Classical and Quantum Gravity Highlights of 2009–2010 Classical and Quantum Gravity

iopscience.org/cqg Classical and Quantum Gravity Highlights of 2009–2010 Classical and Quantum Gravity

Contents

page Editor-in-Chief's letter 3 Fast track communications 5 Topical reviews 6 Special issues 7 Regular papers 9 •Cosmology 9 •Strings, branes, supergravity and gauge theory 10 •Quantum gravity 10 •Quantum cosmology 11 •Geometry and topology 12 •Black holes 12 •Mathematical relativity 13 •Numerical relativity 15 •Experimental gravity 16 Prizes sponsored by Classical and Quantum Gravity 18 Top 20 most downloaded articles in 2010 20 Top 20 most cited articles in 2010 21

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 •Quantum gravity •Supergravity, superstrings and supersymmetry Image taken from: Gravitating discs around black •Mathematical physics relevant to gravitation holes V Karas et al 2004 Class. Quantum Grav. 21 R1–51. More information on each of these areas can be found at iopscience.org/cqg.

Cover image: The effect of shear on an idealized circular image M Bartelmann 2010 Class. Quantum Grav. 27 233001 An artistic interpretation by Fred Swist

2 Highlights of 2009–2010 Classical and Quantum Gravity

Dear colleagues, 2010 has been a very special year for Classical and Quantum Gravity (CQG). The journal received more submissions in 2010 than in any year in its history. CQG also tallied a record number of downloads in 2010, proving the journal to be more popular with authors and readers than ever before. In March 2010, CQG launched its inaugural Advisory Panel (AP). This small panel consists of some of the best referees in gravitational physics. The role of the panel is to provide top-level expert advice on papers submitted to the journal. The AP has already begun to have a positive effect on the quality and speed of CQG’s already outstanding peer review. In the summer of 2010, CQG published its first focus issue on the subject of Non-linear Cosmological Perturbations as a section of CQG Volume 27 Issue 12. Impact Factor This very special issue has been extremely popular. Watch for forthcoming CQG 3.029* focus issues on the subjects of ‘Averaging and Backreaction’ and ‘String Cosmology’ * As listed in ISI®’s 2009 Science to be published in 2011. Citation Index Journal citation reports It was a great pleasure this year to see the award of the first Bergmann--Wheeler

Regular paper submissions Thesis Prize, sponsored by CQG, to Victor Taveras of Louisiana State University. The prize is awarded under the auspices of the International Society for General 900 Relativity and Gravitation (ISGRG) once every three years for outstanding work in 880 Quantum Gravity. Further details about the prizes that CQG sponsors, including how 860 to make nominations, can be found on p18.

840

submissions In June 2010, the CQG Editorial Board convened in London for CQG’s annual 820 board meeting. There, the best papers published in the journal over the previous

800 12 months were selected as the CQG Highlights of 2009–2010. This collection 2007 2008 2009 2010 year represents the best recent work in gravitational physics. This brochure contains the abstracts of the CQG Highlights and I invite you to browse the collection. You are Full-text downloads sure to find highly interesting work relevant to your research. 200 000 I take this opportunity to thank all of the authors, referees, Guest Editors, Editorial 160 000 Board members and the new Advisory Panel members who have made this

120 000 record-breaking year possible for CQG.

80 000 downloads

40 000

0 2007 2008 2009 2010 year Editor-in-Chief Clifford M Will

Highlights of 2009–2010 3 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 P R L V Moniz Universidade da Beira Interior, Covilhã, Portugal V Balasubramanian University of Pennsylvania, Philadelphia, USA B Mours Laboratoire d’Annecy-le-Vieux de Physique des Particules, France A A Coley Dalhousie University, Halifax, NS, Canada S Mukohyama IPMU, University of Tokyo, Japan A Corichi Universidad Nacional Autónoma de México (UNAM), Morelia, N Ó Murchadha University College Cork, Ireland México E Poisson University of Guelph, Ontario, Canada (Book Reviews Editor) S Dhurandhar Inter-University Centre for Astronomy and Astrophysics, Pune, L Rezzolla Max-Planck-Institut für Gravitationsphysik, Golm, Germany India V O Rivelles Universidade de Sao Paulo (USP), Sao Paulo, Brazil V P Frolov University of Alberta, Edmonton, Canada S F Ross Durham University, Durham, UK D Garfinkle Oakland University, Rochester, MI, USA C Rovelli Université de la Mediterranée–Aix-Marseille II, France G Gonzalez Louisiana State University, Baton Rouge, 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 L Lehner Perimeter Institute for Theoretical Physics and University of Guelph, D Sudarsky Universidad Nacional Autónoma de México (UNAM), México D F, Ontario, Canada México R B Mann University of Waterloo, Ontario, Canada R-S Tung Shanghai Normal University, Shanghai, China M Mars Universidad de Salamanca, Spain

Advisory Panel C Barcelo Instituto de Astrofisica de Andalucia, CSIC, Spain J Louko University of Nottingham, UK R-G Cai Chinese Academy of Sciences, China D Marolf University of California, Santa Barbara, USA S Carlip University of California, Davis, USA W-T Ni Shanghai Normal University, China M Cavaglia University of Mississippi, USA B Nolan Dublin City University, Ireland P Chrus´ciel University of Vienna, Austria S Odintsov Space Research Inst and ICREA, Spain C Chryssomalakos Universidad Nacional Autonoma de Mexico, Mexico G Papadopoulos King’s College London, University of London, UK S Deser California Institute of Technology, USA T Pawlowski Consejo Superior de Investigaciones Cientificas, Spain F Dowker Imperial College London, UK H Pfeiffer Canadian Institute for Theoretical Astrophysics, Canada S Fairhurst Cardiff University, UK J Pullin Louisiana State University, USA V Ferrari Universita di Roma ‘La Sapienza’, Italy 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 R Gregory Durham University, UK M Sakellariadou University of Athens, Greece S Hartnoll Stanford University, USA O Sarbach Universidad Michoacana de San Nicolas de Hidalgo, Mexico J M Heinzle Universitaet Wien, Austria S Scott Australian National University, Australia G T Horowitz University of California, Santa Barbara, USA J Simon Edinburgh University, UK W Israel University of Victoria, Canada 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, L B Szabados Research Institute for Particle and Nuclear Physics, Hungary Czech Republic N Van den Bergh Ghent University, Belgium C Kiefer Universität zu Köln, Germany L Vanzo Universita 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

4 Highlights of 2009–2010 Classical and Quantum Gravity

velocity accumulated up to the merger of two non-spinning black holes, Highlighted Fast Track (ii) a ‘close-limit approximation’ calculation of the radiation emitted during Communications the ringdown phase, and based on a solution of the Regge–Wheeler and Zerilli equations using initial data accurate to second post-Newtonian The Fast Track Communications (FTCs) section order. We prove that ringdown radiation produces a significant ‘anti-kick’. is a wonderful opportunity to have your best Adding the contributions due to inspiral, merger and ringdown phases, our work published quickly and recognized by the results for the net kick velocity agree with those from numerical relativity community. FTCs are free to read and are often Jose M M Senovilla to 10–15% over a wide range of mass ratios, with a maximum velocity of selected for the CQG Highlights. A length of Editorial Board 180 km s−1 at a mass ratio of 0.38. member less than 8 pages and a high level of quality are requirements for acceptance to this section.

