AUSTRALASIAN SOCIETY FOR GENERAL RELATIVITY AND GRAVITATION

Electronic Newsletter -- #20, Spring 2013


Items for this newsletter should be emailed to the editor: asgrg *AT* hotmail *DOT* com

The deadline for the next issue is 31 October, 2014.


CONTENTS:


REPORT ON ACGRG6, Rydges Lakeside Resort, Queenstown, February 9-11 2012

The 6th Australasian Conference on General Relativity and Gravitation (ACGRG6) was held at the Rydges Lakeside Resort in Queenstown, New Zealand from 9 to 11 February 2012. ACGRG6, hosted by the University of Otago, Dunedin, was the sixth in a series of approximately biennial conferences on general relativity and gravitation organised by the ASGRG.

A total of 39 presentations were given over the three days of ACGRG6, including plenary talks from Steve Maddox (Galaxy surveys and large-scale structure in the universe), Rachel Webster (Microlensing quasars), Richard Easther (Gravitational waves from the early Universe), Andrew Melatos (Nuclear astrophysics with gravitational wave telescopes), Daniel Shaddock (From LISA to GRACE: Space-based gravitational observations), Renate Meyer (Markov chain Monte Carlo methods for Bayesian gravitational radiation data analysis), Florian Beyer (Guided tour through AVTD regions of the BKL world), and Todd Oliynyk (Lagrange coordinates for the Einstein-Euler equations).

The more specialised talks at ACGRG6 covered topics as diverse as galactic mini-halos, the cosmological constant problem, inhomogeneous cosmological models and cosmic back-reaction, techniques and technologies for gravitational wave detection , rotating fluid stars, thin shell gravastars, ELKO dark matter, three black hole configurations, relativistic spinning particles, entropic forces, analogue spacetimes, Lorentz symmetry-breaking, generalized Taub-NUT solutions, De Sitter solutions, horizons in Szekers models, spacelike singularities, the abstract boundary construction, Witten spinors, and conformal space-time vectors.

The final event on the closing day, February 11, was the second award of the Kerr Prize, to mark the best student presentation at an ACGRG. As in 2009, the judges decided to split the prize between two students: Kyle Tate of Victoria University of Wellington who spoke about Simplicial quantum gravity in Lorentzian signature, and Philip Threlfall of the Australian National University, whose talk was entitled Gravitational entropy for anisotropic futures within the quiescent cosmology framework. The two winners shared the A$200 prize.

The conference banquet was held at the Rydges Lakeside Resort on the evening of February 10, and was a very convivial occasion. Particular thanks should go to the local organisers (Joerg Frauendiener, Ben Whale, Marguerite Hunter) for making ACGRG6 such a great success.


MINUTES OF THE 6TH BIENNIAL GENERAL MEETING OF THE ASGRG held at the Rydges Lakeside Resort, Queenstown, Thursday 9 February, 2012

The meeting opened at 6.10 p.m.

[13 ASGRG members were present. At the time of the BGM 52 of the members were financial, so the meeting was quorate - a minimum of 25% of the financial members needed to be present to make a quorum.]

Apologies: John Schutz, Leo Brewin, Ju Li, David McClelland 1. The minutes of the 6th Biennial General Meeting, held at University of Christchurch, Canterbury, 16 December, 2009, were presented to the meeting. David Wiltshire moved that the minutes be accepted, and Matt Visser seconded. The motion was approved.

2. President's Report: David Wiltshire gave only a brief report on the Society?s recent activities, as little of note had happened over the last two years. A publisher had not been found for the proceedings of ACGRG5. Matt Visser suggested the Journal of Physics Conference series or the Proceedings of Science, but the consensus was that we should not publish the proceedings if we need to pay. The same consideration would apply to the proceedings of ACGRG6. If the ASGRG ever holds a conference on a special theme this policy might change.

David also mentioned that a decision on the siting of the Square Kilometer Array seemed to be imminent, but it was always 3 weeks away.

3. Treasurer's Report: John Steele reported that immediately before the start of ACGRG6 the Society had a total of 49 financial members, of which 42 were life members, 4 were ordinary members and 3 were student members. 3 more members joined at the Meeting. The Society's current account contained $13,770, which represented an increase of almost $700 from 2009. All the Society?s income is derived from membership fees.

As on 2009, John reported that the main expense of the Society was bank fees, which amounted to $12 a month and $40 per annum for the Society?s Commonwealth Bank account. John had investigated alternative options for housing the Society?s money, but none was cheaper than the current arrangement. On a suggestion from the floor, John said he would ring eWay and eMatters, but David Wiltshire advised that eWay was not cheaper than PayPal. A comment would be inserted on the membership form stating that membership fees can be paid directly into John?s account, and John promised that he would look into the possibility of setting up a term deposit for the Society.

4. Auditor's Report: The Auditor, John Schutz, certified in an email that he was satisfied with the Society's accounts. David Wiltshire moved that the ASGRG accept the accounts, and Sue Scott seconded. The motion was approved.

5. Appointment of Auditor for the next session: Sue Scott nominated John Schutz to remain the Auditor of the Society's accounts, and David Wiltshire seconded. The motion was approved.

6. Date and venue for ACGRG7: The meeting decided that ACGRG7 would be hosted by the Australian National University in December 2013. Daniel Shaddock was appointed as chair of the Local Organising Committee.

7. Election of officers: The following people were elected officers of the ASGRG Committee by acclamation (the mover and seconder are shown in brackets):

Daniel Shaddock and Joerg Frauendiener were also co-opted as Committee members.

8. Other business: David Wiltshire reminded the Meeting that cognate societies of the Australian Institute of Physics (of which the ASGRG is one) must offer a discounted membership fee for member s of the AIP. He moved that the annual membership fees for the ASGRG be increased from $30 ordinary/$15 student to $40 ordinary/$20 student, with a 10% discount for AIP members. John Steele seconded. The motion was approved.

David also mentioned that he attended the AIP Council meeting in Melbourne as the representative of the ASGRG in February 2011, but there was nothing noteworthy to report.

Robert Ward informed the Meeting that the American Mathematical Society has a mathematical physics special interest group.

The meeting closed at 7.05 pm.


7TH AUSTRALASIAN CONFERENCE ON GENERAL RELATIVITY AND GRAVITATION (ACGRG7)

Hamilton Island, Queensland, 9-11 December 2013

ACGRG7 is the seventh 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 is only days away, and will be held at the Reef View Hotal on Hamilton Island, Queensland.

The plenary speakers will be:


8th BIENNIAL GENERAL MEETING OF THE ASGRG

The 2013 Biennial General Meeting of the ASGRG will be held in conjunction with ACGRG7, on the evening of Tuesday 10 December 2013.

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

Nominations must be given to the Returning Officer, Matt Visser, by 12 noon on Monday 9 December 2013.


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.html

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://www2.phys.canterbury.ac.nz/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


MEMBERS' ABSTRACTS at gr-qc, July 2011 - November 2013

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 crosslinked 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 174 papers listed here and in the LIGO section represent 1.75% of the 9954 papers posted or cross-linked to gr-qc between July 2011 and November 2013.


arXiv:1211.1466

Pseudo-Riemannian Ricci-flat and Flat Warped Geometries and New Coordinates for the Minkowski metric

Authors: M. M. Akbar (Submitted on 7 Nov 2012)

Abstract: It is well-known that the Einstein condition on warpedgeometries requires the fibres to be necessarily Einstein. However, exact warped solutions have often been obtained using one- and two-dimensional bases. In this paper, keeping the dimensions and signatures of the base and the fibre independently arbitrary, we obtain all Ricci-flat warped metrics with flat base in closed form and show that the number of free parameters is one less than the dimension of the base. Without any assumptions on the base and fibre geometry, we then show that a warped geometry is flat, i.e, has vanishing Riemann curvature, only if its base is flat and its fibre is maximally symmetric, i.e. of constant curvature. Applying this result systematically all possible warped forms of the Euclidean, Minkowski, and flat metrics of arbitrary signature can be obtained in closed form up to disjoint diffemorphisms of the base and fiber metrics. In particular, we obtained four new time-dependent forms of the Minkowski metric in four dimensions in addition to reproducing all of its known warped forms.


arXiv:1107.5746

All extremal instantons in Einstein-Maxwell-dilaton-axion theory

Authors: Mustapha Azreg-Aďnou, Gérard Clément, Dmitri V. Gal'tsov (Submitted on 28 Jul 2011)

Abstract: We construct explicitly all extremal instanton solutions to $\cN=4,\, D=4$ supergravity truncated to one vector field (Einstein-Maxwell-dilaton-axion (EMDA) theory). These correspond to null geodesics of the target space of the sigma-model $G/H=Sp(4,\mathbb{R})/GL(2,\R)$ obtained by compactification of four-dimensional Euclidean EMDA on a circle. They satisfy a no-force condition in terms of the asymptotic charges and part of them (corresponding to nilpotent orbits of the $Sp(4,\mathbb{R})$ U-duality) are presumably supersymmetric. The space of finite action solutions is found to be unexpectedly large and includes, besides the Euclidean versions of known Lorentzian solutions, a number of new asymptotically locally flat (ALF) instantons endowed with electric, magnetic, dilaton and axion charges. We also describe new classes of charged asymptotically locally Euclidean (ALE) instantons as well as some exceptional solutions. Our classification scheme is based on the algebraic classification of matrix generators according to their rank, according to the nature of the charge vectors and according to the number of independent harmonic functions with unequal charges. Besides the nilpotent orbits of $G$, we find solutions which satisfy the asymptotic no-force condition, but are not supersymmetric. The renormalized on-shell action for instantons is calculated using the method of matched background subtraction.

Journal reference: Phys. Rev. D 84 (2011) 104042


arXiv:1206.1408

Rotation and twist regular modes for trapped ghosts

Authors: Mustapha Azreg-Aďnou (Submitted on 7 Jun 2012)

Abstract: A parameter-independent notion of stationary slow motion is formulated then applied to the case of stationary rotation of massless trapped ghosts. The excitations correspond to a rotation mode with angular momentum $J\neq 0$ and twist modes. It is found that the rotation mode, which has no parity, causes excess in the angular velocity of dragged distant coordinate frames in one sheet of the wormhole while in the other sheet the angular velocity of the ghosts is that of rotating stars: $2J/r^3$. As to the twist modes, which all have parity, they cause excess in the angular velocity of one of the throat's poles with respect to the other.


arXiv:1209.5232

Light paths of normal and phantom Einstein-Maxwell-dilaton black holes

Authors: Mustapha Azreg-Aďnou (Submitted on 24 Sep 2012 (v1), last revised 7 Jan 2013 (this version, v3))

Abstract: Null geodesics of normal and phantom Einstein-Maxwell-dilaton black holes are determined analytically by the Weierstrass elliptic functions. The black hole parameters other than the mass enter, with the appropriate signs, the formula for the angle of deflection to the second order in the inverse of the impact parameter allowing for the identification of the nature of matter (phantom or normal). Such identification is also possible via the time delay formula and observation of relativistic images. Scattering experiencesmay favor black holes of Einstein-anti-Maxwell-dilatonic theory for their high relative discrepancy with respect to the Schwarzschild value. For the cases we restrict ourselves to, phantom black holes are characterized by the absence of many-world and two-world null geodesics.

Journal reference: Phys. Rev. D 87, 024012 (2013)


arXiv:1211.5909

Thermodynamical, geometrical and Poincaré methods for charged black holes in presence of quintessence Authors: Mustapha Azreg-Aďnou, Manuel E. Rodrigues

(Submitted on 26 Nov 2012 (v1), last revised 4 Sep 2013 (this version, v2))

Abstract: Properties pertaining to thermodynamical local stability of Reissner-Nordstr\"om black holes surrounded by quintessence as well as adiabatic invariance, adiabatic charging and a generalized Smarr formula are discussed. Limits for the entropy, temperature and electric potential ensuring stability of canonical ensembles are determined by the classical thermodynamical and Poincar\'e methods. By the latter approach we show that microcanonical ensembles (isolated black holes) are stable. Two geometrical approaches lead to determine the same states corresponding to second order phase transitions.


arXiv:1302.6963

Numerical solutions to the cosmological 3-fluid problem

Authors: Mustapha Azreg-Aďnou (Submitted on 27 Feb 2013 (v1), last revised 24 Sep 2013 (this version, v4))

Abstract: We show that, for the scalar field cosmology with exponential potential, the set of values of the coupling parameter for which the solutions undergo a transient period of acceleration is much larger than the set discussed in the literature. The gradual inclusion of ordinary and dark matters results in an everywhere, but near the origin, smoother and right shifted (along the time axis) acceleration curve. For the 3-fluid problem, the energy density need not exhibit a plateau during the acceleration period. Much excess in the dark matter and/or ordinary matter energy densities would lead the universe to undergo an eternal deceleration expansion. For the 3-fluid problem with a single exponential potential we conclude that the Big Bang Nucleosynthesis constraint is not fulfilled if the universe is to undergo a transient period of acceleration. The 3-fluid model remains a good approximation for the description of large scale structures.

Comments:

To appear in General Relativity and Gravitation


arXiv:1304.7470

Phase-space analysis of the cosmological 3-fluid problem: Families of attractors and repellers

Authors: Mustapha Azreg-Aďnou (Submitted on 28 Apr 2013 (v1), last revised 2 Sep 2013 (this version, v3))

Abstract: We perform a phase-space analysis of the cosmological 3-fluid problem consisting of a barotropic fluid with an equation-of-state parameter $\gamma-1$, a pressureless dark matter fluid, plus a scalar field $\phi$ (representing dark energy) coupled to exponential potential $V=V_0\exp{(-\kappa\lambda\phi)}$. Besides the potential-kinetic-scaling solutions, which are not the unique late-time attractors whenever they exist for $\lambda^2\geq 3\ga$, we derive new attractors where both dark energy and dark matter coexist and the final density is shared in a way independent of the value of $\gamma >1$. The case of a pressureless barotropic fluid ($\gamma =1$) has a one-parameter family of attractors where all components coexist. New one-parameter families of matter-dark matter saddle points and kinetic-matter repellers exist. We investigate the stability of the ten critical points by linearization and/or Lyapunov's Theorems and a variant of the theorems formulated in this paper. A solution with two transient periods of acceleration and two transient periods of deceleration is derived.

Comments:

To appear in Classical and Quantum Gravity


arXiv:1108.3186

Summed Parallel Infinite Impulse Response (SPIIR) Filters For Low-Latency Gravitational Wave Detection Authors: Shaun Hooper (UWA), Shin Kee Chung (UWA), Jing Luan (Caltech), David Blair (UWA), Yanbei Chen (Caltech), Linqing Wen (UWA)

(Submitted on 16 Aug 2011)

Abstract: With the upgrade of current gravitational wave detectors, the first detection of gravitational wave signals is expected to occur in the next decade. Low-latency gravitational wave triggers will be necessary to make fast follow-up electromagnetic observations of events related to their source, e.g., prompt optical emission associated with short gamma-ray bursts. In this paper we present a new time-domain low-latency algorithm for identifying the presence of gravitational waves produced by compact binary coalescence events in noisy detector data. Our method calculates the signal to noise ratio from the summation of a bank of parallel infinite impulse response (IIR) filters. We show that our summed parallel infinite impulse response (SPIIR) method can retrieve the signal to noise ratio to greater than 99% of that produced from the optimal matched filter. We emphasise the benefits of the SPIIR method for advanced detectors, which will require larger template banks.

Journal reference: Phys. Rev. D 86, 024012 (2012)


arXiv:1209.0595

On the gravitational wave background from compact binary coalescences in the band of ground-based interferometers

Authors: Xing-Jiang Zhu, Eric J. Howell, David G. Blair, Zong-Hong Zhu (Submitted on 4 Sep 2012 (v1), last revised 10 Mar 2013 (this version, v4))

Abstract: This paper reports a comprehensive study on the gravitational wave (GW) background from compact binary coalescences. We consider in our calculations newly available observation-based neutron star and black hole mass distributions and complete analytical waveforms that include post-Newtonian amplitude corrections. Our results show that: (i) post-Newtonian effects cause a small reduction in the GW background signal; (ii) below 100 Hz the background depends primarily on the local coalescence rate $r_0$ and the average chirp mass and is independent of the chirp mass distribution; (iii) the effects of cosmic star formation rates and delay times between the formation and merger of binaries are linear below 100 Hz and can be represented by a single parameter within a factor of ~ 2; (iv) a simple power law model of the energy density parameter $\Omega_{GW}(f) ~ f^{2/3}$ up to 50-100 Hz is sufficient to be used as a search template for ground-based interferometers. In terms of the detection prospects of the background signal, we show that: (i) detection (a signal-to-noise ratio of 3) within one year of observation by the Advanced LIGO detectors (H1-L1) requires a coalescence rate of $r_0 = 3 (0.2) Mpc^{-3} Myr^{-1}$ for binary neutron stars (binary black holes); (ii) this limit on $r_0$ could be reduced 3-fold for two co-located detectors, whereas the currently proposed worldwide network of advanced instruments gives only ~ 30% improvement in detectability; (iii) the improved sensitivity of the planned Einstein Telescope allows not only confident detection of the background but also the high frequency components of the spectrum to be measured. Finally we show that sub-threshold binary neutron star merger events produce a strong foreground, which could be an issue for future terrestrial stochastic searches of primordial GWs.

Journal reference: MNRAS (2013) 431: 882-899


arXiv:1109.1178

A novel approach to the dynamics of Szekeres dust models

Authors: Roberto A. Sussman, Krzysztof Bolejko (Submitted on 6 Sep 2011 (v1), last revised 8 Mar 2012 (this version, v2))

Abstract: We obtain an elegant and useful description of the dynamics of Szekeres dust models (in their full generality) by means of `quasi-local' scalar variables constructed by suitable integral distributions that can be interpreted as weighed proper volume averages of the local covariant scalars. In terms of these variables, the field equations and basic physical and geometric quantities are formally identical to their corresponding expressions in the spherically symmetric LTB dust models. Since we can map every Szekeres model to a unique LTB model, rigorous results valid for the latter models can be readily generalized to a non-spherical Szekeres geometry. The new variables lead naturally to an initial value formulation in which all scalars are expressed as scaling laws in terms of their values at an arbitrary initial space slice. These variables also yield a significant simplification of numerical work, since the fluid flow evolution equations become a system of autonomous ordinary differential equations subjected to algebraic constraints containing the information on the deviations from spherical symmetry. As an example of how this formalism can be applied, we show that spherical symmetry is stable against small dipole-like perturbations. This new approach to the dynamics of the Szekeres solutions has an enormous potential for dealing with a wide variety of theoretical issues and for constructing non-spherical models of cosmological inhomogeneities to fit observational data.

Journal reference: Class. Q. Grav. 29 (2012) 065018


arXiv:1202.5970

Apparent horizons in the quasi-spherical Szekeres models

Authors: Andrzej Krasi?ski, Krzysztof Bolejko (Submitted on 27 Feb 2012 (v1), last revised 12 Jun 2012 (this version, v2))

Abstract: The notion of an apparent horizon (AH) in a collapsing object can be carried over from the Lema\^{\i}tre -- Tolman (L--T) to the quasispherical Szekeres models in three ways: 1. Literally by the definition -- the AH is the boundary of the region, in which every bundle of null geodesics has negative expansion scalar. 2. As the locus, at which null lines that are as nearly radial as possible are turned toward decreasing areal radius $R$. These lines are in general nongeodesic. The name "absolute apparent horizon" (AAH) is proposed for this locus. 3. As the boundary of a region, where null \textit{geodesics} are turned toward decreasing $R$. The name "light collapse region" (LCR) is proposed for this region (which is 3-dimensional in every space of constant $t$); its boundary coincides with the AAH. The AH and AAH coincide in the L--T models. In the quasispherical Szekeres models, the AH is different from (but not disjoint with) the AAH. Properties of the AAH and LCR are investigated, and the relations between the AAH and the AH are illustrated with diagrams using an explicit example of a Szekeres metric. It turns out that an observer who is already within the AH is, for some time, not yet within the AAH. Nevertheless, no light signal can be sent through the AH from the inside. The analogue of the AAH for massive particles is also considered.

Journal reference: Phys. Rev. D85, 124016 (2012)


arXiv:1204.0909

Ricci focusing, shearing, and the expansion rate in an almost homogeneous Universe

Authors: Krzysztof Bolejko, Pedro G. Ferreira (Submitted on 4 Apr 2012 (v1), last revised 3 May 2012 (this version, v2))

Abstract: The Universe is inhomogeneous, and yet it seems to be incredibly well-characterised by a homogeneous relativistic model. One of the current challenges is to accurately characterise the properties of such a model. In this paper we explore how inhomogeneities may affect the overall optical properties of the Universe by quantifying how they can bias the redshift-distance relation in a number of toy models that mimic the real Universe. The models that we explore are statistically homogeneous on large scales. We find that the effect of inhomogeneities is of order of a few percent, which can be quite important in precise estimation of cosmological parameters. We discuss what lessons can be learned to help us tackle a more realistic inhomogeneous universe.

Journal reference: JCAP05(2012)003


arXiv:1212.4697

Exact inhomogeneous models and the drift of light rays induced by nonsymmetric flow of the cosmic medium

Authors: Andrzej Krasi?ski, Krzysztof Bolejko (Submitted on 19 Dec 2012 (v1), last revised 7 Feb 2013 (this version, v3))

Abstract: After introducing the Szekeres and Lema\^{\i}tre--Tolman cosmological models, the real-time cosmology program is briefly mentioned. Then, a few widespread misconceptions about the cosmological models are pointed out and corrected. Investigation of null geodesic equations in the Szekeres models shows that observers in favourable positions would see galaxies drift across the sky at a rate of up to $10^{-6}$ arc seconds per year. Such a drift would be possible to measure using devices that are under construction; the required time of monitoring would be $\approx10$ years. This effect is zero in the FLRW models, so it provides a measure of inhomogeneity of the Universe. In the Szekeres models, the condition for zero drift is zero shear. But in the shearfree normal models, the condition for zero drift is that, in the comoving coordinates, the time dependence of the metric completely factors out.


arXiv:1309.5695

Intermediate homogenization of the Universe and the problem of gravitational entropy

Authors: Krzysztof Bolejko, William R. Stoeger (Submitted on 23 Sep 2013)

Abstract: This paper studies intermediate homogenization of inhomogeneous cosmological models. It shows that spherically symmetric models, regardless of the equation of state, can undergo intermediate homogenization, i.e. a model can approach a homogeneous and isotropic state (which acts as a saddle point) from a relatively wide range of initial inhomogeneous conditions. The homogenization is not permanent - just temporary. Eventually the model evolves toward a future inhomogeneous state. We also looked at the problem of the gravitational entropy. All definitions of entropy that we checked give decreasing gravitational entropy during the homogenization process. Thus, we should either accept that gravitational entropy can decrease or try to define it in other ways than just via density gradients, as these decrease during homogenization.

