iopscience.org/cqg Classical and Quantum Gravity Highlights of 2010–2011

Read about selected research including: Cosmology Quantum gravity Numerical relativity

…and read about thesis prizes sponsored by Classical and Quantum Gravity Classical and Quantum Gravity Highlights of 2010–2011 All of the articles featured in this brochure are free to read online until 31 December 2012 at iopscience.org/cqg.

Front cover image: An artistic interpretation of the boundary of a 3-vertex foam obtained by merging three graphs Marcin Kisielowski, Jerzy Lewandowski and Jacek Puchta 2012 Class. Quantum Grav. 29 015009. Classical and Quantum Gravity

ISSN 0264-9381 Classical and Quantum Gravity Classical and Quantum Gravity Classicaland Dear colleagues, Volume 28 Number 21 7 November 2011

TOPICAL REVIEW QuantumGravity 213001 Loop quantum cosmology: a status report Abhay Ashtekar and Parampreet Singh Throughout 2011, the quality of Classical and Quantum Gravity (CQG) has PAPERS 215001 Finsler branes and quantum gravity phenomenology with Lorentz symmetry violations SergiuIVacaru 215002 Boundary state stability under spinfoam evolution for the quantum 4-simplex Ma¨ıte´ Dupuis and EteraRLivine

Vol 28, No 21 213001–215023 continued to rise. The journal’s focus issue programme has built considerable 215003 Puncture black hole initial data in the conformal thin-sandwich formalism ThomasWBaumgarte Volume 28 Number 21 7 November 2011 215004 Correspondence between the contracted BTZ solution of cosmological topological massive gravity and two-dimensional Galilean conformal algebra MRSetare andVKamali An international journal of gravitational physics, 215005 The vanishing volume of D =4superspace cosmology, geometry and field theory momentum, publishing an increasing volume of highly interesting work selected Guillaume Bossard,PSHowe,KSStelle and Pierre Vanhove 215006 An all-sky search algorithm for continuous gravitational wavesfrom spinning neutron stars in binary systems Topical review E Goetz andKRiles Loop quantum cosmology: a status report 215007 Quintessence interacting dark energy andascalar dark fluid from 5D vacuum Abhay Ashtekar and Parampreet Singh by the Editorial Board and other top researchers. The average number of LMReyes and Jose´ Edgar Madriz Aguilar 215008 Regular hyperbolicity, dominant energy condition and causality for Lagrangian theories of maps Willie Wai-Yeung Wong 215009 A spacetime not characterized by its invariants is of aligned type II Sigbjørn Hervik downloads per CQG article is greater than ever and CQG’s current impact factor is 215010 Cosmological perturbation analysis inascale invariant model of gravity Pankaj Jain, Purnendu Karmakar, Subhadip Mitra, Sukanta Panda and NaveenKSingh 215011 Matter bounce cosmology with the f(T ) gravity Yi-Fu Cai, Shih-Hung Chen, JamesBDent, Sourish Dutta and EmmanuelNSaridakis 215012 Bertotti–Robinson and Melvin spacetimes at an all-time high of 3.099. D Garfinkle andENGlass 215013 Schrodinger¨ –Newton ‘collapse’ of the wavefunction JRvan Meter 215014 The thermodynamic limit and black hole entropy in the area ensemble J Fernando BarberoGand EduardoJSVillasenor˜ 215015 Entanglement entropy for non-coplanar regions in quantum field theory DavidDBlanco and Horacio Casini 215016 Charge mitigation techniques using glow and corona discharges for advanced We were delighted to see the IOP Gravitational Physics Group’s Thesis gravitational wave detectors P Campsie,LCunningham,MHendry,JHough,SReid,SRowan andGDHammond 7 November 2011 (Continued on inside backcover) iopscience.org/cqg Prize, which is sponsored by CQG, awarded to Barry Wardell in 2011 for the Bibliographic codes CODEN: CQGRDG 28 (21) 213001–215023 (2011) ISSN: 0264-9381 development of groundbreaking computational techniques for obtaining highly CQG28-21cover-UK.indd 2-4 18/10/2011 13:41 accurate expressions for Green functions on curved spacetime. Dr Wardell Impact Factor completed his PhD at University College Dublin under the supervision of * Prof. Adrian C Ottewill. An interview with Dr Wardell can be found on p12. 3.099 CQG maintains a rigorous and fair peer review process that is second to none *As listed in ISI®’s 2010 Science Citation Index Journal citation reports in the field. The quality of the peer review continues to improve as the journal develops its advisory panel of senior referees, the list of whom can be found on p5. Regular paper submissions Never one to shy away from the limelight, the journal even made a brief cameo 900 appearance in CBS’s excellent multi-award-winning sitcom The Big Bang Theory 880 in early 2012 (see p22 for more details).

860

840 submissions

820 Editor-in-Chief 800 Clifford M Will 2008 2009 2010 2011 year

Average full-text downloads per Paper acceptance by region in 2011 Articles published in 2011 by subject area article in year of publication

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70 Africa 1% Asia 9% Black holes 17% Numerical relativity Central/ North Cosmology 22% (and other approximation 0 South America 8% America 23% methods) 4% 2011 Experimental 2008 2009 2010 Western year Australasia 2% studies of gravity 1% Quantum fields in a Eastern Europe 10% Europe 42% General relativity 28% curved spacetime 2% Quantum gravity 9% Middle East 5% Gravitational wave detectors 7% String theory and Higher-dimensional supergravity 7% gravity and other theories of gravity 3%

Highlights of 2010–2011 3 Classical and Quantum Gravity

Contents

page Regular papers 6 •Cosmology 6 •Strings, branes, supergravity and gauge theory 7 •Bergmann Wheeler Thesis Prize 8 •Quantum gravity 10 •Numerical relativity 11 •IOP Gravitational Physics Group Thesis Prize 12 •Mathematical relativity 13 •Geometry and topology 14 •Black holes 14 •Experimental gravity 16 Top 20 most downloaded articles published in 2011 20 Top 20 most cited articles published in 2011 21 Reasons to publish with Classical and Quantum Gravity 22

Journal scope

Published twice monthly (24 issues per year), Classical and Quantum Gravity publishes original research articles on the subjects of gravitational physics and the theory of spacetime. The readership comprises gravitational theorists and experimentalists in physics, mathematics and cosmology.

Papers are published under the following areas: •Classical general relativity •Applications of relativity •Experimental gravitation •Cosmology and the early universe Colour-coded TM reflectivity of a waveguide Quantum gravity structure versus groove depth and waveguide • thickness O Burmeister, D Friedrich, •Supergravity, superstrings and supersymmetry K Danzmann and R Schnabel 2006 Class. •Mathematical physics relevant to gravitation Quantum Grav. 23 7297 The journal publishes the following types of articles: •Research papers. Reports of novel and interesting research work; not normally more than 8500 words (14 journal pages). •Fast Track Communications. Short, timely papers presenting important new developments. Fast Track Communications (FTCs) have a strict length limit of eight journal pages (5000 words). FTCs are given prime importance in the journal. Authors submitting an FTC should accompany their manuscript with a short statement outlining why they believe the work merits high-priority publication. •Special issue articles. Invited papers of high interest, which are included in special issues of the journal. •Topical reviews. Comprehensive review articles commissioned by the Editorial Board. •Brief reviews. Short review articles reviewing rapid, recent progress commissioned by the Editorial Board. •Comments. Brief articles that comment on a previously published Classical and Quantum Gravity article. The authors of the criticized paper may be invited to submit a Reply in response. Comments and Replies are not normally longer than 2500 words each. •Notes. Brief articles that make a short, interesting point, which would not normally merit publication as a full paper but still make a useful and novel addition to the literature. They may highlight an interesting point, clarify a common misunderstanding or confusion, or present a short new result. Unlike Comments, they do not relate to a specific published article. Notes are not normally longer than 2500 words.

More information on each of these areas can be found at iopscience.org/cqg.

