Nguyen, Bram J.J. Slagmolen, Conor M. Mow-Lowry, John Miller, Adam Mullavey, Stefan Goßler, Paul A. Altin, Daniel A. Shaddock, David E. McClelland

(Submitted on 20 Feb 2015 (v1), last revised 3 Jun 2015 (this version, v3))

We present measurements of thermal noise in niobium and aluminium flexures. Our measurements cover the audio frequency band from 10Hz to 10kHz, which is of particular relevance to ground-based interferometric gravitational wave detectors, and span up to an order of magnitude above and below the fundamental flexure resonances at 50Hz - 300Hz. Our results are well-explained by a simple model in which both structural and thermoelastic loss play a role. The ability of such a model to explain this interplay is important for investigations of quantum-radiation-pressure noise and the standard quantum limit.

----------------------------------------------------------------

arXiv:1410.3470 [astro-ph.CO]

Cosmic microwave background anisotropies in the timescape cosmology

M. Ahsan Nazer, David L. Wiltshire

(Submitted on 13 Oct 2014 (v1), last revised 13 Mar 2015 (this version, v3))

We analyze the spectrum of cosmic microwave background (CMB) anisotropies in the timescape cosmology: a potentially viable alternative to homogeneous isotropic cosmologies without dark energy. We exploit the fact that the timescape cosmology is extremely close to the standard cosmology at early epochs to adapt existing numerical codes to produce CMB anisotropy spectra, and to match these as closely as possible to the timescape expansion history. A variety of matching methods are studied and compared. We perform Markov Chain Monte Carlo analyses on the parameter space, and fit CMB multipoles 50≤ℓ≤2500 to the Planck satellite data. Parameter fits include a dressed Hubble constant, H0=61.0 km/s/Mpc (± 1.3% stat) (± 8% sys), and a present void volume fraction fv0=0.627 (± 2.3% stat) (± 13% sys). We find best fit likelihoods which are comparable to that of the best fit Lambda CDM cosmology in the same multipole range. In contrast to earlier results, the parameter constraints afforded by this analysis no longer admit the possibility of a solution to the primordial lithium abundance anomaly. This issue is related to a strong constraint between the ratio of baryonic to nonbaryonic dark matter and the ratio of heights of the second and third acoustic peaks, which cannot be changed as long as the standard cosmology is assumed up to the surface of last scattering. These conclusions may change if backreaction terms are also included in the radiation-dominated primordial plasma.

Journal reference: Phys.Rev.D91:063519,2015

----------------------------------------------------------------

arXiv:1408.6613 [hep-th]

Gauss-Bonnet assisted braneworld inflation in light of BICEP2 and Planck data

Ishwaree P. Neupane (CERN)

(Submitted on 28 Aug 2014 (v1), last revised 10 Dec 2014 (this version, v2))

Motivated by the idea that quantum gravity corrections usually suppress the power of the scalar primordial spectrum (E-mode) more than the power of the tensor primordial spectrum (B-mode), in this paper we construct a concrete gravitational theory in five-dimensions for which V(ϕ)∝ϕn-type inflation (n≥1) generates an appropriate tensor-to-scalar ratio that may be compatible with the BICEP2 and Planck data together. The true nature of gravity is five-dimensional and described by the action S=∫d5x|g|−−√M3(−6λM2+R+αM−2R2) where M is the five-dimensional Planck mass and R2=R2−4RabRab+RabcdRabcd is the Gauss-Bonnet (GB) term. The five-dimensional "bulk" spacetime is anti-de Sitter (λ<0) for which inflation ends naturally. The effects of R2 term on the magnitudes of scalar and tensor fluctuations and spectral indices are shown to be important at the energy scale of inflation. For GB-assisted m2ϕ2-inflation, inflationary constraints from BICEP2 and Planck, such as, ns≃0.9603(±0.0073), r=0.16(+0.06−0.05) and V1/4∗∼1.5×1016GeV are all satisfied for (−λα)≃(3−300)×10−5.

Journal reference: Phys.Rev. D90 (2014) 123534

----------------------------------------------------------------

arXiv:1409.8647 [astro-ph.CO]

Natural Braneworld Inflation in Light of Recent Results from Planck and BICEP2

Ishwaree P. Neupane

(Submitted on 29 Sep 2014 (v1), last revised 14 Nov 2014 (this version, v2))

In this paper we report on a major theoretical observation in cosmology. We present a concrete cosmological model for which inflation has natural beginning and natural ending. Inflation is driven by a cosine-form potential, V(ϕ)=Λ4(1−cos(ϕ/f)), which begins at ϕ≲πf and ends at ϕ=ϕend≲5f/3. The distance traversed by the inflaton field ϕ is sub-Planckian. The Gauss-Bonnet term R2 arising as leading curvature corrections in the action S=∫d5x√−g5M3(−6λM2+R+αM−2R2)+∫d4x√−g4 (ϕ˙2/2−V(ϕ)−σ+Lmatter) (where α and λ are constants and M is the five-dimensional Planck mass) plays a key role to terminate inflation. The model generates appropriate tensor-to-scalar ratio r and inflationary perturbations that are consistent with results from Planck and BICEP2. For example, for N∗=50−60 and ns∼0.960±0.005, the model predicts that M∼5.64×1016GeV and r∼(0.14−0.21) [N∗ is the number of {\it e}--folds of inflation and ns (nt) is the scalar (tensor) spectrum spectral index]. The ratio −nt/r is (13% -- 24%) less than its value in 4D Einstein gravity, −nt/r=1/8. The upper bound on the energy scale of inflation V1/4=2.37×1016GeV (r<0.27) implies that (−λα)≳75×10−5 and Λ<2.17×1016GeV, which thereby rule out the case α=0 (Randall-Sundrum model). The true nature of gravity is holographic as implied by braneworld realization of string and M theory. The model correctly predicts a late epoch cosmic acceleration with the dark energy equation of state wDE≈−1.

Journal reference: Phys. Rev. D 90, 123502 (2014)

----------------------------------------------------------------

arXiv:1401.0277 [math.AP]

A transmission problem for quasi-linear wave equations

Lars Andersson, Todd A. Oliynyk

(Submitted on 1 Jan 2014)

We prove the local existence and uniqueness of solutions to a system of quasi-linear wave equations involving a jump discontinuity in the lower order terms. A continuation principle is also established.

Journal reference: J. Differential Equations 256 (2014), 2023-2078

----------------------------------------------------------------

arXiv:1406.6104 [gr-qc]

The Newtonian limit on cosmological scales

Todd A. Oliynyk

(Submitted on 23 Jun 2014 (v1), last revised 10 Feb 2015 (this version, v2))

We establish the existence of a wide class of inhomogeneous relativistic solutions to the Einstein-Euler equations that are well approximated on cosmological scales by solutions of Newtonian gravity. Error estimates measuring the difference between the Newtonian and relativistic solutions are provided.

----------------------------------------------------------------

arXiv:1410.4894 [gr-qc]

Dynamical compact elastic bodies in general relativity

Lars Andersson, Todd A. Oliynyk, Bernd G. Schmidt

(Submitted on 18 Oct 2014)

We prove the local existence of solutions to the Einstein-Elastic equations that represent self-gravitating, relativistic elastic bodies with compact support.