Journal reference: Phys. Rev. D 88, 063529 (2013)


arXiv:1209.3142

Anti-lensing: the bright side of voids

Authors: Krzysztof Bolejko, Chris Clarkson, Roy Maartens, David Bacon, Nikolai Meures, Emma Beynon (Submitted on 14 Sep 2012 (v1), last revised 17 Jan 2013 (this version, v3))

Abstract: More than half of the volume of our Universe is occupied by cosmic voids. The lensing magnification effect from those under-dense regions is generally thought to give a small dimming contribution: objects on the far side of a void are supposed to be observed as slightly smaller than if the void were not there, which together with conservation of surface brightness implies net reduction in photons received. This is predicted by the usual weak lensing integral of the density contrast along the line of sight. We show that this standard effect is swamped at low redshifts by a relativistic Doppler term that is typically neglected. Contrary to the usual expectation, objects on the far side of a void are brighter than they would be otherwise. Thus the local dynamics of matter in and near the void is crucial and is only captured by the full relativistic lensing convergence. There are also significant nonlinear corrections to the relativistic linear theory, which we show actually under-predicts the effect. We use exact solutions to estimate that these can be more than 20% for deep voids. This remains an important source of systematic errors for weak lensing density reconstruction in galaxy surveys and for supernovae observations, and may be the cause of the reported extra scatter of field supernovae located on the edge of voids compared to those in clusters.

Journal reference: Phys. Rev. Lett. 110, 021302 (2013)


arXiv:1208.2604

Geometry of the quasi-hyperbolic Szekeres models

Authors: Andrzej Krasi?ski, Krzysztof Bolejko (Submitted on 13 Aug 2012 (v1), last revised 19 Dec 2012 (this version, v3))

Abstract: Geometric properties of the quasi-hyperbolic Szekeres models are discussed and related to the quasi-spherical Szekeres models. Typical examples of shapes of various classes of 2-dimensional coordinate surfaces are shown in graphs; for the hyperbolically symmetric subcase and for the general quasi-hyperbolic case. An analysis of the mass function $M(z)$ is carried out in parallel to an analogous analysis for the quasi-spherical models. This leads to the conclusion that $M(z)$ determines the density of rest mass averaged over the whole space of constant time.

Journal reference: Phys. Rev. D86, 104036 (2012)


arXiv:1201.4241

Shear-free perfect fluids with a solenoidal electric curvature

Authors: Norbert Van den Bergh, John Carminati, Hamid Reza Karimian, Peter Huf (Submitted on 20 Jan 2012 (v1), last revised 24 Jan 2012 (this version, v2))

Abstract: We prove that the vorticity or the expansion vanishes for any shear-free perfect fluid solution of the Einstein field equations where the pressure satisfies a barotropic equation of state and the spatial divergence of the electric part of the Weyl tensor is zero.


arXiv:1209.1581

The regular conducting fluid model for relativistic thermodynamics

Authors: Brandon Carter (Submitted on 7 Sep 2012)

Abstract: The "regular" model presented here can be considered to be the most natural solution to the problem of constructing the simplest possible relativistic analogue of the category of classical Fourier--Euler thermally conducting fluid models as characterised by a pair of equations of state for just two dependent variables (an equilibrium density and a conducting scalar). The historically established but causally unsatisfactory solution to this problem due to Eckart is shown to be based on an ansatz that is interpretable as postulating a most unnatural relation between the (particle and entropy) velocities and their associated momenta, which accounts for the well known bad behaviour of that model which has recently been shown to have very pathological mixed-elliptic-hyperbolic comportments. The newer (and more elegant) solution of Landau and Lifshitz has a more mathematically respectable parabolic-hyperbolic comportment, but is still compatible with a well posed initial value problem only in such a restricted limit-case such as that of linearised perturbations of a static background. For mathematically acceptable behaviour undermore general circumstances, and a fortiori for the physically motivated requirement of subluminal signal propagation, only strictly hyperbolic behaviour is acceptable. Attention is drawn here to the availability of a more modern "regular" solution which, is fully satisfactory as far as all these requirements are concerned. This "regular" category of relativistic conducting fluid models arises naturally within a recently developed variational approach, in which the traditionally important stress--momentum-energy density tensor is relegated to a secondary role, while the relevant covariant 4-momentum co-vectors are instead brought to the fore.


arXiv:1203.0952

Classical Anthropic Everett model: indeterminacy in a preordained multiverse

Authors: Brandon Carter (Submitted on 5 Mar 2012)

Abstract: Although ultimately motivated by quantum theoretical considerations, Everett's many-world idea remains valid, as an approximation, in the classical limit. However to be applicable it must in any case be applied in conjunction with an appropriate anthropic principle, whose precise formulation involves an anthropic quotient that can be normalised to unity for adult humans but that would be lower for infants and other animals. The outcome is a deterministic multiverse in which the only function of chance is the specification of one's particular identity.

Reference: J. Cosmology 14. Also in Consciousness and the Universe: Quantum Physics, Evolution, Brain and Mind, ed. R. Penrose, S. Hameroff, S. Kak (Cosmology Science Publishers, Cambridge, Mass., 2011) 1077-1086


arXiv:1204.2000

Gravitational Wave Astronomy: Needle in a Haystack

Authors: Neil J. Cornish (Submitted on 9 Apr 2012)

Abstract: A world-wide array of highly sensitive interferometers stands poised to usher in a new era in astronomy with the first direct detection of gravitational waves. The data from these instruments will provide a unique perspective on extreme astrophysical phenomena such as neutron stars and black holes, and will allow us to test Einstein's theory of gravity in the strong field, dynamical regime. To fully realize these goals we need to solve some challenging problems in signal processing and inference, such as finding rare and weak signals that are buried in non-stationary and non-Gaussian instrument noise, dealing with high-dimensional model spaces, and locating what are often extremely tight concentrations of posterior mass within the prior volume. Gravitational wave detection using space based detectors and Pulsar Timing Arrays bring with them the additional challenge of having to isolate individual signals that overlap one another in both time and frequency. Promising solutions to these problems will be discussed, along with some of the challenges that remain.

Journal reference: Phil. Trans. R. Soc. A. 371, 20110540 (2013)


arXiv:1204.2585

Model-Independent Test of General Relativity: An Extended post-Einsteinian Framework with Complete Polarization Content

Authors: Katerina Chatziioannou, Nicolas Yunes, Neil Cornish (Submitted on 11 Apr 2012 (v1), last revised 29 Aug 2012 (this version, v2))

Abstract: We develop a model-independent test of General Relativity that allows for the constraint of the gravitational wave (GW) polarization content with GW detections of binary compact object inspirals. We first consider three modified gravity theories (Brans-Dicke theory, Rosen's theory and Lightman-Lee theory) and calculate the response function of ground-based detectors to gravitational waves in the inspiral phase. This allows us to see how additional polarizations predicted in these theories modify the General Relativistic prediction of the response function. We then consider general power-law modifications to the Hamiltonian and radiation-reaction force and study how these modify the time-domain and Fourier response function when all polarizations are present. From these general arguments and specific modified gravity examples, we infer an improved parameterized post-Einsteinian template family with complete polarization content. This family enhances General Relativity templates through the inclusion of new theory parameters, reducing to the former when these parameters acquire certain values, and recovering modified gravity predictions for other values, including all polarizations. We conclude by discussing detection strategies to constrain these new, polarization theory parameters by constructing certain null channels through the combination of output from multiple detectors.

Journal Reference: PhysRevD.86.022004


arXiv:1206.2939

Constraints on the Topology of the Universe: Extension to General Geometries

Authors: Pascal M. Vaudrevange, Glenn D. Starkman, Neil J. Cornish, David N. Spergel (Submitted on 13 Jun 2012)

Abstract: We present an update to the search for a non-trivial topology of the universe by searching for matching circle pairs in the cosmic microwave background using the WMAP 7 year data release. We extend the exisiting bounds to encompass a wider range of possible topologies by searching for matching circle pairs with opening angles 10 degree < \alpha < 90 degree and separation angles 11 degree < \theta < 180 degree. The extended search reveal two small anomalous regions in the CMB sky. Numerous pairs of well-matched circles are found where both circles pass through one or the other of those regions. As this is not the signature of any known manifold, but is a likely consequence of contamination in those sky regions, we repeat the search excluding circle pairs where both pass through either of the two regions. We then find no statistically significant pairs of matched circles, and so no hints of a non-trivial topology. The absence of matched circles increases the lower limit on the length of the shortest closed null geodesic that self-intersects at our location in the universe (equivalently the injectivity radius at our location) to 98.5% of the diameter of the last scattering surface or approximately 26 Gpc. It extends the limit to any manifolds in which the intersecting arcs of said geodesic form an angle greater than 10^o.

Journal Reference: PhysRevD.86.083526


arXiv:1207.4848

Prospects for observing ultra-compact binaries with space-based gravitational wave interferometers and optical telescopes

Authors: Tyson B. Littenberg, Shane L. Larson, Gijs Nelemans, Neil J. Cornish (Submitted on 20 Jul 2012 (v1), last revised 24 Aug 2012 (this version, v2))

Abstract: Space-based gravitational wave interferometers are sensitive to the galactic population of ultra-compact binaries. An important subset of the ultra-compact binary population are those stars that can be individually resolved by both gravitational wave interferometers and electromagnetic telescopes. The aim of this paper is to quantify the multi-messenger potential of space-based interferometers with arm-lengths between 1 and 5 Gm. The Fisher Information Matrix is used to estimate the number of binaries from a model of the Milky Way which are localized on the sky by the gravitational wave detector to within 1 and 10 square degrees and bright enough to be detected by a magnitude limited survey. We find, depending on the choice of GW detector characteristics, limiting magnitude, and observing strategy, that up to several hundred gravitational wave sources could be detected in electromagnetic follow-up observations.


arXiv:1209.6286

Astrophysical Model Selection in Gravitational Wave Astronomy

Authors: Matthew Adams, Neil Cornish, Tyson Littenberg (Submitted on 27 Sep 2012)

Abstract: Theoretical studies in gravitational wave astronomy have mostly focused on the information that can be extracted from individual detections, such as the mass of a binary system and its location in space. Here we consider how the information from multiple detections can be used to constrain astrophysical population models. This seemingly simple problem is made challenging by the high dimensionality and high degree of correlation in the parameter spaces that describe the signals, and by the complexity of the astrophysical models, which can also depend on a large number of parameters, some of which might not be directly constrained by the observations. We present a method for constraining population models using a Hierarchical Bayesian modeling approach which simultaneously infers the source parameters and population model and provides the joint probability distributions for both. We illustrate this approach by considering the constraints that can be placed on population models for galactic white dwarf binaries using a future space based gravitational wave detector. We find that a mission that is able to resolve ~5000 of the shortest period binaries will be able to constrain the population model parameters, including the chirp mass distribution and a characteristic galaxy disk radius to within a few percent. This compares favorably to existing bounds, where electromagnetic observations of stars in the galaxy constrain disk radii to within 20%.

Journal Reference: PhysRevD.86.124032


arXiv:1209.6428

Applying Bayesian Inference to the first International Pulsar Timing Array data challenge

Authors: Neil J. Cornish (Submitted on 28 Sep 2012)

Abstract: This is a very brief summary of the techniques I used to analyze the IPTA challenge 1 data sets. I tried many things, and more failed than succeeded, but in the end I found two approaches that appear to work based on tests done using the open data sets. One approach works directly with the time domain data, and the other works with a specially constructed Fourier transform of the data. The raw data was run through TEMPO2 to produce reduced timing residuals for the analysis. Standard Markov Chain Monte Carlo techniques were used to produce samples from the posterior distribution function for the model parameters. The model parameters include the gravitational wave amplitude and spectral slope, and the white noise amplitude for each pulsar in the array. While red timing noise was only included in Dataset 3, I found that it was necessary to include effective red noise in all the analyses to account for some of the spurious effects introduced by the TEMPO2 timing fit. This added an additional amplitude and slope parameter for each pulsar, so my overall model for the 36 pulsars residuals has 110 parameters. As an alternative to using an effective red noise model, I also tried to simultaneously re-fit the timing model model while looking for the gravitational wave signal, but for reasons that are not yet clear, this approach was not very successful. I comment briefly on ways in which the algorithms could be improved. My best estimates for the gravitational wave amplitudes in the three closed (blind) data sets are: (1) $A=(7.3\pm 1.0)\times 10^{-15}$; (2) $A=(5.7\pm 0.6)\times 10^{-14}$; and (3) $A=(4.6\pm 1.3)\times 10^{-15}$.


arXiv:1303.1185

Gravitational Wave Tests of Strong Field General Relativity with Binary Inspirals: Realistic Injections and Optimal Model Selection

Authors: Laura Sampson, Neil Cornish, Nicolas Yunes (Submitted on 5 Mar 2013)

Abstract: We study generic tests of strong-field General Relativity using gravitational waves emitted during the inspiral of compact binaries. Previous studies have considered simple extensions to the standard post-Newtonian waveforms that differ by a single term in the phase. Here we improve on these studies by (i) increasing the realism of injections and (ii) determining the optimal waveform families for detecting and characterizing such signals. We construct waveforms that deviate from those in General Relativity through a series of post-Newtonian terms, and find that these higher-order terms can affect our ability to test General Relativity, in some cases by making it easier to detect a deviation, and in some cases by making it more difficult. We find that simple single-phase post-Einsteinian waveforms are sufficient for detecting deviations from General Relativity, and there is little to be gained from using more complicated models with multiple phase terms. The results found here will help guide future attempts to test General Relativity with advanced ground-based detectors.

Journal Reference: PhysRevD.87.102001


arXiv:1305.0326

Pulsar Timing Array Analysis for Black Hole Backgrounds

Authors: Neil J. Cornish, A. Sesana (Submitted on 2 May 2013)

Abstract: An astrophysical population of supermassive black hole binaries is thought to be the strongest source of gravitational waves in the frequency range covered by Pulsar Timing Arrays (PTAs). A potential cause for concern is that the standard cross-correlation method used in PTA data analysis assumes that the signals are isotropically distributed and Gaussian random, while the signals from a black hole population are likely to be anisotropic and deterministic. Here we argue that while the conventional analysis is not optimal, it is not hopeless either, as the standard Hellings-Downs correlation curve turns out to hold for point sources, and the small effective number of signal samples blurs the distinction between Gaussian and deterministic signals. Possible improvements to the standard cross-correlation analysis that account for the anisotropy of the signal are discussed.


arXiv:1305.1932

Gravitational Waveforms for Precessing, Quasi-circular Binaries via Multiple Scale Analysis and Uniform Asymptotics: The Near Spin Alignment Case

Authors: Antoine Klein, Neil Cornish, Nicolás Yunes (Submitted on 8 May 2013)

Abstract: We calculate analytical gravitational waveforms in the time- and frequency-domain for precessing quasi-circular binaries with spins of arbitrary magnitude, but nearly aligned with the orbital angular momentum. We first derive an analytical solution to the precession equations by expanding in the misalignment angle and using multiple scale analysis to separate timescales. We then use uniform asymptotic expansions to analytically Fourier transform the time-domain waveform, thus extending the stationary-phase approximation, which fails when precession is present. The resulting frequency-domain waveform family has a high overlap with numerical waveforms obtained by direct integration of the post-Newtonian equations of motion and discrete Fourier transformations. Such a waveform family lays the foundations for the accurate inclusion of spin precession effects in analytical gravitational waveforms, and thus, it can aid in the detection and parameter estimation of gravitational wave signals from the inspiral phase of precessing binary systems.


arXiv:1305.2934

Towards a unified treatment of gravitational-wave data analysis

Authors: Neil J.Cornish, Joseph D. Romano (Submitted on 13 May 2013 (v1), last revised 29 May 2013 (this version, v2))

Abstract: We present a unified description of gravitational-wave data analysis that unites the template-based analysis used to detect deterministic signals from well-modeled sources, such as binary-black-hole mergers, with the cross-correlation analysis used to detect stochastic gravitational-wave backgrounds. We also discuss the connection between template-based analyses and those that target poorly-modeled bursts of gravitational waves, and suggest a new approach for detecting burst signals.


arXiv:1307.4116

Detecting a Stochastic Gravitational Wave Background in the presence of a Galactic Foreground and Instrument Noise

Authors: Matthew R. Adams, Neil J. Cornish (Submitted on 15 Jul 2013 (v1), last revised 7 Oct 2013 (this version, v2))

Abstract: Detecting a stochastic gravitational wave background requires that we first understand and model any astrophysical foregrounds. In the millihertz frequency band, the predominate foreground signal will be from unresolved white dwarf binaries in the galaxy. We build on our previous work to show that a stochastic gravitational wave background can be detected in the presence of both instrument noise and a galactic confusion foreground. The key to our approach is accurately modeling the spectra for each of the various signal components. We simulate data for a gigameter Laser Interferometer Space Antenna (LISA) operating in the mHz frequency band detector operating with both 6- and 4-links. We obtain posterior distribution functions for the instrument noise parameters, the galaxy level and modulation parameters, and the stochastic background energy density. We find that we are able to detect a scale-invariant stochastic background with energy density as low as Omega_gw = 2e-13 for a 6-link interferometer and Omega_gw = 5e-13 for a 4-link interferometer with one year of data.


arXiv:1307.4418

Gravitational Waveforms for Precessing, Quasicircular Compact Binaries with Multiple Scale Analysis: Small Spin Expansion

Authors: Katerina Chatziioannou, Antoine Klein, Nicolas Yunes, Neil Cornish (Submitted on 16 Jul 2013 (v1), last revised 29 Sep 2013 (this version, v2))

Abstract: We obtain analytical gravitational waveforms in the frequency-domain for precessing, quasi-circular compact binaries with small spins, applicable, for example, to binary neutron star inspirals. We begin by calculating an analytic solution to the precession equations, obtained by expanding in the dimensionless spin parameters and using multiple-scale analysis to separate timescales. We proceed by analytically computing the Fourier transform of time-domain waveform through the stationary phase approximation. We show that the latter is valid for systems with small spins. Finally, we show that these waveforms have a high overlap with numerical waveforms obtained through direct integration of the precession equations and discrete Fourier transformations. The resulting, analytic waveform family is ideal for detection and parameter estimation of gravitational waves emitted by inspiraling binary neutron stars with ground-based detectors.


arXiv:1307.8144

A Rosetta Stone for Parameterized Tests of Gravity

Authors: Laura Sampson, Nicolas Yunes, Neil Cornish (Submitted on 30 Jul 2013)

Abstract: Several model-independent parameterizations of deviations from General Relativity have been developed to test Einstein's theory. Although these different parameterizations were developed for different gravitational observables, they ultimately all test the same underlying physics. In this paper, we develop connections between the parameterized post-Newtonian, parameterized post-Keplerian, and the parameterized post-Einsteinian frameworks, developed to carry out tests of General Relativity with Solar System, binary pulsar, and gravitational wave observations respectively. These connections allow us to use knowledge gained from one framework to inform and guide tests using the others. Relating these parameterizations and combining the results from each approach strengthens our tests of General Relativity.


arXiv:1108.2332

Covariant Derivatives on Null Submanifolds

Authors: Don Hickethier, Tevian Dray (Submitted on 11 Aug 2011)

Abstract: The degenerate nature of the metric on null hypersurfaces makes it difficult to define a covariant derivative on null submanifolds. Recent approaches using decomposition to define a covariant derivative on null hypersurfaces are investigated, with examples demonstrating the limitations of the methods. Motivated by Geroch's work on asymptotically flat spacetimes, conformal transformations are used to construct a covariant derivative on null hypersurfaces, and a condition on the Ricci tensor is given to determine when this construction can be used. Several examples are given, including the construction of a covariant derivative operator for the class of spherically symmetric hypersurfaces.

Journal reference: Gen. Rel. Grav. 44, 225-238 (2012)


arXiv:1301.5929

Toy model studies of tuning and typicality with an eye toward cosmology

Authors: Aaron Hernley, Andreas Albrecht, Tevian Dray (Submitted on 24 Jan 2013 (v1), last revised 29 May 2013 (this version, v3))

Abstract: We investigate a number of simple toy models to explore interesting relationships between dynamics and typicality. We start with an infinite model that has been proposed as an illustration of how non-ergodic dynamics can produce interesting features that are suggestive for cosmological applications. We consider various attempts to define the infinite model more rigorously as a limit of a finite system. None of our attempts at such rigor were able to preserve the attractive properties. We hope our work will challenge others to find more successful toy models. The difficulty of finding such models suggests that connections between dynamics and typicality we hope for in cosmological theories such as eternal inflation may not be so easy to achieve.


arXiv:1108.2789

Cosmography in f(T)-gravity

Authors: S. Capozziello, V. F. Cardone, H. Farajollahi, A. Ravanpak (Submitted on 13 Aug 2011)

Abstract: Being based on the only assumption that the universe is homogenous and isotropic on large scales, cosmography is an ideal tool to investigate the cosmic expansion history in a almost model-independent way. Fitting the data on the luminosity distance and Baryon Acoustic Oscillations allows to determine the confidence ranges for the cosmographic parameters hence giving some quantitative constraints that a whatever theory has to fulfill. As an application, we consider here the case of teleparallel gravity (TEGR) also referred to as f(T)-gravity. To this end, we first work out analytical expressions to express the present day values of f(T)-derivatives as a function of the cosmographic parameters which hold under quite general and physically motivated conditions. We then use the constraints coming from cosmography to find out the confidence ranges for f(T)-derivatives up to the fifth order and show how these can be used to check the viability of given TEGR models without the need to explicitly solve the second order dynamic equations.

Journal reference: Phys.Rev.D84:043527,2011


arXiv:1109.6317

Observational constraints in scalar tensor theory with tachyonic potential

Authors: H. Farajollahi, A. Salehi, A. Shahabi (Submitted on 28 Sep 2011)

Abstract: We study the dynamics of the scalar tensor cosmological model in the presence of tachyon field. In an alternative approach, in two exponential and power law form of the scalar field functions in the model, field equations are solved by simultaneously best fitting the model parameters with the most recent observational data. This approach gives us an observationally verified interpretation of the dynamics of the universe. We then discuss the best fitted of equation of state parameter, the statefinder parameters and the reconstructed scalar field in the model.

Journal reference: JCAP10(2011)014


arXiv:1109.6358

Stability analysis and Observational Measurement in Chameleonic Generalised Brans--Dicke Cosmology

Authors: H.Farajollahi, A. Salehi (Submitted on 27 Sep 2011)

Abstract: We investigate the dynamics of the chameleonic Generalised Brans--Dicke model in flat FRW cosmology. In a new approach, a framework to study stability and attractor solutions in the phase space is developed for the model by simultaneously best fitting the stability and model parameters with the observational data. The results show that for an accelerating universe the phantom crossing does not occur in the past and near future.