Fast Track Communications

Horˇava–Lifshitz holography

Tatsuma Nishioka

2009 Class. Quantum Grav. 26 242001

We derive the detailed balance condition as a solution to the Hamilton– Jacobi equation in the Horˇava–Lifshitz gravity. This result leads us to Figure 2. Magnitude of the recoil velocity V generated during the ringdown phase as a propose the existence of the d-dimensional quantum field theory on the ringdown function of the symmetric mass ratio η for different initial separations r . future boundary of the (d + 1)-dimensional Horˇava–Lifshitz gravity from the 12 viewpoint of the holographic renormalization group. We also obtain a Ricci flow equation of the boundary theory as the holographic RG flow, which is the Hamilton equation in the bulk gravity, by tuning parameters in the theory. A counterexample to a recent version of the Penrose conjecture The gravitational-wave recoil from the ring- Alberto Carrasco and Marc Mars down phase of coalescing black hole binaries 2010 Class. Quantum Grav. 27 062001

Alexandre Le Tiec, Luc Blanchet and Clifford M Will By considering suitable axially symmetric slices on the Kruskal spacetime, 2010 Class. Quantum Grav. 27 012001 we construct a counterexample to a recent version of the Penrose inequality in terms of so-called generalized apparent horizons.

The gravitational recoil or ‘kick’ of a black hole formed from the merger of two orbiting black holes, and caused by the anisotropic emission of gravitational radiation, is an astrophysically important phenomenon. We combine (i) an earlier calculation, using post-Newtonian theory, of the kick

The new Hyperspace service hosted by the Albert Einstein Institute (the new home of the ‘GRG List’ formerly maintained by M MacCallum) is an excellent source of information regarding the latest job opportunities, forthcoming conferences and other news relating to the gravitational physics community. You can also find rapid notification of high-quality publications in CQG on the Hyperspace site.

Luciano Rezzolla, Editor Browse the website and sign up for the newsletter at http://hyperspace.aei.mpg.de/.

Highlights of 2009–2010 5 Classical and Quantum Gravity

Highlighted Topical Reviews Present status of the Penrose inequality

The Topical Reviews published in Classical and Marc Mars Quantum Gravity cover all areas of gravitational physics. They are among the most popular articles 2009 Class. Quantum Grav. 26 193001 in CQG and, along with the FTCs, are frequently found in the ‘most read’ tab on CQG’s homepage. Joseph Samuel Editorial Board All Topical Reviews are invited by the Editorial Board The Penrose inequality gives a lower bound for the total mass of a member with authors and subjects chosen such that CQG spacetime in terms of the area of suitable surfaces that represent black Topical Reviews always provide expert coverage of holes. Its validity is supported by the cosmic censorship conjecture, and the most interesting topics. Look out for the new therefore its proof (or disproof) is an important problem in relation with ‘Brief Review’ articles, which are short reviews of gravitational collapse. The Penrose inequality is a very challenging problem rapidly developing sub-fields. The first Brief Reviews in mathematical relativity and it has received continuous attention since were published in late 2010. its formulation by Penrose in the early seventies. Important breakthroughs have been made in the last decade or so, with the complete resolution of the so-called Riemannian Penrose inequality and a very interesting proposal to address the general case by Bray and Khuri. In this review, the most The tensor-vector-scalar theory and its important results on this field will be discussed and the main ideas behind their proofs will be summarized, with the aim of presenting what is the status cosmology of our present knowledge in this topic.

Constantinos Skordis

2009 Class. Quantum Grav. 26 143001 Gravitational self-force in extreme mass-ratio

Over the last few decades, astronomers and cosmologists have inspirals accumulated vast amounts of data clearly demonstrating that our current Leor Barack theories of fundamental particles and of gravity are inadequate to explain the observed discrepancy between the dynamics and the distribution of the 2009 Class. Quantum Grav. 26 213001 visible matter in the universe. The modified Newtonian dynamics (MOND) proposal aims at solving the problem by postulating that Newton’s second –10 –2 law of motion is modified for accelerations smaller than ~10 m s . This This review is concerned with the gravitational self-force acting on a mass simple amendment, has had tremendous success in explaining galactic particle in orbit around a large black hole. Renewed interest in this old rotation curves. However, being non-relativistic, it cannot make firm problem is driven by the prospects of detecting gravitational waves from predictions for cosmology. A relativistic theory called tensor-vector-scalar strongly gravitating binaries with extreme mass ratios. We begin here with (TeVeS) has been proposed by Bekenstein building on earlier work of a summary of recent advances in the theory of gravitational self-interaction Sanders which has a MOND limit for non-relativistic systems. In this review in curved spacetime, and proceed to survey some of the ideas and I give a short introduction to TeVeS theory and focus on its predictions for computational strategies devised for implementing this theory in the case of cosmology as well as some non-cosmological studies. a particle orbiting a Kerr black hole. We review in detail two of these methods: (i) the standard mode-sum method, in which the metric perturbation is regularized mode-by-mode in a multipole decomposition, and (ii) m-mode regularization, whereby individual azimuthal modes of the metric perturbation are regularized in 2+1 dimensions. We discuss several practical issues that Quasinormal modes of black holes and black arise, including the choice of gauge, the numerical representation of the branes particle singularity, and how high-frequency contributions near the particle are dealt with in frequency-domain calculations. As an example of a full end-to- Emanuele Berti, Vitor Cardoso and Andrei O Starinets end implementation of the mode-sum method, we discuss the computation of the gravitational self-force for eccentric geodesic orbits in Schwarzschild, 2009 Class. Quantum Grav. 26 163001 using a direct integration of the Lorenz-gauge perturbation equations in the time domain. With the computational framework now in place, researchers have recently turned to explore the physical consequences of the gravitational Quasinormal modes are eigenmodes of dissipative systems. Perturbations self-force; we will describe some preliminary results in this area. An of classical gravitational backgrounds involving black holes or branes appendix to this review presents, for the first time, a detailed derivation of naturally lead to quasinormal modes. The analysis and classification of the the ‘regularization parameters’ necessary for implementing the mode-sum quasinormal spectra require solving non-Hermitian eigenvalue problems for method in Kerr spacetime. the associated linear differential equations. Within the recently developed gauge-gravity duality, these modes serve as an important tool for determining the near-equilibrium properties of strongly coupled quantum field theories, in particular their transport coefficients, such as viscosity, conductivity and diffusion constants. In astrophysics, the detection of Median receipt to first decision times quasinormal modes in gravitational wave experiments would allow precise measurements of the mass and spin of black holes as well as new tests of Regular papers FTCs general relativity. This review is meant as an introduction to the subject, with a focus on the recent developments in the field. 49 DAYS 23 DAYS

6 Highlights of 2009–2010 Classical and Quantum Gravity

In this special issue of the journal we feature the very best research work Special issues from the conference. To find out more, see CQG Volume 26 Issue 20. The CQG Special Issue programme has developed rapidly over the last few years. CQG no longer publishes conference proceedings. However, conferences still form the basis for some special issues with articles being invited to the journal Clifford M Will following a selection process. The journal has Editor-in-Chief begun publishing focus issues that are not based on conferences, but rather on specific topics of high Mario Diaz interest. The quality of CQG’s first focus issue, on Guest Editor cosmological perturbations, was extremely high. Look out for more excellent research in CQG’s future Highlights of this issue: focus issues. Gamma-ray burst afterglow plateaus and gravitational waves Alessandra Corsi and Peter Mészáros 2009 Class. Quantum Grav. 26 204016