----------------------------------------------------------------

arXiv:1501.00045 [math.AP]

A priori estimates for relativistic liquid bodies

Todd A. Oliynyk

(Submitted on 30 Dec 2014)

We demonstrate that a sufficiently smooth solution of the relativistic Euler equations that represents a dynamical compact liquid body, when expressed in Lagrangian coordinates, determines a solution to a system of non-linear wave equations with acoustic boundary conditions. Using this wave formulation, we prove that these solutions satisfy energy estimates without loss of derivatives. Importantly, our wave formulation does not require the liquid to be irrotational, and the energy estimates do not rely on divergence and curl type estimates employed in previous works.

----------------------------------------------------------------

arXiv:1505.00857 [gr-qc]

Future stability of the FLRW fluid solutions in the presence of a positive cosmological constant

Todd A. Oliynyk

(Submitted on 5 May 2015)

We introduce a new method for establishing the future non-linear stability of perturbations of FLRW solutions to the Einstein-Euler equations with a positive cosmological constant and a linear equation of state of the form ρ=Kp. The method is based on a conformal transformation of the Einstein-Euler equations that compactifies the time domain and can handle the equation of state parameter values 0<K≤1/3 in a uniform manner. It also determines the asymptotic behavior of the perturbed solutions in the far future.

----------------------------------------------------------------

arXiv:1410.3881 [gr-qc]

Universe in a black hole with spin and torsion

Nikodem J. Poplawski

(Submitted on 14 Oct 2014)

The conservation law for the angular momentum in curved spacetime requires that the antisymmetric part of the affine connection (the torsion tensor) is a variable in the principle of least action. The coupling between spin and torsion generates gravitational repulsion in fermionic matter at extremely high densities and avoids the formation of singularities in black holes. We show that every black hole in the presence of torsion forms a nonsingular, closed, nearly flat, homogeneous, and isotropic universe on the other side of its event horizon. Quantum particle production in such a universe can generate a period of exponential expansion which creates an enormous amount of matter in that universe. Accordingly, our Universe may have originated from the interior of a black hole existing in another universe.

----------------------------------------------------------------

arXiv:1501.01355 [gr-qc]

A new form of the C-metric with cosmological constant

Yu Chen, Yen-Kheng Lim, Edward Teo

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

The new form of the C-metric proposed by Hong and Teo, in which the two structure functions are factorised, has proved useful in its analysis. In this paper, we extend this form to the case when a cosmological constant is present. The new form of this solution has two structure functions which are partially factorised; moreover, the roots of the structure functions are now regarded as fundamental parameters. This leads to a natural representation of the solution in terms of its so-called domain structure, in which the allowed coordinate range can be visualised as a "box" in a two-dimensional plot. The solution is then completely parameterised by the locations of the edges of this box, at least in the uncharged case. We also briefly analyse other possible domain structures---in the shape of a triangle and trapezoid---that might describe physically interesting space-times within the AdS C-metric.

----------------------------------------------------------------

arXiv:1504.01235 [gr-qc]

A five-parameter class of solutions to the vacuum Einstein equations

Yu Chen, Edward Teo

(Submitted on 6 Apr 2015)

We present a new five-parameter class of Ricci-flat solutions in four dimensions with Euclidean signature. The solution is asymptotically locally flat (ALF), and contains a finite asymptotic NUT charge. When this charge is sent to infinity, the solution becomes asymptotically locally Euclidean (ALE), and one in fact obtains the Ricci-flat Plebanski-Demianski solution. The solution we have found can thus be regarded as an ALF generalisation of the latter solution. We also show that it can be interpreted as a system consisting of two touching Kerr-NUTs: the south pole of one Kerr-NUT touches the north pole of the other. The total NUT charge of such a system is then identified with the asymptotic NUT charge. Setting the asymptotic NUT charge to zero gives a four-parameter asymptotically flat (AF) solution, and contained within this subclass is the completely regular two-parameter AF instanton previously discovered by the present authors. Various other limits are also discussed, including that of the triple-collinearly-centered Gibbons-Hawking solution, and an ALF generalisation of the C-metric.

----------------------------------------------------------------

Robert T. Thompson, Mohsen Fathi

(Submitted on 29 Jun 2015)

The fully covariant formulation of transformation optics is used to find the configuration of a cloaking device operating in an expanding universe modelled by a Friedmann-Lema\^itre-Robertson-Walker spacetime. This spacetime cloak is used as a platform for probing the covariant formulation of transformation optics, thereby rigorously enhancing the conceptual understanding of the theory. By studying the problem in both comoving and physical coordinates we explicitly demonstrate the preservation of general covariance of electrodynamics under the transformation optics procedure. This platform also enables a detailed study of the various transformations that arise in transformation optics. We define a corporeal transformation as the "transformation" of transformation optics, and distinguish it from coordinate and frame transformations. We find that corporeal transformations considered in the literature have generally been restricted to a subset of all possible corporeal transformations, providing a potential mechanism for increased functionality of transformation optics.

----------------------------------------------------------------

arXiv:1410.2334 [gr-qc]

Gravitational Wave Detection with High Frequency Phonon Trapping Acoustic Cavities

Maxim Goryachev, Michael E. Tobar

(Submitted on 9 Oct 2014 (v1), last revised 31 Oct 2014 (this version, v2))

There are a number of theoretical predictions for astrophysical and cosmological objects, which emit high frequency (106−109~Hz) Gravitation Waves (GW) or contribute somehow to the stochastic high frequency GW background. Here we propose a new sensitive detector in this frequency band, which is based on existing cryogenic ultra-high quality factor quartz Bulk Acoustic Wave cavity technology, coupled to near-quantum-limited SQUID amplifiers at 20~mK. We show that spectral strain sensitivities reaching 10−22 per √Hz per mode is possible, which in principle can cover the frequency range with multiple (>100) modes with quality factors varying between 106−1010 allowing wide bandwidth detection. Due to its compactness and well established manufacturing process, the system is easily scalable into arrays and distributed networks that can also impact the overall sensitivity and introduce coincidence analysis to ensure no false detections.

Journal reference: Phys. Rev. D 90, 102005 (2014)

----------------------------------------------------------------

arXiv:1412.2142 [gr-qc]

Testing the isotropy of space using rotating quartz oscillators

Anthony Lo, Philipp Haslinger, Eli Mizrachi, Loic Anderegg, Holger Müller, Michael Hohensee, Maxim Goryachev, Michael E Tobar

(Submitted on 4 Dec 2014 (v1), last revised 28 Jan 2015 (this version, v2))

Violations of Lorentz invariance by matter and light can generate direction- and frame-dependent anisotropies in particles inertial masses and, hence, a measurable modulation of the oscillation frequency of rotating quartz crystal oscillators. This allows simple and low maintenance experiments that are ideally suited for long-term data taking. Using the Standard Model Extension (SME) as a parameterizing framework, we study the magnitude of this putative frequency modulation. A preliminary experiment with room-temperature SC-cut crystals yields a frequency resolution in the 10−15 range with ∼120 hours of data and a limit of c~Q=(−1.8±2.2)×10−14\,GeV on the most weakly constrained neutron-sector c−coefficient of the SME. Future experiments with cryogenic oscillators promise additional improvements in accuracy, opening up the potential for improved tests of Lorentz symmetry in the neutron, proton, electron and photon sector.