Journal reference: JCAP 07(2011)036


arXiv:1110.3668

Cosmological observations in non-local $F(R)$ cosmology

Authors: H. Farajollahi, F. Tayebi, F. Milani, M. Enayati (Submitted on 17 Oct 2011)

Abstract: In this article in a generalization of our previous work, we investigate the dynamics of the non-local $F(R)$ gravity after casting it into local form. The non-singular bouncing behavior and quintom model of dark energy are achieved without involving negative kinetic energy fields. Two cosmological tests are performed to constrain the model parameters. In case of phantom crossing the distance modulus predicted by the model best-fits the observational data. In comparison with the CPL parametrization for drift velocity, the model in some redshift intervals is in good agreement with the data.

Journal reference: Astrophysics and Space Science, 337, 2 773-778 (2012)


arXiv:1111.0232

Geometric and thermodynamic properties in Gauss-Bonnet gravity

Authors: Hossein Farajollahi, Amin Salehi (Submitted on 1 Nov 2011)

Abstract: In this paper, the generalized second law (GSL) of thermodynamics and entropy is revisited in the context of cosmological models in Gauss-Bonnet gravity with the boundary of the universe is assumed to be enclosed by the dynamical apparent horizon. The model is best fitted with the observational data for distance modulus. The best fitted geometric and thermodynamic parameters such as equation of state parameter, deceleration parameter and entropy are derived. To link between thermodynamic and geometric parameters, the "entropy rate of change multiplied by the temperature" as a model independent thermodynamic state parameter is also derived. The results show that the model is in good agreement with the observational analysis.

Journal reference: Astrophysics and Space Science, 338, 1 187-193 (2012)


arXiv:1111.0467

Stability analysis and observational constraints in scalar tensor theory

Authors: Hossein Farajollahi, Amin Salehi, Mohammad Nasiri (Submitted on 2 Nov 2011)

Abstract: We study FRW cosmology for scalar tensor theory where two scalar functions nonminimally coupled to the geometry and matter Lagrangian. In a framework to study stability and attractor solutions of the model in the phase space, we simultaneously solve the dynamical system and best fit the stability and model parameters with the observational data. The approach imposes restrictions on the model constraints while providing information about the universe dynamics. The model predict current accelerating universe, with a phantom crossing in near future.

Journal reference: JCAP11(2011)018


arXiv:1112.5386

Logarithmic entropy-corrected holographic dark energy with non-minimal kinetic coupling

Authors: Ali R. Amani, J. Sadeghi, H. Farajollahi, M. Pourali (Submitted on 22 Dec 2011)

Abstract: In this paper, we have considered a cosmological model with the non--minimal kinetic coupling terms and investigated its cosmological implications with respect to the logarithmic entropy-- corrected holographic dark energy (LECHDE). The correspondence between LECHDE in flat FRW cosmology and the phantom dark energy model with the aim to interpret the current universe acceleration is also examined.


arXiv:1201.0007

Stability analysis of agegraphic dark energy in Brans-Dicke cosmology

Authors: H. Farajollahi, J. Sadeghi, M. Pourali, A. Salehi (Submitted on 29 Dec 2011)

Abstract: Stability analysis of agegraphic dark energy in Brans-Dicke theory is presented in this paper. We constrain the model parameters with the observational data and thus the results become broadly consistent with those expected from experiment. Stability analysis of the model without best fitting shows that universe may begin from an unstable state passing a saddle point and finally become stable in future. However, with the best fitted model, There is no saddle intermediate state. The agegraphic dark energy in the model by itself exhibits a phantom behavior. However, contribution of cold dark matter on the effective energy density modifies the state of teh universe from phantom phase to quintessence one. The statefinder diagnosis also indicates that the universe leaves an unstable state in the past, passes the LCDM state and finally approaches the sable state in future.

Journal reference: Astrophysics and Space Science, 339, 1 79-85 (2012)


arXiv:1201.0080

Constraints on scalar-tensor theories from observations

Authors: Hossein Farajollahi, Amin Salehi, Mohammad Nasiri (Submitted on 30 Dec 2011)

Abstract: We study the dynamical description of scalar-tensor gravity by performing the best-fit analysis for two cases of exponential and power-law form of the potential and scalar field function coupled to the curvature. The models are then tested against observational data. The results show that in both scenarios the Universe undergoes an acceleration expansion period and the geometrical equivalent of dark energy is associated with a time-dependent equation of state.

Journal reference: Phys. Rev. D 84, 124045 (2011)


arXiv:1202.0069

Universe acceleration and fine structure constant variation in BSBM theory

Authors: H. Farajollahi, A. Salehi (Submitted on 1 Feb 2012)

Abstract: In this work we investigate the utility of using SNe Ia observations in constraining the cosmological parameters in BSBM theory where a scalar field is responsible for both fine structure constant variation and late time universe acceleration. The model is discussed in the presence of an exponential self potential for the scalar field. Stability and phase space analysis of the solutions are studied. The model is tested against observational data for Hubble parameter and quasar absorption spectra. With the best fitted model parameters, the theory predicts a good match with the experimental results and exhibits fine structure constant variation. The analysis also shows that for the equation of state parameter, recent universe acceleration and possible phantom crossing in future is forecasted.

Journal reference: JCAP02(2012)041


arXiv:1206.5797

Chameleon gravity on cosmological scales

Authors: H. Farajollahi, A. Salehi (Submitted on 25 Jun 2012)

Abstract: In conventional approach to the chameleon mechanism, by assuming a static and spherically symmetric solutions in which matter density and chameleon field are given by $\rho=\rho(r)$ and $\phi=\phi(r)$, it has been shown that mass of chameleon field is matter density-dependent. In regions of high matter density such as earth, chameleon field is massive, in solar system it is low and in cosmological scales it is very low. In this article we revisit the mechanism in cosmological scales by assuming a redshift dependence of the matter density and chameleon field, i.e. $\rho=\rho(z)$, $\phi=\phi(z)$. To support our analysis, we best fit the model parameters with the observational data. The result shows that in cosmological scales, the mass of chameleon field increases with the redshift, i.e. more massive in higher redshifts. We also find that in both cases of power-law and exponential potential function, the current universe acceleration can be explained by the low mass chameleon field. In comparison with the high redshift observational data, we also find that the model with power-law potential function is in better agreement with the observational data.

Journal reference: Phys. Rev. D 85, 083514 (2012)


arXiv:1207.1642

Observational constraint in FRW cosmology with a nonminimal scalar field-matter coupling

Authors: H. Farajollahi, A. Salehi (Submitted on 5 Jul 2012)

Abstract: In this paper within the scope of FRW cosmology for k = 0, \pm1 we study the dynamics of the universe for a cosmological model with a scalar field nonminimally coupled to matter. By best- fitting the model parameters with the observational data for the direct interaction between the dark sectors in the model we obtain observational constraints on cosmological parameters. The result shows that with the best fitted model parameters, only in flat universe, the phantom crossing occurs twice in the past and once in the future, whereas no crossing occurs for open and closed models of the universe.


arXiv:1209.3763

Varying alpha and cosmic acceleration in Brans-Dicke-BSBM theory: stability analysis and observational tests

Authors: H. Farajollahi, A. Salehi (Submitted on 16 Sep 2012 (v1), last revised 9 Oct 2012 (this version, v2))

Abstract: By integration of generalized BSBM and Brans-Dicke cosmological models, in this article, we investigate the theoretical framework of fine structure constant variation and current cosmic acceleration. We first develop a mathematical formalism to analyse the stability of the model. By employing observational data to constrain the model parameters, phase space is performed and the attractor solutions of the model are detected. We then examine the model against observational data such as observational Hubble parameter dataset and quasar absorption spectra. The results confirms current universe acceleration and also predicts fine structure constant variation. Furthermore, extrapolation of the best fitted model in high redshift ($z> 15$) illustrates a significantly larger variation of fine structure constant in earlier epoch of the universe.


arXiv:1206.4259 Interactive visualization of a thin disc around a Schwarzschild black hole Authors: Thomas Müller, Jörg Frauendiener (Submitted on 19 Jun 2012) Abstract: In the first course of general relativity, the Schwarzschild spacetime is the most discussed analytic solution to Einstein's field equations. Unfortunately, there is rarely enough time to study the optical consequences of the bending of light for some advanced examples. In this paper, we present how the visual appearance of a thin disc around a Schwarzschild black hole can be determined interactively by means of an analytic solution to the geodesic equation processed on current high performance graphical processing units. This approach can, in principle, be customized for any other thin disc in a spacetime with geodesics given in closed form. The interactive visualization discussed here can be used either in a first course of general relativity for demonstration purposes only or as a thesis for an enthusiastic student in an advanced course with some basic knowledge of OpenGL and a programming language.

Journal reference: Eur. J. Phys. 33:955, 2012


arXiv:1207.5854

Numerical space-times near space-like and null infinity. The spin-2 system on Minkowski space

Authors: Florian Beyer, Georgios Doulis, Jörg Frauendiener, Ben Whale (Submitted on 24 Jul 2012 (v1), last revised 3 Dec 2012 (this version, v2))

Abstract: In this paper we demonstrate for the first time that it is possible to solve numerically the Cauchy problem for the linearisation of the general conformal field equations near spacelike infinity, which is only well-defined in Friedrich's cylinder picture. We have restricted ourselves here to the "core" of the equations - the spin-2 system - propagating on Minkowski space. We compute the numerical solutions for various classes of initial data, do convergence tests and also compare to exact solutions. We also choose initial data which intentionally violate the smoothness conditions and then check the analytical predictions about singularities. This paper is the first step in a long-term investigation of the use of conformal methods in numerical relativity.

Journal reference: Class. Quantum Grav. 29 (2012) 245013


arXiv:1301.4286

The second order spin-2 system in flat space near space-like and null-infinity

Authors: Georgios Doulis, Joerg Frauendiener (Submitted on 18 Jan 2013 (v1), last revised 23 Apr 2013 (this version, v2))

Abstract: In previous work, the numerical solution of the linearized gravitational field equations near space-like and null-infinity was discussed in the form of the spin-2 zero-rest-mass equation for the perturbations of the conformal Weyl curvature. The motivation was to study the behavior of the field and properties of the numerical evolution of the system near infinity using Friedrich's conformal representation of space-like infinity as a cylinder. It has been pointed out by H.O. Kreiss and others that the numerical evolution of a system using second order wave equations has several advantages compared to a system of first order equations. Therefore, in the present paper we derive a system of second order wave equations and prove that the solution spaces of the two systems are the same if appropriate initial and boundary data are given. We study the properties of this system of coupled wave equations in the same geometric setting and discuss the differences between the two approaches.


arXiv:1302.0043

Linearized gravitational waves near space-like and null infinity

Authors: Florian Beyer, Georgios Doulis, Jörg Frauendiener, Ben Whale (Submitted on 1 Feb 2013)

Abstract: Linear perturbations on Minkowski space are used to probe numerically the remote region of an asymptotically flat space-time close to spatial infinity. The study is undertaken within the framework of Friedrich's conformal field equations and the corresponding conformal representation of spatial infinity as a cylinder. The system under consideration is the (linear) zero-rest-mass equation for a spin-2 field. The spherical symmetry of the underlying background is used to decompose the field into separate non-interacting multipoles. It is demonstrated that it is possible to reach null-infinity from initial data on an asymptotically Euclidean hyper-surface and that the physically important radiation field can be extracted accurately on $\scri^+$.


arXiv:1304.6458

The Spin-2 Equation on Minkowski Background

Authors: Florian Beyer, Georgios Doulis, Jörg Frauendiener, Ben Whale (Submitted on 24 Apr 2013)

Abstract: The linearised general conformal field equations in their first and second order form are used to study the behaviour of the spin-2 zero-rest-mass equation on Minkowski background in the vicinity of space-like infinity.


arXiv:1308.4729

Numerical evolutions of fields on the 2-sphere using a spectral method based on spin-weighted spherical harmonics

Authors: Florian Beyer, Boris Daszuta, Jörg Frauendiener, Ben Whale (Submitted on 21 Aug 2013)

Abstract: Many applications in science call for the numerical simulation of systems on manifolds with spherical topology. Through use of integer spin weighted spherical harmonics we present a method which allows for the implementation of arbitrary tensorial evolution equations. Our method combines two numerical techniques that were originally developed with different applications in mind. The first is Huffenberger and Wandelt's spectral decomposition algorithm to perform the mapping from physical to spectral space. The second is the application of Luscombe and Luban's method, to convert numerically divergent linear recursions into stable nonlinear recursions, to the calculation of reduced Wigner d-functions. We give a detailed discussion of the theory and numerical implementation of our algorithm. The properties of our method are investigated by solving the scalar and vectorial advection equation on the sphere, as well as the 2+1 Maxwell equations on a deformed sphere.


arXiv:1208.1502

A cosmological solution of Regge calculus

Authors: Adrian P. Gentle (Submitted on 7 Aug 2012)

Abstract: We revisit the Regge calculus model of the Kasner cosmology first considered by S. Lewis. One of the most highly symmetric applications of lattice gravity in the literature, Lewis' discrete model closely matched the degrees of freedom of the Kasner cosmology. As such, it was surprising that Lewis was unable to obtain the full set of Kasner-Einstein equations in the continuum limit. Indeed, an averaging procedure was required to ensure that the lattice equations were even consistent with the exact solution in this limit. We correct Lewis' calculations and show that the resulting Regge model converges quickly to the full set of Kasner-Einstein equations in the limit of very fine discretization. Numerical solutions to the discrete and continuous-time lattice equations are also considered.


arXiv:1107.1689

Are gravitational waves from giant magnetar flares observable?

Authors: Burkhard Zink, Paul D. Lasky, Kostas D. Kokkotas (Submitted on 8 Jul 2011 (v1), last revised 18 Jul 2011 (this version, v3))

Abstract: Are giant flares in magnetars viable sources of gravitational radiation? Few theoretical studies have been concerned with this problem, with the small number using either highly idealized models or assuming a magnetic field orders of magnitude beyond what is supported by observations. We perform nonlinear general-relativistic magnetohydrodynamics simulations of large-scale hydromagnetic instabilities in magnetar models. We utilise these models to find gravitational wave emissions over a wide range of energies, from 10^40 to 10^47 erg. This allows us to derive a systematic relationship between the surface field strength and the gravitational wave strain, which we find to be highly nonlinear. In particular, for typical magnetar fields of a few times 10^15 G, we conclude that a direct observation of f-modes excited by global magnetic field reconfigurations is unlikely with present or near-future gravitational wave observatories, though we also discuss the possibility that modes in a low-frequency band up to 100 Hz could be sufficiently excited to be relevant for observation.

Journal reference: Phys. Rev. D 85, 024030 (2012)


arXiv:1203.3590

Gravitational Waves and Hydromagnetic Instabilities in Rotating Magnetized Neutron Stars

Authors: Paul D. Lasky, Burkhard Zink, Kostas D. Kokkotas (Submitted on 15 Mar 2012)

Abstract: We perform nonlinear general relativistic ideal magnetohydrodynamic simulations of poloidal magnetic fields in rotating polytropic neutron stars. We have three primary goals: i) to understand the nature of magnetohydrodynamic instabilities inherent to poloidal magnetic fields in non-rotating and rotating neutron stars, ii) to explore the possible space of stable equilibrium configurations and iii) to understand gravitational wave emissions caused by the catastrophic reconfiguration of magnetic fields associated with giant magnetar flares. Our key physical contributions can be summarized as follows: i) gravitational waves from f-modes caused by magnetar flares are unlikely to be detected in the current or near-future generation of gravitational waves observatories, ii) gravitational waves from Alfven waves propagating inside the neutron star are more likely candidates, although this interpretation relies on the unknown damping time of these modes, iii) any magnetic field equilibria derived from our simulations are characterized as non-axisymmetric, with approximately 65% of their magnetic energy in the poloidal field, iv) rotation acts to separate the timescales of different instabilities in our system, with the varicose mode playing a more major role due to a delayed kink instability and v) despite the slowing growth rate of the kink mode, it is always present in our simulations, even for models where the rotational period is of the same order as the Alfven timescale.


arXiv:1302.6033

Stochastic gravitational wave background from hydrodynamic turbulence in differentially rotating neutron stars

Authors: Paul D. Lasky, Mark F. Bennett, Andrew Melatos (Submitted on 25 Feb 2013)

Abstract: Hydrodynamic turbulence driven by crust-core differential rotation imposes a fundamental noise floor on gravitational wave observations of neutron stars. The gravitational wave emission peaks at the Kolmogorov decoherence frequency which, for reasonable values of the crust-core shear, \Delta\Omega, occurs near the most sensitive part of the frequency band for ground-based, long-baseline interferometers. We calculate the energy density spectrum of the stochastic gravitational wave background from a cosmological population of turbulent neutron stars generalising previous calculations for individual sources. The spectrum resembles a piecewise power law, \Omega_{gw}(\nu)=\Omega_{\alpha}\nu^{\alpha}, with \alpha=-1 and 7 above and below the decoherence frequency respectively, and its normalisation scales as \Omega_{\alpha}\propto(\Delta\Omega)^{7}. Non-detection of a stochastic signal by Initial LIGO implies an upper limit on \Delta\Omega and hence by implication on the internal relaxation time-scale for the crust and core to come into co-rotation, \tau_{d}=\Delta\Omega/\dot{\Omega}, where \dot{\Omega} is the observed electromagnetic spin-down rate, with \tau_{d}\lesssim 10^{7} yr for accreting millisecond pulsars and \tau_{d}\lesssim 10^{5} yr for radio-loud pulsars. Target limits on \tau_{d} are also estimated for future detectors, namely Advanced LIGO and the Einstein Telescope, and are found to be astrophysically interesting.


arXiv:1310.7633

Tilted torus magnetic fields in neutron stars and their gravitational wave signatures

Authors: Paul D. Lasky, Andrew Melatos (Submitted on 28 Oct 2013)

Abstract: Gravitational-wave diagnostics are developed for discriminating between varieties of mixed poloidal-toroidal magnetic fields in neutron stars, with particular emphasis on differentially rotating protoneutron stars. It is shown that tilted torus magnetic fields, defined as the sum of an internal/external poloidal component, whose axis of symmetry is tilted with respect to the rotation axis, and an internal toroidal component, whose axis of symmetry is aligned with the rotation axis, deform the star triaxially, unlike twisted torus fields, which deform the star biaxially. Utilizing an analytic tilted torus example, we show that these two topologies can be distinguished by their gravitational wave spectrum and polarization phase portraits. For example, the relative amplitudes and frequencies of the spectral peaks allows one to infer the relative strengths of the toroidal and poloidal components of the field, and the magnetic inclination angle. Finally, we show how a tilted torus field arises naturally from magnetohydrodynamic simulations of differentially rotating neutron stars, and how the gravitational wave spectrum evolves as the internal toroidal field winds up. These results point to the sorts of experiments that may become possible once gravitational wave interferometers detect core-collapse supernovae routinely.


arXiv:1311.1352

Nuclear Equation of State from Observations of Short Gamma-Ray Burst Remnants

Authors: Paul D. Lasky, Brynmor Haskell, Vikram Ravi, Eric J. Howell, David M. Coward (Submitted on 6 Nov 2013)

Abstract: The favoured progenitor model for short $\gamma$-ray bursts (SGRBs) is the merger of two neutron stars that triggers an explosion with a burst of collimated $\gamma$-rays. Following the initial prompt emission, some SGRBs exhibit a plateau phase in their $X$-ray light curves that indicates additional energy injection from a central engine, believed to be a rapidly rotating, highly magnetised neutron star. The collapse of this `protomagnetar' to a black hole is likely to be responsible for a steep decay in $X$-ray flux observed at the end of the plateau. In this letter, we show that these observations can be used to effectively constrain the equation of state of dense matter. In particular, we show that the known distribution of masses in binary neutron star systems, together with fits to the $X$-ray light curves, provide constraints that exclude the softest and stiffest plausible equations of state. We further illustrate how a future gravitational wave observation with Advanced LIGO/Virgo can place tight constraints on the equation of state, by adding into the picture a measurement of the chirp mass of the SGRB progenitor.

Report number: LIGO Document P1300195


arXiv:1205.2142

Generating matter inhomogeneities in general relativity

Authors: Alan Coley, Woei Chet Lim (Submitted on 10 May 2012)

Abstract: In this Letter we discuss a natural general relativistic mechanism that causes inhomogeneities and hence generates matter perturbations in the early universe. We concentrate on spikes, both incomplete spikes and recurring spikes, that naturally occur in the initial oscillatory regime of general cosmological models. In particular, we explicitly show that spikes occurring in a class of G_2 models lead to inhomogeneities that, due to gravitational instability, leave small residual imprints on matter in the form of matter perturbations. The residual matter overdensities from recurring spikes are not local but form on surfaces. We discuss the potential physical consequences of the residual matter imprints and their possible effect on the subsequent formation of large scale structure.

Journal reference: Phys. Rev. Lett. 108, 191101 (2012)


arXiv:1206.0932 Spike Oscillations Authors: J. Mark Heinzle, Claes Uggla, Woei Chet Lim (Submitted on 5 Jun 2012) Abstract: According to Belinskii, Khalatnikov and Lifshitz (BKL), a generic spacelike singularity is characterized by asymptotic locality: Asymptotically, toward the singularity, each spatial point evolves independently from its neighbors, in an oscillatory manner that is represented by a sequence of Bianchi type I and II vacuum models. Recent investigations support a modified conjecture: The formation of spatial structures (`spikes') breaks asymptotic locality. The complete description of a generic spacelike singularity involves spike oscillations, which are described by sequences of Bianchi type I and certain inhomogeneous vacuum models. In this paper we describe how BKL and spike oscillations arise from concatenations of exact solutions in a Hubble-normalized state space setting, suggesting the existence of hidden symmetries and showing that the results of BKL are part of a greater picture.


arXiv:1304.2964

The electromagnetic spike solutions

Authors: Ernesto Nungesser, Woei Chet Lim (Submitted on 10 Apr 2013 (v1), last revised 28 Sep 2013 (this version, v3))

Abstract: The aim of this paper is to use the existing relation between polarized electromagnetic Gowdy spacetimes and vacuum Gowdy spacetimes to find explicit solutions for electromagnetic spikes by a procedure which has been developed by one of the authors for gravitational spikes. We present new inhomogeneous solutions which we call the EME and MEM electromagnetic spike solutions.

