Classical and Quantum Gravity

The observational evidence that our universe is statistically isotropic is strong, but can never preclude a sufficiently small residual anisotropy. This work re-examines the ‘Bianchi’ models, a class of homogeneous but potentially anisotropic universes which include the normal, isotropic ‘Friedmann–Robertson–Walker’ models as a subset. We give a novel derivation of the Bianchi universes’ geometrical properties by enumerating ways to break the isotropy of maximally symmetric spaces. Then we find a systematic way to express dynamical behaviour of small anisotropies by linearising the Einstein equations in variables such as the shear. This exposes ways to evade traditional constraints on anisotropy (particularly those from nucleosynthesis, for instance).

The observational evidence that our universe is statistically isotropic is strong, but can never preclude a sufficiently small residual anisotropy.This work re-examines the 'Bianchi' models, a class of homogeneous but potentially anisotropic universes which include the normal, isotropic 'Friedmann-Robertson-Walker' models as a subset.We give a novel derivation of the Bianchi universes' geometrical properties by enumerating ways to break the isotropy of maximally symmetric spaces.Then we find a systematic way to express dynamical behaviour of small anisotropies by linearising the Einstein equations in variables such as the shear.This exposes ways to evade traditional constraints on anisotropy (particularly those from nucleosynthesis, for instance).

Inhomogeneity effects in cosmology
George F R Ellis 2011 Class.Quantum Grav.28 164001 This article looks at how inhomogeneous spacetime models may be significant for cosmology.First it looks at how the averaging process may affect large scale dynamics, with backreaction effects leading to effective contributions to the averaged energy-momentum tensor.Secondly it considers how local inhomogeneities may affect cosmological observations in cosmology, possibly significantly affecting the concordance model parameters.Thirdly it presents the possibility that the universe is spatially inhomogeneous on Hubble scales, with a violation of the Copernican principle leading to an apparent acceleration of the universe.This could perhaps even remove the need for the postulate of dark energy.
A comprehensive overview of the averaging problem and inhomogeneous models in cosmology.

Comment from Editorial Board
An interesting and very readable approach to homogeneous but anisotropic Bianchi cosmological models.A gauge-invariant treatment of general-relativistic higher-order perturbations on generic background spacetime is proposed.After reviewing a general framework of the second-order gauge-invariant perturbation theory, we show the fact that the linear-order metric perturbation is decomposed into gauge-invariant and gauge-variant parts, which was the important premise of this general framework.This means that the development of the higher-order gauge-invariant perturbation theory on generic background spacetime is possible.A remaining issue to be resolved is also discussed.This is a result of the author's series of works on gauge-invariant perturbation theories.

Statistical anisotropy from anisotropic inflation
Jiro Soda 2011 Class.Quantum Grav.28 083001 This is a review of an inflationary scenario with the anisotropic expansion and vector hair, named anisotropic inflation.It is the first counter example to the cosmic no-hair conjecture.It turns out that anisotropic inflation is an attractor solution in the presence of gauge kinetic functions and ubiquitous in supergravity.Remarkably, the anisotropy is at most of the order of the slow roll parameter.Nevertheless, anisotropic couplings induced by the vector hair give rise to the sizable statistical anisotropy in primordial fluctuations and the cross correlation between curvature perturbations and primordial gravitational waves, which can be tested using the CMB.

COSMOLOGy
Presents an interesting discussion of a topical issue in gauge-invariant perturbations.