Report number: LLNL-JRNL-664409-DRAFT

----------------------------------------------------------------

arXiv:1504.03280 [gr-qc]

Search for Lorentz invariance violation through tests of the gravitational inverse square law at short-ranges

Cheng-Gang Shao, Yu-Jie Tan, Wen-Hai Tan, Shan-Qing Yang, Jun Luo, Michael Edmund Tobar

(Submitted on 13 Apr 2015)

A search for sidereal variations in the non-Newtonian force between two tungsten plates separated at millimeter ranges sets experimental limits on Lorentz invariance violation involving quadratic couplings of Riemann curvature. We show that the Lorentz invariance violation force between two finite flat plates is dominated by the edge effects, which includes a suppression effect leading to lower limits than previous rough estimates. From this search, we determine the current best constraints of the Lorentz invariance violating coefficients at a level of 10−8 m2.

----------------------------------------------------------------

arXiv:1506.05801 [gr-qc]

Quasilocal energy and thermodynamic equilibrium conditions

Nezihe Uzun, David L. Wiltshire

(Submitted on 18 Jun 2015)

Equilibrium thermodynamic laws are typically applied to horizons in general relativity without stating the conditions that bring them into equilibrium. We fill this gap by applying a new thermodynamic interpretation to a generalized Raychaudhuri equation for a closed spacelike 2-surface, the "screen", which encompasses a system of arbitrary size in nonequilibrium with its surroundings in general. In the case of spherical symmetry this enables us to identify quasilocal thermodynamic potentials directly related to standard quasilocal energy definitions. Quasilocal thermodynamic equilibrium is defined by minimizing the mean extrinsic curvature of the screen. Moreover, without any direct reference to surface gravity, we find that the system comes into quasilocal thermodynamic equilibrium when the screen is located at a generalized apparent horizon. Examples of the Schwarzschild, Friedmann-Lemaitre and Lemaitre-Tolman geometries are investigated and compared. Conditions for the quasilocal thermodynamic and hydrodynamic equilibrium states to coincide are also discussed, and a quasilocal virial relation is suggested as a potential application of this approach.

Comments: 27 pages, no figures. Accepted by Classical and Quantum Gravity.

----------------------------------------------------------------

arXiv:1504.03266 [physics.optics]

Modelling Thermoelastic Distortion of Optics Using Elastodynamic Reciprocity

Eleanor King, Yuri Levin, David Ottaway, Peter Veitch

(Submitted on 13 Apr 2015 (v1), last revised 17 Apr 2015 (this version, v2))

Thermoelastic distortion resulting from optical absorption by transmissive and reflective optics can cause unacceptable changes in optical systems that employ high power beams. In advanced-generation laser-interferometric gravitational wave detectors for example, optical absorption is expected to result in wavefront distortions that would compromise the sensitivity of the detector; thus necessitating the use of adaptive thermal compensation. Unfortunately, these systems have long thermal time constants and so predictive feed-forward control systems could be required - but the finite-element analysis is computationally expensive. We describe here the use of the Betti-Maxwell elastodynamic reciprocity theorem to calculate the response of linear elastic bodies (optics) to heating that has arbitrary spatial distribution. We demonstrate using a simple example, that it can yield accurate results in computational times that are significantly less than those required for finite-element analyses.

----------------------------------------------------------------

arXiv:1312.0405 [gr-qc]

Ray tracing Einstein-Æther black holes: Universal versus Killing horizons

Bethan Cropp, Stefano Liberati, Arif Mohd, Matt Visser

(Submitted on 2 Dec 2013 (v1), last revised 2 Mar 2014 (this version, v2))

Violating Lorentz-invariance, and so implicitly permitting some form of superluminal communication, necessarily alters the notion of a black hole. Nevertheless, in both Einstein-{\AE}ther gravity, and Ho\v{r}ava-Lifshitz gravity, there is still a causally disconnected region in black-hole solutions; now being bounded by a "Universal horizon", which traps excitations of arbitrarily high velocities. To better understand the nature of these black holes, and their Universal horizons, we study ray trajectories in these spacetimes. We find evidence that Hawking radiation is associated with the Universal horizon, while the "lingering" of ray trajectories near the Killing horizon hints at reprocessing there. In doing this we solve an apparent discrepancy between the surface gravity of the Universal horizon and the associated temperature derived by tunneling method. These results advance the understanding of these exotic horizons, and provide hints for a full understanding of black-hole thermodynamics in Lorentz-violating theories.

Journal reference: Phys. Rev. D 89, 064061 (2014)

----------------------------------------------------------------

arXiv:1401.0568 [gr-qc]

Bounding the greybody factors for scalar excitations of the Kerr-Newman spacetime

Petarpa Boonserm (Chulalongkorn University), Tritos Ngampitipan (Chulalongkorn University), Matt Visser (Victoria University of Wellington)

(Submitted on 3 Jan 2014)

Finding exact solutions for black-hole greybody factors is generically impractical; typically one resorts either to making semi-analytic or numerical estimates, or alternatively to deriving rigorous analytic bounds. Indeed, rigorous bounds have already been established for the greybody factors of Schwarzschild and Riessner-Nordstrom black holes, and more generally for those of arbitrary static spherically symmetric asymptotically flat black holes. Adding rotation to the problem greatly increases the level of difficulty, both for purely technical reasons (the Kerr or Kerr-Newman black holes are generally much more difficult to work with than the Schwarzschild or Reissner-Nordstrom black holes), but also at a conceptual level (due to the generic presence of super-radiant modes). In the current article we analyze bounds on the greybody factors for scalar excitations of the Kerr-Newman geometry in some detail, first for zero-angular-momentum modes, then for the non-super-radiant modes, and finally for the super-radiant modes.

Journal reference: JHEP 1403 (2014) 113

----------------------------------------------------------------

arXiv:1405.5678 [gr-qc]

Greybody factors for Myers-Perry black holes

Petarpa Boonserm (Chulalongkorn University), Auttakit Chatrabhuti (Chulalongkorn University), Tritos Ngampitipan (Chulalongkorn University), Matt Visser (Victoria University of Wellington)

(Submitted on 22 May 2014)

The Myers-Perry black holes are higher-dimensional generalizations of the usual (3+1)-dimensional rotating Kerr black hole. They are of considerable interest in Kaluza-Klein models, specifically within the context of brane-world versions thereof. In the present article we shall consider the greybody factors associated with scalar field excitations of the Myers-Perry spacetimes, and develop some rigorous bounds on these greybody factors. These bounds are of relevance for characterizing both the higher-dimensional Hawking radiation, and the super-radiance, that is expected for these spacetimes.

----------------------------------------------------------------

arXiv:1407.5678 [gr-qc]

Super-radiance and flux conservation

Petarpa Boonserm (Chulalongkorn University), Tritos Ngampitipan (Chulalongkorn University), Matt Visser (Victoria University of Wellington)

(Submitted on 21 Jul 2014 (v1), last revised 28 Jul 2014 (this version, v2))

The theoretical foundations of the phenomenon known as super-radiance still continues to attract considerable attention. Despite many valiant attempts at pedagogically clear presentations, the effect nevertheless still continues to generate some significant confusion. Part of the confusion arises from the fact that super-radiance in a quantum field theory [QFT] context is not the same as super-radiance (super-fluorescence) in some condensed matter contexts; part of the confusion arises from traditional but sometimes awkward normalization conventions, and part is due to sometimes unnecessary confusion between fluxes and probabilities. We shall argue that the key point underlying the effect is flux conservation, (and, in the presence of dissipation, a controlled amount of flux non-conservation), and that attempting to phrase things in terms of reflection and transmission probabilities only works in the absence of super-radiance. To help clarify the situation we present a simple exactly solvable toy model exhibiting both super-radiance and damping.