Journal reference: Class. Quantum Grav. 30 (2013) 235020


arXiv:1308.0902

Spherically symmetric cosmological spacetimes with dust and radiation - numerical implementation

Authors: Woei Chet Lim, Marco Regis, Chris Clarkson (Submitted on 5 Aug 2013)

Abstract: We present new numerical cosmological solutions of the Einstein Field Equations. The spacetime is spherically symmetric with a source of dust and radiation approximated as a perfect fluid. The dust and radiation are necessarily non-comoving due to the inhomogeneity of the spacetime. Such a model can be used to investigate non-linear general relativistic effects present during decoupling or big-bang nucleosynthesis, as well as for investigating void models of dark energy with isocurvature degrees of freedom. We describe the full evolution of the spacetime as well as the redshift and luminosity distance for a central observer. After demonstrating accuracy of the code, we consider a few example models, and demonstrate the sensitivity of the late time model to the degree of inhomogeneity of the initial radiation contrast.


arXiv:1311.1857

General relativistic density perturbations

Authors: Woei Chet Lim, Alan Coley (Submitted on 8 Nov 2013)

Abstract: We investigate a general relativistic mechanism in which spikes generate matter overdensities in the early universe. When the cosmological fluid is tilted, the tilt provides another mechanism in generating matter inhomogeneities. We numerically investigate the effect of a sign change in the tilt, when there is a spike but the tilt does not change sign, and when the spike and the sign change in the tilt coincide. We find that the tilt plays the primary role in generating matter inhomogeneities, and it does so by creating both local overdensities and underdensities. We discuss of the physical implications of the work.


arXiv:1305.6374

Electrostatic Potential of a Point Charge in a Brans-Dicke Reissner-Nordstrom Field

Authors: Maya Watanabe, A.W.C Lun (Submitted on 28 May 2013 (v1), last revised 12 Aug 2013 (this version, v3))

Abstract: We consider the Brans-Dicke Reissner-Nordstrom spacetime in isotropic coordinates and the electrostatic field of an electric point charge placed outside its surface of inversion. We treat the static electric point charge as a linear perturbation on the Brans-Dicke Reissner-Nordstrom background. We develop a method based upon the Copson method to convert the governing Maxwell equation on the electrostatic potential generated by the static electric point charge into a solvable linear second order ordinary differential equation. We obtain a closed form fundamental solution of the curved space Laplace equation arising from the background metric, which is shown to be regular everywhere except at the point charge and its image point inside the surface of inversion. We also develop a method that demonstrates that the solution does not contain any other charge that may creep into the region that lies beyond the surface of inversion and which is not covered by the isotropic coordinates. The Brans-Dicke Reissner-Nordstrom spacetime therefore is linearly stable under electrostatic perturbations. This stability result includes the three degenerate cases of the fundamental solution that correspond to the Brans Type 1, the Reissner-Nordstrom and the Schwarzschild background spacetimes.

Journal reference: Phys. Rev. D 88, 045007 (2013)


arXiv:1107.4627

Isotropic extensions of the vacuum solutions in general relativity

Authors: C. Molina, Prado Martín-Moruno, Pedro F. González-Díaz (Submitted on 22 Jul 2011 (v1), last revised 29 Oct 2011 (this version, v2))

Abstract: In this work, we obtain isotropic extensions of the usual spherically symmetric vacuum geometries in general relativity. Exact and perturbative solutions are derived. The classes of geometries obtained include black holes in compact and noncompact universes, wormholes in the interior region of cosmological horizons, and anti-de Sitter geometries with excess/deficit solid angle. The tools developed here are applicable in more general contexts.

Journal reference: Phys. Rev. D 84, 104013 (2011)


arXiv:1112.5253

Generic thin-shell gravastars

Authors: Prado Martin-Moruno (Victoria University of Wellington), Nadiezhda Montelongo Garcia (CINVESTAV, IPN, Universidade de Lisboa), Francisco S. N. Lobo (Universidade de Lisboa), Matt Visser (Victoria University of Wellington)

(Submitted on 22 Dec 2011 (v1), last revised 27 Mar 2012 (this version, v2))

Abstract: We construct generic spherically symmetric thin-shell gravastars by using the cut-and-paste procedure. We take considerable effort to make the analysis as general and unified as practicable; investigating both the internal physics of the transition layer and its interaction with "external forces" arising due to interactions between the transition layer and the bulk spacetime. Furthermore, we discuss both the dynamic and static situations. In particular, we consider "bounded excursion" dynamical configurations, and probe the stability of static configurations. For gravastars there is always a particularly compelling configuration in which the surface energy density is zero, while surface tension is nonzero.

Journal reference: JCAP 03 (2012) 034


arXiv:1201.6091

Could a foliation by constant mean curvature hypersurfaces cover the existence of most observers in our part of spacetime?

Authors: Prado Martin-Moruno (Submitted on 29 Jan 2012)

Abstract: We present a foliation by constant mean curvature hypersurfaces of a de Sitter space with a thin-wall Coleman-De Luccia bubble of de Sitter space inside, which covers the existence of most observers in our part of spacetime if we are placed in the region outside the bubble.

Journal reference: AIP Conf.Proc. 1458 (2011) 471-474


arXiv:1205.2158

Massive gravity from bimetric gravity

Authors: Valentina Baccetti (Victoria University of Wellington), Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 10 May 2012 (v1), last revised 11 Nov 2012 (this version, v3))

Abstract: We discuss the subtle relationship between massive gravity and bimetric gravity, focusing particularly on the manner in which massive gravity may be viewed as a suitable limit of bimetric gravity. The limiting procedure is more delicate than currently appreciated. Specifically, this limiting procedure should not unnecessarily constrain the background metric, which must be externally specified by the theory of massive gravity itself. The fact that in bimetric theories one always has two sets of metric equations of motion continues to have an effect even in the massive gravity limit, leading to additional constraints besides the one set of equations of motion naively expected. Thus, since solutions of bimetric gravity in the limit of vanishing kinetic term are also solutions of massive gravity, but the contrary statement is not necessarily true, there is not complete continuity in the parameter space of the theory. In particular, we study the massive cosmological solutions which are continuous in the parameter space, showing that many interesting cosmologies belong to this class.

Journal reference: Class. Quantum Grav. 30 (2013) 015004


arXiv:1206.3814

Null Energy Condition violations in bimetric gravity

Authors: Valentina Baccetti (Victoria University of Wellington), Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 18 Jun 2012 (v1), last revised 27 Jun 2012 (this version, v2))

Abstract: We consider the effective stress-energy tensors for the foreground and background sectors in ghost-free bimetric gravity. By considering the symmetries of the theory, we show that the foreground and background null energy conditions (NECs) are strongly anti-correlated. In particular, the NECs can only be simultaneously fulfilled when they saturate, corresponding to foreground and background cosmological constants. In all other situations, either the foreground or the background is subject to a NEC-violating contribution to the total stress-energy.

Journal reference: JHEP 1208 (2012) 148


arXiv:1206.4720

Gordon and Kerr-Schild ansatze in massive and bimetric gravity

Authors: Valentina Baccetti (Victoria University of Wellington), Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 20 Jun 2012)

Abstract: We develop the "generalized Gordon ansatz" for the ghost-free versions of both massive and bimetric gravity, an ansatz which is general enough to include almost all spacetimes commonly considered to be physically interesting, and restricted enough to greatly simplify calculations. The ansatz allows explicit calculation of the matrix square root gamma = sqrt{g^{-1} f} appearing as a central feature of the ghost-free analysis. In particular, this ansatz automatically allows us to write the effective stress-energy tensor as that corresponding to a perfect fluid. A qualitatively similar "generalized Kerr-Schild ansatz" can also be easily considered, now leading to an effective stress-energy tensor that corresponds to a null fluid. Cosmological implications are considered, as are consequences for black hole physics. Finally we have a few words to say concerning the null energy condition in the framework provided by these ansatze.

Journal reference: JHEP 1208 (2012) 108


arXiv:1211.0214 Bounces, turnarounds and singularities in bimetric gravity Authors: Salvatore Capozziello, Prado Martin-Moruno (Submitted on 1 Nov 2012 (v1), last revised 18 Jan 2013 (this version, v2)) Abstract: In this letter, we consider cosmological solutions of bimetric theory without assuming that only one metric is coupled to gravity. We conclude that any cosmology can be described by fixing the matter content of the space that we are not inhabiting. On the other hand, we show that some conclusions can still be extracted independently of the matter content filling both spaces. In particular, we can conclude the occurrence of some extremality events in one universe if we know that they take place in the other space.


arXiv:1211.0605

Linearised stability analysis of generic thin shells

Authors: Francisco S.N. Lobo, Prado Martin-Moruno, Nadiezhda Montelongo Garcia, Matt Visser (Submitted on 3 Nov 2012)

Abstract: We construct generic spherically symmetric thin shells by using the cut-and-paste procedure. We take considerable effort to make the analysis as general and unified as practicable; investigating both the internal physics of the transition layer and its interaction with "external forces" arising due to interactions between the transition layer and the bulk spacetime. We demonstrate in full generality that stability of the thin shell is equivalent to choosing suitable properties for the material residing on the junction interface. Applications to gravastars and wormhole geometries are also explored.


arXiv:1301.2334

Is there vacuum when there is mass? Vacuum and non-vacuum solutions for massive gravity

Authors: Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 10 Jan 2013 (v1), last revised 27 Jun 2013 (this version, v3))

Abstract: Massive gravity is a theory which has a tremendous amount of freedom to describe different cosmologies; but at the same time the various solutions one encounters must fulfill some rather nontrivial constraints. Most of the freedom comes not from the Lagrangian, which contains only a small number of free parameters (typically 3 depending on counting conventions), but from the fact that one is in principle free to choose the background reference metric almost arbitrarily --- which effectively introduces a non-denumerable infinity of free parameters. In the current paper we stress that although changing the reference metric would lead to a different cosmological model, this does not mean that the dynamics of the universe can be entirely divorced from its matter content. That is, while the choice of reference metric certainly influences the evolution of the physically observable foreground metric, the effect of matter cannot be neglected. Nevertheless, the relation between matter and geometry can be significantly changed in some specific models; effectively since the graviton would be able to curve the spacetime by itself, without the need of matter. Thus, even the set of vacuum solutions for massive gravity can have significant structure. On the other hand, in some cases the effect of the reference metric could be so strong that no conceivable material content would be able to drastically affect the cosmological evolution.

Journal reference: Class.Quant.Grav. 30 (2013) 155021


arXiv:1302.2687

Massive gravity as a limit of bimetric gravity

Authors: Prado Martin-Moruno (Victoria University of Wellington), Valentina Baccetti (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 12 Feb 2013)

Abstract: Massive gravity may be viewed as a suitable limit of bimetric gravity. The limiting procedure can lead to an interesting interplay between the "background" and "foreground" metrics in a cosmological context. The fact that in bimetric theories one always has two sets of metric equations of motion continues to have an effect even in the massive gravity limit. Thus, solutions of bimetric gravity in the limit of vanishing kinetic term are also solutions of massive gravity, but the contrary statement is not necessarily true.


arXiv:1305.1993

Classical and quantum flux energy conditions for quantum vacuum states

Authors: Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 9 May 2013 (v1), last revised 17 Sep 2013 (this version, v3))

Abstract: The classical energy conditions are known to not be fundamental physics -- they are typically violated by semiclassical quantum effects. Consequently, some effort has gone into finding possible semiclassical replacements for the classical energy conditions -- the most well developed being the Ford-Roman quantum inequalities. In the current article we shall instead develop classical and quantum versions of a "flux energy condition" (FEC and QFEC) based on the notion of constraining the possible fluxes measured by timelike observers. The naive classical FEC will be seen to be satisfied in some situations, and even for some quantum vacuum states, while its quantum analogue (the QFEC) is satisfied (for naturally defined quantum vacuum states) under a rather wide range of conditions. The situation for completely general (nonvacuum) quantum states is less clear.

Journal reference: Physical Review D 88 (2013) 061701(R)


arXiv:1306.2076

Semiclassical energy conditions for quantum vacuum states

Authors: Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 10 Jun 2013 (v1), last revised 12 Sep 2013 (this version, v3))

Abstract: We present and develop several nonlinear energy conditions suitable for use in the semiclassical regime. In particular, we consider the recently formulated "flux energy condition" (FEC), and the novel "trace-of-square" (TOSEC) and "determinant" (DETEC) energy conditions. As we shall show, these nonlinear energy conditions behave much better than the classical linear energy conditions in the presence of semiclassical quantum effects. Moreover, whereas the quantum extensions of these nonlinear energy conditions seem to be quite widely satisfied as one enters the quantum realm, analogous quantum extensions are generally not useful for the linear classical energy conditions.

Journal reference: JHEP 1309 (2013) 050


arXiv:1111.5825

The Gravitational Wave International Committee Roadmap: The future of gravitational wave astronomy

Authors: Jay Marx, Karsten Danzmann, James Hough, Kazuaki Kuroda, David McClelland, Benoit Mours, Sterl Phinney, Sheila Rowan, B. Sathyaprakash, Flavio Vetrano, Stefano Vitale, Stan Whitcomb, Clifford Will (for the Gravitational Wave International Committee) (Submitted on 24 Nov 2011)

Abstract: Gravitational wave science is on the verge of direct observation of the waves predicted by Einstein's General Theory of Relativity and opening the exciting new field of gravitational wave astronomy. In the coming decades, ultra-sensitive arrays of ground-based instruments and complementary spaced-based instruments will observe the gravitational wave sky, inevitably discovering entirely unexpected phenomena while providing new insight into many of the most profound astrophysical phenomena known. in July 2007 the Gravitational Wave International Committee (GWIC) initiated the development of a strategic roadmap for the field of gravitational wave science with a 30-year horizon. The goal of this roadmap is to serve the international gravitational wave community and its stakeholders as a tool for the development of capabilities and facilities needed to address the exciting scientific opportunities on the intermediate and long-term horizons.


arXiv:1112.3118

Arm-length stabilisation for interferometric gravitational-wave detectors using frequency-doubled auxiliary lasers

Authors: Adam J. Mullavey, Bram J. J. Slagmolen, John Miller, Matthew Evans, Peter Fritschel, Daniel Sigg, Sam J. Waldman, Daniel A. Shaddock, David E. McClelland (Submitted on 14 Dec 2011)

Abstract: Residual motion of the arm cavity mirrors is expected to prove one of the principal impediments to systematic lock acquisition in advanced gravitational-wave interferometers. We present a technique which overcomes this problem by employing auxiliary lasers at twice the fundamental measurement frequency to pre-stabilise the arm cavities' lengths. Applying this approach, we reduce the apparent length noise of a 1.3 m long, independently suspended Fabry-Perot cavity to 30 pm rms and successfully transfer longitudinal control of the system from the auxiliary laser to the measurement laser.

Journal reference: Optics Express, Vol. 20, Issue 1, pp. 81-89 (2012)


arXiv:1112.1542

Updated gravitational-wave upper limits on the internal magnetic field strength of recycled pulsars

Authors: Alpha Mastrano, Andrew Melatos (Submitted on 7 Dec 2011)

Abstract: Recent calculations of the hydromagnetic deformation of a stratified, non-barotropic neutron star are generalized to describe objects with superconducting interiors, whose magnetic permeability \mu is much smaller than the vacuum value \mu_0. It is found that the star remains oblate if the poloidal magnetic field energy is \gtrsim 40% of total magnetic field energy, that the toroidal field is confined to a torus which shrinks as \mu decreases, and that the deformation is much larger (by a factor \sim \mu_0/\mu) than in a non-superconducting object. The results are applied to the latest direct and indirect upper limits on gravitational-wave emission from Laser Interferometer Gravitational Wave Observatory (LIGO) and radio pulse timing (spin-down) observations of 81 millisecond pulsars, to show how one can use these observations to infer the internal field strength. It is found that the indirect spin-down limits already imply astrophysically interesting constraints on the poloidal-toroidal field ratio and diamagnetic shielding factor (by which accretion reduces the observable external magnetic field, e.g. by burial). These constraints will improve following gravitational-wave detections, with implications for accretion-driven magnetic field evolution in recycled pulsars and the hydromagnetic stability of these objects' interiors.


arXiv:1302.2635

Reanalysis of F-statistic gravitational-wave searches with the higher criticism statistic

Authors: M. F. Bennett, A. Melatos, A. Delaigle, P. Hall (Submitted on 11 Feb 2013)

Abstract: We propose a new method of gravitational wave detection using a modified form of higher criticism, a statistical technique introduced by Donoho & Jin (2004). Higher criticism is designed to detect a group of sparse, weak sources, none of which are strong enough to be reliably estimated or detected individually. We apply higher criticism as a second-pass method to synthetic F-statistic and C-statistic data for a monochromatic periodic source in a binary system and quantify the improvement relative to the first-pass methods. We find that higher criticism on C-statistic data is more sensitive by ~6% than the C-statistic alone under optimal conditions (i.e. binary orbit known exactly) and the relative advantage increases as the error in the orbital parameters increases. Higher criticism is robust even when the source is not monochromatic (e.g. phase wandering in an accreting system). Applying higher criticism to a phase-wandering source over multiple time intervals gives a >30% increase in detectability with few assumptions about the frequency evolution. By contrast, in all-sky searches for unknown periodic sources, which are dominated by the brightest source, second-pass higher criticism does not provide any benefits over a first pass search.


arXiv:1311.1379

Implementation of the frequency-modulated sideband search method for gravitational waves from low mass X-ray binaries

Authors: Letizia Sammut, Christopher Messenger, Andrew Melatos, Benjamin J. Owen (Submitted on 6 Nov 2013)

Abstract: We describe the practical implementation of the sideband search, a search for periodic gravitational waves from neutron stars in binary systems. The orbital motion of the source in its binary system causes frequency-modulation in the combination of matched filters known as the $\mathcal{F}$-statistic. The sideband search is based on the incoherent summation of these frequency-modulated $\mathcal{F}$-statistic sidebands. It provides a new detection statistic for sources in binary systems, called the $\mathcal{C}$-statistic. The search is well suited to low-mass X-ray binaries, the brightest of which, called Sco X-1, is an ideal target candidate. For sources like Sco X-1, with well constrained orbital parameters, a slight variation on the search is possible. The extra orbital information can be used to approximately demodulate the data from the binary orbital motion in the coherent stage, before incoherently summing the now reduced number of sidebands. We investigate this approach and show that it improves the sensitivity of the standard Sco X-1 directed sideband search. Prior information on the neutron star inclination and gravitational wave polarization can also be used to improve upper limit sensitivity. We estimate the sensitivity of a Sco X-1 directed sideband search on 10 days of LIGO data and show that it can beat previous upper limits in current LIGO data, with a possibility of constraining theoretical upper limits using future advanced instruments.


arXiv:1211.3410

Cosmology of a Friedmann-Lamaître-Robertson-Walker 3-brane, Late-Time Cosmic Acceleration, and the Cosmic Coincidence

Authors: Ciaran Doolin (Canterbury University), Ishwaree P. Neupane (Canterbury University, Tribhuvan University, and CERN) (Submitted on 14 Nov 2012 (v1), last revised 7 Mar 2013 (this version, v2))

Abstract: A late epoch cosmic acceleration may be naturally entangled with cosmic coincidence -- the observation that at the onset of acceleration the vacuum energy density fraction nearly coincides with the matter density fraction. In this Letter we show that this is indeed the case with the cosmology of a Friedmann-Lama\^itre-Robertson-Walker (FLRW) 3-brane in a five-dimensional anti-de Sitter spacetime. We derive the four-dimensional effective action on a FLRW 3-brane, which helps define a general reduction formula, namely, $M_P^{2}=\rho_{b}/|\Lambda_5|$, where $M_{P}$ is the effective Planck mass, $\Lambda_5$ is the 5-dimensional cosmological constant, and $\rho_b$ is the sum of the 3-brane tension $V$ and the matter density $\rho$. The behavior of the background solution is consistent with the results based on the form of the 4D effective potential. Although the range of variation in $\rho_{b}$ is strongly constrained, the big bang nucleosynthesis bound on the time variation of the renormalized Newton constant $G_N = (8\pi M_P^2)^{-1}$ is satisfied when the ratio $V/\rho \gtrsim {O} (10^2)$ on cosmological scales. The same bound leads to an effective equation of state close to -1 at late epochs in accordance with current astrophysical and cosmological observations.

Journal Reference: PhysRevLett.110.141301


arXiv:1307.6278

Lagrange coordinates for the Einstein-Euler equations

Authors: Todd A. Oliynyk (Submitted on 24 Jul 2013)

Abstract: We derive a new symmetric hyperbolic formulation of the Einstein-Euler equations in Lagrange coordinates that are adapted to the Frauendiener-Walton formulation of the Euler equations. As an application, we use this system to show that the densitized lapse and zero shift coordinate systems for the vacuum Einstein equations are equivalent to Lagrange coordinates for a fictitious fluid with a specific equation of state.