4 Highlights of 2010–2011 Classical and Quantum Gravity

Editorial Board

Editor-in-Chief C M Will Washington University, St Louis, USA

Editorial Board L Andersson Max-Planck-Institut für Gravitationsphysik, Golm, Germany S Mukohyama IPMU, University of Tokyo, Japan M-A Bizouard IN2P3-CNRS et Université de Paris-Sud, France N Ó Murchadha University College Cork, Ireland M Campanelli Rochester Institute of Technology, USA E Poisson University of Guelph, Ontario, Canada (Book Reviews Editor) A A Coley Dalhousie University, Halifax, NS, Canada L Rezzolla Max-Planck-Institut für Gravitationsphysik, Golm, Germany A Corichi Universidad Nacional Autónoma de México (UNAM), Morelia, Mexico V O Rivelles Universidade de Sao Paulo (USP), Sao Paulo, Brazil S Dhurandhar Inter-University Centre for Astronomy and Astrophysics, Pune, S F Ross Durham University, Durham, UK India C Rovelli Université de la Mediterranée–Aix-Marseille II, France D Garfinkle Oakland University, Rochester, MI, USA J Samuel Raman Research Institute, Bangalore, India J Isenberg University of Oregon, Eugene, USA M Sasaki Kyoto University, Japan K Kuroda University of Tokyo, Japan J M M Senovilla Universidad del País Vasco, Bilbao, Spain D Langlois Université Paris 7 – Denis Diderot, Paris, France C C Speake University of Birmingham, Birmingham, UK R B Mann University of Waterloo, Ontario, Canada D Sudarsky Universidad Nacional Autónoma de México (UNAM), Mexico D F, D Marolf University of California, Santa Barbara, USA México M Mars Universidad de Salamanca, Spain R-S Tung Nanyang Technological University, Singapore P R L V Moniz Universidade da Beira Interior, Covilhã, Portugal D L Wiltshire University of Canterbury, New Zealand

Advisory Panel C Barcelo Instituto de Astrofisica de Andalucia, CSIC, Spain L Lehner Perimeter Institute for Theoretical Physics and University of Guelph, R-G Cai Chinese Academy of Sciences, China Ontario, Canada S Carlip University of California, Davis, USA J Louko University of Nottingham, UK M Cavaglia University of Mississippi, USA B Mours Laboratoire d’Annecy-le-vieux de Physique des Particules, France P Chrus´ciel University of Vienna, Austria W-T Ni Tsing Hua National University, Hsinchu, Taiwan C Chryssomalakos Universidad Nacional Autónoma de México, Mexico B Nolan Dublin City University, Ireland S Deser California Institute of Technology, USA S Odintsov Space Research Inst and ICREA, Spain B Dittrich Max-Planck-Institut für Gravitationsphysik, Potsdam, Germany G Papadopoulos King’s College London, University of London, UK F Dowker Imperial College London, UK T Pawlowski Consejo Superior de Investigaciones Cientificas, Spain S Fairhurst Cardiff University, UK H Pfeiffer Canadian Institute for Theoretical Astrophysics, Canada V Ferrari Università di Roma ‘La Sapienza’, Italy J Pullin Louisiana State University, USA V P Frolov University of Alberta, Edmonton, Canada A D Rendall Max-Planck-Institut für Gravitationsphysik, Golm, Germany J Gair Cambridge University, UK N Robertson California Institute of Technology, USA and University of M Giovannini CERN, Switzerland Glasgow, UK G Gonzalez Louisiana State University, Baton Rouge, USA M Sakellariadou King’s College London, University of London, UK R Gregory Durham University, UK O Sarbach Universidad Michoacana de San Nicolas de Hidalgo, Mexico S Hartnoll Stanford University, USA S Scott Australian National University, Australia J M Heinzle Universität Wien, Austria J Simon Edinburgh University, UK G T Horowitz University of California, Santa Barbara, USA C F Sopuerta Institut de Ciències de l’Espai (CSIC-IEEC), Spain O Jennrich European Space Agency/ESTEC, Netherlands A Starobinsky Russian Academy of Sciences, Russia V Karas Astronomical Institute, Academy of Sciences, Prague, Czech L B Szabados Research Institute for Particle and Nuclear Physics, Hungary Republic N Van den Bergh Ghent University, Belgium C Kiefer Universität zu Köln, Germany L Vanzo Università Degli Studi di Trento, Italy K Koyama University of Portsmouth, UK R M Wald University of Chicago, USA K Krasnov University of Nottingham, UK D Wands University of Portsmouth, UK

Editorial Board and Advisory Panel The CQG Editorial Board is a cross-section of the best gravitational physicists from around the world. The Board oversees the scientific strategy of the journal and provides expert advice on the CQG peer review.

The CQG Advisory Panel (AP) is a selection of the best referees in gravitational physics. Members are invited to the panel after showing excellent judgment as regular referees for CQG. The AP members provide quick and authoritative advice, which adds enormous value to the CQG peer review. Editor-in-Chief Clifford M Will

Highlights of 2010–2011 5 Classical and Quantum Gravity

q Cosmology Horˇava–Lifshitz cosmology: Review Article a review Effective constraints and physical coherent states in quantum cosmology: a numerical Shinji Mukohyama comparison 2010 Class. Quantum Grav. 27 223101

Here we review the basic construction and cosmological implications of a Martin Bojowald and Artur Tsobanjan power-counting renormalizable theory of gravitation, recently proposed by Horˇava. We explain that (i) at low energy this theory does not exactly recover 2010 Class. Quantum Grav. 27 145004 general relativity but instead mimics general relativity plus dark matter; (ii) higher spatial curvature terms allow bouncing and cyclic universes as In an earlier work, the authors have extended a recently developed regular solutions; (iii) the anisotropic scaling with the dynamical critical approximate method for computing back-reaction of a sharply peaked exponent z = 3 solves the horizon problem and leads to scale-invariant wavefunction on the trajectory traced by the expectation values of cosmological perturbations even without inflation. We also comment on coordinates and momenta. The method is applicable to quantum systems issues related to an extra scalar degree of freedom called scalar graviton. In governed by a Hamiltonian constraint equation such as the Wheeler- particular, for spherically-symmetric, static, vacuum configurations we prove DeWitt equation of quantum cosmology. Here, this technique is applied non-perturbative continuity of the λ → 1 + 0 limit, where λ is a parameter in alongside a direct method, which relies on explicitly evolving coherent the kinetic action and general relativity has the value λ = 1. We also derive the wave-packets, and compared using numerical predictions in the context condition under which linear instability of the scalar graviton does not show up. of a cyclic cosmological model. The comparison indicates that, as long as the wave-packet remains sharply peaked, the approximate technique accurately captures the leading order behavior of expectation values and their spreads, while avoiding several cumbersome details associated with Conservation of the nonlinear curvature the explicit evolution of wave-packets. perturbation in generic single-field inflation Figure 1. Classical (dotted), coherent state (solid) and effective (dashed) phase space trajectories, Atsushi Naruko and Misao Sasaki evolved for 0 G β+ G 5α0. Fast Track Communications

2011 Class. Quantum Grav. 28 072001

In this paper, we answer the following question: Is the conservation of the curvature perturbation on superhorizon scales universal?. It is known that the curvature perturbation on uniform energy density slices on superhorizon scales is conserved to full nonlinear order if the pressure is only a function of the energy density, independent of the gravitational theory. We explicitly Atsushi Naruko show that the same conservation holds for a universe Aperiodic oscillatory asymptotic behavior for dominated by a single scalar field provided that the field is in an attractor regime, for a very general class of scalar-field theories. However, we also show some Bianchi spacetimes that if the scalar-field equation contains a second time derivative of the metric, one has to invoke the gravitational-field equations to show the conservation. François Béguin

2010 Class. Quantum Grav. 27 185005

We study the asymptotic behavior of vacuum Bianchi type A spacetimes Focus section: inhomogeneous cosmological models close to their singularity. It has been conjectured that this behavior is driven by a certain circle map, called the Kasner map. As a step toward In this focus section, we concentrate on the physical this conjecture, we prove that some orbits of the Kasner map do indeed state of the current universe and the problem of going attract some solutions of the system of ODEs which describes the behavior beyond perturbation theory. The following topics are of vacuum Bianchi type A spacetimes. The orbits of the Kasner map for covered: a general overview and a discussion of the which we can prove such a result are those which are not periodic and do relevant issues, inhomogeneous cosmological models, not accumulate on any periodic orbit. This shows the existence of Bianchi Lars Andersson the current observations and physics of the universe, and spacetimes with aperiodic oscillatory asymptotic behavior. Guest Editor averaging and backreaction. We note that some of the backreaction discussion may be applicable to quantum gravity. A number of fundamental issues in inhomogeneous cosmology have been addressed in this focus section. We invite you to read the articles in CQG Volume 28 Issue 16.

6 Highlights of 2010–2011 Classical and Quantum Gravity q Strings, branes, supergravity and On the thermodynamics of moving bodies gauge theory Jorge G Russo and Paul K Townsend

Emergent geometry and gravity from matrix 2010 Class. Quantum Grav. 27 175005 models: an introduction In precise analogy with Unruh’s result for black holes, the temperature registered by a particle-detector near a Harold Steinacker black p-brane horizon can be explained as a quantum kinematical effect due to acceleration in a (2+p)- 2010 Class. Quantum Grav. 27 133001 dimensional Minkowski space. For p=2 this picture allows a simple computation of the detector response to An introductory review to emergent noncommutative gravity within constant velocity motion in a heat bath, as a function of Jorge G Russo Yang–Mills matrix models is presented. Spacetime is described as a velocity, angle and ‘photon’ frequency. noncommutative brane solution of the matrix model, i.e. as a submanifold of For non-relativistic velocities and ultra-violet frequencies, where the thermal RD . Fields and matter on the brane arise as fluctuations of the bosonic resp. excitations of a quantum field behave like a classical gas of particles, we fermionic matrices around such a background, and couple to an effective recover the standard formula obtained by Peebles and Wilkinson in the metric interpreted in terms of gravity. Suitable tools are provided for the context of the Cosmic Microwave Background Radiation (CMBR). However, we description of the effective geometry in the semi-classical limit. The relation find a deviation from their formula away from the UV limit due to a frequency- to non-commutative gauge theory and the role of UV/IR mixing are explained. dependence of the dipole contribution, and very different behaviour at Several types of geometries are identified, in particular ‘harmonic’ and relativistic velocities. We discuss possible implications for the CMBR. ‘Einstein’ types of solutions. The physics of the harmonic branch is discussed in some detail, emphasizing the non-standard role of vacuum energy. This may provide a new approach to some of the big puzzles in this context. The IKKT model with D = 10 and close relatives are singled out as promising candidates for quantum theory of fundamental interactions including gravity.