----------------------------------------------------------------

arXiv:1407.7295 [gr-qc]

Physical observability of horizons

Matt Visser (Victoria University of Wellington)

(Submitted on 28 Jul 2014 (v1), last revised 25 Nov 2014 (this version, v3))

Event horizons are (generically) not physically observable. In contrast, apparent horizons (and the closely related trapping horizons) are generically physically observable --- in the sense that they can be detected by observers working in finite-size regions of spacetime. Consequently event horizons are inappropriate tools for defining astrophysical black holes, or indeed for defining any notion of evolving}black hole, (evolving either due to accretion or Hawking radiation). The only situation in which an event horizon becomes physically observable is for the very highly idealized stationary or static black holes, when the event horizon is a Killing horizon which is degenerate with the apparent and trapping horizons; and then it is the physical observability of the apparent/trapping horizons that is fundamental -- the event horizon merely comes along for the ride.

----------------------------------------------------------------

arXiv:1409.7754 [gr-qc]

Thermality of the Hawking flux

Matt Visser (Victoria University of Wellington)

(Submitted on 27 Sep 2014 (v1), last revised 6 May 2015 (this version, v3))

Is the Hawking flux "thermal"? Unfortunately, the answer to this seemingly innocent question depends on a number of often unstated, but quite crucial, technical assumptions built into modern (mis-)interpretations of the word "thermal". The original 1850's notions of thermality -- based on classical thermodynamic reasoning applied to idealized "black bodies" or "lamp black surfaces" -- when supplemented by specific basic quantum ideas from the early 1900's, immediately led to the notion of the black-body spectrum, (the Planck-shaped spectrum), but "without" any specific assumptions or conclusions regarding correlations between the quanta. Many (not all) modern authors (often implicitly and unintentionally) add an extra, and quite unnecessary, assumption that there are no correlations in the black-body radiation; but such usage is profoundly ahistorical and dangerously misleading. Specifically, the Hawking flux from an evaporating black hole, (just like the radiation flux from a leaky furnace or a burning lump of coal), is only "approximately" Planck-shaped over a bounded frequency range. Standard physics (phase space and adiabaticity effects) explicitly bound the frequency range over which the Hawking flux is "approximately" Planck-shaped from both above and below -- the Hawking flux is certainly not exactly Planckian, and there is no compelling physics reason to assume the Hawking photons are uncorrelated.

----------------------------------------------------------------

arXiv:1501.01619 [gr-qc]

Energy conditions in the epoch of galaxy formation

Matt Visser

(Submitted on 7 Jan 2015)

The energy conditions of Einstein gravity (classical general relativity) do not require one to fix a specific equation of state. In a Friedmann-Robertson-Walker universe where the equation of state for the cosmological fluid is uncertain, the energy conditions provide simple, model-independent, and robust bounds on the behaviour of the density and look-back time as a function of red-shift. Current observations suggest that the "strong energy condition" is violated sometime between the epoch of galaxy formation and the present. This implies that no possible combination of "normal" matter is capable of fitting the observational data.

Journal reference: Science 276 (1997) 88-90

----------------------------------------------------------------

arXiv:1501.07044 [gr-qc]

Modelling anisotropic fluid spheres in general relativity

Petarpa Boonserm (Chulalongkorn University), Tritos Ngampitipan (Chulalongkorn University), Matt Visser (Victoria University of Wellington)

(Submitted on 28 Jan 2015 (v1), last revised 3 Feb 2015 (this version, v2))

We argue that an arbitrary general relativistic anisotropic fluid sphere, (spherically symmetric but with transverse pressure not equal to radial pressure), can nevertheless be successfully modelled by suitable linear combinations of quite ordinary classical matter: an isotropic perfect fluid, a classical electromagnetic field, and a classical (minimally coupled) scalar field. While the most general decomposition is not unique, a preferred minimal decomposition can be constructed that is unique. We show how the classical energy conditions for the anisotropic fluid sphere can be related to energy conditions for the isotropic perfect fluid, electromagnetic field, and scalar field components of the model. Furthermore we show how this decomposition relates to the distribution of electric charge density and scalar charge density throughout the model that is used to mimic the anisotropic fluid sphere. Consequently, we can build physically reasonable matter models for almost any spherically symmetric spacetime.

----------------------------------------------------------------

arXiv:1502.02758 [gr-qc]

Conformally Friedmann-Lemaitre-Robertson-Walker cosmologies

Matt Visser (Victoria University of Wellington)

(Submitted on 10 Feb 2015 (v1), last revised 29 Apr 2015 (this version, v3))

In a universe where, according to the standard cosmological models, some 97% of the total mass-energy is still "missing in action" it behooves us to spend at least a little effort critically assessing and exploring radical alternatives. Among possible, (dare we say plausible), nonstandard but superficially viable models, those spacetimes conformal to the standard Friedmann-Lemaitre-Robertson-Walker class of cosmological models play a very special role -- these models have the unique and important property of permitting large non-perturbative geometric deviations from Friedmann-Lemaitre-Robertson-Walker cosmology without unacceptably distorting the cosmic microwave background. Performing a "cosmographic" analysis, (that is, temporarily setting aside the Einstein equations, since the question of whether or not the Einstein equations are valid on galactic and cosmological scales is essentially the same question as whether or not dark matter/dark energy actually exist), and using both supernova data and information about galactic structure, one can nevertheless place some quite significant observational constraints on any possible conformal mode -- however there is still an extremely rich range of phenomenological possibilities for both cosmologists and astrophysicists to explore.

Comments: This version accepted for publication in Classical and Quantum Gravity

----------------------------------------------------------------

arXiv:1506.03975 [gr-qc]

The Hawking cascade from a black hole is extremely sparse

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

(Submitted on 12 Jun 2015)

The Hawking flux from a black hole, (at least as seen from large distances), is extremely sparse and thin, with the average time between emission of successive Hawking quanta being hundreds of times larger than the natural timescale set by the energies of the emitted quanta. Some aspects of this result have been known for over 30 years, but have been largely forgotten, possibly because authors focussed mainly on the late-time high-temperature regime. We shall instead focus on the early-stage low-temperature regime, and shall both quantify and significantly extend these observations in a number of different ways.

First we shall identify several natural dimensionless figures of merit, and thereby compare the mean time between emission of successive Hawking quanta to several quite natural timescales that can be associated with the emitted quanta, demonstrating that ratios of 300 or more are typical for emission of photons or gravitons from a Schwarzschild black hole. Furthermore these ratios are independent of the mass of the black hole as it slowly evolves. The situation for fermion emission (massless neutrinos) is actually worse. Second, we shall then show that the situation for Reissner-Nordstrom, Kerr, Kerr-Newman and "dirty" black holes is even worse. Third, we consider the effects of particle rest mass. Overall, the Hawking quanta are seen to be dribbling out of the black hole one at a time, in an extremely slow cascade of 3-body decays. This implies that the Hawking flux is subject to "shot noise". Observationally, the Planck spectrum of the Hawking flux can only be determined by collecting and integrating data over a very long time. We conclude by connecting these points back to various kinematic aspects of the Hawking evaporation process.