Journal reference: Phys. Rev. D 85, 044019 (2012)


arXiv:1307.6281

On the Cosmological Newtonian limit

Authors: Todd A. Oliynyk (Submitted on 24 Jul 2013)

Abstract: We describe a wide class of inhomogeneous relativistic solutions that are well approximated on cosmological scales by solutions of Newtonian gravity. Error estimates measuring the difference between the Newtonian and relativistic solutions are given. The solutions presented here unambiguously demonstrate that it is possible for Newtonian gravity to provide a good approximation to General Relativity on cosmological scales.


arXiv:1108.0211

Conformal time in a black-hole universe with torsion

Authors: Nikodem J. Poplawski (Submitted on 31 Jul 2011)

Abstract: In the Einstein-Cartan-Sciama-Kibble theory of gravity, the intrinsic spin of fermionic matter generates spacetime torsion and induces gravitational repulsion at extremely high densities. This repulsion prevents the collapsing spin-fluid matter in a black hole from forming a singularity. Instead, the interior of a black hole with a stiff equation of state becomes a new universe, which contracts until a (big) bounce and then expands. We derive the equations describing the dynamics of our Universe, formed in such a scenario, in terms of the conformal time which is a convenient variable for testing signatures of the contracting phase in the Cosmic Microwave Background radiation.


arXiv:1108.6100

A Maxwell field minimally coupled to torsion

Authors: Nikodem J. Poplawski (Submitted on 31 Aug 2011)

Abstract: We consider the Lagrangian density for a free Maxwell field, in which the electromagnetic field tensor minimally couples to the affine connection, in the Einstein-Cartan-Sciama-Kibble theory of gravity. We derive the formulae for the torsion and electromagnetic field tensors in terms of the electromagnetic potential. The divergence of the magnetic field does not vanish: the photon-torsion coupling acts like an effective magnetic monopole density. Such a coupling, which breaks U(1) gauge invariance, is significant only at extremely high energies existing in the very early Universe or inside black holes. It may, however, provide a mechanism for Dirac's quantization of electric charge.


arXiv:1110.0051

On the polarization of nonlinear gravitational waves

Authors: Nikodem J. Poplawski (Submitted on 30 Sep 2011)

Abstract: We derive a relation between the two polarization modes of a plane, linear gravitational wave in the second-order approximation. Since these two polarizations are not independent, an initially monochromatic gravitational wave loses its periodic character due to the nonlinearity of the Einstein field equations. Accordingly, real gravitational waves may differ from solutions of the linearized field equations, which are being assumed in gravitational-wave detectors.


arXiv:1110.5019

Mass of the universe in a black hole

Authors: Nikodem J. Poplawski (Submitted on 23 Oct 2011)

Abstract: If spacetime torsion couples to the intrinsic spin of matter according to the Einstein-Cartan-Sciama-Kibble theory of gravity, then the resulting gravitational repulsion at supranuclear densities prevents the formation of singularities in black holes. Consequently, the interior of every black hole becomes a new universe that expands from a nonsingular bounce. We consider gravitational collapse of fermionic spin-fluid matter with the stiff equation of state in a stellar black hole. Such a collapse increases the mass of the matter, which occurs through the Parker-Zel'dovich-Starobinskii quantum particle production in strong, anisotropic gravitational fields. The subsequent pair annihilation changes the stiff matter into an ultrarelativistic fluid. We show that the universe in a black hole of mass $M_\textrm{BH}$ at the bounce has a mass $M_\textrm{b}\sim M^2_\textrm{BH} m^{1/2}_\textrm{n}/m^{3/2}_\textrm{Pl}$, where $m_\textrm{n}$ is the mass of a neutron and $m_\textrm{Pl}$ is the reduced Planck mass. For a typical stellar black hole, $M_\textrm{b}$ is about $10^{32}$ solar masses, which is $10^6$ larger than the mass of our Universe. As the relativistic black-hole universe expands, its mass decreases until the universe becomes dominated by nonrelativistic heavy particles.


arXiv:1111.4595

Nonsingular, big-bounce cosmology from spinor-torsion coupling

Authors: Nikodem Poplawski (Submitted on 20 Nov 2011 (v1), last revised 4 Jul 2012 (this version, v2))

Abstract: The Einstein-Cartan-Sciama-Kibble theory of gravity removes the constraint of general relativity that the affine connection be symmetric by regarding its antisymmetric part, the torsion tensor, as a dynamical variable. The minimal coupling between the torsion tensor and Dirac spinors generates a spin-spin interaction which is significant in fermionic matter at extremely high densities. We show that such an interaction averts the unphysical big-bang singularity, replacing it with a cusp-like bounce at a finite minimum scale factor, before which the Universe was contracting. This scenario also explains why the present Universe at largest scales appears spatially flat, homogeneous and isotropic.

Journal reference: Phys. Rev. D 85, 107502 (2012)


arXiv:1201.0316

Thermal fluctuations in Einstein-Cartan-Sciama-Kibble-Dirac bouncing cosmology

Authors: Nikodem J. Poplawski (Submitted on 31 Dec 2011)

Abstract: We study cosmological perturbations arising from thermal fluctuations in the big-bounce cosmology in the Einstein-Cartan-Sciama-Kibble theory of gravity. We show that such perturbations cannot have a scale-invariant spectrum if fermionic matter minimally coupled to the torsion tensor is macroscopically averaged as a spin fluid, but have a scale-invariant spectrum if the Dirac form of the spin tensor of the fermionic matter is used.


arXiv:1203.0294

Affine theory of gravitation

Authors: Nikodem Poplawski (Submitted on 1 Mar 2012 (v1), last revised 3 Aug 2012 (this version, v2))

Abstract: We propose a new theory of gravitation, in which the affine connection is the only dynamical variable describing the gravitational field. We construct the simplest dynamical Lagrangian density that is entirely composed from the connection, via its curvature and torsion, and is an algebraic function of its derivatives. It is given by the contraction of the Ricci tensor with a tensor which is inverse to the symmetric, contracted square of the torsion tensor, $k_{\mu\nu}=S^\rho_{\lambda\mu}S^\lambda_{\rho\nu}$. We vary the total action for the gravitational field and matter with respect to the affine connection, assuming that the matter fields couple to the connection only through $k_{\mu\nu}$. We derive the resulting field equations and show that they are identical with the Einstein equations of general relativity with a nonzero cosmological constant, if the tensor $k_{\mu\nu}$ is regarded as the metric tensor. The cosmological constant is simply a constant of proportionality between the two tensors, which together with $c$ and $G$ provides a natural system of units in gravitational physics. This theory therefore provides a physically valid construction of the metric as an algebraic function of the connection, and naturally explains dark energy as an intrinsic property of spacetime. The observed accelerating expansion of the Universe may be the strongest evidence for torsion.


arXiv:1209.5772

Gravity with spin excludes fermionic strings

Authors: Nikodem Poplawski (Submitted on 25 Sep 2012)

Abstract: The existence of intrinsic spin of matter requires the metric-affine formulation of gravity, in which the affine connection is not constrained to be symmetric and its antisymmetric part (torsion tensor) is a dynamical variable. We show that the cyclic identity for the curvature tensor in the metric-affine formulation forbids fermions represented by Dirac spinors to form point or string configurations. Consequently, fermionic strings contradict the gravitational field equations in the presence of spin. Superstring theory is therefore incorrect.


arXiv:1304.0047

Intrinsic spin requires gravity with torsion and curvature

Authors: Nikodem Poplawski (Submitted on 29 Mar 2013)

Abstract: We show that the intrinsic angular momentum of matter in curved spacetime requires the metric-affine formulation of gravity, in which the antisymmetric part of the affine connection (the torsion tensor) is not constrained to be zero but is a variable in the principle of stationary action. Regarding the tetrad and spin connection (or the metric and torsion tensors) as independent variables gives the correct generalization of the conservation law for the total (orbital plus intrinsic) angular momentum to the presence of the gravitational field. The metric-affine formulation extends general relativity to the simplest theory of gravity with intrinsic spin: the Einstein-Cartan-Sciama-Kibble theory. We also show that teleparallel gravity, which constrains the connection by setting the curvature tensor to zero, is inconsistent with the conservation of the total angular momentum.


arXiv:1305.6977

Energy and momentum of the Universe

Authors: Nikodem Poplawski (Submitted on 29 May 2013)

Abstract: The Einstein-Cartan-Sciama-Kibble theory of gravity naturally extends general relativity to include quantum-mechanical, intrinsic angular momentum of matter by equipping spacetime with torsion. Using the Einstein energy-momentum pseudotensor for the gravitational field in this theory, we show that the energy and momentum of the closed Universe are equal to zero. Since the positive energy from mass and motion of the observed matter in the Universe exceeds in magnitude the negative energy from gravity, the Universe must contain another form of matter whose energy is negative. This form, which cannot be composed from particles, may be the observed dark matter.


arXiv:1310.8014

Schwinger's principle in Einstein-Cartan gravity

Authors: Nikodem Poplawski (Submitted on 30 Oct 2013)

Abstract: By applying Schwinger's variational principle to the Einstein-Cartan action for the gravitational field, we derive quantum commutation relations between the metric and torsion tensors.


arXiv:1109.3925

Interacting classical and quantum particles

Authors: Alvin J. K. Chua, Michael J. W. Hall, C. M. Savage (Submitted on 19 Sep 2011)

Abstract: We apply Hall and Reginatto's theory of interacting classical and quantum ensembles to harmonically coupled particles, with a view to understanding its experimental implications. This hybrid theory has no free parameters and makes distinctive predictions that should allow it to be experimentally distinguished from quantum mechanics. It also bears on the questions of quantum measurement and quantum gravity.

Journal reference: Phys. Rev. A 85, 022110 (2011)


arXiv:1207.5305

Nonlocal signaling in the configuration space model of quantum-classical interactions

Authors: Michael J. W. Hall, Marcel Reginatto, C. M. Savage (Submitted on 23 Jul 2012)

Abstract: When interactions are turned off, the theory of interacting quantum and classical ensembles due to Hall and Reginatto is shown to suffer from a nonlocal signaling effect that is effectively action at a distance. This limits the possible applicability of the theory. In its present form, it is restricted to those situations in which interactions are always on, such as classical gravity interacting with quantized matter.

Journal reference: Physical Review A 86, 054101 (2012)


arXiv:1211.6156

Conformal Structures Admitted by a Class of FRW Cosmologies

Authors: Philip Threlfall, Susan M. Scott (Submitted on 26 Nov 2012 (v1), last revised 16 Dec 2012 (this version, v2))

Abstract: In this paper we demonstrate that there are large classes of Friedmann-Robertson-Walker (FRW) cosmologies that admit isotropic conformal structures of Quiescent Cosmology. FRW models have long been known to admit singularities such as Big Bangs and Big Crunches [1, 2] but recently it has been shown that there are other cosmological structures that these solutions contain. These structures are Big Rips, Sudden Singularities and Extremality Events [1, 2]. Within the Quiescent Cosmology framework [3] there also exist structures consistent with a cosmological singularity known as the Isotropic Past Singularity (IPS) [4, 5]. There also exists a cosmological final state known as a Future Isotropic Universe (FIU) [4], which strictly speaking, doesn't fit with the fundamental ideals of Quiescent Cosmology. In this paper, we compare the cosmological events of a large class of FRW solutions to the conformal structures of Quiescent Cosmology [4]. In the first section of this paper we present the relevant background information and our motivation. In the second section of this paper we construct conformal relationships for relevant FRW models. The third section contains a thorough discussion of a class of FRW solutions that cannot represent any of the previously constructed isotropic conformal structures from Quiescent Cosmology. The final section contains our remarks and future outlook for further study of this field.


arXiv:1211.6157

The Monotonicity of the Gravitational Entropy Scalar within Quiescent Cosmology

Authors: Philip Threlfall, Susan M. Scott (Submitted on 26 Nov 2012 (v1), last revised 17 Dec 2012 (this version, v2))

Abstract: In this paper we show that Quiescent Cosmology [1, 2, 3] is consistent with Penrose's Weyl Curvature Hypothesis and the notion of gravitational entropy [4]. Gravitational entropy, from a conceptual point of view, acts in an opposite fashion to the more familiar notion of entropy. A closed system of gravitating particles will coalesce whereas a collection of gas particles will tend to diffuse; regarding increasing entropy, these two scenarios are identical. What has been shown previously [2, 3] is that gravitational entropy at the initial singularity predicted by Quiescent Cosmology - the Isotropic Past Singularity (IPS) - tends to zero. The results from this paper show that not only is this the case but that gravitational entropy increases as this singularity evolves.

In the first section of this paper we present relevant background information and motivation. In the second section of this paper we present the main results of this paper. Our third section contains a discussion of how this result will inspire future research before we make concluding remarks in our final section.


arXiv:1107.2665

Toward Early-Warning Detection of Gravitational Waves from Compact Binary Coalescence

Authors: Kipp Cannon, Romain Cariou, Adrian Chapman, Mireia Crispin-Ortuzar, Nickolas Fotopoulos, Melissa Frei, Chad Hanna, Erin Kara, Drew Keppel, Laura Liao, Stephen Privitera, Antony Searle, Leo Singer, Alan Weinstein (Submitted on 13 Jul 2011 (v1), last revised 4 Apr 2012 (this version, v3))

Abstract: Rapid detection of compact binary coalescence (CBC) with a network of advanced gravitational-wave detectors will offer a unique opportunity for multi-messenger astronomy. Prompt detection alerts for the astronomical community might make it possible to observe the onset of electromagnetic emission from (CBC). We demonstrate a computationally practical filtering strategy that could produce early-warning triggers before gravitational radiation from the final merger has arrived at the detectors.

Journal reference: 2012 ApJ 748 136


arXiv:1110.2815

DC readout experiment in Enhanced LIGO

Authors: Tobin T. Fricke, Nicolas D. Smith-Lefebvre, Richard Abbott, Rana Adhikari, Katherine L. Dooley, Matthew Evans, Peter Fritschel, Valery V. Frolov, Keita Kawabe, Jeffrey S. Kissel, Bram J. J. Slagmolen, Sam J. Waldman (Submitted on 13 Oct 2011 (v1), last revised 10 Feb 2012 (this version, v2))

Abstract: The two 4 km long gravitational wave detectors operated by the Laser Interferometer Gravitational-wave Observatory (LIGO) were modified in 2008 to read out the gravitational wave channel using the DC readout form of homodyne detection and to include an optical filter cavity at the output of the detector. As part of the upgrade to Enhanced LIGO, these modifications replaced the radio-frequency (RF) heterodyne system used previously. We describe the motivations for and the implementation of DC readout and the output mode cleaner in Enhanced LIGO. We present characterizations of the system, including measurements and models of the couplings of the noises from the laser source to the gravitational wave readout channel. We show that noise couplings using DC readout are improved over those for RF readout, and we find that the achieved shot-noise-limited sensitivity is consistent with modeled results.

Journal reference: Class. Quantum Grav. 29 (2012) 065005


arXiv:1308.2074

Low-Frequency Terrestrial Gravitational-Wave Detectors

Authors: Jan Harms, Bram J. J. Slagmolen, Rana X. Adhikari, M. Coleman Miller, Matthew Evans, Yanbei Chen, Holger Müller, Masaki Ando (Submitted on 9 Aug 2013)

Abstract: Direct detection of gravitational radiation in the audio band is being pursued with a network of kilometer-scale interferometers (LIGO, Virgo, KAGRA). Several space missions (LISA, DECIGO, BBO) have been proposed to search for sub-Hz radiation from massive astrophysical sources. Here we examine the potential sensitivity of three ground-based detector concepts aimed at radiation in the 0.1 -- 10\,Hz band. We describe the plethora of potential astrophysical sources in this band and make estimates for their event rates and thereby, the sensitivity requirements for these detectors. The scientific payoff from measuring astrophysical gravitational waves in this frequency band is great. Although we find no fundamental limits to the detector sensitivity in this band, the remaining technical limits will be extremely challenging to overcome.


arXiv:1107.5596

Gamma ray burst distances and the timescape cosmology

Authors: Peter R. Smale (Submitted on 27 Jul 2011 (v1), last revised 22 Aug 2011 (this version, v2))

Abstract: Gamma ray bursts can potentially be used as distance indicators, providing the possibility of extending the Hubble diagram to redshifts ~7. Here we follow the analysis of Schaefer (2007), with the aim of distinguishing the timescape cosmological model from the \LambdaCDM model by means of the additional leverage provided by GRBs in the range 2 < z < 7. We find that the timescape model fits the GRB sample slightly better than the \LambdaCDM model, but that the systematic uncertainties are still too little understood to distinguish the models.

Journal reference: Mon. Not. R. Astron. Soc. 418 (2011) 2779-2784


arXiv:1201.5371

Hubble flow variance and the cosmic rest frame

Authors: David L. Wiltshire, Peter R. Smale, Teppo Mattsson, Richard Watkins (Submitted on 25 Jan 2012 (v1), last revised 25 Oct 2013 (this version, v5))

Abstract: We characterize the radial and angular variance of the Hubble flow in the COMPOSITE sample of 4534 galaxies, on scales in which much of the flow is in the nonlinear regime. With no cosmological assumptions other than the existence of a suitably averaged linear Hubble law, we find with decisive Bayesian evidence (ln B >> 5) that the Hubble constant averaged in independent spherical radial shells is closer to its asymptotic value when referred to the rest frame of the Local Group, rather than the standard rest frame of the Cosmic Microwave Background. An exception occurs for radial shells in the range 40/h-60/h Mpc. Angular averages reveal a dipole structure in the Hubble flow, whose amplitude changes markedly over the range 32/h-62/h Mpc. Whereas the LG frame dipole is initially constant and then decreases significantly, the CMB frame dipole initially decreases but then increases. The map of angular Hubble flow variation in the LG rest frame is found to coincide with that of the residual CMB temperature dipole, with correlation coefficient -0.92. These results are difficult to reconcile with the standard kinematic interpretation of the motion of the Local Group in response to the clustering dipole, but are consistent with a foreground non-kinematic anisotropy in the distance-redshift relation of 0.5% on scales up to 65/h Mpc. Effectively, the differential expansion of space produced by nearby nonlinear structures of local voids and denser walls and filaments cannot be reduced to a local boost. This hypothesis suggests a reinterpretation of bulk flows, which may potentially impact on calibration of supernovae distances, anomalies associated with large angles in the CMB anisotropy spectrum, and the dark flow inferred from the kinematic Sunyaev-Zel'dovich effect. It is consistent with recent studies that find evidence for a non-kinematic dipole in the distribution of distant radio sources.

Journal reference: Phys. Rev. D 88, 083529 (2013)


arXiv:1109.4197

The general homothetic equations

Authors: John D. Steele (Submitted on 20 Sep 2011)

Abstract: In an earlier paper [6] the author wrote the homothetic equations for vacuum solutions in a first order formalism allowing for arbitrary alignment of the dyad. This paper generalises that method to homothetic equations in non-vacuum spaces and also provides useful second integrability conditions. An application to the well-known Petrov type O pure radiation solutions is given.


arXiv:1212.1179

Conformal vectors in general space-times

Authors: John D. Steele (Submitted on 5 Dec 2012)

Abstract: In an earlier paper (Class. Quantum Grav. 19 (2002) p.259) the author wrote the homothetic equations for vacuum solutions in a first order formalism allowing for arbitrary alignment of the dyad. This paper generalises that method to conformal vectors in non-vacuum spaces. The method is applied to metrics admitting a three parameter motion group on non-null orbits.


arXiv:1107.0763

A new AF gravitational instanton

Authors: Yu Chen, Edward Teo (Submitted on 5 Jul 2011)

Abstract: It has long been conjectured that the Euclidean Schwarzschild and Euclidean Kerr instantons are the only non-trivial asymptotically flat (AF) gravitational instantons. In this letter, we show that this conjecture is false by explicitly constructing a new two-parameter AF gravitational instanton with a U(1)xU(1) isometry group, using the inverse-scattering method. It has Euler number \chi=3 and Hirzebruch signature \tau=1, and its global topology is CP^2 with a circle S^1 removed appropriately. Various other properties of this gravitational instanton are also discussed.

Journal reference: Phys.Lett.B703:359-362,2011


arXiv:1108.1849

Unbalanced Pomeransky-Sen'kov black ring

Authors: Yu Chen, Kenneth Hong, Edward Teo (Submitted on 9 Aug 2011 (v1), last revised 20 Sep 2011 (this version, v2))

Abstract: The Pomeransky-Sen'kov solution is well known to describe an asymptotically flat doubly rotating black ring in five dimensions, whose self-gravity is exactly balanced by the centrifugal force arising from the rotation in the ring direction. In this paper, we generalise this solution to the unbalanced case, in which there is in general a conical singularity in the space-time. Unlike a previous form of this solution presented in the literature, our form is much more compact. We describe in detail how this solution can be derived using the inverse-scattering method, and study its various properties. In particular, we show how various known limits can be recovered as special cases of this solution.

Journal Reference: PhysRevD.84.084030


arXiv:1204.3116

Rotating black rings on Taub-NUT

Authors: Yu Chen, Edward Teo (Submitted on 13 Apr 2012)

Abstract: In this paper, we construct new solutions describing rotating black rings on Taub-NUT using the inverse-scattering method. These are five-dimensional vacuum space-times, generalising the Emparan-Reall and extremal Pomeransky-Sen'kov black rings to a Taub-NUT background space. When reduced to four dimensions in Kaluza-Klein theory, these solutions describe (possibly rotating) electrically charged black holes in superposition with a finitely separated magnetic monopole. Various properties of these solutions are studied, from both a five- and four-dimensional perspective.


arXiv:1204.5785

A doubly rotating black ring with dipole charge

Authors: Yu Chen, Kenneth Hong, Edward Teo (Submitted on 25 Apr 2012)

Abstract: We present a dipole-charged generalisation of the Pomeransky-Sen'kov black ring in five-dimensional Kaluza-Klein theory. It rotates in two independent directions, although one of the rotations has been tuned to achieve balance, so that the space-time does not contain any conical singularities. This solution was constructed using the inverse-scattering method in six-dimensional vacuum gravity. We then study various physical properties of this solution, with particular emphasis on the new features that the dipole charge introduces.


arXiv:1208.0415

Balanced electric-magnetic dihole in Kaluza-Klein theory

Authors: Yu Chen, Edward Teo (Submitted on 2 Aug 2012 (v1), last revised 3 Sep 2012 (this version, v2))

Abstract: We present a four-dimensional double-black-hole (or dihole) solution in Kaluza-Klein theory, describing a superposition of an electrically charged and a magnetically charged black hole. This system can be balanced for appropriately chosen parameters, and the resulting space-time is completely regular on and outside the event horizons. This solution was constructed using the inverse-scattering method in five-dimensional vacuum gravity, in which it describes a rotating black ring surrounding a static black hole on a Taub-NUT background space. Various properties of this solution are studied, from both a four- and five-dimensional perspective.


arXiv:1309.2054

Generalised Planar Black Holes and the Holography of Hydrodynamic Shear

Authors: Brett McInnes, Edward Teo (Submitted on 9 Sep 2013 (v1), last revised 19 Nov 2013 (this version, v2))

Abstract: AdS black holes with planar event horizon topology play a central role in AdS/CFT holography, and particularly in its applications. Generalisations of the known planar black holes can be found by considering the Plebanski--Demianski metrics, a very general family of exactly specified solutions of the Einstein equations. These generalised planar black holes may be useful in applications. We give a concrete example of this in the context of the holographic description of the Quark-Gluon Plasma (QGP). We argue that our generalised planar black holes allow us to construct a model of the internal shearing motion generated when the QGP is produced in peripheral heavy-ion collisions. When embedded in string theory, the bulk physics is in fact unstable. We find however that this instability may develop too slowly to affect the evolution of the plasma, except possibly for high values of the quark chemical potential, such as will be studied in experimental scans of the quark matter phase diagram.


arXiv:1108.5414

Testing Lorentz Invariance Using an Odd-Parity Asymmetric Optical Resonator

Authors: Fred Baynes, Andre Luiten, Michael Tobar (Submitted on 27 Aug 2011)

Abstract: We present the first experimental test of Lorentz invariance using the frequency difference between counter-propagating modes in an asymmetric odd-parity optical resonator. This type of test is $\sim10^{4}$ more sensitive to odd-parity and isotropic (scalar) violations of Lorentz invariance than equivalent conventional even-parity experiments due to the asymmetry of the optical resonator. The disadvantages of odd parity resonators have been negated by the use of counter-propagating modes, delivering a high level of immunity to environmental fluctuations. With a non-rotating experiment our result limits the isotropic Lorentz violating parameter $\tilde{\kappa}_{tr}$ to 3.4 $\pm$ 6.2 x $10^{-9}$, the best reported constraint from direct measurements. Using this technique the bounds on odd-parity and scalar violations of Lorentz invariance can be improved by many orders of magnitude.


arXiv:1306.1571

Testing local position and fundamental constant invariance due to periodic gravitation and boost using long-term comparison of the SYRTE atomic fountains and H-masers

Authors: M. E. Tobar, P. L. Stanwix, J.J. McFerran, J. Guéna, M. Abgrall, S. Bize, A. Clairon, Ph. Laurent, P. Rosenbusch, D. Rovera, G. Santarelli (Submitted on 6 Jun 2013)

Abstract: The frequencies of three separate Cs fountain clocks and one Rb fountain clock have been compared to various hydrogen masers to search for periodic changes correlated with the changing solar gravitational potential at the Earth and boost with respect to the Cosmic Microwave Background (CMB) rest frame. The data sets span over more than eight years. The main sources of long-term noise in such experiments are the offsets and linear drifts associated with the various H-masers. The drift can vary from nearly immeasurable to as high as 1.3*10^-15 per day. To circumvent these effects we apply a numerical derivative to the data, which significantly reduces the standard error when searching for periodic signals. We determine a standard error for the putative Local Position Invariance (LPI) coefficient with respect to gravity for a Cs-Fountain H-maser comparison of 4.8*10^-6 and 10^-5 for a Rb-Fountain H-maser comparison. From the same data the putative boost LPI coefficients were measured to a precision of up to parts in 10^11 with respect to the CMB rest frame. By combining these boost invariance experiments to a Cryogenic Sapphire Oscillator versus H-maser comparison, independent limits on all nine coefficients of the boost violation vector with respect to fundamental constant invariance (fine structure constant, electron mass and quark mass respectively), were determined to a precision of parts up to 10^10.


arXiv:1108.4965

Fixed-Topology Lorentzian Triangulations: Quantum Regge Calculus in the Lorentzian Domain

Authors: Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 24 Aug 2011 (v1), last revised 9 Nov 2011 (this version, v2))

Abstract: A key insight used in developing the theory of Causal Dynamical Triangulations (CDTs) is to use the causal (or light-cone) structure of Lorentzian manifolds to restrict the class of geometries appearing in the Quantum Gravity (QG) path integral. By exploiting this structure the models developed in CDTs differ from the analogous models developed in the Euclidean domain, models of (Euclidean) Dynamical Triangulations (DT), and the corresponding Lorentzian results are in many ways more "physical".