Probing the core-collapse supernova mechanism with gravitational waves Christian D Ott NRDA 2008 2009 Class. Quantum Grav. 26 204015 The 2008 Numerical Relativity Data Analysis (NRDA) Meeting was hosted by the Department of Physics at Syracuse University, New York, on 11–14 August 2008. The 2009 CERN Winter School

The purpose of this groundbreaking meeting was to bring together two The Winter School on Strings, Supergravity and Gauge Theories took place in communities with a vested interest in gravitational-wave observations: CERN, Switzerland, from 9–13 February 2009. the data analysis and numerical relativity communities. This was the second in the NRDA series of meetings and marked the beginning of real This event was a series of lectures and discussions on areas of recent collaborations between the two communities, even featuring presentations progress and open problems in string theory. One of the most active areas in based on joint work by numerical relativists and data analysts. string theory in recent years has been the AdS/CFT correspondence which was a key focus of the conference. CQG Volume 26 Issue 11 was dedicated to this important meeting and includes the best work presented at the conference. For more information on this, and many other key developments in the field, please see CQG Volume 26 Issue 22. Remarkably, all three papers published in this small issue were selected for the CQG Highlights.

Deirdre Shoemaker Patrick Sutton Guest Editor Guest Editor Angel Uranga Highlights of this issue: Guest Editor Status of black-hole-binary simulations for gravitational-wave detection Mark Hannam Highlights of this issue: 2009 Class. Quantum Grav. 26 114001 The information paradox: a pedagogical introduction Samir D Mathur The path to the enhanced and advanced LIGO gravitational-wave detectors 2009 Class. Quantum Grav. 26 224001 J R Smith for the LIGO Scientific Collaboration 2009 Class. Quantum Grav. 26 114013 Lectures on holographic methods for condensed matter physics Sean A Hartnoll 2009 Class. Quantum Grav. 26 224002 GWDAW 13 Gravity and hydrodynamics: lectures on the fluid-gravity correspondence The 13th Gravitational Wave Data Analysis Workshop took place in San Mukund Rangamani Juan, Puerto Rico, from 19–22 January 2009. This annual event has 2009 Class. Quantum Grav. 26 224003 become the established venue for presenting and discussing new results and techniques in this crucial subfield of gravitational wave astronomy.

A primary focus of the meeting was the improved sensitivity of instrumentation and detection equipment. It is hoped that such improvements will play a key and necessary role in the first observation of gravitational waves.

Highlights of 2009–2010 7 Classical and Quantum Gravity

nonlinear dynamics of isolated compact stars, to binary systems, to new Amaldi 8 approaches to the study of relativistic compact objects.

The 8th Edoardo Amaldi Conference on Gravitational Waves was held at We invite you to read the special issue, CQG Volume 27 Issue 11, which Columbia University, New York, from 22–26 June 2009. In the two years features selected high-quality papers from both the NRDA 09 and MICRA 09 leading up to the conference, significant advances had been made in meetings. gravitational-wave detector technology, data analysis and theory. As such, these proved to be the primary focal point of the meeting.

Selected high-quality papers were published in CQG with the remainder of the proceedings being published as Volume 228 of Journal of Physics: Conference Series (JPCS).

Read high-quality articles from the conference, including updates on the status of projects such as Advanced LIGO, LCGT and DECIGO, in this special Sascha Husa Christian Ott Christopher Pethick Luciano Rezzolla issue: CQG Volume 27 Issue 8. Guest Editor Guest Editor Guest Editor Guest Editor

Highlights of this issue: Numerical relativity confronts compact neutron star binaries: a review and status report Matthew D Duez 2010 Class. Quantum Grav. 27 114002

Zsuzsa Márka Szabolcs Márka The current status of binary black hole simulations in numerical relativity Guest Editor Guest Editor Ian Hinder 2010 Class. Quantum Grav. 27 114004 Highlights of this issue: Advanced LIGO: the next generation of gravitational wave detectors Gregory M Harry and the LIGO Scientific Collaboration 2010 Class. Quantum Grav. 27 084006 Focus section: Cosmological Perturbations

Verification of time-delay interferometry techniques using the University of In recent years there has been a resurgence of interest in the study of non- Florida LISA interferometry simulator linear perturbations of cosmological models. This has been the result of Shawn J Mitryk, Vinzenz Wand and Guido Mueller both theoretical developments and observational advances. 2010 Class. Quantum Grav. 27 084012 This focus section, dedicated to the memory of Lev Kofman, brings together Status update of the Parkes pulsar timing array a collection of very high-quality invited papers, which will be of interest to J P W Verbiest et al theoretical physicists and astrophysicists alike interested in understanding 2010 Class. Quantum Grav. 27 084015 and interpreting recent developments in cosmological perturbation theory and models of the early universe. Thermal-noise-limited underground interferometer CLIO Kazuhiro Agatsuma et al We invite you to read these specially commissioned articles, which were 2010 Class. Quantum Grav. 27 084022 published as a section of CQG Volume 27 Issue 12.

Alignment sensing and control in advanced LIGO L Barsotti, M Evans and P Fritschel 2010 Class. Quantum Grav. 27 084026

The GEO 600 squeezed light source Henning Vahlbruch et al 2010 Class. Quantum Grav. 27 084027 Misao Sasaki David Wands Guest Editor Guest Editor

Highlights of this issue: NRDA 2009 and MICRA 2009 Nonlinear dynamics and primordial curvature perturbations from preheating The 2009 Numerical Relativity and Data Analysis Meeting (NRDA) was held Andrei V Frolov at the Albert Einstein Institute in Potsdam, Germany, from 6–9 July 2009. 2010 Class. Quantum Grav. 27 124006 This was the third in an annual series of meetings seeking to build bridges between numerical relativists and gravitational wave data analysts. A geometrical approach to nonlinear perturbations in relativistic cosmology David Langlois and Filippo Vernizzi The following month saw a meeting entitled ‘Microphysics In Computational 2010 Class. Quantum Grav. 27 124007 Relativistic Astrophysics – (MICRA) 2009’, which took place at the Niels Bohr International Academy, in Copenhagen, from 24–28 August 2009. The Inhomogeneity and the foundations of concordance cosmology talks presented at the workshop covered all of the most salient aspects of Chris Clarkson and Roy Maartens the subject, ranging from stellar collapse and supernova dynamics, to the 2010 Class. Quantum Grav. 27 124008