----------------------------------------------------------------

arXiv:1411.1662 [gr-qc]

Rotating black holes in a draining bathtub: superradiant scattering of gravity waves

Mauricio Richartz, Angus Prain, Stefano Liberati, Silke Weinfurtner

(Submitted on 6 Nov 2014 (v1), last revised 5 Jun 2015 (this version, v3))

In a draining rotating fluid flow background, surface perturbations behave as a scalar field on a rotating effective black hole spacetime. We propose a new model for the background flow which takes into account the varying depth of the water. Numerical integration of the associated Klein-Gordon equation using accessible experimental parameters shows that gravity waves in an appropriate frequency range are amplified through the mechanism of superradiance. Our numerical results suggest that the observation of this phenomenon in a common fluid mechanical system is within experimental reach. Unlike the case of wave scattering around Kerr black holes, which depends only on one dimensionless background parameter (the ratio a/M between the specific angular momentum and the mass of the black hole), our system depends on two dimensionless background parameters, namely the normalized angular velocity and surface gravity at the effective black hole horizon.

Journal reference: Phys. Rev. D 91, 124018 (2015)

----------------------------------------------------------------

arXiv:1401.1287 [gr-qc]

The chart based approach to studying the global structure of a spacetime induces a coordinate invariant boundary

Ben Whale

(Submitted on 7 Jan 2014 (v1), last revised 25 Feb 2014 (this version, v2))

I demonstrate that the chart based approach to the study of the global structure of Lorentzian manifolds induces a homeomorphism of the manifold into a topological space as an open dense set. The topological boundary of this homeomorphism is a chart independent boundary of ideal points equipped with a topological structure and a physically motivated classification. I show that this new boundary contains all other boundaries that can be presented as the topological boundary of an envelopment. Hence, in particular, it is a generalisation of Penrose's conformal boundary. I provide three detailed examples: the conformal compactification of Minkowski spacetime, Scott and Szekeres' analysis of the Curzon singularity and Beyer and Hennig's analysis of smooth Gowdy symmetric generalised Taub-NUT spacetimes.

Journal reference: General Relativity and Gravitation, Vol. 46, No. 1. (2014), pp. 1-43

----------------------------------------------------------------

arXiv:1412.5652 [math.DG]

Generalised time functions and finiteness of the Lorentzian distance

Adam Rennie, Ben E. Whale

(Submitted on 17 Dec 2014 (v1), last revised 30 Jan 2015 (this version, v2))

We show that finiteness of the Lorentzian distance is equivalent to the existence of generalised time functions with gradient uniformly bounded away from light cones. To derive this result we introduce new techniques to construct and manipulate achronal sets. As a consequence of these techniques we obtain a functional description of the Lorentzian distance extending the work of Franco and Moretti.

----------------------------------------------------------------

arXiv:1312.1952 [gr-qc]

Finding high-order analytic post-Newtonian parameters from a high-precision numerical self-force calculation

Abhay G Shah, John L Friedman, Bernard F Whiting

(Submitted on 6 Dec 2013 (v1), last revised 11 Mar 2014 (this version, v2))

We present a novel analytic extraction of high-order post-Newtonian (pN) parameters that govern quasi-circular binary systems. Coefficients in the pN expansion of the energy of a binary system can be found from corresponding coefficients in an extreme-mass-ratio inspiral (EMRI) computation of the change ΔU in the redshift factor of a circular orbit at fixed angular velocity. Remarkably, by computing this essentially gauge-invariant quantity to accuracy greater than one part in 10225, and by assuming that a subset of pN coefficients are rational numbers or products of π and a rational, we obtain the exact analytic coefficients. We find the previously unexpected result that the post-Newtonian expansion of ΔU (and of the change ΔΩ in the angular velocity at fixed redshift factor) have conservative terms at half-integral pN order beginning with a 5.5 pN term. This implies the existence of a corresponding 5.5 pN term in the expansion of the energy of a binary system.

Coefficients in the pN series that do not belong to the subset just described are obtained to accuracy better than 1 part in 10265−23n at nth pN order. We work in a radiation gauge, finding the radiative part of the metric perturbation from the gauge-invariant Weyl scalar ψ0 via a Hertz potential. We use mode-sum renormalization, and find high-order renormalization coefficients by matching a series in L=ℓ+1/2 to the large-L behavior of the expression for ΔU. The non-radiative parts of the perturbed metric associated with changes in mass and angular momentum are calculated in the Schwarzschild gauge.

----------------------------------------------------------------

arXiv:1312.2975 [gr-qc]

Half-integral conservative post-Newtonian approximations in the redshift factor of black hole binaries

Luc Blanchet, Guillaume Faye, Bernard F. Whiting

(Submitted on 10 Dec 2013 (v1), last revised 12 Mar 2014 (this version, v2))

Recent perturbative self-force computations (Shah, Friedman & Whiting, submitted to Phys. Rev. {\bf D}, arXiv:1312.1952 [gr-qc]), both numerical and analytical, have determined that half-integral post-Newtonian terms arise in the conservative dynamics of black-hole binaries moving on exactly circular orbits. We look at the possible origin of these terms within the post-Newtonian approximation, find that they essentially originate from non-linear "tail-of-tail" integrals and show that, as demonstrated in the previous paper, their first occurrence is at the 5.5PN order. The post-Newtonian method we use is based on a multipolar-post-Minkowskian treatment of the field outside a general matter source, which is re-expanded in the near zone and extended inside the source thanks to a matching argument. Applying the formula obtained for generic sources to compact binaries, we obtain the redshift factor of circular black hole binaries (without spins) at 5.5PN order in the extreme mass ratio limit. Our result fully agrees with the determination of the 5.5PN coefficient by means of perturbative self-force computations reported in the previously cited paper.

Journal reference: Phys. Rev. D 89, 064026,

----------------------------------------------------------------

----------------------------------------------------------------

arXiv:1503.02618 [gr-qc]

Raising and Lowering operators of spin-weighted spheroidal harmonics

Abhay G. Shah, Bernard F. Whiting

(Submitted on 9 Mar 2015)

In this paper we generalize the spin-raising and lowering operators of spin-weighted spherical harmonics to linear-in-γ spin-weighted spheroidal harmonics where γ is an additional parameter present in the second order ordinary differential equation governing these harmonics. One can then generalize these operators to higher powers in γ. Constructing these operators required calculating the ℓ-, s- and m-raising and lowering operators (and various combinations of them) of spin-weighted spherical harmonics which have been calculated and shown explicitly in this paper.