In this paper we use this insight to formulate a Lorentzian signature model that is analogous to the Quantum Regge Calculus (QRC) approach to Euclidean Quantum Gravity. We exploit another crucial fact about the structure of Lorentzian manifolds, namely that certain simplices are not constrained by the triangle inequalities present in Euclidean signature. We show that this model is not related to QRC by a naive Wick rotation; this serves as another demonstration that the sum over Lorentzian geometries is not simply related to the sum over Euclidean geometries. By removing the triangle inequality constraints, there is more freedom to perform analytical calculations, and in addition numerical simulations are more computationally efficient.

We first formulate the model in 1+1 dimensions, and derive scaling relations for the pure gravity path integral on the torus using two different measures. It appears relatively easy to generate "large" universes, both in spatial and temporal extent. In addition, loop-to-loop amplitudes are discussed, and a transfer matrix is derived. We then also discuss the model in higher dimensions.

Journal reference: JHEP 1111: 072,2011


arXiv:1108.5240

Conservative entropic forces

Authors: Matt Visser (Victoria University of Wellington) (Submitted on 26 Aug 2011 (v1), last revised 20 Oct 2011 (this version, v4))

Abstract: Entropic forces have recently attracted considerable attention as ways to reformulate, retrodict, and perhaps even "explain'" classical Newtonian gravity from a rather specific thermodynamic perspective. In this article I point out that if one wishes to reformulate classical Newtonian gravity in terms of an entropic force, then the fact that Newtonian gravity is described by a conservative force places significant constraints on the form of the entropy and temperature functions. (These constraints also apply to entropic reinterpretations of electromagnetism, and indeed to any conservative force derivable from a potential.)

The constraints I will establish are sufficient to present real and significant problems for any reasonable variant of Verlinde's entropic gravity proposal, though for technical reasons the constraints established herein do not directly impact on either Jacobson's or Padmanabhan's versions of entropic gravity. In an attempt to resolve these issues, I will extend the usual notion of entropic force to multiple heat baths with multiple "temperatures'" and multiple "entropies".


arXiv:1109.2710

Entropy bounds in terms of the w parameter

Authors: Gabriel Abreu (Victoria University of Wellington), Carlos Barcelo (Instituto de Astrofisica de Andalucia, IAA-CSIC), Matt Visser (Victoria University of Wellington) (Submitted on 13 Sep 2011 (v1), last revised 8 Dec 2011 (this version, v3))

Abstract: In a pair of recent articles [PRL 105 (2010) 041302 - arXiv:1005.1132; JHEP 1103 (2011) 056 - arXiv:1012.2867] two of the current authors have developed an entropy bound for equilibrium uncollapsed matter using only classical general relativity, basic thermodynamics, and the Unruh effect. An odd feature of that bound, S <= A/2, was that the proportionality constant, 1/2, was weaker than that expected from black hole thermodynamics, 1/4. In the current article we strengthen the previous results by obtaining a bound involving the (suitably averaged) w parameter. Simple causality arguments restrict this averaged parameter to be <= 1. When equality holds, the entropy bound saturates at the value expected based on black hole thermodynamics. We also add some clarifying comments regarding the (net) positivity of the chemical potential. Overall, we find that even in the absence of any black hole region, we can nevertheless get arbitrarily close to the Bekenstein entropy.


arXiv:1110.5694

Realizability of the Lorentzian (n,1)-Simplex

Authors: Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 26 Oct 2011 (v1), last revised 12 Jan 2012 (this version, v3))

Abstract: In a previous article [JHEP 1111 (2011) 072; arXiv:1108.4965] we have developed a Lorentzian version of the Quantum Regge Calculus in which the significant differences between simplices in Lorentzian signature and Euclidean signature are crucial. In this article we extend a central result used in the previous article, regarding the realizability of Lorentzian triangles, to arbitrary dimension. This technical step will be crucial for developing the Lorentzian model in the case of most physical interest: 3+1 dimensions.

We first state (and derive in an appendix) the realizability conditions on the edge-lengths of a Lorentzian n-simplex in total dimension n=d+1, where d is the number of space-like dimensions. We then show that in any dimension there is a certain type of simplex which has all of its time-like edge lengths completely unconstrained by any sort of triangle inequality. This result is the d+1 dimensional analogue of the 1+1 dimensional case of the Lorentzian triangle.

Journal reference: JHEP 1201 (2012) 028


arXiv:1111.0214

Quantum vacuum radiation in optical glass

Authors: Stefano Liberati, Angus Prain, Matt Visser (Submitted on 1 Nov 2011)

Abstract: A recent experimental claim of the detection of analogue Hawking radiation in an optical system [PRL 105 (2010) 203901] has led to some controversy [PRL 107 (2011) 149401, 149402]. While this experiment strongly suggests some form of particle creation from the quantum vacuum (and hence it is per se very interesting), it is also true that it seems difficult to completely explain all features of the observations by adopting the perspective of a Hawking-like mechanism for the radiation. For instance, the observed photons are emitted parallel to the optical horizon, and the relevant optical horizon is itself defined in an unusual manner by combining group and phase velocities. This raises the question: Is this really Hawking radiation, or some other form of quantum vacuum radiation? Naive estimates of the amount of quantum vacuum radiation generated due to the rapidly changing refractive index --- sometimes called the dynamical Casimir effect --- are not encouraging. However we feel that naive estimates could be misleading depending on the quantitative magnitude of two specific physical effects: "pulse steepening" and "pulse cresting". Plausible bounds on the maximum size of these two effects results in estimates much closer to the experimental observations, and we argue that the dynamical Casimir effect is now worth additional investigation.

Journal reference: Phys. Rev. D 85(2012) 084014


arXiv:1111.6340

Lorentz violating kinematics: Threshold theorems

Authors: Valentina Baccetti (Victoria University of Wellington), Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 28 Nov 2011 (v1), last revised 5 Dec 2011 (this version, v2))

Abstract: Recent tentative experimental indications, and the subsequent theoretical speculations, regarding possible violations of Lorentz invariance have attracted a vast amount of attention. An important technical issue that considerably complicates detailed calculations in any such scenario, is that once one violates Lorentz invariance the analysis of thresholds in both scattering and decay processes becomes extremely subtle, with many new and naively unexpected effects. In the current article we develop several extremely general threshold theorems that depend only on the existence of some energy momentum relation E(p), eschewing even assumptions of isotropy or monotonicity. We shall argue that there are physically interesting situations where such a level of generality is called for, and that existing (partial) results in the literature make unnecessary technical assumptions. Even in this most general of settings, we show that at threshold all final state particles move with the same 3-velocity, while initial state particles must have 3-velocities parallel/anti-parallel to the final state particles. In contrast the various 3-momenta can behave in a complicated and counter-intuitive manner.


arXiv:1112.1466

Inertial frames without the relativity principle

Authors: Valentina Baccetti (Victoria University of Wellington), Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 7 Dec 2011 (v1), last revised 26 Apr 2012 (this version, v3))

Abstract: Ever since the work of von Ignatowsky circa 1910 it has been known (if not always widely appreciated) that the relativity principle, combined with the basic and fundamental physical assumptions of locality, linearity, and isotropy, leads almost uniquely to either the Lorentz transformations of special relativity or to Galileo's transformations of classical Newtonian mechanics. Thus, if one wishes to entertain the possibility of Lorentz symmetry breaking within the context of the class of local physical theories, then it seems likely that one will have to abandon (or at the very least grossly modify) the relativity principle. Working within the framework of local physics, we reassess the notion of spacetime transformations between inertial frames in the absence of the relativity principle, arguing that significant and nontrivial physics can still be extracted as long as the transformations are at least linear. An interesting technical aspect of the analysis is that the transformations now form a groupoid/pseudo-group --- it is this technical point that permits one to evade the von Ignatowsky argument. Even in the absence of a relativity principle we can nevertheless deduce clear and compelling rules for the transformation of space and time, rules for the composition of 3-velocities, and rules for the transformation of energy and momentum. As part of the analysis we identify two particularly elegant and physically compelling models implementing "minimalist" violations of Lorentz invariance --- in the first of these minimalist models all Lorentz violations are confined to carefully delineated particle physics sub-sectors, while the second minimalist Lorentz-violating model depends on one free function of absolute velocity, but otherwise preserves as much as possible of standard Lorentz invariant physics.


arXiv:1112.2057

Generic spherically symmetric dynamic thin-shell traversable wormholes in standard general relativity

Authors: Nadiezhda Montelongo Garcia, Francisco S. N. Lobo, Matt Visser (Submitted on 9 Dec 2011 (v1), last revised 15 Aug 2012 (this version, v3))

Abstract: We consider the construction of generic spherically symmetric thin-shell traversable wormhole spacetimes in standard general relativity. By using the cut-and-paste procedure, we comprehensively analyze the stability of arbitrary spherically symmetric thin-shell wormholes to linearized spherically symmetric perturbations around static solutions. While a number of special cases have previously been dealt with in scattered parts of the literature, herein we take considerable effort to make the analysis as general and unified as practicable. We demonstrate in full generality that stability of the wormhole is equivalent to choosing suitable properties for the exotic material residing on the wormhole throat.

Journal reference: Phys.Rev.D86:044026,2012


arXiv:1204.3138

Quantization of area for event and Cauchy horizons of the Kerr-Newman black hole

Authors: Matt Visser (Victoria University of Wellington) (Submitted on 14 Apr 2012 (v1), last revised 22 May 2012 (this version, v3))

Abstract: Based on various string theoretic constructions, and various string-inspired generalizations thereof, there have been repeated suggestions that the areas of black hole event horizons might be quantized in a quite specific manner, in terms of linear combinations of square roots of positive integers. It is important to realise that there are significant physical constraints on such integer-based proposals when one (somewhat speculatively) attempts to extend them outside their original extremal and supersymmetric framework. Specifically, in their most natural and direct physical interpretations, some of the more speculative integer-based proposals for the quantization of horizon areas fail for the ordinary Kerr-Newman black holes in (3+1) dimensions, essentially because the fine structure constant is not an integer. A more baroque interpretation involves asserting the fine structure constant is the square root of a rational number; but such a proposal has its own problems. Insofar as one takes (3+1) general relativity (plus the usual quantization of angular momentum and electric charge) as being paramount, the known explicitly calculable spectra of horizon areas for the physically compelling Kerr-Newman spacetimes indicate that some caution is called for when assessing the universality of some of the more speculative integer-based string-inspired proposals.

Journal reference: JHEP 1206 (2012) 023


arXiv:1205.6814

Area products for stationary black hole horizons

Authors: Matt Visser (Victoria University of Wellington) (Submitted on 30 May 2012 (v1), last revised 15 Aug 2013 (this version, v3))

Abstract: Area products for multi-horizon stationary black holes often have intriguing properties, and are often (though not always) independent of the mass of the black hole itself (depending only on various charges, angular momenta, and moduli). Such products are often formulated in terms of the areas of inner (Cauchy) horizons and outer (event) horizons, and sometimes include the effects of unphysical "virtual" horizons. But the conjectured mass-independence sometimes fails. Specifically, for the Schwarzschild-de Sitter [Kottler] black hole in (3+1) dimensions it is shown by explicit exact calculation that the product of event horizon area and cosmological horizon area is not mass independent. (Including the effect of the third "virtual" horizon does not improve the situation.) Similarly, in the Reissner-Nordstrom-anti-de Sitter black hole in (3+1) dimensions the product of inner (Cauchy) horizon area and event horizon area is calculated (perturbatively), and is shown to be not mass independent. That is, the mass-independence of the product of physical horizon areas is not generic. In spherical symmetry, whenever the quasi-local mass m(r) is a Laurent polynomial in aerial radius, r=sqrt{A/4\pi}, there are significantly more complicated mass-independent quantities, the elementary symmetric polynomials built up from the complete set of horizon radii (physical and virtual). Sometimes it is possible to eliminate the unphysical virtual horizons, constructing combinations of physical horizon areas that are mass independent, but they tend to be considerably more complicated than the simple products and related constructions currently being mooted in the literature.

Journal reference: Physical Review D 88 (2013) 044014


arXiv:1206.1092

Modelling gravity on a hyper-cubic lattice

Authors: Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 5 Jun 2012)

Abstract: We present an elegant and simple dynamical model of symmetric, non-degenerate (n x n) matrices of fixed signature defined on a n-dimensional hyper-cubic lattice with nearest-neighbor interactions. We show how this model is related to General Relativity, and discuss multiple ways in which it can be useful for studying gravity, both classical and quantum. In particular, we show that the dynamics of the model when all matrices are close to the identity corresponds exactly to a finite-difference discretization of weak-field gravity in harmonic gauge. We also show that the action which defines the full dynamics of the model corresponds to the Einstein-Hilbert action to leading order in the lattice spacing, and use this observation to define a lattice analogue of the Ricci scalar and Einstein tensor. Finally, we perform a mean-field analysis of the statistical mechanics of this model.

Journal reference: Physical Review D 86 (2012) 124003


arXiv:1206.2397

Survey of analogue spacetimes

Authors: Matt Visser (Victoria University of Wellington) (Submitted on 11 Jun 2012 (v1), last revised 19 Jun 2012 (this version, v2))

Abstract: Analogue spacetimes, (and more boldly, analogue models both of and for gravity), have attracted significant and increasing attention over the last decade and a half. Perhaps the most straightforward physical example, which serves as a template for most of the others, is Bill Unruh's model for a dumb hole, (mute black hole, acoustic black hole), wherein sound is dragged along by a moving fluid --- and can even be trapped behind an acoustic horizon. This and related analogue models for curved spacetimes are useful in many ways: Analogue spacetimes provide general relativists with extremely concrete physical models to help focus their thinking, and conversely the techniques of curved spacetime can sometimes help improve our understanding of condensed matter and/or optical systems by providing an unexpected and countervailing viewpoint. In this introductory chapter, I shall provide a few simple examples of analogue spacetimes as general background for the rest of the contributions.

Journal reference: Lecture Notes in Physics Volume 870 (2013) 31-50


arXiv:1207.5525

Unruh-DeWitt detector event rate for trajectories with time-dependent acceleration

Authors: Luis C. Barbado, Matt Visser (Submitted on 23 Jul 2012 (v1), last revised 25 Oct 2012 (this version, v2))

Abstract: We analyse the response function of an Unruh--DeWitt detector moving with time-dependent acceleration along a one-dimensional trajectory in Minkowski spacetime. To extract the physics of the process, we propose an adiabatic expansion of this response function. This expansion is also a useful tool for computing the click rate of detectors in general trajectories. The expansion is done in powers of the time derivatives of the acceleration (jerk, snap, and higher derivatives). At the lowest order, we recover a Planckian spectrum with temperature proportional to the acceleration of the detector at each instant of the trajectory. Higher orders in the expansion involve powers of the derivatives of the acceleration, with well-behaved spectral coefficients with different shapes. Finally, we illustrate this analysis in the case of an initially inertial trajectory that acquires a given constant acceleration in a finite time.

Journal reference: Physical Review D 86 (2012) 084011


arXiv:1302.2383

Surface gravities for non-Killing horizons

Authors: Bethan Cropp (SISSA/INFN), Stefano Liberati (SISSA/INFN), Matt Visser (Victoria University of Wellington) (Submitted on 11 Feb 2013)

Abstract: There are many logically and computationally distinct characterizations of the surface gravity of a horizon, just as there are many logically rather distinct notions of horizon. Fortunately, in standard general relativity, for stationary horizons, most of these characterizations are degenerate. However, in modified gravity, or in analogue spacetimes, horizons may be non-Killing or even non-null, and hence these degeneracies can be lifted. We present a brief overview of the key issues, specifically focusing on horizons in analogue spacetimes and universal horizons in modified gravity.

Journal reference: Classical and Quantum Gravity 30 (2013) 125001


arXiv:1302.5989

Inertial frames without the relativity principle: breaking Lorentz symmetry

Authors: Valentina Baccetti (Victoria University of Wellington), Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 25 Feb 2013)

Abstract: We investigate inertial frames in the absence of Lorentz invariance, reconsidering the usual group structure implied by the relativity principle. We abandon the relativity principle, discarding the group structure for the transformations between inertial frames, while requiring these transformations to be at least linear (to preserve homogeneity). In theories with a preferred frame (aether), the set of transformations between inertial frames forms a groupoid/pseudogroup instead of a group, a characteristic essential to evading the von Ignatowsky theorems. In order to understand the dynamics, we also demonstrate that the transformation rules for energy and momentum are in general affine. We finally focus on one specific and compelling model implementing a minimalist violation of Lorentz invariance.


arXiv:1303.3185

Clausius entropy for arbitrary bifurcate null surfaces

Authors: Valentina Baccetti (Victoria University of Wellington), Matt Visser (Victoria University of Wellington) (Submitted on 13 Mar 2013 (v1), last revised 9 Apr 2013 (this version, v2))

Abstract: Is entropy objectively "real"? Or is entropy in some sense subjective and observer-dependent? These innocent questions open a Pandora's box of often inconclusive debate. A consensus opinion, though certainly not universally held, seems to be that the Clausius entropy (thermodynamic entropy, defined via a Clausius relation d S = dQ/T) should be objectively real, but that the ontological status of the statistical entropy (the Shannon or von Neumann entropy) is much more ambiguous, and much more likely to be observer-dependent. This question is particularly pressing when it comes to understanding Bekenstein entropy (black hole entropy). To perhaps further add to the confusion, we shall argue that even Clausius entropy can often be observer-dependent. We shall demonstrate that one can meaningfully assign a notion of Clausius entropy to the matter crossing arbitrary bifurcate null surfaces --- effectively defining a "virtual Clausius entropy" for arbitrary "virtual causal horizons".


arXiv:1305.1416

Regge-Wheeler equation, linear stability, and greybody factors for dirty black holes

Authors: Petarpa Boonserm (Chulalongkorn University), Tritos Ngampitipan (Chulalongkorn University), Matt Visser (Victoria University of Wellington) (Submitted on 7 May 2013 (v1), last revised 15 Aug 2013 (this version, v4))

Abstract: So-called "dirty" black holes are those surrounded by non-zero stress-energy, rather than vacuum. The presence of the non-zero stress-energy modifies key features of the black hole, such as the surface gravity, Regge-Wheeler equation, linear stability, and greybody factors in a rather nontrivial way. Working within the inverse-Cowling approximation, (effectively the test-field limit), we shall present general forms for the Regge-Wheeler equation for linearized spin 0, spin 1, and axial spin 2 perturbations on an arbitrary static spherically symmetric background spacetime. Using very general features of the background spacetime, (in particular the classical energy conditions for the stress-energy surrounding the black hole), we extract several interesting and robust bounds on the behaviour of such systems, including rigorous bounds on the greybody factors for dirty black holes.

Journal reference: Physical Review D 88 (2013) 041502(R)


arXiv:1208.3601

Superradiant scattering of dispersive fields

Authors: Maurício Richartz, Angus Prain, Silke Weinfurtner, Stefano Liberati (Submitted on 17 Aug 2012 (v1), last revised 25 Mar 2013 (this version, v3))

Abstract: Motivated by analogue models of classical and quantum field theory in curved spacetimes and their recent experimental realizations, we consider wave scattering processes of dispersive fields exhibiting two extra degrees of freedom. In particular, we investigate how standard superradiant scattering processes are affected by subluminal or superluminal modifications of the dispersion relation. We analyze simple 1-dimensional toy-models based on fourth-order corrections to the standard second order wave equation and show that low-frequency waves impinging on generic scattering potentials can be amplified during the process. In specific cases, by assuming a simple step potential, we determine quantitatively the deviations in the amplification spectrum that arise due to dispersion, and demonstrate that the amplification can be further enhanced due to the presence of extra degrees of freedom. We also consider dispersive scattering processes in which the medium where the scattering takes place is moving with respect to the observer and show that superradiance can also be manifest in such situations.