Figure 1. Sketch of an embedding function g(x) with short-scale perturbations and long- distance oscillations.

Figure 1. Spectra at (a) v = 0.04 and (b) 0.8, and ϕ = 0. The dashed line represents the Doppler- shifted spectrum.

Integrability of five-dimensional minimal Focus issue: string cosmology supergravity and charged rotating black holes This special issue appraises recent applications of string-theoretic and Pau Figueras, Ella Jamsin, Jorge V Rocha and Amitabh Virmani string-inspired ideas to the cosmos. The issue features, among other topics: 2010 Class. Quantum Grav. 27 135011 inflationary scenarios, alternatives to conventional inflation, dark matter/ We explore the integrability of five-dimensional minimal supergravity in Vijay Paulo R L V energy models, string gas cosmology, the presence of three commuting Killing vectors. We argue that to see Balasubramanian Moniz and cosmic scenarios arising from the Guest Editor Guest Editor the integrability structure of the theory one necessarily has to perform an landscape of string vacua. Ehlers reduction to two dimensions. A direct dimensional reduction to two As cosmology develops into a rigorous, data-driven scientific discipline, dimensions does not allow us to see the integrability of the theory in an windows into earlier epochs and higher energies are becoming available. easy way. This situation is in contrast with vacuum five-dimensional gravity. A particular focus of this issue is the exciting possibility of using We derive the Belinski–Zakharov (BZ) Lax pair for minimal supergravity cosmological experiments to probe string theory. based on a symmetric 7 × 7 coset representative matrix for the coset R R G2(2)/(SL (2, ) x SL (2, )). We elucidate the relationship between our BZ Read the invited articles in CQG Volume 28 Issue 20. Lax pair and the group theoretic Lax pair previously known in the literature. The BZ Lax pair allows us to generalize the well-known BZ dressing method to five-dimensional minimal supergravity. We show that the action of the three-dimensional hidden symmetry transformations on the BZ dressing method is simply the group action on the BZ vectors. As an illustration of Forthcoming special issue our formalism, we obtain the doubly spinning five-dimensional Myers–Perry black hole by applying solitonic transformations on the Schwarzschild black hole. We also derive the Cveticˇ–Youm black hole by applying solitonic Autumn 2012 transformations on the Reissner–Nordström black hole. Focus issue: applications of holography to condensed matter systems Guest Editor: Simon F Ross

Highlights of 2010–2011 7 Classical and Quantum Gravity

The Bergmann-Wheeler Thesis Prize

The Bergmann-Wheeler Thesis Prize, sponsored by Classical and Quantum Gravity, was instituted in 2008 by the International Society for General Relativity and Gravitation. It celebrates two great pioneers of quantum gravity: Peter Bergmann (1915–2002) and John Wheeler (1911–2008).

The prize is awarded every three years for the best PhD thesis in the broad area encompassing all approaches to quantum gravity. The winner of the next prize will receive a cheque for $1640 and a certificate.

The full guidelines for nominating a candidate for the prize can be found at http://isgrg.org.

The first winner of the prize, in 2010, was Dr Victor Taveras, who completed his PhD under the supervision of Prof. Abhay Ashtekar at Pennsylvania State University. The award was made in recognition of Dr Taveras’s contributions to loop quantum cosmology.

Carlo Rovelli Fellow of the International Society for General Relativity and Gravitation

Interview with Dr Victor Taveras Bergmann-Wheeler Thesis Prize winner

What led you into science and your chosen area of What are you working on at the moment? research? I’m teaching full time but I’m also working on extending part of the work I was always interested in astronomy at an early in my thesis. Part of the work in my thesis involved finding quantum age and read every book that I could get. In college corrections to the Friedmann equations. Those results were for a I was fortunate enough to be able to learn quantum massless scalar field. I’m extending this work for arbitrary matter. field theory on curved spacetimes. When I went to graduate school it was natural to continue and join What do you think will be the next big breakthrough in gravitational the loop quantum gravity group. physics? I’m really excited about Advanced LIGO. I think their results should be What do you find most interesting about this subject? interesting and I hope to see some evidence of gravitational waves. I really like that it is trying to reconcile quantum theory with gravity. These have always been my two favourite subjects in physics. They each have What advice would you give to young scientists in the early stages of their own regimes of applicability and have issues when we try to push their career? them too far. I would suggest that they focus on their coursework. A solid foundation in the fundamentals is necessary in order to be able to do advanced work. What was the title of your thesis and is it available online? I would also suggest that they find a faculty mentor or someone in their “Loop Gravity: An Application and an Extension”. It is available from field that they can meet with regularly to help keep them on course. the Pennsylvania State University electronic thesis repository (http:// etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-4378/ What has been the most exciting moment in your career so far? index.html). It sounds kind of cheesy but I’ve had a bunch of moments where I have run into some of my old students who ended up being scientists or engineers and they’ve actually thanked me for teaching them physics.

8 Highlights of 2010–2011 Classical and Quantum Gravity

at zero temperature. This solution has vanishing entropy density, and the New stability results for Einstein scalar gravity charge density in the bulk is carried entirely by fluxes. The dimensionless magnetic field to charge density ratio for these solutions is bounded Thomas Faulkner, Gary T Horowitz and Matthew M Roberts from below, with a quantum critical point appearing at the lower bound. Using matched asymptotic expansions, we extract the low temperature 2010 Class. Quantum Grav. 27 205007 thermodynamics of the system. Above the critical magnetic field, the low temperature entropy density takes a simple form, linear in the temperature, We consider asymptotically anti de Sitter gravity coupled to a scalar field and with a specific heat coefficient diverging at the critical point. At the 1/3 with mass slightly above the Breitenlohner–Freedman bound. This theory critical magnetic field, we derive the scaling law s ~ T inferred previously admits a large class of consistent boundary conditions characterized by from the numerical analysis. We also compute the full scaling function an arbitrary function W. An important open question is to determine which describing the region near the critical point and identify the dynamical W admit stable ground states. It has previously been shown that the total critical exponent: z = 3. These solutions are expected to holographically energy is bounded from below if W is bounded from below, and the bulk represent boundary theories in which strongly interacting fermions are filling scalar potential V(z) admits a suitable superpotential. We extend this result up a Fermi sea. They are fully top–down constructions in which both the bulk and show that the energy remains bounded even in some cases where W can and boundary theories have well-known embeddings in the string theory. become arbitrarily negative. As one application, this leads to the possibility Figure 1. Schematic phase that in gauge/gravity duality, one can add a double trace operator with diagram illustrating the various negative coefficient to the dual field theory and still have a stable vacuum. behaviors of the entropy density versus temperature and magnetic field. The region inside the dashed line is controlled by the quantum critical point at (ˆT = 0, Bˆ = Bˆ ), and the entropy Invited Article c Lectures on N = 2 gauge theory Special density can be expressed in terms of a single scaling function ˆ ˆ 2/3 f of (B − Bc)/T . We move Davide Gaiotto around inside this region by changing the temperature ˆT and the relevant coupling Bˆ − Bˆ . The 2010 Class. Quantum Grav. 27 214002 c boundary of the region is defined to be where irrelevant operators become important. The yellow region denotes a regime where In these two lectures I will review classic results on four-dimensional N = 2 temperature is the largest energy scale, corresponding to the argument of the scaling function supersymmetric gauge theory, which are a useful prerequisite to learn more f being small. Outside the yellow region the low temperature behavior of the entropy density, for fixed Bˆ, is either constant or linear in ˆT, depending on whether the quantum critical point is recent advances in the field. The lectures were given at the CERN Winter ˆ ˆ approached from below or from above Bc as T → 0. School on Supergravity, Strings and Gauge Theory (CERN, 25–29 January 2010).

The fluid/gravity InvitedSpecial Article correspondence:a new perspective Special issue: 2010 CERN Winter School on the membrane paradigm The Winter School on Supergravity, Strings and Gauge Theory, took place at CERN on 25–29 January, 2010. Veronika E Hubeny This school was meant primarily for training doctoral students and young postdoctoral researchers in recent 2011 Class. Quantum Grav. 28 114007 Johannes Walcher developments in theoretical high-energy physics and Guest Editor string theory. The topics of the lecture series were This paper gives an overview of the recently-formulated fluid/gravity chosen to represent a broad section of current research, from cosmology correspondence, which was developed in the context of gauge/gravity and string phenomenology to mathematical physics, and gauge theory duality. Mathematically, it posits that Einstein’s equations (with negative to properties of quantum black holes. cosmological constant) in d + 1 dimensions capture the (generalized) For further information and to read the invited articles see Navier–Stokes equations in d dimensions. Given an arbitrary fluid CQG Volume 27 Issue 21. dynamical solution, we can systematically construct a corresponding asymptotically AdS black hole spacetime with a regular horizon whose properties mimic that of the fluid flow. Apart from an overview of this construction, we describe some of its applications and implications.