8 Highlights of 2009–2010 Classical and Quantum Gravity

In this paper, we discuss the renormalizability of Horˇava–Lifshitz-type Watch for the following special issues in CQG: gravity theories. Using the fact that Horˇava–Lifshitz (HL) gravity is very closely related to the stochastic quantization of topologically massive GWDAW 14 gravity, we show that the renormalizability of HL gravity only depends on the Selected articles from the 14th Gravitational Wave Data Analysis renormalizability of topologically massive gravity. This is a consequence of Workshop (GWDAW-14), University of Rome Sapienza, Rome, Italy, the BRST and time-reversal symmetries pertinent to theories satisfying the 26–29 January 2010 detailed balance condition. CQG Volume 27 Issue 19 The CERN Winter School 2010 Lectures from the CERN Winter School on Strings, Supergravity and Gauge Theory, CERN, 25–29 January 2010 An analysis of the phase space of CQG Volume 27 Issue 21 Horˇava–Lifshitz cosmologies LISA 8 Selected articles from the 8th International LISA Symposium, Stanford S Carloni, E Elizalde and P J Silva University, California, USA, 28 June – 2 July 2010 Spring 2011 2010 Class. Quantum Grav. 27 045004 GR19 Selected articles from the 19th International Conference on General Using the dynamical system approach, the properties of cosmological Relativity and Gravitation, Mexico City, Mexico, 5–9 July 2010 models based on the Horˇava–Lifshitz gravity are systematically studied. Spring – summer 2011 In particular, the cosmological phase space of the Horˇava–Lifshitz Focus section: 25 years of Quantum Gravity model is characterized. The analysis allows us to compare some key Guest Editor: J Samuel physical consequences of the imposition (or not) of detailed balance. Spring – summer 2011 The result of the investigation is that in the detailed balance case one of the attractors in the theory corresponds to an oscillatory behavior. Such NRDA/Capra 2010 oscillations can be associated with a bouncing universe, as previously Selected articles from ‘Theory Meets Data Analysis at Comparable and described by Brandenberger, and will prevent a possible evolution toward Extreme Mass Ratios’, Perimeter Institute, Canada, 20–26 June 2010 a de Sitter universe. Other results obtained show that the cosmological Summer 2011 models generated by Horˇava–Lifshitz gravity without the detailed balance Focus section: Averaging and Backreaction assumption have indeed the potential to describe the transition between Summer – autumn 2011 the Friedmann and the dark-energy eras. The whole analysis leads to the plausible conclusion that a cosmology compatible with the present Focus section: String Cosmology observations of the universe can be achieved only if the detailed balance Autumn – winter 2011 condition is broken.

Highlighted regular papers The motion of galaxy clusters in CQG is an excellent journal in which to publish your inhomogeneous cosmologies original and high-quality research. Our peer review has a strong reputation for fairness and efficiency. David Garfinkle Published articles are highly visible and, as an added bonus, the best submissions are selected by 2010 Class. Quantum Grav. 27 065002 Gabriela Gonzalez the Editorial Board as CQG Highlights and are freely Editorial Board accessible throughout the following year. I am sure member that you will agree that this collection represents Lemaître–Tolman–Bondi inhomogeneous spacetimes can be used as a a great deal of excellent and valuable work across cosmological model to account for the type Ia supernova data. However, gravitational physics, and I look forward to reading such models also give rise to large velocities of galaxy clusters with respect your articles in the CQG Highlights soon. to the cosmic microwave background. These velocities can be measured using the kinematic Sunyaev–Zeldovich effect. This paper presents a calculation of galaxy cluster velocities as a function of redshift for such a model. q Cosmology Perturbative instabilities in Horˇava gravity The renormalizability of Horˇava–Lifshitz-type gravities Charalampos Bogdanos and Emmanuel N Saridakis 2010 Class. Quantum Grav. 27 075005 Domenico Orlando and Susanne Reffert

2009 Class. Quantum Grav. 26 155021 We investigate the scalar and tensor perturbations in Horˇava gravity, with and without detailed balance, around a flat background. Once both types

Highlights of 2009–2010 9 Classical and Quantum Gravity of perturbations are taken into account, it is revealed that the theory is The value of the cosmological constant is one of the major puzzles of plagued by ghost-like scalar instabilities in the range of parameters which modern cosmology: it is tiny but nonzero. Sorkin predicted, from the Causet would render it power-counting renormalizable, that cannot be overcome approach to quantum gravity, that the cosmological constant has quantum by simple tricks such as analytic continuation. Implementing a consistent fluctuations. The predicted order of magnitude of the fluctuations agrees flow between the UV and IR limits seems thus more challenging than initially with the subsequently observed value of the cosmological constant. We presumed, regardless of whether the theory approaches general relativity had earlier developed an analogy between the cosmological constant of the at low energies or not. Even in the phenomenologically viable parameter universe and the surface tension of fluid membranes. Here we demonstrate space, the tensor sector leads to additional potential problems, such as by computer simulations that the surface tension of a fluid membrane has fine-tunings and super-luminal propagation. statistical fluctuations stemming from its discrete molecular structure. Our analogy enables us to view these numerical experiments as probing a small and fluctuating cosmological constant. Deriving insights from our analogy, we show that a fluctuating cosmological constant is a generic feature of quantum gravity models and is far more general than the specific context q Strings, branes, supergravity and in which it was originally proposed. We pursue and refine the idea of a fluctuating cosmological constant and work towards making further testable gauge theory predictions.

The Bernstein conjecture, minimal cones and critical dimensions

Gary W Gibbons, Kei-Ichi Maeda and Umpei Miyamoto

2009 Class. Quantum Grav. 26 185008

Minimal surfaces and domain walls play important roles in various contexts of spacetime physics as well as material science. In this paper, we first review the Bernstein conjecture, which asserts that a plane is the only globally well-defined solution of the minimal surface equation which is a single valued graph over a hyperplane in flat spaces, and its failure in higher dimensions. Then, we review how minimal cones in four- and higher- Figure 1. The figure shows a system of 400 lipid molecules forming a symmetric bilayer with 200 molecules on each side. The system is subject to periodic boundary conditions in the dimensional spacetimes, which are curved and even singular at the apex, x–y–z-directions. may be used to provide counterexamples to the conjecture. The physical implications of these counterexamples in curved spacetimes are discussed from various points of view, ranging from classical general relativity, brane physics and holographic models of fundamental interactions. Schrödinger picture of quantum gravitational collapse Charged rotating black holes in T Vachaspati six-dimensional gauged supergravity 2009 Class. Quantum Grav. 26 215007

David D K Chow The functional Schrödinger equation is used to study the quantum collapse 2010 Class. Quantum Grav. 27 065004 of a gravitating, spherical domain wall and a massless scalar field coupled to the metric. The approach includes backreaction of pre-Hawking radiation on the gravitational collapse. Truncating the degrees of freedom to a We obtain non-extremal charged rotating black holes in six-dimensional minisuperspace leads to an integro-differential Schrödinger equation. SU(2) gauged supergravity with two independent angular momenta and We define a ‘black hole’ operator and find its eigenstates. The black hole one U(1) charge. These include supersymmetric black holes without naked operator does not commute with the Hamiltonian, leading to an energy- closed timelike curves. black holeness uncertainty relation. We discuss energy eigenstates and also obtain a partial differential equation for the time-dependent gravitational collapse problem. q Quantum gravity Figure 1. A picture of the system under consideration. A spherical domain wall with The universe in a soap film radius R(t) collapses towards its Schwarzschild radius, RS, and emits scalar radiation in

the process. Due to the emission, RS also Rohit Katti, Joseph Samuel and Supurna Sinha decreases.