Journal reference: Class. Quantum Grav. 30, 085009 (2013)


arXiv:1210.5182

Reducing Spacetime to Binary Information

Authors: Silke Weinfurtner, Gemma De las Cuevas, Miguel Angel Martin-Delgado, Hans J. Briegel (Submitted on 18 Oct 2012)

Abstract: We present a new description of discrete space-time in 1+1 dimensions in terms of a set of elementary geometrical units that represent its independent classical degrees of freedom. This is achieved by means of a binary encoding that is ergodic in the class of space-time manifolds respecting coordinate invariance of general relativity. Space-time fluctuations can be represented in a classical lattice gas model whose Boltzmann weights are constructed with the discretized form of the Einstein-Hilbert action. Within this framework, it is possible to compute basic quantities such as the Ricci curvature tensor and the Einstein equations, and to evaluate the path integral of discrete gravity. The description as a lattice gas model also provides a novel way of quantization and, at the same time, to quantum simulation of fluctuating space-time.


arXiv:1302.0375

Classical aspects of Hawking radiation verified in analogue gravity experiment

Authors: Silke Weinfurtner, Edmund W.Tedford, Matthew C.J.Penrice, William G.Unruh, Gregory A.Lawrenc (Submitted on 2 Feb 2013)

Abstract: There is an analogy between the propagation of fields on a curved spacetime and shallow water waves in an open channel flow. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include wave horizons. Long (shallow water) waves propagating upstream towards this region are blocked and converted into short (deep water) waves. This is the analogue of the stimulated Hawking emission by a white hole (the time inverse of a black hole). The measurements of amplitudes of the converted waves demonstrate that they appear in pairs and are classically correlated; the spectra of the conversion process is described by a Boltzmann-distribution; and the Boltzmann-distribution is determined by the determined by the change in flow across the white hole horizon.


arXiv:1305.3867

On the robustness of entanglement in analogue gravity systems

Authors: David Edward Bruschi, Nicolai Friis, Ivette Fuentes, Silke Weinfurtner (Submitted on 16 May 2013 (v1), last revised 7 Nov 2013 (this version, v2))

Abstract: We investigate the possibility to generate quantum-correlated quasi-particles utilizing analogue gravity systems. The quantumness of these correlations is a key aspect of analogue gravity effects and their presence allows for a clear separation between classical and quantum analogue gravity effects. However, experiments in analogue systems, such as Bose-Einstein condensates, and shallow water waves, are always conducted at non-ideal conditions, in particular, one is dealing with dispersive media at nonzero temperatures. We analyze the influence of the initial temperature on the entanglement generation in analogue gravity phenomena. We lay out all the necessary steps to calculate the entanglement generated between quasi-particle modes and we analytically derive an upper bound on the maximal temperature at which given modes can still be entangled. We further investigate a mechanism to enhance the quantum correlations. As a particular example we analyze the robustness of the entanglement creation against thermal noise in a sudden quench of an ideally homogeneous Bose-Einstein condensate, taking into account the super-sonic dispersion relations.

Journal reference: New J. Phys. 15, 113016 (2013)


arXiv:1201.6414

The dependence of the abstract boundary classification on a set of curves II: How the classification changes when the bounded parameter property satisfying set of curves changes

Authors: B. E. Whale (Submitted on 31 Jan 2012)

Abstract: The abstract boundary uses sets of curves with the bounded parameter property (b.p.p.) to classify the elements of the abstract boundary into regular points, singular points, points at infinity and so on. Building on the material of Part one of this two part series, we show how this classification changes when the set of b.p.p. satisfying curves changes.


arXiv:1111.5378

The First Law of Binary Black Hole Mechanics in General Relativity and Post-Newtonian Theory

Authors: Alexandre Le Tiec, Luc Blanchet, Bernard F. Whiting (Submitted on 23 Nov 2011 (v1), last revised 2 Apr 2012 (this version, v2))

Abstract: First laws of black hole mechanics, or thermodynamics, come in a variety of different forms. In this paper, from a purely post-Newtonian (PN) analysis, we obtain a first law for binary systems of point masses moving along an exactly circular orbit. Our calculation is valid through 3PN order and includes, in addition, the contributions of logarithmic terms at 4PN and 5PN orders. This first law of binary point-particle mechanics is then derived from first principles in general relativity, and analogies are drawn with the single and binary black hole cases. Some consequences of the first law are explored for PN spacetimes. As one such consequence, a simple relation between the PN binding energy of the binary system and Detweiler's redshift observable is established. Through it, we are able to determine with high precision the numerical values of some previously unknown high order PN coefficients in the circular-orbit binding energy. Finally, we propose new gauge invariant notions for the energy and angular momentum of a particle in a binary system.

Journal reference: Phys.Rev.D85:064039,2012


arXiv:1306.3208

Timescape cosmology with radiation fluid

Authors: James A.G. Duley, M. Ahsan Nazer, David L. Wiltshire (Submitted on 13 Jun 2013 (v1), last revised 7 Jul 2013 (this version, v2))

Abstract: The timescape cosmology represents a potentially viable alternative to the standard homogeneous cosmology, without the need for dark energy. Although average cosmic evolution in the timescape scenario only differs substantially from that of Friedmann-Lemaitre model at relatively late epochs when the contribution from the energy density of radiation is negligible, a full solution of the Buchert equations to incorporate radiation is necessary to smoothly match parameters to the epoch of photon decoupling and to obtain constraints from cosmic microwave background data. Here we extend the matter-dominated solution found in earlier work to include radiation, providing series solutions at early times and an efficient numerical integration strategy for generating the complete solution. The numerical solution is used to directly calculate the scale of the sound horizon at decoupling, and at the baryon drag epoch. The constraints on these scales from the Planck satellite data yield bounds on the timescape cosmological parameters, which are found to also agree with the best-fit values from a recent analysis of SDSS-II supernova data, while avoiding the problem of a primordial lithium-7 abundance anomaly.

Journal reference: Class.Quant.Grav.30:175006, 2013


arXiv:1311.3787

Cosmic structure, averaging and dark energy

Authors: David L. Wiltshire (Submitted on 15 Nov 2013)

Abstract: These lecture notes review the theoretical problems associated with coarse-graining the observed inhomogeneous structure of the universe at late epochs, of describing average cosmic evolution in the presence of growing inhomogeneity, and of relating average quantities to physical observables. In particular, a detailed discussion of the timescape scenario is presented. In this scenario, dark energy is realized as a misidentification of gravitational energy gradients which result from gradients in the kinetic energy of expansion of space, in the presence of density and spatial curvature gradients that grow large with the growth of structure. The phenomenology and observational tests of the timescape model are discussed in detail, with updated constraints from Planck satellite data. In addition, recent results on the variation of the Hubble expansion on < 100/h Mpc scales are discussed. The spherically averaged Hubble law is significantly more uniform in the rest frame of the Local Group of galaxies than in the conventional rest frame assumed for the Cosmic Microwave Background. This unexpected result supports a fundamental revision of the notion of the cosmic rest frame, consistent with the expectations of the timescape scenario.


ABSTRACTS FROM THE LIGO SCIENTIFIC COLLABORATION at gr-qc, July 2011 - November 2013

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, Daniel Shaddock, Susan Scott, Antony Searle, Bram Slagmolen, Michael Stefszky, Peter Veitch, Bernard Whiting and Chunnong Zhao.

Listed below are all the abstracts listed on gr-qc from July 2011 to November 2013 from consortia that include one ASGRG member as a co-author ? these are mostly LIGO abstracts, but there are some from eLISA.


------------- arXiv:1108.1521

Identification of long-duration noise transients in LIGO and Virgo

Authors: LIGO Scientific Collaboration, Virgo Collaboration: Michael W. Coughlin (Submitted on 7 Aug 2011 (v1), last revised 13 Nov 2011 (this version, v2))

Abstract: The LIGO and Virgo detectors are sensitive to a variety of noise sources, such as instrumental artifacts and environmental disturbances. The Stochastic Transient Analysis Multi-detector Pipeline (STAMP) has been developed to search for long-duration (t$\gtrsim$1s) gravitational-wave (GW) signals. This pipeline can also be used to identify environmental noise transients. Here we present an algorithm to determine when long-duration noise sources couple into the interferometers, as well as identify what these noise sources are. We analyze the cross-power between a GW strain channel and an environmental sensor, using pattern recognition tools to identify statistically significant structure in cross-power time-frequency maps. We identify interferometer noise from airplanes, helicopters, thunderstorms and other sources. Examples from LIGO's sixth science run, S6, and Virgo's third scientific run, VSR3, are presented.


arXiv:1109.0330

Noise Line Identification in LIGO S6 and Virgo VSR2

Authors: LIGO Scientific Collaboration, Virgo Collaboration: Michael W. Coughlin (Submitted on 2 Sep 2011)

Abstract: An important goal for LIGO (the Laser Interferometer Gravitational-Wave Observatory) and Virgo is to find periodic sources of gravitational waves. The LIGO and Virgo detectors are sensitive to a variety of noise of non-astrophysical origin, such as instrumental artifacts and environmental disturbances. These artifacts make it difficult to know when a signal is due to a gravitational wave or noise. A continuous wave search algorithm, Fscan, and the calculation of the coherence between the gravitational wave channels and auxiliary channels has been developed to identify the source of noise lines. The programs analyze data from the gravitational wave channels as well as environmental sensors, searching for significant lines that appear in coincidence (using various thresholds and frequency windows) in the gravitational wave channel as well the environmental monitors. By this method, the source of powerful signals at specific frequencies in the gravitational wave channel caused by noise can be determined. Examples from LIGO's sixth science run, S6, and Virgo' second scientific run, VSR2, are presented.

Journal reference: 2010 J. Phys.: Conf. Ser. 243 012010


arXiv:1109.3498

Implementation and testing of the first prompt search for gravitational wave transients with electromagnetic counterparts

Authors: The LIGO Scientific Collaboration, Virgo Collaboration (Submitted on 15 Sep 2011 (v1), last revised 12 Jan 2012 (this version, v2))

Abstract: Aims. A transient astrophysical event observed in both gravitational wave (GW) and electromagnetic (EM) channels would yield rich scientific rewards. A first program initiating EM follow-ups to possible transient GW events has been developed and exercised by the LIGO and Virgo community in association with several partners. In this paper, we describe and evaluate the methods used to promptly identify and localize GW event candidates and to request images of targeted sky locations.

Methods. During two observing periods (Dec 17 2009 to Jan 8 2010 and Sep 2 to Oct 20 2010), a low-latency analysis pipeline was used to identify GW event candidates and to reconstruct maps of possible sky locations. A catalog of nearby galaxies and Milky Way globular clusters was used to select the most promising sky positions to be imaged, and this directional information was delivered to EM observatories with time lags of about thirty minutes. A Monte Carlo simulation has been used to evaluate the low-latency GW pipeline's ability to reconstruct source positions correctly.

Results. For signals near the detection threshold, our low-latency algorithms often localized simulated GW burst signals to tens of square degrees, while neutron star/neutron star inspirals and neutron star/black hole inspirals were localized to a few hundred square degrees. Localization precision improves for moderately stronger signals. The correct sky location of signals well above threshold and originating from nearby galaxies may be observed with ~50% or better probability with a few pointings of wide-field telescopes.

Report number: LIGO-P1000061


arXiv:1110.0208

All-sky Search for Periodic Gravitational Waves in the Full S5 LIGO Data

Authors: J. Abadie et al. (Submitted on 2 Oct 2011)

Abstract: We report on an all-sky search for periodic gravitational waves in the frequency band 50-800 Hz and with the frequency time derivative in the range of 0 through -6e-9 Hz/s. Such a signal could be produced by a nearby spinning and slightly non-axisymmetric isolated neutron star in our galaxy. After recent improvements in the search program that yielded a 10x increase in computational efficiency, we have searched in two years of data collected during LIGO's fifth science run and have obtained the most sensitive all-sky upper limits on gravitational wave strain to date. Near 150 Hz our upper limit on worst-case linearly polarized strain amplitude $h_0$ is 1e-24, while at the high end of our frequency range we achieve a worst-case upper limit of 3.8e-24 for all polarizations and sky locations. These results constitute a factor of two improvement upon previously published data. A new detection pipeline utilizing a Loosely Coherent algorithm was able to follow up weaker outliers, increasing the volume of space where signals can be detected by a factor of 10, but has not revealed any gravitational wave signals. The pipeline has been tested for robustness with respect to deviations from the model of an isolated neutron star, such as caused by a low-mass or long-period binary companion.

Journal Reference: PhysRevD.85.022001

Report number: LIGO-P1100029-v36


arXiv:1111.7314

Search for Gravitational Waves from Low Mass Compact Binary Coalescence in LIGO's Sixth Science Run and Virgo's Science Runs 2 and 3

Authors: the LIGO Scientific Collaboration, the Virgo Collaboration (Submitted on 30 Nov 2011 (v1), last revised 18 Jan 2012 (this version, v4))

Abstract: We report on a search for gravitational waves from coalescing compact binaries using LIGO and Virgo observations between July 7, 2009 and October 20, 2010. We searched for signals from binaries with total mass between 2 and 25 solar masses; this includes binary neutron stars, binary black holes, and binaries consisting of a black hole and neutron star. The detectors were sensitive to systems up to 40 Mpc distant for binary neutron stars, and further for higher mass systems. No gravitational-wave signals were detected. We report upper limits on the rate of compact binary coalescence as a function of total mass, including the results from previous LIGO and Virgo observations. The cumulative 90%-confidence rate upper limits of the binary coalescence of binary neutron star, neutron star- black hole and binary black hole systems are 1.3 x 10^{-4}, 3.1 x 10^{-5} and 6.4 x 10^{-6} Mpc^{-3}yr^{-1}, respectively. These upper limits are up to a factor 1.4 lower than previously derived limits. We also report on results from a blind injection challenge.

Journal reference: Phys. Rev. D 85, 082002 (2012)


arXiv:1112.1057

Astronomy and astrophysics with gravitational waves in the Advanced Detector Era

Authors: Alan J. Weinstein, for the LIGO Scientific Collaboration, for the Virgo Collaboration (Submitted on 5 Dec 2011 (v1), last revised 12 Mar 2012 (this version, v3))

Abstract: With the advanced gravitational wave detectors coming on line in the next 5 years, we expect to make the first detections of gravitational waves from astrophysical sources, and study the properties of the waves themselves as tests of General Relativity. In addition, these gravitational waves will be powerful tools for the study of their astrophysical sources and source populations. They carry information that is quite complementary to what can be learned from electromagnetic or neutrino observations, probing the central gravitational engines that power the electromagnetic emissions. Preparations are being made to enable near-simultaneous observations of both gravitational wave and electromagnetic observations of transient sources, using low-latency search pipelines and rapid sky localization. We will review the many opportunities for multi-messenger astronomy and astrophysics with gravitational waves enabled by the advanced detectors, and the preparations that are being made to quickly and fully exploit them.

Journal reference: Class. Quantum Grav. 29 (2012) 124012

Report number: LIGO Document P1100189


arXiv:1112.1637

Search for gravitational waves associated with the InterPlanetary Network short gamma ray bursts

Authors: V. Predoi, for the LIGO Scientific Collaboration, for the Virgo Collaboration, K. Hurley, for IPN (Submitted on 7 Dec 2011)

Abstract: We outline the scientific motivation behind a search for gravitational waves associated with short gamma ray bursts detected by the InterPlanetary Network (IPN) during LIGO's fifth science run and Virgo's first science run. The IPN localisation of short gamma ray bursts is limited to extended error boxes of different shapes and sizes and a search on these error boxes poses a series of challenges for data analysis. We will discuss these challenges and outline the methods to optimise the search over these error boxes.


arXiv:1112.5004

Upper limits on a stochastic gravitational-wave background using LIGO and Virgo interferometers at 600-1000 Hz

Authors: J. Abadie et al. (Submitted on 21 Dec 2011 (v1), last revised 23 Feb 2012 (this version, v4))

Abstract: A stochastic background of gravitational waves is expected to arise from a superposition of many incoherent sources of gravitational waves, of either cosmological or astrophysical origin. This background is a target for the current generation of ground-based detectors. In this article we present the first joint search for a stochastic background using data from the LIGO and Virgo interferometers. In a frequency band of 600-1000 Hz, we obtained a 95% upper limit on the amplitude of $\Omega_{\rm GW}(f) = \Omega_3 (f/900 \mathrm{Hz})^3$, of $\Omega_3 < 0.33$, assuming a value of the Hubble parameter of $h_{100}=0.72$. These new limits are a factor of seven better than the previous best in this frequency band.

Journal Reference: PhysRevD.85.122001

Report number: LIGO-P1000128-v22


arXiv:1112.6005

First Low-Latency LIGO+Virgo Search for Binary Inspirals and their Electromagnetic Counterparts

Authors: The LIGO Scientific Collaboration, The Virgo Collaboration (Submitted on 27 Dec 2011 (v1), last revised 13 Jan 2012 (this version, v4))

Abstract: Aims. The detection and measurement of gravitational-waves from coalescing neutron-star binary systems is an important science goal for ground-based gravitational-wave detectors. In addition to emitting gravitational-waves at frequencies that span the most sensitive bands of the LIGO and Virgo detectors, these sources are also amongst the most likely to produce an electromagnetic counterpart to the gravitational-wave emission. A joint detection of the gravitational-wave and electromagnetic signals would provide a powerful new probe for astronomy.

Methods. During the period between September 19 and October 20, 2010, the first low-latency search for gravitational-waves from binary inspirals in LIGO and Virgo data was conducted. The resulting triggers were sent to electromagnetic observatories for followup. We describe the generation and processing of the low-latency gravitational-wave triggers. The results of the electromagnetic image analysis will be described elsewhere.

Results. Over the course of the science run, three gravitational-wave triggers passed all of the low-latency selection cuts. Of these, one was followed up by several of our observational partners. Analysis of the gravitational-wave data leads to an estimated false alarm rate of once every 6.4 days, falling far short of the requirement for a detection based solely on gravitational-wave data.


arXiv:1201.3176

Searches for continuous gravitational wave signals and stochastic backgrounds in LIGO and Virgo data

Authors: C. Palomba, for the LIGO Scientific Collaboration, for the Virgo Collaboration (Submitted on 16 Jan 2012)

Abstract: We present results from searches of recent LIGO and Virgo data for continuous gravitational wave signals (CW) from spinning neutron stars and for a stochastic gravitational wave background (SGWB). The first part of the talk is devoted to CW analysis with a focus on two types of searches. In the targeted search of known neutron stars a precise knowledge of the star parameters is used to apply optimal filtering methods. In the absence of a signal detection, in a few cases, an upper limit on strain amplitude can be set that beats the spindown limit derived from attributing spin-down energy loss to the emission of gravitational waves. In contrast, blind all-sky searches are not directed at specific sources, but rather explore as large a portion of the parameter space as possible. Fully coherent methods cannot be used for these kind of searches which pose a non trivial computational challenge. The second part of the talk is focused on SGWB searches. A stochastic background of gravitational waves is expected to be produced by the superposition of many incoherent sources of cosmological or astrophysical origin. Given the random nature of this kind of signal, it is not possible to distinguish it from noise using a single detector. A typical data analysis strategy relies on cross-correlating the data from a pair or several pairs of detectors, which allows discriminating the searched signal from instrumental noise. Expected sensitivities and prospects for detection from the next generation of interferometers are also discussed for both kind of sources.


arXiv:1201.3621

eLISA: Astrophysics and cosmology in the millihertz regime

Authors: Pau Amaro-Seoane et al. (Submitted on 17 Jan 2012)

Abstract: This document introduces the exciting and fundamentally new science and astronomy that the European New Gravitational Wave Observatory (NGO) mission (derived from the previous LISA proposal) will deliver. The mission (which we will refer to by its informal name "eLISA") will survey for the first time the low-frequency gravitational wave band (about 0.1 mHz to 1 Hz), with sufficient sensitivity to detect interesting individual astrophysical sources out to z = 15. The eLISA mission will discover and study a variety of cosmic events and systems with high sensitivity: coalescences of massive black holes binaries, brought together by galaxy mergers; mergers of earlier, less-massive black holes during the epoch of hierarchical galaxy and black-hole growth; stellar-mass black holes and compact stars in orbits just skimming the horizons of massive black holes in galactic nuclei of the present era; extremely compact white dwarf binaries in our Galaxy, a rich source of information about binary evolution and about future Type Ia supernovae; and possibly most interesting of all, the uncertain and unpredicted sources, for example relics of inflation and of the symmetry-breaking epoch directly after the Big Bang. eLISA's measurements will allow detailed studies of these signals with high signal-to-noise ratio, addressing most of the key scientific questions raised by ESA's Cosmic Vision programme in the areas of astrophysics and cosmology. They will also provide stringent tests of general relativity in the strong-field dynamical regime, which cannot be probed in any other way. This document not only describes the science but also gives an overview on the mission design and orbits.


arXiv:1201.4413

Implications For The Origin Of GRB 051103 From LIGO Observations

Authors: The LIGO Scientific Collaboration (Submitted on 21 Jan 2012 (v1), last revised 17 Apr 2012 (this version, v3))

Abstract: We present the results of a LIGO search for gravitational waves (GWs) associated with GRB 051103, a short-duration hard-spectrum gamma-ray burst (GRB) whose electromagnetically determined sky position is coincident with the spiral galaxy M81, which is 3.6 Mpc from Earth. Possible progenitors for short-hard GRBs include compact object mergers and soft gamma repeater (SGR) giant flares. A merger progenitor would produce a characteristic GW signal that should be detectable at the distance of M81, while GW emission from an SGR is not expected to be detectable at that distance. We found no evidence of a GW signal associated with GRB 051103. Assuming weakly beamed gamma-ray emission with a jet semi-angle of 30 deg we exclude a binary neutron star merger in M81 as the progenitor with a confidence of 98%. Neutron star-black hole mergers are excluded with > 99% confidence. If the event occurred in M81 our findings support the the hypothesis that GRB 051103 was due to an SGR giant flare, making it the most distant extragalactic magnetar observed to date.


arXiv:1201.5999

Search for Gravitational Waves from Intermediate Mass Binary Black Holes

Authors: the LIGO Scientific Collaboration, the Virgo Collaboration (Submitted on 28 Jan 2012 (v1), last revised 25 Apr 2012 (this version, v3))

Abstract: We present the results of a weakly modeled burst search for gravitational waves from mergers of non-spinning intermediate mass black holes (IMBH) in the total mass range 100--450 solar masses and with the component mass ratios between 1:1 and 4:1. The search was conducted on data collected by the LIGO and Virgo detectors between November of 2005 and October of 2007. No plausible signals were observed by the search which constrains the astrophysical rates of the IMBH mergers as a function of the component masses. In the most efficiently detected bin centered on 88+88 solar masses, for non-spinning sources, the rate density upper limit is 0.13 per Mpc^3 per Myr at the 90% confidence level.

Journal Reference: PhysRevD.85.102004

Report number: P1100068


arXiv:1202.0839 Low-frequency gravitational-wave science with eLISA/NGO

Authors: Pau Amaro-Seoane et al. (Submitted on 3 Feb 2012 (v1), last revised 12 Sep 2012 (this version, v2))

Abstract: We review the expected science performance of the New Gravitational-Wave Observatory (NGO, a.k.a. eLISA), a mission under study by the European Space Agency for launch in the early 2020s. eLISA will survey the low-frequency gravitational-wave sky (from 0.1 mHz to 1 Hz), detecting and characterizing a broad variety of systems and events throughout the Universe, including the coalescences of massive black holes brought together by galaxy mergers; the inspirals of stellar-mass black holes and compact stars into central galactic black holes; several millions of ultracompact binaries, both detached and mass transferring, in the Galaxy; and possibly unforeseen sources such as the relic gravitational-wave radiation from the early Universe. eLISA's high signal-to-noise measurements will provide new insight into the structure and history of the Universe, and they will test general relativity in its strong-field dynamical regime.