Comment from Editorial Board
A well written review of anisotropic inflation and its observable consequences, a topic that has been very active recently.A key problem for gravitational theory is the discrepancy between quantum field theoretic predictions for the vacuum energy density, and the dark energy density observed in cosmology.It is known that this discrepancy can be resolved by replacing the Einstein Field Equations by their trace-free part, independently assuming energy-momentum conservation.However, one has to check that, as well as preserving the standard cosmological equations, this does not destroy the junction conditions that underlie the use of standard stellar models.We confirm that no problems arise here: hence, the Trace-Free Einstein Equations are indeed viable for cosmological and astrophysical applications.
A We critically assess the twin prospects of describing the observed universe in string theory, and using cosmological experiments to probe string theory.For the purposes of this short review, we focus on the limitations imposed by our incomplete understanding of string theory.After presenting an array of significant obstacles, we indicate a few areas that may admit theoretical progress in the near future.
Loop quantum cosmology: a status report Over the last decade, quantum geometry effects underlying loop quantum cosmology have been shown to resolve cosmological singularities, replacing them with quantum bounces.The rich Planck scale physics that results provides a novel conceptual framework for the early universe.When combined with specific scenarios, such as inflation, it provides concrete avenues to confront loop quantum gravity with observations.This topical review provides an overview of this rapidly evolving field.Special effort was made to make the subject readily accessible to three distinct communities: young researchers, quantum gravity experts and cosmologists.
Excellent review providing a clear and alluring description of the current status and challenges in string cosmology.

Comment from Editorial Board
An authoritative presentation of loop quantum cosmology.An important reference.
Comment from Editorial Board

Forthcoming focus issue
Autumn 2013

Massive gravity
Guest editor: Shinji Mukohyama

Forthcoming focus issue
Autumn 2013 The Galileons are a set of terms within four-dimensional effective field theories, obeying symmetries that can be derived from the dynamics of a (3 + 1)-dimensional flat brane embedded in a 5-dimensional Minkowski bulk.These theories have some intriguing properties, including freedom from ghosts and a non-renormalization theorem that hints at possible applications in both particle physics and cosmology.In this brief paper,we will summarize our attempts over the last year to extend the Galileon idea in two important ways.We will discuss the effective field theory construction arising from flat branes, of co-dimension greater than 1, embedded in a flat background-the multi-Galileons-and we will then describe symmetric covariant versions of the Galileons, more suitable for general cosmological applications.While all these Galileons can be thought of as interesting four-dimensional field theories in their own rights, the work described here may also make it easier to embed them into string theory, with its multiple extra dimensions and more general gravitational backgrounds.

QUANTUM COSMOLOGy
An elegant summary of the literature on Galileon theories.

Rising Impact Factor
CQG's Impact Factor has risen steadily for several years.The most recent Impact Factor of 3.320 puts the journal at an all-time high.

High visibility
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An excellent review of string theory, from its early days trying to understand its perturbative formulation, up to the current focus on non-perturbative issues.

Comment from Editorial Board
A very detailed analysis of the application of TsT transformations to string and M-theory.This review provides some historical background and then reviews developments in string theory over the last 25 years or so.The structure of the theory is explained in largely intuitive terms, highlighting how it incorporates our present understanding of particles and gravity and how it proposes to address the outstanding problems.Both perturbative and non-perturbative approaches to string theory are surveyed.Each of the four principal chapters concludes with a section "New Insights about Quantum Gravity" that summarises the concrete lessons learned about gravity at each stage.
The geometry and phase structure of non-relativistic branes Nabamita Banerjee, Suvankar Dutta and Dileep P Jatkar 2011 Class.Quantum Grav.28 165002 The AdS/CFT correspondence, an example of the holographic paradigm, suggests an equivalence between a d-dimensional gravity theory and a (d-1)-dimensional gauge theory.Non-relativistic generalization of this correspondence is an interesting aspect of current research.The dual theories with Schrödinger isometry are useful for studying interesting condensed matter phenomena.
We use a solution generating technique, the TsT-transformation, to obtain new solutions, which enjoys asymptotic Schrödinger symmetry, in type-II string theory and M-theory.We work out examples of rotating M2-branes and D1-D5-p-system, which have interesting field theory duals.Finally, we have devised a noble method to study the thermodynamics and phase structure of these newly-found stringy solutions.  .