----------------------------------------------------------------

arXiv:1503.02638 [gr-qc]

Experimental mathematics meets gravitational self-force

Nathan K. Johnson-McDaniel, Abhay G. Shah, Bernard F. Whiting

(Submitted on 9 Mar 2015)

It is now possible to compute linear in mass-ratio terms in the post-Newtonian (PN) expansion for compact binaries to very high orders using black hole perturbation theory applied to various invariants. For instance, a computation of the redshift invariant of a point particle in a circular orbit about a black hole in linear perturbation theory gives the linear-in-mass-ratio portion of the binding energy of a circular binary with arbitrary mass ratio. This binding energy, in turn, encodes the system's conservative dynamics. We give a method for extracting the analytic forms of these PN coefficients from high-accuracy numerical data using experimental mathematics techniques, notably an integer relation algorithm. Such methods should be particularly important when the calculations progress to the considerably more difficult case of perturbations of the Kerr metric. As an example, we apply this method to the redshift invariant in Schwarzschild. Here we obtain analytic coefficients to 12.5PN, and higher-order terms in mixed analytic-numerical form to 21.5PN, including analytic forms for the complete 13.5PN coefficient, and all the logarithmic terms at 13PN. At these high orders, an individual coefficient can have over 30 terms, including a wide variety of transcendental numbers, when written out in full. We are still able to obtain analytic forms for such coefficients from the numerical data through a careful study of the structure of the expansion. The structure we find also allows us to predict certain "leading logarithm"-type contributions to all orders. The additional terms in the expansion we obtain improve the accuracy of the PN series for the redshift observable, even in the very strong-field regime inside the innermost stable circular orbit, particularly when combined with exponential resummation.

Report number: ICTS/2015/3

----------------------------------------------------------------

arXiv:1503.04192 [astro-ph.CO]

Determing the frame of minimum Hubble expansion variance

James H. McKay, David L. Wiltshire

(Submitted on 13 Mar 2015 (v1), last revised 20 Mar 2015 (this version, v2))

We characterize a cosmic rest frame in which the variation of the spherically averaged Hubble expansion is most uniform, under local Lorentz boosts of the central observer. Using the COMPOSITE sample of 4534 galaxies, we identify a degenerate set of candidate minimum variance frames, which includes the rest frame of the Local Group (LG) of galaxies, but excludes the standard Cosmic Microwave Background (CMB) frame. Candidate rest frames defined by a boost from the LG frame close to the plane of the galaxy have a statistical likelihood similar to the LG frame. This may result from a lack of constraining data in the Zone of Avoidance in the COMPOSITE sample. We extend our analysis to the Cosmicflows-2 (CF2) sample of 8,162 galaxies. While the signature of a systematic boost offset between the CMB and LG frames averages is still detected, the spherically averaged expansion variance in all rest frames is significantly larger in the CF2 sample than would be reasonably expected. We trace this to the CF2 distances being reported without a correction for inhomogeneous distribution Malmquist bias. Systematic differences in the inclusion of the large SFI++ subsample into the COMPOSITE and CF2 catalogues are analysed. Our results highlight the importance of a careful treatment of Malmquist biases for future peculiar velocities studies, including tests of the hypothesis of Wiltshire et al [arXiv:1201.5371] that a significant fraction of the CMB temperature dipole may be nonkinematic in origin.

----------------------------------------------------------------

arXiv:1505.07800 [gr-qc]

Is there proof that backreaction of inhomogeneities is irrelevant in cosmology?

T. Buchert, M. Carfora, G.F.R. Ellis, E.W. Kolb, M.A.H. MacCallum, J.J. Ostrowski, S. Räsänen, B.F. Roukema, L. Andersson, A.A. Coley, D.L. Wiltshire

(Submitted on 28 May 2015)

No. In a number of papers Green and Wald argue that the standard FLRW model approximates our Universe extremely well on all scales, except in the immediate vicinity of very strong field astrophysical objects. In particular, they argue that the effect of inhomogeneities on average properties of the Universe (backreaction) is irrelevant. We show that their claims are not valid. Specifically, we demonstrate, referring to their recent review paper, that (i) their two-dimensional example used to illustrate the fitting problem differs from the actual problem in important respects, and it assumes what is to be proven; (ii) the proof of the trace-free property of backreaction is unphysical and the theorem about it is mathematically flawed; (iii) the scheme that underlies the trace-free theorem does not involve averaging and therefore does not capture crucial non-local effects; (iv) their arguments are to a large extent coordinate-dependent, and (v) many of their criticisms of backreaction frameworks do not apply to the published definitions of these frameworks.

Report number: HIP-2015-17/TH

******************************************************************************

ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc,

December 2013 - June 2015

The LIGO Scientific Collaboration is a consortium of scientific institutions doing work on the Laser Interferometer Gravitational-Wave Observatory (LIGO), which consists of two laser interferometers 3030 km apart, one at Hanford, Washington State and the other at Livingston, Louisiana. The LIGO Scientific Collaboration includes ASGRG members Pablo Barriga, David Blair, Philip Charlton, Neil Cornish, Ra Inta, Ju Li, David McClelland, Andrew Melatos, Jesper Munch, Susan Scott, Antony Searle, Daniel Shaddock, Bram Slagmolen, Michael Stefszky, Peter Veitch, Bernard Whiting and Chunnong Zhao.

Listed below are all the abstracts listed on gr-qc from December 2013 to June 2015 from consortia that include one ASGRG member as a co-author – these are mostly LIGO abstracts, but there are occasionally some from eLISA and Virgo.

-----------------------------------------------------------------------------

arXiv:1401.0939 [gr-qc]

The NINJA-2 project: Detecting and characterizing gravitational waveforms modelled using numerical binary black hole simulations

The LIGO Scientific Collaboration, the Virgo Collaboration, the NINJA-2 Collaboration

(Submitted on 5 Jan 2014)

The Numerical INJection Analysis (NINJA) project is a collaborative effort between members of the numerical relativity and gravitational-wave astrophysics communities. The purpose of NINJA is to study the ability to detect gravitational waves emitted from merging binary black holes and recover their parameters with next-generation gravitational-wave observatories. We report here on the results of the second NINJA project, NINJA-2, which employs 60 complete binary black hole hybrid waveforms consisting of a numerical portion modelling the late inspiral, merger, and ringdown stitched to a post-Newtonian portion modelling the early inspiral. In a "blind injection challenge" similar to that conducted in recent LIGO and Virgo science runs, we added 7 hybrid waveforms to two months of data recolored to predictions of Advanced LIGO and Advanced Virgo sensitivity curves during their first observing runs. The resulting data was analyzed by gravitational-wave detection algorithms and 6 of the waveforms were recovered with false alarm rates smaller than 1 in a thousand years. Parameter estimation algorithms were run on each of these waveforms to explore the ability to constrain the masses, component angular momenta and sky position of these waveforms. We also perform a large-scale monte-carlo study to assess the ability to recover each of the 60 hybrid waveforms with early Advanced LIGO and Advanced Virgo sensitivity curves. Our results predict that early Advanced LIGO and Advanced Virgo will have a volume-weighted average sensitive distance of 300Mpc (1Gpc) for 10M⊙+10M⊙ (50M⊙+50M⊙) binary black hole coalescences. We demonstrate that neglecting the component angular momenta in the waveform models used in matched-filtering will result in a reduction in sensitivity for systems with large component angular momenta. [Abstract abridged for ArXiv, full version in PDF]

Journal reference: Class. Quantum Grav. 31 115004, 2014

Report number: LIGO-P1300199

----------------------------------------------------------------

arXiv:1402.4974 [gr-qc]

Implementation of an F-statistic all-sky search for continuous gravitational waves in Virgo VSR1 data

Authors: J. Aasi et al.