2009 Class. Quantum Grav. 26 135018

10 Highlights of 2009–2010 Classical and Quantum Gravity

Manifestly gauge-invariant general relativistic Black hole determinants and quasinormal perturbation theory: I. Foundations modes

K Giesel, S Hofmann, T Thiemann and O Winkler Frederik Denef, Sean A Hartnoll and Subir Sachdev

2010 Class. Quantum Grav. 27 055005 2010 Class. Quantum Grav. 27 125001

Linear cosmological perturbation theory is pivotal to a theoretical We derive an expression for functional determinants in thermal spacetimes understanding of current cosmological experimental data provided e.g. by as a product over the corresponding quasinormal modes. As simple cosmic microwave anisotropy probes. A key issue in that theory is to applications we give efficient computations of scalar determinants in extract the gauge-invariant degrees of freedom which allow unambiguous thermal AdS, BTZ black hole and de Sitter spacetimes. We emphasize the comparison between theory and experiment. When one goes beyond conceptual utility of our formula for discussing ‘1/N’ corrections to strongly first (linear) order, the task of writing the Einstein equations expanded coupled field theories via the holographic correspondence. to nth order in terms of quantities that are gauge-invariant up to terms of higher orders becomes highly non-trivial and cumbersome. This fact has prevented progress for instance on the issue of the stability of linear perturbation theory and is a subject of current debate in the literature. In this series of papers we circumvent these difficulties by passing to a q Quantum cosmology manifestly gauge-invariant framework. In other words, we only perturb gauge-invariant, i.e. measurable quantities, rather than gauge variant ones. Thus, gauge invariance is preserved non-perturbatively while we Accelerating universe from warped extra construct the perturbation theory for the equations of motion for the gauge- dimensions invariant observables to all orders. In this first paper we develop the general framework which is based on a seminal paper due to Brown and Kucharˇ as Ishwaree P Neupane well as the relational formalism due to Rovelli. In the second, companion, paper we apply our general theory to FRW cosmologies and derive the 2009 Class. Quantum Grav. 26 195008 deviations from the standard treatment in linear order. As it turns out, these deviations are negligible in the late universe, thus our theory is in agreement with the standard treatment. However, the real strength of our formalism Accelerating universe or the existence of a small and positive cosmological is that it admits a straightforward and unambiguous, gauge-invariant constant is probably the most pressing obstacle as well as opportunity to generalization to higher orders. This will also allow us to settle the stability significantly improving the models of four-dimensional cosmology from issue in a future publication. fundamental theories of gravity, including string theory. In seeking to resolve this problem, one naturally wonders if the real world can somehow be interpreted as an inflating de Sitter brane embedded in a higher dimensional spacetime described by warped geometry. In this scenario, Spin-foams for all loop quantum gravity the four-dimensional cosmological constant may be uniquely determined in terms of two length scales: one is a scale associated with the size of extra Wojciech Kamin´ski, Marcin Kisielowski and Jerzy Lewandowski dimensions and the other is a scale associated with the expansion rate of our universe. 2010 Class. Quantum Grav. 27 095006

The simplicial framework of Engle–Pereira–Rovelli–Livine spin-foam models is generalized to match the diffeomorphism invariant framework of loop Casting loop quantum cosmology in the spin quantum gravity. The simplicial spin-foams are generalized to arbitrary foam paradigm linear 2-cell spin-foams. The resulting framework admits all the spin-network states of loop quantum gravity, not only those defined by triangulations Abhay Ashtekar, Miguel Campiglia and Adam Henderson (or cubulations). In particular, the notion of embedded spin-foam we use allows us to consider knotting or linking spin-foam histories. Also the 2010 Class. Quantum Grav. 27 135020 main tools, the vertex structure and the vertex amplitude, are naturally generalized to an arbitrary valency case. The correspondence between all the SU(2) intertwiners and the SU(2)×SU(2) EPRL intertwiners is proved to The goal of spin foam models is to provide a viable path integral formulation be 1–1 in the case of the Barbero–Immirzi parameter |γ| ≥ 1. of quantum gravity. Because of background independence, their underlying framework has certain novel features that are not shared by path integral Figure 1. A spin-network. formulations of familiar field theories in Minkowski space. As a simple viability test, these features were recently examined through the lens of loop quantum cosmology (LQC). Results of that analysis, reported in a brief communication (Ashtekar A et al 2009 Phys. Lett. B 681 347–52), turned out to provide concrete arguments in support of the spin foam paradigm. We now present detailed proofs of those results. Since the quantum theory of LQC models is well understood, this analysis also serves to shed new light on some long standing issues in the spin foam and group field theory literature. In particular, it suggests an intriguing possibility for addressing the question of why the cosmological constant is positive and small.

Highlights of 2009–2010 11 Classical and Quantum Gravity

Friedrich’s proofs for the global existence results of de Sitter-like spacetimes q Black holes and of semi-global existence of Minkowski-like spacetimes (Friedrich 1986 Commun. Math. Phys. 107 587) are re-examined and discussed, making use of the extended conformal field equations and a gauge based Hawking radiation of an apparent horizon in a on conformal geodesics. In this gauge, the location of the conformal FRW universe boundary of the spacetimes is known a priori once the initial data have been prescribed. Thus, it provides an analysis which is conceptually and Rong-Gen Cai, Li-Ming Cao and Ya-Peng Hu calculationally simpler.

2009 Class. Quantum Grav. 26 155018 Static near-horizon geometries in five Hawking radiation is an important quantum phenomenon of a black hole, which is closely related to the existence of an event horizon of a black dimensions hole. The cosmological event horizon of de Sitter space is also of Hawking radiation with a thermal spectrum. By use of the tunneling approach, we Hari K Kunduri and James Lucietti show that there is indeed a Hawking radiation with temperature,

T = 1/2πr~A, for a locally defined apparent horizon of a Friedmann– 2009 Class. Quantum Grav. 26 245010

Robertson–Walker universe with any spatial curvature, where ~r A is the apparent horizon radius. Thus we fill in the gap existing in the literature investigating the relation between the first law of thermodynamics and We consider the classification of static near-horizon geometries of stationary Friedmann equations; there the apparent horizon is assumed to have such extremal (not necessarily BPS) black hole solutions of five-dimensional a temperature without any proof. In addition, we stress the implication of Einstein–Maxwell theory coupled to a Chern–Simons term with coupling the Hawking temperature associated with the apparent horizon. ξ (with ξ = 1 corresponding to supergravity). Assuming that the black holes have two rotational symmetries, we show that their near-horizon 2 geometries are either the direct product AdS3 × S or a warped product of

AdS2 and compact 3D space. In the AdS2 case we are able to classify all possible near-horizon geometries with no magnetic fields. There are two Enthalpy and the mechanics of AdS black 3 such solutions: the direct product AdS2 × S as well as a warped product 3 holes of AdS2 and an inhomogeneous S . The latter solution turns out to be a near-horizon limit of an extremal Reissner–Nordström black hole in an

David Kastor, Sourya Ray and Jennie Traschen external electric field. In the AdS2 case with magnetic fields, we reduce the problem (in all cases) to a single nonlinear ODE. We show that if there are 2009 Class. Quantum Grav. 26 195011 any purely magnetic solutions of this kind, they must have S1 × S2 horizon topology, and for ξ2 < 1/4 we find examples of solutions with both electric and magnetic fields. We present geometric derivations of the Smarr formula for static AdS black holes and an expanded first law that includes variations in the cosmological constant. These two results are further related by a scaling argument based on Euler’s theorem. The key new ingredient in the constructions is a two-form potential for the static Killing field. Surface integrals of the Killing potential determine the coefficient of the variation of Λ in the first law. This coefficient is proportional to a finite, effective volume for the region outside the AdS black hole horizon, which can also be interpreted as minus the volume excluded from a spatial slice by the black hole horizon. This effective volume also contributes to the Smarr formula. Since Λ is naturally thought of as a pressure, the new term in the first law has the form of effective volume times change in pressure that arises in the variation of the enthalpy in classical thermodynamics. This and related arguments suggest that the mass of an AdS black hole should be interpreted as the enthalpy of the spacetime.