TOPICAL REVIEW
The prize is awarded every three years for the best PhD thesis in quantum gravity.The winner of the prize will receive $1500 and a certificate.Nominations can be made at any time.Full guidelines can be found at isgrg.org.
The winner of the prize, to be awarded at the 20th General Relativity and Gravitation meeting in Warsaw in July 2013, is Dr Aron C Wall.The award was made for his original and insightful proof of a local form of the generalized second law of thermodynamics.Dr Wall completed his PhD under the supervision of Prof. Ted Jacobson at the University of Maryland.

Interview with Aron C Wall, Bergmann-Wheeler Thesis Prize winner
What led you into science and your chosen area of research?When I was seven, I read a physics book from the children's section of the library.At the end it said that subatomic particles were made out of quarks of three different "colors", and that each particle was made of "one quark of each color".I asked my Dad whether the book meant one, or at least one.So he took me to the grown-up section of the library to look at pop particle-physics books.In high school I was inspired by the online writings of John Baez to study quantum gravity.

What do you find most interesting about this subject?
There The loop approach to quantum gravity has developed very rapidly in the last 25 years.
The paper gives a synthetic presentation of the present state of the theory and attempts a critical evaluation of its successes and its present limits.

Holography for asymptotically locally Lifshitz spacetimes
Simon F Ross 2011 Class.Quantum Grav.28 215019 Detailed analysis of the holographic correspondence has mostly focused on asymptotically anti-de Sitter spacetimes, which are related to relativistic conformal field theories.The extension to field theories with anisotropic scaling symmetry is of interest for condensed matter physics; a holographic dual is provided by the Lifshitz spacetimes.This paper extends key features of the holographic dictionary to these cases by defining asymptotically locally Lifshitz spacetimes and showing how holographic renormalization can be implemented for arbitrary boundary data.We can thus study these field theories on general backgrounds, and we also obtain a deeper understanding of a new example of holography.
Well written introduction to the covariant definition of loop quantum gravity.Gravity may be the one force of nature we are intuitively most familiar with, but its theoretical understanding-despite the beauty of general relativity and string theory-is still shrouded in surprisingly many layers of mystery.Perhaps we already have all the pieces of the puzzle and just need to find the correct way of putting them together, or perhaps new ideas are needed.In this context, the idea of gravity with Lifshitz-type anisotropic scaling has attracted a lot of attention recently.
In this paper, we briefly review some of the main features of quantum gravity with anisotropic scaling, in its original formulation, and comment on its possible relation to the causal dynamical triangulations (CDT) approach to lattice quantum gravity.We then explain the construction of gravity with anisotropic scaling with an extended gauge symmetry-essentially a nonrelativistic version of general covariance.This extra symmetry eliminates the scalar graviton polarization, and thus brings the theory closer to general relativity at long distances.In this article I find a generalisation to the full Lorentz group.I take the phase space SL(2,C) of complex Ashtekar variables on a spin-network graph, and decompose it in terms of twistors.To every link there are two twistors attached, one for each boundary point.This setting embeds the non-linear phase space SL(2,C) into a vector space with canonical Darboux coordinates, and provides a new derivation of the solution space of the simplicity constraints of LQG.

Excellent review about the author's proposal
An introduction of some twistorial methods into loop quantum gravity.

Comment from Editorial Board
Simon Ross Guest editor Focus issue: Applications of holography to condensed matter physics Guest editor: Simon Ross The focus issue brings together original research contributions at the forefront of the rapidly developing use of holography to study the dynamics of strongly coupled systems in condensed matter physics.Work in this direction has led to new perspectives on fundamental topics such as superconductors and Fermi liquids.The aim of the focus issue is to highlight some of the recent developments and strengthen the connections between this direction and other gravitational research.