Figure 3. The causal structure of the Magnetic field-induced quantum criticality spacetimes dual to fluid mechanics illustrating the tube structure. The via new asymptotically AdS5 solutions dashed line denotes the future event horizon H+ generated by ξA, while the shaded tube indicates the region Eric D’Hoker and Per Kraus of spacetime over which the solution is well approximated by a tube of the 2010 Class. Quantum Grav. 27 215022 uniform black brane.

Using analytical methods, we derive and extend previously obtained numerical results on the low temperature properties of holographic duals to four-dimensional gauge theories at finite density in a nonzero magnetic field. We find a new asymptotically AdS5 solution representing the system

Highlights of 2010–2011 9 Classical and Quantum Gravity

Surprising connections between InvitedSpecial Article Quantum fluids, Josephson tunneling and general relativity and condensed gravitational waves matter Francesco Sorge

Gary T Horowitz 2010 Class. Quantum Grav. 27 225001

2011 Class. Quantum Grav. 28 114008 We study Josephson tunnelling for a quantum fluid (Bose-Einstein condensate) in the presence of a weak This article is intended to introduce gravitational physicists to recent gravitational wave. Working in the linearized gravity we developments in which general relativity is being used to describe certain derive the Gross–Pitaevskii equation for the quantum aspects of condensed matter systems, e.g., superconductivity. fluid. We apply such an equation to a quantum fluid confined in a double-well trap, made of two identical Francesco Sorge boxes placed orthogonally to each other and weakly Special issue: GR19 coupled through a Josephson junction. We show that the trap geometry The 19th International Conference on influences the coupling between the gravitational wave and the field modes, General Relativity and Gravitation (GR19) giving rise to a relative phase shift of the condensate wavefunction in the was held in Mexico City on 6–9 July 2010. two sides of the trap. This leads to a gravitationally induced ac-Josephson effect. We discuss possible experimental detection of such an effect for This is the principal international superconductors. Daniel Sudarsky Don Marolf conference for scientists working Guest Editor Guest Editor Figure 1. A sketch of the double- in the areas of relativity, gravitation trap considered in the text, with and related topics. The conference covered a wide range of subjects the adopted reference frame. Two including classical relativity, relativistic astrophysics and cosmology, identical coupled traps (1) and experimental gravity, and quantum issues in gravitation. (2) are rotated by π/2 about the z-axis with respect to each other. Read the invited articles in CQG Volume 28 Issue 11. The gravitational wave propagates along the z-direction. The gray region between the traps represents the small ‘channel’ allowing for Josephson tunneling. q Quantum gravity

Polarization diffusion from spacetime An alternative approach for general covariant uncertainty Horˇava–Lifshitz gravity and matter coupling

Carlo R Contaldi, Fay Dowker and Lydia Philpott Alan M da Silva 2010 Class. Quantum Grav. 27 172001 2011 Class. Quantum Grav. 28 055011 A model of Lorentz invariant random fluctuations in photon polarization is presented. The effects are frequency dependent and affect the polarization Recently, Horˇava and Melby-Thompson proposed of photons as they propagate through space. We test for this effect by a nonrelativistic gravity theory with extended gauge confronting the model with the latest measurements of polarization of symmetry that is free of the spin-0 graviton. We propose cosmic microwave background photons. a minimal substitution recipe to implement this extended gauge symmetry which reproduces the results obtained by them. Our prescription has the advantage Alan M da Silva of being manifestly gauge invariant and immediately Probing the quantum-gravity realm with slow generalizable to other fields, like matter. We briefly discuss the coupling of atoms gravity with scalar and vector fields found by our method. We show also that the extended gauge invariance in gravity does not force the value of λ to be λ = 1. However, the spin-0 graviton is eliminated even for general λ. Flavio Mercati, Diego Mazón, Giovanni Amelino-Camelia, José Manuel Carmona, José Luis Cortés, Javier Induráin, Claus Lämmerzahl and Guglielmo M Tino

2010 Class. Quantum Grav. 27 215003 Focus section: 25 years of quantum gravity This focus section features invited review articles from For the study of Planck-scale modifications of the energy–momentum physicists who have long been associated with loop dispersion relation, which had been previously focused on the implications quantum gravity and string theory. for ultrarelativistic particles, we consider the possible role of experiments involving nonrelativistic particles, and particularly atoms. We extend a The focus section gives a comparative perspective on recent result establishing that measurements of ‘atom-recoil frequency’ Joseph Samuel these programmes. Reviewers were asked to critically can provide an insight that is valuable for some theoretical models. From a Guest Editor evaluate progress in their field over the last 25 years. broader perspective we analyze the complementarity of the nonrelativistic Read the topical reviews in CQG Volume 28 Issue 15. and the ultrarelativistic regimes in this research area.

10 Highlights of 2010–2011 Classical and Quantum Gravity

Operator spin foam models q Numerical relativity

Benjamin Bahr, Frank Hellmann, Wojciech Kamin´ski, Marcin Kisielowski and Jerzy Lewandowski Numerical viscosity in hydrodynamics simulations in general relativity 2011 Class. Quantum Grav. 28 105003

In this article we introduce a systematic approach to Pablo Cerdá-Durán state-sum models for quantum gravity, called spin foam operators. These are discretizations of the path 2010 Class. Quantum Grav. 27 205012 integral for gravity on a 2-complex, and a state is given by an assignment of operators to edges and group We present an alternative method to estimate the numerical viscosity in representations to faces of the complex. A large class simulations of astrophysical objects, which is based on the damping of of models falls into this framework, including the spin fluid oscillations. We apply the method to general relativistic hydrodynamic Benjamin Bahr foam models underlying loop quantum gravity, where simulations using spherical coordinates. We perform 1D spherical and 2D the choice of quantization of the simplicity constraints is directly translated axisymmetric simulations of radial oscillations in spherical systems. We first into the choice of edge-operators. We pay particular attention to models calibrate the method with simulations with physical bulk viscosity and study which are invariant under subdivision of edges and faces, behave nicely the differences between several numerical schemes. We apply the method to under gluing of 2-complexes, and discuss the EPRL-FK model and the BC- radial oscillations of neutron stars and we conclude that the main source of model, understood in this framework. numerical viscosity in this case is the surface of the star. We expect that this method could be useful to compute the resolution requirements and limitations of the numerical simulations in different astrophysical scenarios in the future.

Two-dimensional quantum black holes: numerical methods

Fethi M Ramazanoglu and Frans Pretorius

Figure 6. Invariance under the edge subdivision. 2010 Class. Quantum Grav. 27 245027

We present details of a new numerical code designed to study the formation Curvature in spinfoams and evaporation of two-dimensional black holes within the Callan–Giddings– Harvey–Strominger model. We explain several elements of the scheme Elena Magliaro and Claudio Perini that are crucial to resolve the late-time behavior of the spacetime, including regularization of the field variables, compactification of the coordinates, the 2011 Class. Quantum Grav. 28 145028 algebraic form of the discretized equations of motion and the use of a modified Richardson extrapolation scheme to achieve high-order convergence. Physical In spinfoam quantum gravity, the transition amplitudes interpretation of our results will be discussed in detail elsewhere. can be defined in a truncation of the theory to a finite spacetime triangulation. The semiclassical limit of the truncated amplitudes is currently understood in terms of the large spin expansion. We apply this expansion Special issue: NRDA/Capra 2011 in order to show for a simple triangulation that the spinfoam amplitude is peaked on a nontrivial (curved) The Theory Meets Data Claudio Perini classical geometry. The result suggests a regime in Analysis at Comparable which spinfoams reproduce the version of Einstein equations given by and Extreme Mass Regge calculus. Ratios Conference (NRDA/Capra 2010) Figure 1. Spinfoam diagram σ dual to the 3 Luis Lehner Harald Pfeiffer Eric Poisson was held at the simplicial triangulation obtained by glueing Guest Editor Guest Editor Guest Editor three 4-simplices on a common triangle. Perimeter Institute for Two 4-simplices share a tetrahedron. The Theoretical Physics, Waterloo, Canada, on 20–26 June 2010. spinfoam face f dual to the triangle is

‘colored’ with the internal spin jf. This conference brought together researchers from three gravitational wave communities – numerical relativity, perturbation theory and data analysis – who discussed the latest advances and challenges for detecting and exploiting gravitational waves. CQG Volume 28 Issue 13 contains selected articles that feature the best work presented at this conference.

Highlights of 2010–2011 11 Classical and Quantum Gravity

IOP Gravitational Physics Group Thesis Prize

The Gravitational Physics Group Thesis Prize, sponsored by Classical and Quantum Gravity, is awarded for excellence in postgraduate research and communication skills in gravitational physics.