q Geometry and topology

On de Sitter-like and Minkowski-like spacetimes iopscience.org/cqg Christian Lübbe and Juan Antonio Valiente Kroon Image adapted from: Dark energy and cosmological solutions in 2009 Class. Quantum Grav. 26 145012 second-order string gravity G Calcagni et al 2005 Class. Quantum Grav. 22 3977–4006

12 Highlights of 2009–2010 Classical and Quantum Gravity

boundary conditions. We provide the algebraic means for modeling the On the uniqueness of extremal vacuum black multipole expansion of incoming CMB radiation. From the assumption of holes randomness, we derive selection rules, depending on the point symmetry of the manifold. Pau Figueras and James Lucietti

2010 Class. Quantum Grav. 27 095001

We prove uniqueness theorems for asymptotically flat, stationary, extremal, vacuum black hole solutions, in four and five dimensions with one and two commuting rotational Killing fields respectively. As in the non-extremal case, these problems may be cast as boundary value problems on the two-dimensional orbit space. We show that the orbit space for solutions with one extremal horizon is homeomorphic to an infinite strip, where the two boundaries correspond to the rotational axes, and the two asymptotic Figure 1. The cubic manifolds N2 and N3. The cubic prototile and three neighbor tiles sharing its regions correspond to spatial infinity and the near-horizon geometry. In faces F1, F2, F3. The four cubes are replaced by their Euclidean counterparts and separated from one another. Visible faces are denoted by the numbers from figure A5. The actions transforming four dimensions, this allows us to establish the uniqueness of extremal the prototile into its three neighbors generate the deck transformations and the 8-cell tiling of S3. Kerr amongst asymptotically flat, stationary, rotating, vacuum black holes In the tiling, homotopic face gluing takes the form of shared pairs of faces N2: F3 ∪ F1, F4 ∪ F2, with a single extremal horizon. In five dimensions, we show that there is at F6 ∪ F5 and N3: F1 ∪ F6, F2 ∪ F4, F3 ∪ F5. It is marked by heavy lines or arcs. most one asymptotically flat, stationary, extremal vacuum black hole with a connected horizon, two commuting rotational symmetries and given interval structure and angular momenta. We also provide necessary and sufficient conditions for four- and five-dimensional asymptotically flat vacuum black holes with the above symmetries to be static (valid for extremal, non- q Mathematical relativity extremal and even non-connected horizons). Regularity of the Einstein equations at future null infinity Vincent Moncrief and Oliver Rinne

2009 Class. Quantum Grav. 26 125010

When Einstein’s equations for an asymptotically flat, vacuum spacetime are reexpressed in terms of an appropriate conformal metric that is regular at (future) null infinity, they develop apparently singular terms in the associated conformal factor and thus appear to be ill-behaved at this (exterior) boundary. In this paper however we show, through an Figure 1. Orbit space B for non-extremal black holes. Left: B can be mapped to the upper half of the complex plane ζ = z + i ρ. The boundaries and corners correspond to intervals on the real axis. enforcement of the Hamiltonian and momentum constraints to the needed Right: B can also be represented as a semi-infinite strip in the w = r + i x plane. order in a Taylor expansion, that these apparently singular terms are not only regular at the boundary but can in fact be explicitly evaluated there in terms of conformally regular geometric data. Though we employ a rather rigidly constrained and gauge-fixed formulation of the field equations, we Platonic topology and CMB fluctuations: discuss the extent to which we expect our results to have a more ‘universal’ significance and, in particular, to be applicable, after minor modifications, homotopy, anisotropy and multipole to alternative formulations. selection rules Peter Kramer Towards the quasi-localization of canonical 2010 Class. Quantum Grav. 27 095013 general relativity László B Szabados The cosmic microwave background (CMB) originates from an early stage in the history of the universe. Observations show low-multipole contributions 2009 Class. Quantum Grav. 26 125013 of CMB fluctuations. A possible explanation is given by a non-trivial topology of the universe and has motivated the search for topological selection rules. Harmonic analysis on a topological manifold must provide basis sets for A general framework for a systematic quasi-localization of canonical general all functions compatible with a given topology and so is needed to model relativity and a new ingredient, the requirement of the gauge invariance of the the CMB fluctuations. We analyze the fundamental groups of Platonic boundary terms appearing in the calculation of Poisson brackets, are given. tetrahedral, cubic and octahedral manifolds using deck transformations. As a consequence of this it is shown, in particular, that the generator vector From them we construct the appropriate harmonic analysis and fields (built from the lapse and shift) of the quasi-local quantities must be divergence free with respect to a Sen-type connection, and the volume form induced from the spatial metric on the boundary surface must be fixed.

Highlights of 2009–2010 13 Classical and Quantum Gravity

The light-cone theorem Static spacetimes with prescribed multipole

Yvonne Choquet-Bruhat, Piotr T Chrus´ciel and José M Martín-García moments: a proof of a conjecture by Geroch Magnus Herberthson 2009 Class. Quantum Grav. 26 135011 2009 Class. Quantum Grav. 26 215009

We prove that the area of cross-sections of light cones, in spacetimes satisfying suitable energy conditions, is smaller than or equal to that of the In this paper we give sufficient conditions on a sequence of multipole corresponding cross-sections in Minkowski, or de Sitter, or anti-de Sitter moments for a static spacetime to exist with precisely these moments. The spacetime. The equality holds if and proof is constructive in the sense that a metric having prescribed multipole only if the metric coincides with the moments up to a given order can be calculated. Since these sufficient corresponding model in the domain of conditions agree with already known necessary conditions, this completes dependence of the light cone. the proof of a long standing conjecture due to Geroch.

Figure 1. The cross-section A (s) of the light + cone C p ; C (s) is the shaded blue region. Two generators γ and γ are also shown. ℓ1 ℓ2 Mass, angular-momentum and charge inequalities for axisymmetric initial data Future asymptotics of vacuum Bianchi type Piotr T Chrus´ciel and João Lopes Costa 2009 Class. Quantum Grav. 26 235013 VI0 solutions J Mark Heinzle and Hans Ringström We present the key elements of the proof of an upper bound for angular 2009 Class. Quantum Grav. 26 145001 momentum and charge in terms of the mass for electro-vacuum asymptotically flat axisymmetric initial data sets with simply connected orbit space. In this paper, we present a thorough analysis of the future asymptotic dynamics of spatially homogeneous cosmological models of Bianchi type VI0. Each of these models converges to a flat Kasner solution (Taub solution) for late times; we give detailed asymptotic expansions describing 2 1 this convergence. In particular, we prove that the future asymptotics of Regularity of Cauchy horizons in S × S

Bianchi type VI0 solutions cannot be approximated in any way by Bianchi type II solutions, which is in contrast to Bianchi type VIII and IX models Gowdy spacetimes (in the direction toward the singularity). The paper contains an extensive Jörg Hennig and Marcus Ansorg introduction where we put the results into a broader context. The core of these considerations consists in the fact that there exist regions in 2010 Class. Quantum Grav. 27 065010 the phase space of Bianchi type VIII models where solutions can be approximated, to a high degree of accuracy, by type VI0 solutions. The behavior of solutions in these regions is essential for the question of We study general S2 × S1 Gowdy models with a regular past Cauchy horizon ‘locality’, i.e., whether particle horizons form or not. Since Bianchi type and prove that a second (future) Cauchy horizon exists, provided that a VIII models are conjectured to be important role models for generic particular conserved quantity J is not zero. We derive an explicit expression cosmological singularities, our understanding of Bianchi type VI0 dynamics for the metric form on the future Cauchy horizon in terms of the initial data might thus be crucial to help to shed some light on the important question 2 on the past horizon and conclude the universal relation ApAf = (8πJ) where of whether to expect generic singularities to be local or not. Ap and Af are the areas of past and future Cauchy horizons respectively. Figure 1. The Gowdy square. We assume an H4-regular metric and H3-regular time derivatives on all slices t = constant in a neighborhood (gray region) of the H past Cauchy horizon ( p : t = 0) and find by virtue of the results

in [2] that then the metric is H4- regular on all future slices t = constant, 0 ≤ t ≤ π (unless the quantity J introduced in (25) is

zero). In particular, an H4-regular future Cauchy horizon H ( f : t = π) exists.