FOCUS ISSUE
We The holographic principle implies a vast reduction in the number of degrees of freedom of quantum gravity.This idea can be made precise in AdS 3 , where the stringy or gravitational exclusion principle asserts that certain perturbative excitations are not present in the exact quantum spectrum.We show that this effect is visible directly in the bulk gravity theory: the norm of the offending linearized state is zero or negative.When the norm is negative, the theory is signalling its own breakdown as an effective field theory; this provides a perturbative bulk explanation for the stringy exclusion principle.When the norm vanishes the bulk state is null rather than physical.This implies that certain non-trivial diffeomorphisms must be regarded as gauge symmetries rather than spectrum-generating elements of the asymptotic symmetry group.This leads to subtle effects in the computation of one-loop determinants for Einstein gravity, higher spin theories and topologically massive gravity in AdS 3 .In particular, heat kernel methods do not capture the correct spectrum of a theory with null states.
de Sitter invariance of the dS graviton vacuum A recent precise formulation of the hoop conjecture in four spacetime dimensions is that the Birkhoff invariant β (the least maximal length of any sweepout or foliation by circles) of an apparent horizon of energy E and area A should satisfy β G 4πE.This conjecture together with the cosmic censorship or isoperimetric inequality implies that the length  of the shortest non-trivial closed geodesic satisfies  2 G πA.We have tested these conjectures on the horizons of all four-charged rotating black hole solutions of ungauged supergravity theories and found that they always hold.They continue to hold in the presence of a negative cosmological constant, and for multi-charged rotating solutions in gauged supergravity.Surprisingly, they also hold for the Ernst-Wild static black holes immersed in a magnetic field, which are asymptotic to the Melvin solution.
Publisher's note: this abstract has been truncated to fit this page.Einstein's theory of gravity must undergo corrections at microscopic distances.Lovelock gravity is the simplest generalization that incorporates higher dimensions and higher curvature terms as suggested by string theory and quantum mechanics.We derive the Hamiltonian (i.e.energy function) for spherically symmetric Lovelock gravity in terms of geometrical variables.Remarkably, the result is as simple and elegant as that of its Einstein counterpart, supporting the interpretation of Lovelock gravity as the most natural higher-dimensional extension of general relativity.More importantly, this provides a crucial first step towards the study of the quantum mechanics and formation of generic Lovelock black holes.

A conservation law formulation of nonlinear elasticity in general relativity
Carsten Gundlach, Ian Hawke and Stephanie J Erickson 2012 Class.Quantum Grav.29 015005 Although the neutron star crust contributes only a small fraction to the total mass of the star, it is expected to affect the dynamics of systems where interface or crustal modes are excited.To model these, we have rewritten the elasticity formalism of Carter and Quintana in the form of conservation-laws for the stress-energy tensor and a configuration gradient, } A i ; the form is a clear extension of existing methods.We show that, with an appropriate constraint addition, the system is symmetric hyperbolic; this clarifies issues with constraints found in the Newtonian literature.We also perform various strongly non-linear numerical tests.
We expect that this paper will open up a whole new field of understanding critical phenomena in higher-derivative gravity theories.

Comment from Editorial Board
An important first step towards more realistic modelling of astrophysical compact objects.Comment from Editorial Board In this focus section we cover some of the non-astrophysical applications of numerical relativity, with an emphasis on recent uses of numerical relativity in high energy physics and quantum gravity.
Topics covered include higher dimensional black holes, the AdS/CFT correspondence, loop quantum cosmology, singularities, and corrections to general relativity.The articles cover not only the current state of the art in the numerical treatment of these subjects, but also strategies for choosing problems in these areas that can be effectively treated using numerical techniques and for crafting numerical techniques that can be applied to these problems.Find out more at articleevolution.org.