All members of the IOP Gravitational Physics Group (GPG) who passed their PhD viva voce exam between 30 September 2010 and 1 October 2012 are entitled to enter the competition. Nominations should be made by the end of November 2012.

The winner will receive £500 and will be invited to submit a paper to CQG based on the winning thesis, which, if accepted, will be made an “IOP Select” article in the journal.

Nils Andersson Further information about the prize and the nomination procedure can be found on the IOP GPG website at http://gp.iop.org/. Honorary Chair Gravitational The 2010 prize was awarded to Dr Barry Wardell, who completed his PhD at University College Dublin under the supervision Physics Group of Prof. Adrian C Ottewill. The award was made for Dr Wardell’s development of groundbreaking computational techniques for obtaining highly accurate expressions for Green functions on curved spacetime.

Interview with Dr Barry Wardell Gravitational Physics Group Thesis Prize winner

What led you into science and your chosen area of What are you working on at the moment? research? I am involved in a variety of projects mostly related to the study of I have always been interested in physics and black holes as sources of gravitational radiation. I am working on both mathematics. My father is a physicist, so this was a numerical relativity simulations of binary black hole systems and on big inspiration when I was growing up. My specific analytic techniques for studying perturbations of black hole spacetimes. area of research was strongly influenced by my PhD advisor who gave me a lot of direction towards my What do you think will be the next big breakthrough in gravitational current research interests. physics? This will undoubtedly be the detection of gravitational waves by LIGO. What do you find most interesting about this subject? I like the idea that, while it may not be the final theory, general relativity What advice would you give to young scientists in the early stages of is the most complete theory that we have of how our universe works. their career? I also enjoy the fact that the subject lends itself so well to combining Be persistent and never give up when things aren’t going well. Always do theoretical and analytic methods with numerical simulations. what you enjoy doing.

What was the title of your thesis and is it available online? What has been the most exciting moment in your career so far? The title of my thesis is “Green Functions and Radiation Reaction from a Giving my first big conference talk. It was both terrifying and exciting at Spacetime Perspective”. It is available online on the arXiv (http://arxiv. the same time. org/abs/0910.2634).

12 Highlights of 2010–2011 Classical and Quantum Gravity

stress–energy tensor as they appear in its laws of motion. All effects on the Linearized perturbations of the Kerr motion other than these (finite) renormalizations arise from a difference spacetime and outer boundary conditions in field satisfying the vacuum field equations. This vacuum field has a much simpler structure than the physical one in typical applications. numerical relativity

E Deadman and J M Stewart Spacetime extensions II

2011 Class. Quantum Grav. 28 015003 István Rácz

This paper discusses linearized vacuum gravitational perturbations of 2010 Class. Quantum Grav. 27 155007 the Kerr spacetime in a neighbourhood of future null infinity J+ . Unlike earlier discussion of perturbations of the Kerr spacetime we avoid the The global extendibility of smooth causal geodesically incomplete use of spheroidal harmonics and harmonic time dependence. Instead we spacetimes is investigated. Denote by γ one of the incomplete non- develop the theory in terms of Hertz potentials and spherical harmonics with extendible causal geodesics of a causal geodesically incomplete spacetime coupling between modes. The ‘master equation’ is a single complex scalar (M, g ). First, it is shown that it is always possible to select a synchronized wave equation which, in the Minkowski limit, reduces to the Euler–Poisson– ab family of causal geodesics Γ and an open neighbourhood U of a final Darboux equation. We solve this by Picard iteration making extensive use of segment of γ in M such that U comprises members of Γ, and suitable the flat spacetime Riemann–Green function. As an application we consider local coordinates can be defined everywhere on U provided that γ does the problem of outer boundary conditions for numerical relativity and not terminate either on a tidal force tensor singularity or on a topological generalize earlier results of Buchman and Sarbach (2006 Class. Quantum singularity. It is also shown that if, in addition, the spacetime (M, gab) is Grav. 23 6709–44, 2007 Class. Quantum Grav. 24 S307–26) for the globally hyperbolic, and the components of the curvature tensor, and its Schwarzschild case. covariant derivatives up to order k − 1 are bounded on U , and also the line integrals of the components of the kth-order covariant derivatives are finite along the members of Γ—where all the components are meant to be On critical collapse of gravitational waves registered with respect to a synchronized frame field on U —then there k − – – – exists a C extension Φ : (M, gab) → (M, gab) so that for each γ ∈ Γ , which – – – Evgeny Sorkin is inextendible in (M, gab), the image, Φ o γ , is extendible in (M, gab). Finally, it is also proved that whenever γ does terminate on a topological singularity (M, g ) cannot be generic. 2011 Class. Quantum Grav. 28 025011 ab

An axisymmetric collapse of non-rotating gravitational waves is numerically investigated in the subcritical regime where no black holes form but where Einstein gravity as a 3D conformally invariant curvature attains a maximum and decreases, following the dispersion of the initial wave packet. We focus on a curvature invariant with dimensions of theory length, and find that near the threshold for black hole formation it reaches a maximum along concentric rings of finite radius around the axis. In this Henrique Gomes, Sean Gryb and Tim Koslowski regime the maximal value of the invariant exhibits a power-law scaling with the approximate exponent 0.38, as a function of a parametric distance from 2011 Class. Quantum Grav. 28 045005 the threshold. In addition, the variation of the curvature in the critical limit is accompanied by increasing amount of echos, with nearly equal temporal and We give an alternative description of the physical content of general spatial periods. The scaling and the echoing patterns, and the corresponding relativity that does not require a Lorentz invariant spacetime. Instead, we constants, are independent of the initial data and coordinate choices. find that gravity admits a dual description in terms of a theory where local size is irrelevant. The dual theory is invariant under foliation-preserving 3-diffeomorphisms and 3D conformal transformations that preserve q Mathematical relativity the 3-volume (for the spatially compact case). Locally, this symmetry is identical to that of Horˇava–Lifshitz gravity in the high energy limit but our theory is equivalent to Einstein gravity. Specifically, we find that the solutions of general relativity, in a gauge where the spatial hypersurfaces Effective stress–energy tensors, self-force have constant mean extrinsic curvature, can be mapped to solutions of and broken symmetry a particular gauge fixing of the dual theory. Moreover, this duality is not accidental. We provide a general geometric picture for our procedure that allows us to trade foliation invariance for conformal invariance. The dual Abraham I Harte theory provides a new proposal for the theory space of quantum gravity.

2010 Class. Quantum Grav. 27 135002

This paper considers the effects of self-interaction on the motion of charged particles in curved spacetimes. Forthcoming special issue Such problems are difficult mainly because the majority of a body’s own field does not directly affect its motion. Autumn 2012 Building upon previous work, this paper makes such statements precise. A certain solution of the field Focus issue: non-astrophysical numerical relativity Abraham I Harte equations is shown only to exert forces and torques that Guest Editors: David Garfinkle and Luis Lehner effectively shift the multipole moments of a charge’s

Highlights of 2010–2011 13 Classical and Quantum Gravity

Singularity theorems from weakened energy Optical structures, algebraically special conditions spacetimes, and the Goldberg–Sachs theorem in five dimensions Christopher J Fewster and Gregory J Galloway

Arman Taghavi-Chabert 2011 Class. Quantum Grav. 28 125009

We establish analogues of the Hawking and Penrose singularity theorems 2011 Class. Quantum Grav. 28 145010 based on (a) averaged energy conditions with exponential damping; (b) conditions on local stress–energy averages inspired by the quantum Optical (or Robinson) structures are one generalization of four-dimensional energy inequalities satisfied by a number of quantum field theories. As shearfree congruences of null geodesics to higher dimensions. They are particular applications, we establish singularity theorems for the Einstein Lorentzian analogues of complex and CR structures. In this context, we equations coupled to a classical scalar field, which violates the strong extend the Goldberg–Sachs theorem to five dimensions. To be precise, we energy condition, and the nonminimally coupled scalar field, which also find a new algebraic condition on the Weyl tensor, which generalizes the violates the null energy condition. Petrov type II condition, in the sense that it ensures the existence of such congruences on a five-dimensional spacetime, vacuum or under weaker assumptions on the Ricci tensor. This results in a significant simplification of the field equations. We discuss possible degenerate cases, including a five- q Geometry and topology dimensional generalization of the Petrov type D condition. We also show that the vacuum black ring solution is endowed with optical structures, yet fails to be algebraically special with respect to them. We finally explain the generalization of these ideas to higher dimensions, which has been Tomimatsu–Sato geometries, holography checked in six and seven dimensions. and quantum gravity

Jack Gegenberg, Haitao Liu, Sanjeev S Seahra and Benjamin K Tippett q Black holes

2011 Class. Quantum Grav. 28 085004 Uniqueness of static black holes without We analyze the δ = 2 Tomimatsu–Sato spacetime in the context of the proposed Kerr/CFT correspondence. This four-dimensional vacuum analyticity spacetime is not only asymptotically flat and has a well-defined ADM mass and angular momentum but also involves several exotic features including Piotr T Chrus´ciel and Gregory J Galloway a naked ring singularity, and two disjoint Killing horizons separated by a region with closed timelike curves and a rod-like conical singularity. We Fast Track Communications demonstrate that the near-horizon geometry belongs to a general class of Ricci-flat metrics with SL (2, R) × U (1) symmetry that includes both the extremal Kerr and extremal Kerr–Bolt geometries. We calculate the central 2010 Class. Quantum Grav. 27 152001 charge and temperature for the CFT dual to this spacetime and confirm that the Cardy formula reproduces the Bekenstein–Hawking entropy. We find We show that the hypothesis of analyticity in the uniqueness theory that all of the basic parameters of the dual CFT are most naturally expressed of vacuum, or electrovacuum, static black holes is not needed. More in terms of charges defined intrinsically on the horizon, which are distinct generally, we show that prehorizons covering a closed set cannot occur in from the ADM charges in this geometry. well-behaved domains of outer communications.