Journal reference: Pau Amaro-Seoane et al 2012 Class. Quantum Grav. 29 124016


arXiv:1202.2788

All-sky search for gravitational-wave bursts in the second joint LIGO-Virgo run

Authors: the LIGO Scientific Collaboration, the Virgo Collaboration (Submitted on 13 Feb 2012 (v1), last revised 20 Apr 2012 (this version, v3))

Abstract: We present results from a search for gravitational-wave bursts in the data collected by the LIGO and Virgo detectors between July 7, 2009 and October 20, 2010: data are analyzed when at least two of the three LIGO-Virgo detectors are in coincident operation, with a total observation time of 207 days. The analysis searches for transients of duration < 1 s over the frequency band 64-5000 Hz, without other assumptions on the signal waveform, polarization, direction or occurrence time. All identified events are consistent with the expected accidental background. We set frequentist upper limits on the rate of gravitational-wave bursts by combining this search with the previous LIGO-Virgo search on the data collected between November 2005 and October 2007. The upper limit on the rate of strong gravitational-wave bursts at the Earth is 1.3 events per year at 90% confidence. We also present upper limits on source rate density per year and Mpc^3 for sample populations of standard-candle sources. As in the previous joint run, typical sensitivities of the search in terms of the root-sum-squared strain amplitude for these waveforms lie in the range 5 10^-22 Hz^-1/2 to 1 10^-20 Hz^-1/2. The combination of the two joint runs entails the most sensitive all-sky search for generic gravitational-wave bursts and synthesizes the results achieved by the initial generation of interferometric detectors.

Journal Reference: PhysRevD.85.122007

Report number: LIGO-P1100118


arXiv:1203.2674

Sensitivity Achieved by the LIGO and Virgo Gravitational Wave Detectors during LIGO's Sixth and Virgo's Second and Third Science Runs

Authors:The LIGO Scientific Collaboration, The Virgo Collaboration

(Submitted on 12 Mar 2012 (v1), last revised 15 Mar 2012 (this version, v2)) Abstract: We summarize the sensitivity achieved by the LIGO and Virgo gravitational wave detectors for low-mass compact binary coalescence (CBC) searches during LIGO's sixth science run and Virgo's second and third science runs. We present strain noise power spectral densities (PSDs) which are representative of the typical performance achieved by the detectors in these science runs. The data presented here and in the accompanying web-accessible data files are intended to be released to the public as a summary of detector performance for low-mass CBC searches during S6 and VSR2-3.


arXiv:1203.5613

The characterization of Virgo data and its impact on gravitational-wave searches

Authors: J. Aasi et al. (Submitted on 26 Mar 2012 (v1), last revised 18 Jun 2012 (this version, v2))

Abstract: Between 2007 and 2010 Virgo collected data in coincidence with the LIGO and GEO gravitational-wave (GW) detectors. These data have been searched for GWs emitted by cataclysmic phenomena in the universe, by non-axisymmetric rotating neutron stars or from a stochastic background in the frequency band of the detectors. The sensitivity of GW searches is limited by noise produced by the detector or its environment. It is therefore crucial to characterize the various noise sources in a GW detector. This paper reviews the Virgo detector noise sources, noise propagation, and conversion mechanisms which were identified in the three first Virgo observing runs. In many cases, these investigations allowed us to mitigate noise sources in the detector, or to selectively flag noise events and discard them from the data. We present examples from the joint LIGO-GEO-Virgo GW searches to show how well noise transients and narrow spectral lines have been identified and excluded from the Virgo data. We also discuss how detector characterization can improve the astrophysical reach of gravitational-wave searches.


arXiv:1204.2497

Data Quality Studies of Enhanced Interferometric Gravitational Wave Detectors

Authors: Jessica McIver, for the LIGO Scientific Collaboration, for the Virgo Collaboration (Submitted on 11 Apr 2012)

Abstract: Data quality assessment plays an essential role in the quest to detect gravitational wave signals in data from the LIGO and Virgo interferometric gravitational wave detectors. Interferometer data contains a high rate of noise transients from the environment, the detector hardware, and the detector control systems. These transients severely limit the statistical significance of gravitational wave candidates of short duration and/or poorly modeled waveforms. This paper describes the data quality studies that have been performed in recent LIGO and Virgo observing runs to mitigate the impact of transient detector artifacts on the gravitational wave searches.


arXiv:1204.4110

Gravitational waves and gamma-ray bursts

Authors: Alessandra Corsi, for the LIGO Scientific Collaboration, for the Virgo Collaboration (Submitted on 18 Apr 2012 (v1), last revised 11 May 2012 (this version, v2))

Abstract: Gamma-Ray Bursts are likely associated with a catastrophic energy release in stellar mass objects. Electromagnetic observations provide important, but indirect information on the progenitor. On the other hand, gravitational waves emitted from the central source, carry direct information on its nature. In this context, I give an overview of the multi-messenger study of gamma-ray bursts that can be carried out by using electromagnetic and gravitational wave observations. I also underline the importance of joint electromagnetic and gravitational wave searches, in the absence of a gamma-ray trigger. Finally, I discuss how multi-messenger observations may probe alternative gamma-ray burst progenitor models, such as the magnetar scenario.


arXiv:1205.1124

Swift follow-up observations of candidate gravitational-wave transient events

Authors: P. A. Evans et al. (Submitted on 5 May 2012 (v1), last revised 23 Nov 2012 (this version, v2))

Abstract: We present the first multi-wavelength follow-up observations of two candidate gravitational-wave (GW) transient events recorded by LIGO and Virgo in their 2009-2010 science run. The events were selected with low latency by the network of GW detectors and their candidate sky locations were observed by the Swift observatory. Image transient detection was used to analyze the collected electromagnetic data, which were found to be consistent with background. Off-line analysis of the GW data alone has also established that the selected GW events show no evidence of an astrophysical origin; one of them is consistent with background and the other one was a test, part of a "blind injection challenge". With this work we demonstrate the feasibility of rapid follow-ups of GW transients and establish the sensitivity improvement joint electromagnetic and GW observations could bring. This is a first step toward an electromagnetic follow-up program in the regime of routine detections with the advanced GW instruments expected within this decade. In that regime multi-wavelength observations will play a significant role in completing the astrophysical identification of GW sources. We present the methods and results from this first combined analysis and discuss its implications in terms of sensitivity for the present and future instruments.

Journal reference: ApJS, 203, 28 (2012)


arXiv:1205.2216

Search for gravitational waves associated with gamma-ray bursts during LIGO science run 6 and Virgo science runs 2 and 3

Authors: The LIGO Scientific Collaboration, Virgo Collaboration (Submitted on 10 May 2012 (v1), last revised 24 Sep 2012 (this version, v3))

Abstract: We present the results of a search for gravitational waves associated with 154 gamma-ray bursts (GRBs) that were detected by satellite-based gamma-ray experiments in 2009-2010, during the sixth LIGO science run and the second and third Virgo science runs. We perform two distinct searches: a modeled search for coalescences of either two neutron stars or a neutron star and black hole; and a search for generic, unmodeled gravitational-wave bursts. We find no evidence for gravitational-wave counterparts, either with any individual GRB in this sample or with the population as a whole. For all GRBs we place lower bounds on the distance to the progenitor, under the optimistic assumption of a gravitational-wave emission energy of 10^-2 M c^2 at 150 Hz, with a median limit of 17 Mpc. For short hard GRBs we place exclusion distances on binary neutron star and neutron star-black hole progenitors, using astrophysically motivated priors on the source parameters, with median values of 16 Mpc and 28 Mpc respectively. These distance limits, while significantly larger than for a search that is not aided by GRB satellite observations, are not large enough to expect a coincidence with a GRB. However, projecting these exclusions to the sensitivities of Advanced LIGO and Virgo, which should begin operation in 2015, we find that the detection of gravitational waves associated with GRBs will become quite possible.

Report number: LIGO-P1000121


arXiv:1206.6163

Rapid alerts for following up gravitational wave event candidates

Authors: Peter S. Shawhan, for the LIGO Scientific Collaboration, Virgo Collaboration (Submitted on 27 Jun 2012)

Abstract: Gravitational waves carry unique information about high-energy astrophysical events such as the inspiral and merger of neutron stars and black holes, core collapse in massive stars, and other sources. Large gravitational wave (GW) detectors utilizing exquisitely sensitive laser interferometry--namely, LIGO in the United States and GEO 600 and Virgo in Europe--have been successfully operated in recent years and are currently being upgraded to greatly improve their sensitivities. Many signals are expected to be detected in the coming decade. Simultaneous observing with the network of GW detectors enables us to identify and localize event candidates on the sky with modest precision, opening up the possibility of capturing optical transients or other electromagnetic counterparts to confirm an event and obtain complementary information about it. We developed and implemented the first complete low-latency GW data analysis and alert system in 2009-10 and used it to send alerts to several observing partners; the system design and some lessons learned are briefly described. We discuss several operational considerations and design choices for improving this scientific capability for future observations.


arXiv:1209.6533

Search for Gravitational Waves from Binary Black Hole Inspiral, Merger and Ringdown in LIGO-Virgo Data from 2009-2010

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration (Submitted on 28 Sep 2012 (v1), last revised 25 Feb 2013 (this version, v3))

Abstract: We report a search for gravitational waves from the inspiral, merger and ringdown of binary black holes (BBH) with total mass between 25 and 100 solar masses, in data taken at the LIGO and Virgo observatories between July 7, 2009 and October 20, 2010. The maximum sensitive distance of the detectors over this period for a (20,20) Msun coalescence was 300 Mpc. No gravitational wave signals were found. We thus report upper limits on the astrophysical coalescence rates of BBH as a function of the component masses for non-spinning components, and also evaluate the dependence of the search sensitivity on component spins aligned with the orbital angular momentum. We find an upper limit at 90% confidence on the coalescence rate of BBH with non-spinning components of mass between 19 and 28 Msun of 3.3 \times 10^-7 mergers /Mpc^3 /yr.

Journal reference: Phys. Rev. D 87, 022002 (2013)

Report number: LIGO-P1200024


arXiv:1210.7173

Data analysis challenges in transient gravitational-wave astronomy

Authors: Eric Chassande-Mottin, for the LIGO Scientific Collaboration, for the Virgo Collaboration (Submitted on 26 Oct 2012 (v1), last revised 3 Mar 2013 (this version, v2))

Abstract: Gravitational waves are radiative solutions of space-time dynamics predicted by Einstein's theory of General Relativity. A world-wide array of large-scale and highly sensitive interferometric detectors constantly scrutinizes the geometry of the local space-time with the hope to detect deviations that would signal an impinging gravitational wave from a remote astrophysical source. Finding the rare and weak signature of gravitational waves buried in non-stationary and non-Gaussian instrument noise is a particularly challenging problem. We will give an overview of the data-analysis techniques and associated observational results obtained so far by Virgo (in Europe) and LIGO (in the US), along with the prospects offered by the up-coming advanced versions of those detectors.

Report number: LIGO-P1200135; VIR-0379C-12


arXiv:1207.7176

Einstein@Home all-sky search for periodic gravitational waves in LIGO S5 data

Authors: J. Aasi et al. (Submitted on 31 Jul 2012 (v1), last revised 4 Aug 2012 (this version, v2))

Abstract: This paper presents results of an all-sky searches for periodic gravitational waves in the frequency range [50, 1190] Hz and with frequency derivative ranges of [-2 x 10^-9, 1.1 x 10^-10] Hz/s for the fifth LIGO science run (S5). The novelty of the search lies in the use of a non-coherent technique based on the Hough-transform to combine the information from coherent searches on timescales of about one day. Because these searches are very computationally intensive, they have been deployed on the Einstein@Home distributed computing project infrastructure. The search presented here is about a factor 3 more sensitive than the previous Einstein@Home search in early S5 LIGO data. The post-processing has left us with eight surviving candidates. We show that deeper follow-up studies rule each of them out. Hence, since no statistically significant gravitational wave signals have been detected, we report upper limits on the intrinsic gravitational wave amplitude h0. For example, in the 0.5 Hz-wide band at 152.5 Hz, we can exclude the presence of signals with h0 greater than 7.6 x 10^-25 with a 90% confidence level.

Journal reference: Phys. Rev. D 87, 042001 (2013)

Report number: LIGO-P1200026


arXiv:1304.0670

Prospects for Localization of Gravitational Wave Transients by the Advanced LIGO and Advanced Virgo Observatories

Authors: LIGO Scientific Collaboration, Virgo Collaboration (Submitted on 2 Apr 2013)

Abstract: We present a possible observing scenario for the Advanced LIGO and Advanced Virgo gravitational wave detectors over the next decade, with the intention of providing information to the astronomy community to facilitate planning for multi-messenger astronomy with gravitational waves. We determine the expected sensitivity of the network to transient gravitational-wave signals, and study the capability of the network to determine the sky location of the source. For concreteness, we focus primarily on gravitational-wave signals from the inspiral of binary neutron star (BNS) systems, as the source considered likely to be the most common for detection and also promising for multimessenger astronomy. We find that confident detections will likely require at least 2 detectors operating with BNS sensitive ranges of at least 100 Mpc, while ranges approaching 200 Mpc should give at least ~1 BNS detection per year even under pessimistic predictions of signal rates. The ability to localize the source of the detected signals depends on the geographical distribution of the detectors and their relative sensitivity, and can be as large as thousands of square degrees with only 2 sensitive detectors operating. Determining the sky position of a significant fraction of detected signals to areas of 5 sq deg to 20 sq deg will require at least 3 detectors of sensitivity within a factor of ~2 of each other and with a broad frequency bandwidth. Should one of the LIGO detectors be relocated in India as expected, many gravitational-wave signals will be localized to a few square degrees by gravitational-wave observations alone.

Report number: LIGO-P1200087, VIR-0288A-12


arXiv:1304.1775

Parameter estimation for compact binary coalescence signals with the first generation gravitational-wave detector network

Authors: the LIGO Scientific Collaboration, the Virgo Collaboration (Submitted on 5 Apr 2013 (v1), last revised 22 Oct 2013 (this version, v4))

Abstract: Compact binary systems with neutron stars or black holes are one of the most promising sources for ground-based gravitational wave detectors. Gravitational radiation encodes rich information about source physics; thus parameter estimation and model selection are crucial analysis steps for any detection candidate events. Detailed models of the anticipated waveforms enable inference on several parameters, such as component masses, spins, sky location and distance that are essential for new astrophysical studies of these sources. However, accurate measurements of these parameters and discrimination of models describing the underlying physics are complicated by artifacts in the data, uncertainties in the waveform models and in the calibration of the detectors. Here we report such measurements on a selection of simulated signals added either in hardware or software to the data collected by the two LIGO instruments and the Virgo detector during their most recent joint science run, including a "blind injection" where the signal was not initially revealed to the collaboration. We exemplify the ability to extract information about the source physics on signals that cover the neutron star and black hole parameter space over the individual mass range 1 Msun - 25 Msun and the full range of spin parameters. The cases reported in this study provide a snap-shot of the status of parameter estimation in preparation for the operation of advanced detectors.

Journal reference: Phys. Rev. D 88, 062001 (2013)


arXiv:1305.0816

The Transient Gravitational-Wave Sky

Authors: Nils Andersson et al. (Submitted on 3 May 2013)

Abstract: Interferometric detectors will very soon give us an unprecedented view of the gravitational-wave sky, and in particular of the explosive and transient Universe. Now is the time to challenge our theoretical understanding of short-duration gravitational-wave signatures from cataclysmic events, their connection to more traditional electromagnetic and particle astrophysics, and the data analysis techniques that will make the observations a reality. This paper summarizes the state of the art, future science opportunities, and current challenges in understanding gravitational-wave transients.


arXiv:1305.5720

The Gravitational Universe

Authors: The eLISA Consortium (Submitted on 24 May 2013)

Abstract: The last century has seen enormous progress in our understanding of the Universe. We know the life cycles of stars, the structure of galaxies, the remnants of the big bang, and have a general understanding of how the Universe evolved. We have come remarkably far using electromagnetic radiation as our tool for observing the Universe. However, gravity is the engine behind many of the processes in the Universe, and much of its action is dark. Opening a gravitational window on the Universe will let us go further than any alternative. Gravity has its own messenger: Gravitational waves, ripples in the fabric of spacetime. They travel essentially undisturbed and let us peer deep into the formation of the first seed black holes, exploring redshifts as large as z ~ 20, prior to the epoch of cosmic re-ionisation. Exquisite and unprecedented measurements of black hole masses and spins will make it possible to trace the history of black holes across all stages of galaxy evolution, and at the same time constrain any deviation from the Kerr metric of General Relativity. eLISA will be the first ever mission to study the entire Universe with gravitational waves. eLISA is an all-sky monitor and will offer a wide view of a dynamic cosmos using gravitational waves as new and unique messengers to unveil The Gravitational Universe. It provides the closest ever view of the early processes at TeV energies, has guaranteed sources in the form of verification binaries in the Milky Way, and can probe the entire Universe, from its smallest scales around singularities and black holes, all the way to cosmological dimensions.


arXiv:1309.4027 Gravitational waves from known pulsars: results from the initial detector era

Authors: J. Aasi et al. (Submitted on 16 Sep 2013 (v1), last revised 30 Sep 2013 (this version, v2))

Abstract: We present the results of searches for gravitational waves from a large selection of pulsars using data from the most recent science runs (S6, VSR2 and VSR4) of the initial generation of interferometric gravitational wave detectors LIGO (Laser Interferometric Gravitational-wave Observatory) and Virgo. We do not see evidence for gravitational wave emission from any of the targeted sources but produce upper limits on the emission amplitude. We highlight the results from seven young pulsars with large spin-down luminosities. We reach within a factor of five of the canonical spin-down limit for all seven of these, whilst for the Crab and Vela pulsars we further surpass their spin-down limits. We present new or updated limits for 172 other pulsars (including both young and millisecond pulsars). Now that the detectors are undergoing major upgrades, and, for completeness, we bring together all of the most up-to-date results from all pulsars searched for during the operations of the first-generation LIGO, Virgo and GEO600 detectors. This gives a total of 195 pulsars including the most recent results described in this paper.

Report number: LIGO Document No. LIGO-P1200104


arXiv:1309.6160

A search for long-lived gravitational-wave transients coincident with long gamma-ray bursts

Authors: The LIGO Scientific Collaboration, the Virgo Collaboration (Submitted on 24 Sep 2013 (v1), last revised 26 Sep 2013 (this version, v2))

Abstract: Long gamma-ray bursts (GRBs) have been linked to extreme core-collapse supernovae from massive stars. Gravitational waves (GW) offer a probe of the physics behind long GRBs. We investigate models of long-lived (~10-1000s) GW emission associated with the accretion disk of a collapsed star or with its protoneutron star remnant. Using data from LIGO's fifth science run, and GRB triggers from the swift experiment, we perform a search for unmodeled long-lived GW transients. Finding no evidence of GW emission, we place 90% confidence level upper limits on the GW fluence at Earth from long GRBs for three waveforms inspired by a model of GWs from accretion disk instabilities. These limits range from F<3.5 ergs cm^-2 to $F<1200 ergs cm^-2, depending on the GRB and on the model, allowing us to probe optimistic scenarios of GW production out to distances as far as ~33 Mpc. Advanced detectors are expected to achieve strain sensitivities 10x better than initial LIGO, potentially allowing us to probe the engines of the nearest long GRBs.


arXiv:1309.6221

A directed search for continuous Gravitational Waves from the Galactic Center

Authors: The LIGO Scientific Collaboration, The Virgo Collaboration (Submitted on 24 Sep 2013 (v1), last revised 27 Sep 2013 (this version, v2))

Abstract: We present the results of a directed search for continuous gravitational waves from unknown, isolated neutron stars in the Galactic Center region, performed on two years of data from LIGO's fifth science run from two LIGO detectors. The search uses a semi-coherent approach, analyzing coherently 630 segments, each spanning 11.5 hours, and then incoherently combining the results of the single segments. It covers gravitational wave frequencies in a range from 78 to 496 Hz and a frequency-dependent range of first order spindown values down to -7.86 x 10^-8 Hz/s at the highest frequency. No gravitational waves were detected. We place 90% confidence upper limits on the gravitational wave amplitude of sources at the Galactic Center. Placing 90% confidence upper limits on the gravitational wave amplitude of sources at the Galactic Center, we reach ~3.35x10^-25 for frequencies near 150 Hz. These upper limits are the most constraining to date for a large-parameter-space search for continuous gravitational wave signals.

Report number: LIGO-P1300037


arXiv:1310.2384

Constraints on cosmic (super)strings from the LIGO-Virgo gravitational-wave detectors

Authors: J. Aasi et al. (Submitted on 9 Oct 2013)

Abstract: Cosmic string cusps produce powerful bursts of gravitational waves (GWs). These bursts provide the most promising observational signature of cosmic strings. In this letter we report stringent limits on cosmic string models obtained from the analysis of 625 days of observation with the LIGO and Virgo GW detectors. A significant fraction of the cosmic string parameter space is ruled out. This result complements and improves existing limits from searches for a stochastic background of GWs using cosmic microwave background and pulsar timing data. In particular, if the size of loops is given by gravitational back-reaction, we place upper limits on the string tension $G\mu$ below $10^{-8}$ in some regions of the cosmic string parameter space.


arXiv:1311.2409

Application of a Hough search for continuous gravitational waves on data from the 5th LIGO science run

Authors: The LIGO Scientific Collaboration, The Virgo Collaboration (Submitted on 11 Nov 2013 (v1), last revised 27 Nov 2013 (this version, v2))

Abstract: We report on an all-sky search for periodic gravitational waves in the frequency range $\mathrm{50-1000 Hz}$ with the first derivative of frequency in the range $-8.9 \times 10^{-10}$ Hz/s to zero in two years of data collected during LIGO's fifth science run. Our results employ a Hough transform technique, introducing a $\chi^2$ test and analysis of coincidences between the signal levels in years 1 and 2 of observations that offers a significant improvement in the product of strain sensitivity with compute cycles per data sample compared to previously published searches. Since our search yields no surviving candidates, we present results taking the form of frequency dependent, 95$%$ confidence upper limits on the strain amplitude $h_0$. The most stringent upper limit from year 1 is $1.0\times 10^{-24}$ in the $\mathrm{158.00-158.25 Hz}$ band. In year 2, the most stringent upper limit is $\mathrm{8.9\times10^{-25}}$ in the $\mathrm{146.50-146.75 Hz}$ band. This improved detection pipeline, which is computationally efficient by at least two orders of magnitude better than our flagship Einstein$@$Home search, will be important for "quick-look" searches in the Advanced LIGO and Virgo detector era.

Report number: LIGO Document P1300071