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An interesting and provocative idea that has enriched our view on black hole interiors.
Comment from Editorial Board Bound inside rotating or charged black holes, there are stable periodic planetary orbits, which neither come out nor terminate at the central singularity.Stable periodic orbits inside black holes exist even for photons.These bound orbits may be defined as orbits of the third kind, following the Chandrasekhar classification of particle orbits in the black hole gravitational field.The existence domain for the third-kind orbits is rather spacious, and thus there is place for life inside supermassive black holes in the galactic nuclei.
Interiors of the supermassive black holes may be inhabited by civilizations, being invisible from the outside.In principle, one can get information from the interiors of black holes by observing their white hole counterparts.
Focus issue: Relativistic quantum information This focus issue provides the first summary of the state of the art in research in RQI.Some of the articles in this issue review the subject while others provide interesting new results that will stimulate further research.What makes the subject all the more exciting is that it is beginning to enter the stage at which actual experiments can be contemplated, and some of the articles appearing in this issue discuss some of these exciting new developments.
One of the articles in this issue, "Optical Black Hole Lasers" by D Faccio et al, received considerable attention in the science media including the New Scientist news article: "Black-hole laser edges closer to testing Hawking" by Lisa Grossman.

FOCUS ISSUE
For further information and to read the invited articles see CQG One of the limiting noise sources in advanced interferometric gravitational wave detectors is the brownian thermal noise arising within the mirror coatings applied to the test masses.It has been shown that the largest contribution to this thermal noise comes from the high refractive index coating material, tantalum pentoxide (tantala).In this paper, hafnium dioxide (hafnia) is considered as an alternative high-index coating material.The mechanical loss, which is directly related to thermal noise in the interferometer, of various hafnia coatings is measured within the temperature range of 10-310 K.It is observed that the mechanical loss of hafnia is below that of tantala at temperatures below 100 K--a region of interest for future cryogenic detectors.Furthermore, it was found that the hafnia coatings were partially-crystalline, which has been shown to increase mechanical loss in tantala.Silica doping of hafnia has been shown to prevent crystallization in hafnia, indicating an avenue for further improving the mechanical loss of hafnia coatings.
Characterization of the Virgo seismic environment The Virgo gravitational wave detector is an interferometer with 3km arms located in Pisa, Italy.Despite several techniques previously adopted to isolate the interferometer from the environment, seismic noise remains an important issue for Virgo.I stationed a seismometer at various locations around the Virgo site hosting major infrastructure machines.The primary aim of this study was to identify noisy machines which seismically affect the ITF environment and thus require mitigation attention.It was found that noise from several Virgo infrastructure devices, such as water chillers, heaters, and pumps, seismically affects sensitive parts of the interferometer.We further explore the reduced basis (RB) approach for modeling and representing gravitational waves as developed in Field et al (2011 Phys. Rev. Lett. 106 221102).
While Field et al focused on a matched filter search for non-spinning compact binary coalescences in Advanced LIGO, we focus on single and multiple quasi-normal mode (QNM) black hole ringdown matched filter searches as a demonstration of the very compact and high accuracy representations possible in higher dimensional spaces.
Multimode searches would allow one to test the no-hair theorem and to improve parameter estimation among other advantages.For a minimal match of 0.99 of signal with template in a traditional matched filter bank, we find only 737 RB waveforms needed for a GRconstrained two-mode search (1198 for unconstrained two-mode search).These numbers stand in contrast to the 3500 (3400000) traditional templates needed.In fact, we find that the RB waveforms are able to represent any QNM in the original bank with error less than 10 -13 , and any QNM in the continuum of the search space with error less than 9:8 × 10 -10 .
A promising new method for template bank based match filtering analysis.
Comment from Editorial Board