Figure 3. Contour plot of the Kretschmann scalar −4 αβγ δ tanh(σ Rαβγ δR ) in the (ρ, z) plane for p = 1/3. The center of the flower-shaped region is the ring singularity. DID YOU KNOW? All of the papers featured in the Highlights of 2010–2011 brochure are free to read on the CQG website until 31 December 2012. Visit iopscience.org/cqg for more information.

14 Highlights of 2010–2011 Classical and Quantum Gravity

and modifies the phase space of solutions. We recover analytically many Next-to-leading-order spin–orbit effects in the properties first identified by Christodoulou, in particular the loss of regularity motion of inspiralling compact binaries (especially at the center), and then investigate numerically the properties of these spacetimes.

Rafael A Porto

2010 Class. Quantum Grav. 27 205001

Using effective field theory (EFT) techniques, we calculate the next-to- leading-order (NLO) spin–orbit contributions to the gravitational potential of inspiralling compact binaries. We use the covariant spin supplementarity condition (SSC), and explicitly prove the equivalence with previous results by Faye et al (2006 Phys. Rev. D 74 104033). We also show that the direct application of the Newton–Wigner SSC at the level of the action leads to the correct dynamics using a canonical (Dirac) algebra. This paper then completes the calculation of the necessary spin dynamics within the EFT formalism that will be used in a separate paper to compute the spin contributions to the energy flux and phase evolution to NLO.

Figure 2. Projection onto the plane (α = 0; H , ξ , K = 1/2) of streamlines A new cubic theory of gravity in five of the flow vector field of the system (3.11) for σ = 1/3. We have marked the fixed point (a) at H = 0, ξ = 0 and the projections at H = 0, ξ = ±1 of dimensions: black hole, Birkhoff’s theorem the exact solutions mentioned in the text. We have also marked the points H = −K ± √K2 + 1, which are fixed points in Christodoulou’s system, but are and C-function no longer fixed points in our system. Those four points now form part of an unstable projected structure that resembles a limit circle, though we do not know whether a true limit cycle exists in the full phase space. Julio Oliva and Sourya Ray

2010 Class. Quantum Grav. 27 225002 Black hole entropy, loop gravity, InvitedSpecial Article We present a new cubic theory of gravity in five dimensions which has second-order traced field equations, analogous to BHT new massive and polymer physics gravity in three dimensions. Moreover, for static spherically symmetric spacetimes all the field equations are of second order, and the theory Eugenio Bianchi admits a new asymptotically locally flat black hole. Furthermore, we prove the uniqueness of this solution, study its thermodynamical properties and 2011 Class. Quantum Grav. 28 114006 show the existence of a C-function for the theory following the arguments of Anber and Kastor (2008 J. High Energy Phys. JHEP05(2008)061 Loop gravity provides a microscopic derivation of black hole entropy. In this (arXiv:0802.1290 [hep-th])) in pure Lovelock theories. Finally, we include paper, I show that the microstates counted admit a semiclassical description the Einstein–Gauss–Bonnet and cosmological terms and find new in terms of shapes of a tessellated horizon. The counting of microstates and asymptotically AdS black holes at the point where the three maximally the computation of the entropy can be done via a mapping to an equivalent symmetric solutions of the theory coincide. These black holes may also statistical mechanical problem: the counting of conformations of a closed possess a Cauchy horizon. polymer chain. This correspondence suggests a number of intriguing relations between the thermodynamics of black holes and the physics of polymers.

Existence of naked singularities in the Brans–Dicke theory of gravitation. An analytical and numerical study

Nabil Bedjaoui, Philippe G LeFloch, José M Martín-García and Jérôme Novak

2010 Class. Quantum Grav. 27 245010

Within the framework of the scalar–tensor models of gravitation and by Figure 1. Left: tessellated surface determined by the areas and normals of its facets; typical 2 relying on analytical and numerical techniques, we establish the existence configuration belonging to the ensemble with fixed total area discussed in the text (AH ≈ 100LP). of a class of spherically symmetric spacetimes containing a naked Right: closed polymer chain corresponding to the tessellated surface. singularity. Our result relies on and extends a work by Christodoulou on the existence of naked singularities for the Einstein–scalar field equations. We establish that a key parameter in Christodoulou’s construction couples to the Brans–Dicke field and becomes a dynamical variable, which enlarges

Highlights of 2010–2011 15 Classical and Quantum Gravity

1000 Myr−1 MWEG−1 (Kalogera et al 2004 Astrophys. J. 601 L179; Kalogera q Experimental gravity et al 2004 Astrophys. J. 614 L137 (erratum)). We convert coalescence rates into detection rates based on data from the LIGO S5 and Virgo VSR2 science runs and projected sensitivities for our advanced detectors. Using Accurate light-time correction due to a the detector sensitivities derived from these data, we find a likely detection rate of 0.02 per year for Initial LIGO–Virgo interferometers, with a plausible gravitating mass range between 2 × 10−4 and 0.2 per year. The likely binary neutron–star detection rate for the Advanced LIGO–Virgo network increases to 40 events Neil Ashby and Bruno Bertotti per year, with a range between 0.4 and 400 per year.

2010 Class. Quantum Grav. 27 145013

This technical paper of mathematical physics arose as an aftermath of the 2002 Cassini experiment (Bertotti et al 2003 Nature 425 374–6), in −5 which the PPN parameter γ was measured with an accuracy σγ = 2.3 × 10 and found consistent with the prediction γ = 1 of general relativity. The Orbit Determination Program (ODP) of NASA’s Jet Propulsion Laboratory, which was used in the data analysis, is based on an expression (8) for the gravitational delay Δt that differs from the standard formula (2); this difference is of second order in powers of m—the gravitational radius of the Sun—but in Cassini’s case it was much larger than the expected order of magnitude m2/b, where b is the distance of the closest approach of the ray. Since the ODP does not take into account any other second-order terms, it is necessary, also in view of future more accurate experiments, to revisit the whole problem, to systematically evaluate higher order corrections and to determine which terms, and why, are larger than the expected value. We note that light propagation in a static spacetime is equivalent to a problem in ordinary geometrical optics; Fermat’s action functional at its minimum is Figure 2. Noise amplitude spectral densities (ASDs) as a function of frequency. The Initial LIGO noise ASD (solid red curve) corresponds to the typical detector sensitivity as measured from data just the light-time between the two end points A and B. A new and powerful taken during the S5 run. The Advanced LIGO noise ASD (dashed magenta) represents a possible formulation is thus obtained. This method is closely connected with the Advanced LIGO configuration with high laser power and zero detuning. The Initial Virgo noise ASD much more general approach of Le Poncin-Lafitte et al (2004 Class. (dotted blue) was measured during Virgo’s VSR2 run. The Advanced Virgo noise ASD (dash-dotted Quantum Grav. 21 4463–83), which is based on Synge’s world function. green) is based on the Advanced Virgo Baseline Design. Asymptotic power series are necessary to provide a safe and automatic way of selecting which terms to keep at each order. Higher order approximations to the required quantities, in particular the delay and the deflection, are easily obtained. We also show that in a close superior conjunction, when Parametric instabilities in advanced b is much smaller than the distances of A and B from the Sun, say of order R, the second-order correction has an enhanced part of order m2R/b2, gravitational wave detectors which corresponds just to the second-order terms introduced in the ODP. Gravitational deflection of the image of a far away source when observed S Gras, C Zhao, D G Blair and L Ju from a finite distance from the mass is obtained up to O(m2). 2010 Class. Quantum Grav. 27 205019