Figure 3. This figure shows the part Σ+ < −1/3 of the state space BVI0. The outer shell is ∂BVI0, 2 2 1/2 2 1/2 which can be represented as (Σ− +N +) = (1− Σ+) ; in the vicinity of T3 this corresponds to 2 2 1/2 1/2 2 2 1/2 3/4 (Σ− +N +) ≃ √2(1 + Σ+) . The two ‘cones’ are surfaces (Σ− +N +) = a(1 + Σ+) for two different values of the constant a. By (38), a type VI0 orbit that is contained on such a cone initially remains (approximately) on that cone for all later times; in fact, asymptotically, a type VI0 orbit spirals in toward T3 along such a ‘cone’.

14 Highlights of 2009–2010 Classical and Quantum Gravity q Numerical relativity The close-limit approximation for black hole binaries with post-Newtonian initial Axial quasi-normal modes of neutron stars: conditions accounting for the superfluid in the crust Alexandre Le Tiec and Luc Blanchet Lars Samuelsson and Nils Andersson 2010 Class. Quantum Grav. 27 045008 2009 Class. Quantum Grav. 26 155016

The ringdown phase of a black hole formed from the merger of two orbiting We present the results of the first study of global oscillations of relativistic black holes is described by means of the close-limit (CL) approximation stars with both elastic crusts and interpenetrating superfluid components. starting from second-post-Newtonian (2PN) initial conditions. The 2PN For simplicity, we focus on the axial quasi-normal modes. Our results metric of point-particle binaries is formally expanded in CL form and demonstrate that the torsional crust modes are essentially unaffected identified with that of a perturbed Schwarzschild black hole. The multipolar by the coupling to the gravitational field. This is as expected since these coefficients describing the even-parity (polar) and odd-parity (axial) oscillations are known to be weak gravitational-wave sources. In contrast, components of the linear perturbation consistently satisfy the 2PN-accurate the presence of a loosely coupled superfluid neutron component in the perturbative field equations. We use these coefficients to build initial crust can have a significant effect on the oscillation spectrum. We show conditions for the Regge–Wheeler and Zerilli wave equations, which we that the entrainment between the superfluid and the crust nuclei is a key then evolve numerically. The ringdown waveform is obtained in two cases: parameter in the problem. Our analysis highlights the need for a more head-on collision with zero-angular momentum, composed only of even detailed understanding of the coupled crust-superfluid at the microphysical modes, and circular orbits, for which both even and odd modes contribute. level. Our numerical results have, even though we have not considered In a separate work, this formalism is applied to the study of the gravitational magnetized stars, some relevance for efforts to carry out seismology based recoil produced during the ringdown phase of coalescing binary black on quasi-periodic oscillations observed in the tails of magnetar flares. In holes. particular, we argue that the sensitive dependence on the entrainment may have to be accounted for in attempts to match theoretical models to observational data.

A new approach to static numerical relativity and its application to Kaluza–Klein black holes

Matthew Headrick, Sam Kitchen and Toby Wiseman

2010 Class. Quantum Grav. 27 035002

We propose a framework for solving the Einstein equation for static and Euclidean metrics. First, we address the issue of gauge-fixing by borrowing from the Ricci-flow literature the so-called DeTurck trick, which renders the Einstein equation strictly elliptic and generalizes the usual harmonic- μ Figure 1. Two black holes of Schwarzschild masses m1,2 in a Cartesian coordinate system x = {ct, coordinate gauge. We then study two algorithms, Ricci-flow and Newton’s x, y, z} centered on the binary’s center of mass, or the associated spherical coordinate system method, for solving the resulting Einstein–DeTurck equation. We illustrate xα = {ct, r, θ, ϕ}. the use of these methods by studying localized black holes and non-uniform black strings in five-dimensional Kaluza–Klein theory, improving on previous calculations of their thermodynamic and geometric properties. We study spectra of various operators for these solutions, in particular finding the negative modes of the Lichnerowicz operator. We classify the localized solutions into two branches that meet at a minimum temperature. We DID YOU KNOW? find good evidence for a merger between the localized and non-uniform solutions. We also find a narrow window of localized solutions that possess All of the papers featured in the negative modes yet have positive specific heat. Highlights of 2009–2010 brochure are free to read on the CQG website until 31 December 2011. Visit iopscience.org/cqg for more information.

Highlights of 2009–2010 15 Classical and Quantum Gravity q Experimental gravity Comparison of the temperature dependence of the mechanical dissipation in thin films of

Ta2O5 and Ta2O5 doped with TiO2 I W Martin et al

2009 Class. Quantum Grav. 26 155012

Here we report the first results comparing the temperature dependence of the mechanical dissipation in thin films of Ta2O5 and Ta2O5 doped with TiO2, of a type suitable for use in the multilayer optical coatings for advanced gravitational wave detectors. The results indicate that doping Ta2O5 with TiO2 can significantly alter the distribution of activation energies associated with the low-temperature dissipation peak.

Figure 1. Comparison of the loss peak of an

undoped Ta2O5 and a Ta2O5 coating doped with

TiO2 at ~55 Hz.

Figure 1. Summary of all submitted numerical waveforms: r/M Re(h22). The x-axis shows time in units of M and the y-axis shows the real part of the (l,m) = (2, 2) component of the dimensionless

wave strain rh = rh+ − irh×. The top panels show the complete waveforms: the top-left panel includes waveforms that last more than about 700M, and the top-right panel includes waveforms shorter than about 700M. The bottom panel shows an enlargement of the merger phase for all waveforms.