(Submitted on 20 Feb 2014 (v1), last revised 10 Apr 2014 (this version, v3))

We present an implementation of the F-statistic to carry out the first search in data from the Virgo laser interferometric gravitational wave detector for periodic gravitational waves from a priori unknown, isolated rotating neutron stars. We searched a frequency f0 range from 100 Hz to 1 kHz and the frequency dependent spindown f1 range from −1.6(f0/100Hz)×10−9 Hz/s to zero. A large part of this frequency - spindown space was unexplored by any of the all-sky searches published so far. Our method consisted of a coherent search over two-day periods using the F-statistic, followed by a search for coincidences among the candidates from the two-day segments. We have introduced a number of novel techniques and algorithms that allow the use of the Fast Fourier Transform (FFT) algorithm in the coherent part of the search resulting in a fifty-fold speed-up in computation of the F-statistic with respect to the algorithm used in the other pipelines. No significant gravitational wave signal was found. The sensitivity of the search was estimated by injecting signals into the data. In the most sensitive parts of the detector band more than 90% of signals would have been detected with dimensionless gravitational-wave amplitude greater than 5×10−24.

Comments: 27 pages, 10 figures, submitted to CQG.

Report number: LIGO Document No. LIGO-P1300133

----------------------------------------------------------------

arXiv:1403.5306 [gr-qc]

Search for gravitational wave ringdowns from perturbed intermediate mass black holes in LIGO-Virgo data from 2005-2010

The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 20 Mar 2014 (v1), last revised 22 May 2014 (this version, v2))

We report results from a search for gravitational waves produced by perturbed intermediate mass black holes (IMBH) in data collected by LIGO and Virgo between 2005 and 2010. The search was sensitive to astrophysical sources that produced damped sinusoid gravitational wave signals, also known as ringdowns, with frequency 50≤f0/Hz≤2000 and decay timescale 0.0001≲τ/s≲0.1 characteristic of those produced in mergers of IMBH pairs. No significant gravitational wave candidate was detected. We report upper limits on the astrophysical coalescence rates of IMBHs with total binary mass 50≤M/M⊙≤450 and component mass ratios of either 1:1 or 4:1. For systems with total mass 100≤M/M⊙≤150, we report a 90%-confidence upper limit on the rate of binary IMBH mergers with non-spinning and equal mass components of 6.9×10−8Mpc−3yr−1. We also report a rate upper limit for ringdown waveforms from perturbed IMBHs, radiating 1% of their mass as gravitational waves in the fundamental, ℓ=m=2, oscillation mode, that is nearly three orders of magnitude more stringent than previous results.

Report number: LIGO-P1300156

----------------------------------------------------------------

arXiv:1403.6639 [astro-ph.HE]

Search for gravitational waves associated with gamma-ray bursts detected by the InterPlanetary Network

The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 26 Mar 2014 (v1), last revised 17 Apr 2014 (this version, v2))

We present the results of a search for gravitational waves associated with 223 gamma-ray bursts (GRBs) detected by the InterPlanetary Network (IPN) in 2005-2010 during LIGO's fifth and sixth science runs and Virgo's first, second and third science runs. The IPN satellites provide accurate times of the bursts and sky localizations that vary significantly from degree scale to hundreds of square degrees. We search for both a well-modeled binary coalescence signal, the favored progenitor model for short GRBs, and for generic, unmodeled gravitational wave bursts. Both searches use the event time and sky localization to improve the gravitational-wave search sensitivity as compared to corresponding all-time, all-sky searches. We find no evidence of a gravitational-wave signal associated with any of the IPN GRBs in the sample, nor do we find evidence for a population of weak gravitational-wave signals associated with the GRBs. For all IPN-detected GRBs, for which a sufficient duration of quality gravitational-wave data is available, we place lower bounds on the distance to the source in accordance with an optimistic assumption of gravitational-wave emission energy of 10−2M⊙c2 at 150 Hz, and find a median of 13 Mpc. For the 27 short-hard GRBs we place 90% confidence exclusion distances to two source models: a binary neutron star coalescence, with a median distance of 12Mpc, or the coalescence of a neutron star and black hole, with a median distance of 22 Mpc. Finally, we combine this search with previously published results to provide a population statement for GRB searches in first-generation LIGO and Virgo gravitational-wave detectors, and a resulting examination of prospects for the advanced gravitational-wave detectors.

Report number: LIGO-P1300226

----------------------------------------------------------------

arXiv:1404.2199 [gr-qc]

Search for gravitational radiation from intermediate mass black hole binaries in data from the second LIGO-Virgo joint science run

The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 8 Apr 2014 (v1), last revised 19 Jun 2014 (this version, v4))

This paper reports on an unmodeled, all-sky search for gravitational waves from merging intermediate mass black hole binaries (IMBHB). The search was performed on data from the second joint science run of the LIGO and Virgo detectors (July 2009 - October 2010) and was sensitive to IMBHBs with a range up to ∼200 Mpc, averaged over the possible sky positions and inclinations of the binaries with respect to the line of sight. No significant candidate was found. Upper limits on the coalescence-rate density of nonspinning IMBHBs with total masses between 100 and 450 M⊙ and mass ratios between 0.25 and 1 were placed by combining this analysis with an analogous search performed on data from the first LIGO-Virgo joint science run (November 2005 - October 2007). The most stringent limit was set for systems consisting of two 88 M⊙ black holes and is equal to 0.12 Mpc−3 Myr−1 at the 90% confidence level. This paper also presents the first estimate, for the case of an unmodeled analysis, of the impact on the search range of IMBHB spin configurations: the visible volume for IMBHBs with nonspinning components is roughly doubled for a population of IMBHBs with spins aligned with the binary's orbital angular momentum and uniformly distributed in the dimensionless spin parameter up to 0.8, whereas an analogous population with antialigned spins decreases the visible volume by ∼20%.

Report number: LIGO-P1300158

----------------------------------------------------------------

arXiv:1405.7904 [gr-qc]

First all-sky search for continuous gravitational waves from unknown sources in binary systems

The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 30 May 2014 (v1), last revised 17 Sep 2014 (this version, v2))

We present the first results of an all-sky search for continuous gravitational waves from unknown spinning neutron stars in binary systems using LIGO and Virgo data. Using a specially developed analysis program, the TwoSpect algorithm, the search was carried out on data from the sixth LIGO Science Run and the second and third Virgo Science Runs. The search covers a range of frequencies from 20 Hz to 520 Hz, a range of orbital periods from 2 to ~2,254 h and a frequency- and period-dependent range of frequency modulation depths from 0.277 to 100 mHz. This corresponds to a range of projected semi-major axes of the orbit from ~0.6e-3 ls to ~6,500 ls assuming the orbit of the binary is circular. While no plausible candidate gravitational wave events survive the pipeline, upper limits are set on the analyzed data. The most sensitive 95% confidence upper limit obtained on gravitational wave strain is 2.3e-24 at 217 Hz, assuming the source waves are circularly polarized. Although this search has been optimized for circular binary orbits, the upper limits obtained remain valid for orbital eccentricities as large as 0.9. In addition, upper limits are placed on continuous gravitational wave emission from the low-mass x-ray binary Scorpius X-1 between 20 Hz and 57.25 Hz.