As the LIGO interferometric gravitational wave detectors have finished Predictions for the rates of Review Article gathering a large observational data set, an intense effort is underway compact binary coalescences to upgrade these observatories to improve their sensitivity by a factor of ~10. High circulating power in the arm cavities is required, which leads to observable by ground-based the possibility of parametric instability due to three-mode opto-acoustic resonant interactions between the carrier, transverse optical modes gravitational-wave detectors and acoustic modes. Here, we present detailed numerical analysis of parametric instability in a configuration that is similar to Advanced LIGO. LIGO/VIRGO collaboration After examining parametric instability for a single three-mode interaction in detail, we examine instability for the best and worst cases, as determined by the resonance condition of transverse modes in the power and signal 2010 Class. Quantum Grav. 27 173001 recycling cavities. We find that, in the best case, the dual recycling detector is substantially less susceptible to instability than a single We present an up-to-date, comprehensive summary of the rates for all cavity, but its susceptibility is dependent on the signal recycling cavity types of compact binary coalescence sources detectable by the initial and design, and on tuning for narrow band operation. In all cases considered, advanced versions of the ground-based gravitational-wave detectors LIGO the interferometer will experience parametric instability at full power and Virgo. Astrophysical estimates for compact-binary coalescence rates operation, but the gain varies from 3 to 1000, and the number of unstable depend on a number of assumptions and unknown model parameters modes varies between 7 and 30 per test mass. The analysis focuses and are still uncertain. The most confident among these estimates are on understanding the detector complexity in relation to opto-acoustic the rate predictions for coalescing binary neutron stars which are based interactions, on providing insights that can enable predictions of the on extrapolations from observed binary pulsars in our galaxy. These yield detector response to transient disturbances, and of variations in thermal a likely coalescence rate of 100 Myr−1 per Milky Way Equivalent Galaxy compensation conditions. (MWEG), although the rate could plausibly range from 1 Myr−1 MWEG−1 to

16 Highlights of 2010–2011 Classical and Quantum Gravity

Calibration of the LIGO displacement Effect of heat treatment on mechanical actuators via laser frequency modulation dissipation in Ta2O5 coatings

E Goetz and R L Savage Jr I W Martin et al

2010 Class. Quantum Grav. 27 215001 2010 Class. Quantum Grav. 27 225020

The kilometre-scale laser Thermal noise arising from mechanical dissipation in dielectric reflective interferometers currently being used coatings is expected to critically limit the sensitivity of precision to search for cosmic gravitational measurement systems such as high-resolution optical spectroscopy, optical waves are sensitive to differential frequency standards and future generations of interferometric gravitational length variations on the order of wave detectors. We present measurements of the effect of post-deposition 10-19 m. Maximizing the scientific heat treatment on the temperature dependence of the mechanical benefit of future detections will dissipation in ion-beam sputtered tantalum pentoxide between 11 K and E Goetz R L Savage Jr require absolute calibration 300 K. We find that the temperature dependence of the dissipation is accuracies of better than one percent. With most calibration methods, strongly dependent on the temperature at which the heat treatment was forces are applied directly to the interferometer’s mirrors. These calibration carried out, and we have identified three dissipation peaks occurring at forces can deform the mirrors and therefore compromise the calibration different heat treatment temperatures. At temperatures below 200 K, the accuracy. We have implemented a new calibration method that uses magnitude of the loss was found to increase with higher heat treatment laser frequency modulation to create an effective length variation via the temperatures, indicating that heat treatment is a significant factor in dynamic resonance condition for the interferometer’s 4-km-long Fabry-Perot determining the level of coating thermal noise. arm cavities. This technique enables force-free calibration with improved precision using a displacement reference that is fundamentally different from those employed with other calibration methods.

Experimental validation of dual/ InvitedSpecial Article

modified dual arm locking for LISA

Yinan Yu, Shawn J Mitryk and Guido Mueller

2011 Class. Quantum Grav. 28 094009

The laser frequency stabilization subsystem is one of the most significant parts within the interferometric measurement system of LISA. Arm locking as a proposed frequency stabilization technique synthesizes an adequately filtered linear combination of the interferometry signals as a frequency reference. Until now all the benchtop experiments on arm locking verified only the basic single arm locking configuration with unrealistic short signal travel times. At the University of Florida we developed the hardware- based University of Florida LISA Interferometer Simulator (UFLIS) to study and verify laser frequency noise reduction and suppression techniques under realistic LISA-like conditions. These conditions include the Doppler shifts between the spacecraft, LISA-like signal travel times, realistic laser Figure 1. Magnitude and phase of the frequency-to-length transfer function, C(f ), for the LIGO 4 km long arm cavities over a 100 kHz span (left) and over a 10 kHz span (right). frequency and timing noise. In this paper we will report about preliminary experiments on advanced arm locking schemes including dual arm locking and modified dual arm locking with realistic 33 s light travel times. In our experiments the implementation of a dual/modified dual arm locking sensor and controller is realized using several digital signal processing Special issue: GWDAW 14 boards. We demonstrated the closed-loop stability of arm locking setup and measured the noise suppression in these experiments. The 14th Gravitational Wave Data Analysis Workshop (GWDAW 14) was held at the Department of Physics of the University of Rome ‘Sapienza’, Italy, on 26–29 January 2010. Forthcoming special issues Fulvio Ricci This year the workshop was focused on strengthening Guest Editor the connection between the gravitational wave and Summer 2012 the other astrophysical communities. Thus, a significant fraction of Invited articles from Amaldi 9 the workshop was dedicated to exploring the potentialities of multi- Guest Editors: Patrick Sutton, Mark Hannam, Stefan Hild and messenger astronomy and in particular on the emerging neutrino Chris Van Den Broeck observatories in conjunction with GW observations. To find out more and to read the invited articles see CQG Volume 27 Focus issue: tests of the weak equivalence principle Issue 19. Guest Editors: Clifford M Will and Clive C Speake

Highlights of 2010–2011 17 Classical and Quantum Gravity

and a detector in India gives a network with position error ellipses a factor Fundamental physics and cosmology with of 7 smaller in area and boosts the detected event rate a further 2.4 times above the extra-site gain over the original LIGO–VIRGO network. Enlarged LISA advanced networks could look forward to detecting 300–400 neutron star binary coalescences per year. Stanislav Babak, Jonathan R Gair, Antoine Petiteau and Alberto Sesana

2011 Class. Quantum Grav. 28 114001

In this paper, we briefly review some of the applications to fundamental physics and cosmology of the observations that will be made with the future space-based gravitational wave (GW) detector LISA. This includes detection of GW bursts generated by cosmic strings, measurement of a stochastic GW background, mapping the spacetime around massive compact objects in galactic nuclei using extreme-mass-ratio inspirals and testing the predictions of general relativity for the strong dynamical fields generated by inspiralling binaries. We give particular attention to some new results which demonstrated the capability of LISA to constrain cosmological parameters using observations of coalescing massive black hole binaries.

Networks of gravitational wave detectors and three figures of merit

Bernard F Schutz

2011 Class. Quantum Grav. 28 125023

This paper develops a general framework for studying the effectiveness Figure 6. Three network isotropy patterns, which show the parts of the unit sphere where the of networks of interferometric gravitational wave detectors and then uses amplitude sensitivity of the detector is better than √2 of its best sensitivity. it to show that enlarging the existing LIGO–VIRGO network with one or more planned or proposed detectors in Japan (LCGT), Australia, and India brings major benefits, including much larger detection rate increase than previously thought. I focus on detecting bursts, i.e. short-duration signals, with optimal coherent data-analysis methods. I show that the polarization- Special issue: LISA 8 averaged sensitivity of any network of identical detectors to any class of The 8th International Laser sources can be characterized by two numbers—the visibility distance of Interferometric Space Antenna (LISA) the expected source from a single detector and the minimum signal-to- Symposium was held at Stanford noise ratio (SNR) for a confident detection—and one angular function, the University on 28 June to 2 July 2010. antenna pattern of the network. I show that there is a universal probability distribution function (PDF) for detected SNR values, which implies that Sasha Buchman Ke-Xun Sun The conference covered a wide range Guest Editor Guest Editor the most likely SNR value of the first detected event will be 1.26 times the of topics relating to LISA and the search threshold. For binary systems, I also derive the universal PDF for detection of gravitational waves. Featured aspects of LISA included detected values of the orbital inclination, taking into account the Malmquist LISA Science, LISA Interferometry, LISA PathFinder and Data Analysis. bias; this implies that the number of gamma-ray bursts associated with Large gravitational wave detection efforts, DECIGO and LIGO were also detected binary coalescences should be 3.4 times larger than expected presented, as well as a number of other fundamental physics space from just the beaming fraction of the gamma burst. Using network antenna experiments such as GP-B and STEP. patterns, I propose three figures of merit (f.o.m.’s) that characterize Browse the invited articles in CQG Volume 28 Issue 9. the relative performance of different networks. These measure (a) the expected rate of detection by the network and any sub-networks of three or more separated detectors, taking into account the duty cycle of the interferometers, (b) the isotropy of the network antenna pattern, and (c) the accuracy of the network at localizing the positions of events on the sky. I compare various likely and possible networks, based on these f.o.m.’s. Adding any new site to the planned LIGO–VIRGO network can dramatically DID YOU KNOW? increase, by factors of 2–4, the detected event rate by allowing coherent data analysis to reduce the spurious instrumental coincident background. Classical and Quantum Gravity publishes Moving one of the LIGO detectors to Australia additionally improves direction finding by a factor of 4 or more. Adding LCGT to the original fast track communications, which are short LIGO–VIRGO network not only improves direction finding but will further articles, of 8 pages or less, presenting new increase the detection rate over the extra-site gain by factors of almost 2, partly by improving the network duty cycle. Including LCGT, LIGO-Australia, and highly interesting results.