Vibrating systems in Schwarzschild Testing gravitational-wave searches with spacetime: toward new experiments in numerical relativity waveforms: results from gravitation? the first Numerical INJection Analysis (NINJA) L Bergamin, P Delva and A Hees project 2009 Class. Quantum Grav. 26 185006

Benjamin Aylott et al In this paper, the effects of vibrations at high frequencies onto a freely 2009 Class. Quantum Grav. 26 165008 falling two-body system in Schwarzschild spacetime are investigated. As reference motion of the same system without vibrations, a circular orbit around the central body is considered. The vibrations induce a perturbation The Numerical INJection Analysis (NINJA) project is a collaborative effort on this motion, whose period is close to the orbital period, in agreement between members of the numerical relativity and gravitational-wave data with the simpler situation of the Shirokov effect [1]. In general relativity, analysis communities. The purpose of NINJA is to study the sensitivity of the amplitude of the perturbation is dominated by high-velocity effects, existing gravitational-wave search algorithms using numerically generated which grow linearly in the radius r of the circular orbit, while the leading term waveforms and to foster closer collaboration between the numerical surviving the Newtonian limit decays as 1/r. Thus, even for very large radii relativity and data analysis communities. We describe the results of the first a significant difference NINJA analysis which focused on gravitational waveforms from binary black between Newtonian hole coalescence. Ten numerical relativity groups contributed numerical physics and general data which were used to generate a set of gravitational-wave signals. relativity is found. We These signals were injected into a simulated data set, designed to mimic give an estimate of the response of the initial LIGO and Virgo gravitational-wave detectors. this effect for some Nine groups analysed this data using search and parameter-estimation molecular vibrations of a pipelines. Matched filter algorithms, un-modelled-burst searches and system orbiting around Bayesian parameter estimation and model-selection algorithms were the Earth. applied to the data. We report the efficiency of these search methods in detecting the numerical waveforms and measuring their parameters. We Figure 2. Profile of the describe preliminary comparisons between the different search methods constraint. and suggest improvements for future NINJA analyses.

16 Highlights of 2009–2010 Classical and Quantum Gravity

A xylophone configuration for a third-generation gravitational wave detector S Hild, S Chelkowski, A Freise, J Franc, N Morgado, R Flaminio and R DeSalvo

2010 Class. Quantum Grav. 27 015003

Achieving the demanding sensitivity and bandwidth, envisaged for third- generation gravitational wave (GW) observatories, is extremely challenging with a single broadband interferometer. Very high optical powers (megawatts) are required to reduce the quantum noise contribution at high frequencies, while the interferometer mirrors have to be cooled to cryogenic temperatures in order to reduce thermal noise sources at low frequencies. To resolve this potential conflict of cryogenic test masses with high thermal load, we present a conceptual design for a 2-band xylophone configuration for a third-generation GW observatory, composed of a high-power, high-frequency interferometer and a cryogenic low-power, low-frequency instrument. Featuring inspiral ranges of 3200 Mpc and 38 000 Mpc for binary neutron stars and binary black holes coalesences, respectively, we find that the potential sensitivity of xylophone configurations can be significantly wider and better than what is possible in a single broadband interferometer.

iopscience.org/cqg

Image adapted from: Gravitating discs around black holes V Karas et al 2004 Class. Quantum Grav. 21 R1–51

Figure 2. Apparent displacement from individual thermal noise contributions for a single cryogenic silicon test mass of ET-LF.

On the background estimation by time slides in a network of gravitational wave detectors

Michał Was, Marie-Anne Bizouard, Violette Brisson, Fabien Cavalier, Michel Davier, Patrice Hello, Nicolas Leroy, Florent Robinet and Miltiadis Vavoulidis

2010 Class. Quantum Grav. 27 015005

Time shifting the outputs of gravitational wave detectors operating in coincidence is a convenient way to estimate the background in a search for short-duration signals. However, this procedure is limited as increasing indefinitely the number of time shifts does not provide better estimates. We show that the false alarm rate estimation error saturates with the number of time shifts. In particular, for detectors with very different trigger rates, iopscience.org/cqg this error saturates at a large value. Explicit computations are done for two detectors, and for three detectors where the detection statistic relies on the logical ‘OR’ of the coincidences of the three couples in the network. Image adapted from: Adaptive Hough transform for the search of periodic sources C Palomba et al 2005 Class. Quantum Grav. 22 S1255–64

Highlights of 2009–2010 17 Classical and Quantum Gravity q Prizes sponsored by CQG IOP Gravitational Physics Group Thesis prize

The Gravitational Physics Group Thesis Prize, sponsored by Classical and The Bergmann-Wheeler Thesis prize Quantum Gravity, is made for excellence in postgraduate research and communication skills in gravitational physics. All members of the IOP In 2008, the International Society for General Relativity and Gravitation Gravitational Physics Group (GPG) who passed their PhD viva voce exam instituted a Thesis Prize in memory of two great pioneers of quantum gravity: between 30 September 2009 and 1 October 2010 are entitled to enter the Professors Peter Bergmann (1915–2002) and John Wheeler competition. Nominations should be made by the end of November 2011. (1911–2008). This prize is sponsored by Classical and Quantum Gravity. The winner will be awarded £500 and they will be invited to submit a paper The prize is awarded for the best PhD thesis in the broad area encompassing to CQG based on the winning thesis, which, if accepted, will be made a all approaches to quantum gravity. The winner will receive a cheque for ‘select article’ in CQG. Furthermore, the winner will be given the opportunity $1640 and a certificate. to present their work at one of the meetings supported by the GPG, usually the UK ‘BritGrav’ meeting. The first Bergmann-Wheeler prize was awarded to Dr Victor Taveras from Anyone (student or otherwise) can nominate a candidate (including the Louisiana State University at the 19th International Conference on General candidate themselves). If you have someone in mind, please send an e-mail Relativity and Gravitation (GR19) in Mexico City. The award was made in to the GPG secretary (David Burton, [email protected]) containing recognition of Dr Taveras’ contributions to loop quantum cosmology and his the candidate’s contact details and the committee will encourage development of a novel extension of loop quantum gravity. them to enter the competition. The candidate must be a member of the IOP Gravitational Physics Group and will be asked to provide their IOP The governance of the second prize can be summarized as follows. membership number. Nominations can be made at any time. The deadline for receipt of the nomination packet is 30 September 2012. The nominator must be a Membership of the GPG is open to all members of IOP. If you are not an IOP member of the International Society on General Relativity and Gravitation member, it is easy to join. Please see the IOP website for more information at and can make at most one nomination. The nominee need not be a member members.iop.org. of the society but the official defense must have taken place between 1 October 2009 and 30 September 2012. The nomination package will Please note that the student’s external PhD examiner must be willing to consist of: comment on the student’s research, quality of the student’s thesis and the student’s ability to cogently communicate their work (evidenced by their i) A nomination letter summarizing the main results and their importance, viva performance and/or seminars). Students are advised to consult their elaborating the student’s role in case of joint work. external examiner before entering the competition. ii) A PDF file or four hard copies of the thesis (one for each committee member). Copies of published papers that constitute the thesis can be Further information about the prize is available on the IOP GPG webpage at submitted in lieu of the thesis. gp.iop.org/. iii) A CV and the publication list of the nominee. iv) An official university document showing the date of the successful PhD Nils Andersson defense. Honorary Chair v) One or two letters in support from experts if the nominator is a supervisor Gravitational Physics Group of the PhD thesis.

The primary criteria for selection will be the high quality of scientific results, creativity and originality, and the significance of results to the broad area of the prize. The winner for each prize will be chosen by a committee of leading international experts in the field approximately six months before the GR20 conference in Warsaw, Poland. This will provide the winners ample time to plan travel to the conference. (In exceptional cases, the executive committee of the GRG Society may allow the prize to be given in absentia.)

Electronic nominations are preferred. The nomination packet should be sent IOPscience express on your iPhone to me via e-mail at [email protected]. Keep up to date with the latest articles published in IOP journals The mailing address for hard copies is: • Search School of Mathematical Sciences • Read Queen Mary, University of London • Download Mile End Road • E-mail London E1 4NS UK iopscience.org Malcolm A H MacCallum President International Society for General Relativity and Gravitation

18 Highlights of 2009–2010 Classical and Quantum Gravity

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Highlights of 2009–2010 19 Classical and Quantum Gravity