Journal reference: Phys. Rev. D 90 (15 September 2014), 062010

----------------------------------------------------------------

arXiv:1406.4556 [gr-qc]

Improved Upper Limits on the Stochastic Gravitational-Wave Background from 2009-2010 LIGO and Virgo Data

The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 17 Jun 2014 (v1), last revised 12 Aug 2014 (this version, v2))

Gravitational waves from a variety of sources are predicted to superpose to create a stochastic background. This background is expected to contain unique information from throughout the history of the universe that is unavailable through standard electromagnetic observations, making its study of fundamental importance to understanding the evolution of the universe. We carry out a search for the stochastic background with the latest data from LIGO and Virgo. Consistent with predictions from most stochastic gravitational-wave background models, the data display no evidence of a stochastic gravitational-wave signal. Assuming a gravitational-wave spectrum of Omega_GW(f)=Omega_alpha*(f/f_ref)^alpha, we place 95% confidence level upper limits on the energy density of the background in each of four frequency bands spanning 41.5-1726 Hz. In the frequency band of 41.5-169.25 Hz for a spectral index of alpha=0, we constrain the energy density of the stochastic background to be Omega_GW(f)<5.6x10^-6. For the 600-1000 Hz band, Omega_GW(f)<0.14*(f/900 Hz)^3, a factor of 2.5 lower than the best previously reported upper limits. We find Omega_GW(f)<1.8x10^-4 using a spectral index of zero for 170-600 Hz and Omega_GW(f)<1.0*(f/1300 Hz)^3 for 1000-1726 Hz, bands in which no previous direct limits have been placed. The limits in these four bands are the lowest direct measurements to date on the stochastic background. We discuss the implications of these results in light of the recent claim by the BICEP2 experiment of the possible evidence for inflationary gravitational waves.

----------------------------------------------------------------

arXiv:1410.6211 [gr-qc]

Searching for stochastic gravitational waves using data from the two co-located LIGO Hanford detectors

The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 22 Oct 2014 (v1), last revised 3 Dec 2014 (this version, v3))

Searches for a stochastic gravitational-wave background (SGWB) using terrestrial detectors typically involve cross-correlating data from pairs of detectors. The sensitivity of such cross-correlation analyses depends, among other things, on the separation between the two detectors: the smaller the separation, the better the sensitivity. Hence, a co-located detector pair is more sensitive to a gravitational-wave background than a non-co-located detector pair. However, co-located detectors are also expected to suffer from correlated noise from instrumental and environmental effects that could contaminate the measurement of the background. Hence, methods to identify and mitigate the effects of correlated noise are necessary to achieve the potential increase in sensitivity of co-located detectors. Here we report on the first SGWB analysis using the two LIGO Hanford detectors and address the complications arising from correlated environmental noise. We apply correlated noise identification and mitigation techniques to data taken by the two LIGO Hanford detectors, H1 and H2, during LIGO's fifth science run. At low frequencies, 40 - 460 Hz, we are unable to sufficiently mitigate the correlated noise to a level where we may confidently measure or bound the stochastic gravitational-wave signal. However, at high frequencies, 460-1000 Hz, these techniques are sufficient to set a 95 confidence level (C.L.) upper limit on the gravitational-wave energy density of \Omega(f)<7.7 x 10^{-4} (f/ 900 Hz)^3, which improves on the previous upper limit by a factor of ∼180. In doing so, we demonstrate techniques that will be useful for future searches using advanced detectors, where correlated noise (e.g., from global magnetic fields) may affect even widely separated detectors.

----------------------------------------------------------------

arXiv:1410.7764 [gr-qc]

Characterization of the LIGO detectors during their sixth science run

The LIGO Scientific Collaboration, The Virgo Collaboration

(Submitted on 28 Oct 2014 (v1), last revised 18 Nov 2014 (this version, v2))

In 2009-2010, the Laser Interferometer Gravitational-wave Observa- tory (LIGO) operated together with international partners Virgo and GEO600 as a network to search for gravitational waves of astrophysical origin. The sensitiv- ity of these detectors was limited by a combination of noise sources inherent to the instrumental design and its environment, often localized in time or frequency, that couple into the gravitational-wave readout. Here we review the performance of the LIGO instruments during this epoch, the work done to characterize the de- tectors and their data, and the effect that transient and continuous noise artefacts have on the sensitivity of LIGO to a variety of astrophysical sources.

----------------------------------------------------------------

arXiv:1410.8310 [astro-ph.IM]

Narrow-band search of continuous gravitational-wave signals from Crab and Vela pulsars in Virgo VSR4 data

The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 30 Oct 2014)

In this paper we present the results of a coherent narrow-band search for continuous gravitational-wave signals from the Crab and Vela pulsars conducted on Virgo VSR4 data. In order to take into account a possible small mismatch between the gravitational wave frequency and two times the star rotation frequency, inferred from measurement of the electromagnetic pulse rate, a range of 0.02 Hz around two times the star rotational frequency has been searched for both the pulsars. No evidence for a signal has been found and 95% confidence level upper limits have been computed both assuming polarization parameters are completely unknown and that they are known with some uncertainty, as derived from X-ray observations of the pulsar wind torii. For Vela the upper limits are comparable to the spin-down limit, computed assuming that all the observed spin-down is due to the emission of gravitational waves. For Crab the upper limits are about a factor of two below the spin-down limit, and represent a significant improvement with respect to past analysis. This is the first time the spin-down limit is significantly overcome in a narrow-band search.

----------------------------------------------------------------

arXiv:1412.0605 [gr-qc]

A directed search for gravitational waves from Scorpius X-1 with initial LIGO

The LIGO Scientific Collaboration, the Virgo Collaboration

(Submitted on 1 Dec 2014)

We present results of a search for continuously-emitted gravitational radiation, directed at the brightest low-mass X-ray binary, Scorpius X-1. Our semi-coherent analysis covers 10 days of LIGO S5 data ranging from 50-550 Hz, and performs an incoherent sum of coherent F-statistic power distributed amongst frequency-modulated orbital sidebands. All candidates not removed at the veto stage were found to be consistent with noise at a 1% false alarm rate. We present Bayesian 95% confidence upper limits on gravitational-wave strain amplitude using two different prior distributions: a standard one, with no a priori assumptions about the orientation of Scorpius X-1; and an angle-restricted one, using a prior derived from electromagnetic observations. Median strain upper limits of 1.3e-24 and 8e-25 are reported at 150 Hz for the standard and angle-restricted searches respectively. This proof of principle analysis was limited to a short observation time by unknown effects of accretion on the intrinsic spin frequency of the neutron star, but improves upon previous upper limits by factors of ~1.4 for the standard, and 2.3 for the angle-restricted search at the sensitive region of the detector.

----------------------------------------------------------------

arXiv:1412.5942 [astro-ph.HE]

Searches for continuous gravitational waves from nine young supernova remnants

Authors: J. Aasi, et al.

(Submitted on 18 Dec 2014)

We describe directed searches for continuous gravitational waves in data from the sixth LIGO science data run. The targets were nine young supernova remnants not associated with pulsars; eight of the remnants are associated with non-pulsing suspected neutron stars. One target's parameters are uncertain enough to warrant two searches, for a total of ten. Each search covered a broad band of frequencies and first and second frequency derivatives for a fixed sky direction. The searches coherently integrated data from the two LIGO interferometers over time spans from 5.3-25.3 days using the matched-filtering F-statistic. We found no credible gravitational-wave signals. We set 95% confidence upper limits as strong (low) as 4×10−25 on intrinsic strain, 2×10−7 on fiducial ellipticity, and 4×10−5 on r-mode amplitude. These beat the indirect limits from energy conservation and are within the range of theoretical predictions for neutron-star ellipticities and r-mode amplitudes.

Report number: LIGO-P1400182

----------------------------------------------------------------