18 Highlights of 2010–2011 Classical and Quantum Gravity

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Highlights of 2010–2011 19 Classical and Quantum Gravity

Top 20 most downloaded articles published in 2011

1 String theory: a perspective over the last 25 years Sunil Mukhi 2011 Class. Quantum Grav. 28 153001

2 Loop quantum gravity: the first 25 years Carlo Rovelli 2011 Class. Quantum Grav. 28 153002

3 Surprising connections between general relativity and condensed matter Gary T Horowitz 2011 Class. Quantum Grav. 28 114008

4 Conservation of the nonlinear curvature perturbation in generic single-field inflation Atsushi Naruko and Misao Sasaki 2011 Class. Quantum Grav. 28 072001

5 Does an atom interferometer test the gravitational redshift at the Compton frequency? Peter Wolf, Luc Blanchet, Christian J Bordé, Serge Reynaud, Christophe Salomon and Claude Cohen-Tannoudji 2011 Class. Quantum Grav. 28 145017

6 Supergravity-based inflation models: a review Masahide Yamaguchi 2011 Class. Quantum Grav. 28 103001

7 Atom interferometry and the gravitational redshift Supurna Sinha and Joseph Samuel 2011 Class. Quantum Grav. 28 145018

8 Constant mean curvature spacelike hypersurfaces in Lorentzian manifolds with a timelike gradient conformal vector field Magdalena Caballero, Alfonso Romero and Rafael M Rubio 2011 Class. Quantum Grav. 28 145009

9 Networks of gravitational wave detectors and three figures of merit Bernard F Schutz 2011 Class. Quantum Grav. 28 125023

10 Loop quantum cosmology: a status report Abhay Ashtekar and Parampreet Singh 2011 Class. Quantum Grav. 28 213001

11 Measuring the spins of accreting black holes Jeffrey E McClintock, Ramesh Narayan, Shane W Davis, Lijun Gou, Akshay Kulkarni, Jerome A Orosz, Robert F Penna, Ronald A Remillard and James F Steiner 2011 Class. Quantum Grav. 28 114009

12 The fluid/gravity correspondence: a new perspective on the membrane paradigm Veronika E Hubeny 2011 Class. Quantum Grav. 28 114007

13 Sensitivity studies for third-generation gravitational wave observatories S Hild, M Abernathy, F Acernese et al 2011 Class. Quantum Grav. 28 094013

14 Spatial variation of fundamental couplings and Lunar Laser Ranging Thibault Damour and John F Donoghue 2011 Class. Quantum Grav. 28 162001

15 A new look at loop quantum gravity Carlo Rovelli 2011 Class. Quantum Grav. 28 114005

16 Conformal diagrams for the gravitational collapse of a spherical dust cloud Néstor Ortiz and Olivier Sarbach 2011 Class. Quantum Grav. 28 235001

17 The gravitational mechanism to generate mass M Novello 2011 Class. Quantum Grav. 28 035003

18 Gravity from the entropy of light Alessandro Pesci 2011 Class. Quantum Grav. 28 045001

19 The Japanese space gravitational wave antenna: DECIGO S Kawamura et al 2011 Class. Quantum Grav. 28 094011

20 LISA Pathfinder: mission and status F Antonucci et al 2011 Class. Quantum Grav. 28 094001

20 Highlights of 2010–2011 Classical and Quantum Gravity

Top 20 most cited articles published in 2011

1 Observational constraints on inhomogeneous cosmological models without dark energy V Marra and A Notari 2011 Class. Quantum Grav. 28 164004

2 Conservation of the nonlinear curvature perturbation in generic single-field inflation A Naruko and M Sasaki 2011 Class. Quantum Grav. 28 072001

3 An alternative approach for general covariant Horˇava-Lifshitz gravity and matter coupling A M Da Silva 2011 Class. Quantum Grav. 28 055011

4 Orthogonal non-Gaussianities from Dirac-Born-Infeld Galileon inflation S Renaux-Petel 2011 Class. Quantum Grav. 28 182001

5 Toward physical cosmology: Focus on inhomogeneous geometry and its non-perturbative effects T Buchert 2011 Class. Quantum Grav. 28 164007

6 Suitability of post-Newtonian/numerical-relativity hybrid waveforms for gravitational wave detectors I MacDonald, S Nissanke and H P Pfeiffer 2011 Class. Quantum Grav. 28 134002

7 More on massive 3D supergravity E A Bergshoeff, O Hohm, J Rosseel, E Sezgin and P K Townsend 2011 Class. Quantum Grav. 28 015002

8 What is dust? – Physical foundations of the averaging problem in cosmology D L Wiltshire 2011 Class. Quantum Grav. 28 164006

9 Is backreaction really small within concordance cosmology? C Clarkson and O Umeh 2011 Class. Quantum Grav. 28 164010

10 Remarks on the scalar graviton decoupling and consistency of Horˇava gravity M-I Park 2011 Class. Quantum Grav. 28 015004

11 Inhomogeneous cosmological models: Exact solutions and their applications K Bolejko, M-N Celerier and A Krasinski 2011 Class. Quantum Grav. 28 164002

12 Linear kinetic Sunyaev-Zel’dovich effect and void models for acceleration J P Zibin and A Moss 2011 Class. Quantum Grav. 28 164005

13 Cyclic cosmology from Lagrange-multiplier modified gravity Y-F Cai and E N Saridakis 2011 Class. Quantum Grav. 28 035010

14 Matter bounce cosmology with the f(T) gravity Y-F Cai, S-H Chen, J B Dent, S Dutta and E N Saridakis 2011 Class. Quantum Grav. 28 215011

15 Generalizing Galileons M Trodden and K Hinterbichler 2011 Class. Quantum Grav. 28 204003

16 Holographic models of de Sitter QFTs D Marolf, M Rangamani and M Van Raamsdonk 2011 Class. Quantum Grav. 28 105015

17 Acceleration of particles by black holes – A general explanation O B Zaslavskii 2011 Class. Quantum Grav. 28 105010

18 Curvature perturbations and non-Gaussianities from the waterfall phase transition during inflation A A Abolhasani, H Firouzjahi and M H Namjoo 2011 Class. Quantum Grav. 28 075009

19 Exact renormalization group with optimal scale and its application to cosmology B Koch and I Ramirez 2011 Class. Quantum Grav. 28 055008

20 Homogeneous vacua of (generalized) new massive gravity I Bakas and C Sourdis 2011 Class. Quantum Grav. 28 015012

Highlights of 2010–2011 21 Classical and Quantum Gravity

Reasons to publish with Classical and Quantum Gravity

HIGH EDITORIAL STANDARDS ARTICLE EVOLUTION 1 Classical and Quantum Gravity has the most 2 As of 2012, all Classical and Quantum Gravity rigorous peer review in the field. The approval articles are available in a new “enhanced HTML” of two independent referees is required for format. Among other things, the new format publication. In 2011, 41.5% of submissions allows improved rendering of mathematics were accepted. and can be easily viewed on tablets and smartphones.

HIGHLY CITED GLOBAL READERSHIP 3 Classical and Quantum Gravity’s impact factor 4 Classical and Quantum Gravity papers were is at an all-time high. Your research will be in the downloaded by researchers from 2160 right place to be cited by your peers. departments from around the world in 2011.

3.5 Africa 1% Asia 26% Australasia 2% Central/South America 5% 3.0 Eastern Europe 4% Middle East 5% impact factor North America 21% Western Europe 36%

2.5 2006 2007 2008 2009 2010 year

Classical and Quantum Gravity in other media Some high-profile news stories featuring articles in Classical and Quantum Gravity include the following.

Location counts in search for waves www.nature.com/nature/journal/v474/n7349/full/474009b.html Featuring: Bernard F Schutz 2011 Class. Quantum Grav. 28 125023

Time up for relativity table-top test? www.nature.com/news/2011/110609/full/news.2011.358.html Featuring: Peter Wolf et al 2011 Class. Quantum Grav. 28 145017, and Supurna Sinha and Joseph Samuel 2011 Class. Quantum Grav. 28 145018

And finally, your favourite journal even had a brief cameo on CBS’s The Big Bang Theory in early 2012. The appearance can be found in series 5, episode 15, “The Friendship Contraction”, shortly after the title credits. If you watch closely, you can see Leonard putting a copy of CQG into his backpack while in the university cafeteria.

Copyright belongs to CBS and its licensors. Inclusion of this image is not intended to imply that CBS, Warner Bros or anyone else connected with the show endorses CQG.

22 Highlights of 2010–2011 We would like to thank all of our authors, referees, Editorial Board members and supporters across the world for their vital contribution to the work and progress of Classical and Quantum Gravity.

Scan this code with your smartphone to go directly to the online collection. The selected research highlights are free to read until 31 December 2012.

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