Abhay
Ashtekar and Parampreet Singh 2011 Class.Quantum Grav.28 213001 String theory: a perspective over the last 25 years Sunil Mukhi 2011 Class.Quantum Grav.28 153001 's a lot of deep conceptual questions about how we should think about spacetime: is it discrete or continuous, what is it made out of, what things should count as observables, what is the role of thermodynamics and Lorentz invariance, that I find fascinating.Can you tell us a little bit about the work in your thesis?It's a proof of the Second Law of Thermodynamics for black holes and other causal horizons.Even when the quantum fields falling into the black hole are rapidly evolving, the entropy increases at every point on the horizon.It's been published as three separate articles, freely available on the arXiv(0901.3865,1007.1493,1105.3445 to treat gravity with an anisotropic scaling.describes the evolution of "quanta of spacetime" in terms of a local product of interaction vertices (spinfoam), reconstructing a Lorentzian spacetime.The resulting transition amplitudes are computed between states, where the quantum geometry is captured by SU(2)-decorated graphs.These states can describe a homogeneous and isotropic geometry, and for Minkowski and de Sitter space, the resulting transition amplitudes yield the Friedmann equation.In this paper it is shown that this result holds independently of the particular graph used in the boundary states.The first spinfoam calculation indicating that the amplitude may be stable under refinement of the boundary node. of pioneering articles Dupuis, Freidel, Livine, Speziale and Tambornino developed a spinorial description of loop quantum gravity (LQG).They considered the case of real-valued Ashtekar variables, which however, inherited the notorious limitations of the SU(2) formalism.When considering local Lorentz transformations only spatial rotations act linearly.
Atsushi Higuchi, Donald Marolf and Ian A Morrison 2011 Class.Quantum Grav.28 245012The two-point function of linearized gravitons on de Sitter space is infrared divergent in the standard transverse traceless synchronous gauge defined by k =0 cosmological coordinates (also called conformal or Poincaré coordinates).We show that this divergence can be removed by adding a linearized diffeomorphism to each mode function, i.e. by an explicit change of gauge.It follows that the graviton vacuum state is well defined and de Sitter invariant in agreement with various earlier arguments.A very interesting paper about the careful quantization of pure 2+1 AdS gravity when the cosmological constant is Planck scale.Comment from Editorial BoardThe IR behavior of de Sitter invariant graviton has always been a problem.Here it is clearly shown that the IR divergence can be removed by a simple gauge transformation.'s invariant and Thorne's hoop conjecture for horizons M Cvetic ˇ, G W Gibbons and C N Pope 2011 Class.QuantumGrav.28 195001 Ricci solitons, Ricci flow and strongly coupled CFT in the Schwarzschild Unruh or Boulware vacua Pau Figueras, James Lucietti and Toby Wiseman 2011 Class.Quantum Grav.28 215018 In recent years it has become clear that numerical methods will be key in finding new black hole solutions.Building on previous work of one of the authors (TW), in this paper we consider the Einstein-DeTurck equation as an elliptic equation to be solved numerically as a boundary value problem.For static vacuum spacetimes with zero or negative cosmological constant we show that under suitable boundary conditions, all solutions to the Einstein-DeTurck equation must be Einstein.As an example, we numerically construct (using Ricci flow) the gravitational dual of N = 4 super-Yang-Mills in the background of the Schwarzschild black hole.An interesting attempt to seek a precise formulation of Thorne's hoop conjecture.Comment from Editorial Board A very good work on numerically finding stationary solutions, with an application to AdS/CFT.symmetric Lovelock gravity Gabor Kunstatter, Tim Taves and Hideki Maeda 2012 Class.Quantum Grav.29 092001

FTC
field CQG has the best peer review of any gravitational physics journal.The quality ratings of published articles assigned by the referees during peer review clearly show continued improvement.CQG brings you the best online reading experience with our enhanced HTML article format Thanks to the Article Evolution project, CQG offers a fast and simple browsing experience on tablets and smartphones.Other features include the ability to zoom in and out of images and MathJax technology, which enables sharp rendering of mathematics.
Guest Editors: R B Mann and T RalphOver the past few years, a new field of high research intensity has emerged that blends together concepts from gravitational physics and quantum computing.Known as Relativistic Quantum Information, or RQI, the subject pulls together concepts and ideas from special relativity, quantum optics, general relativity, quantum communication, and quantum computation.
Classical and Quantum Gravity iopscience.org/cqg/highlights11-1233 ExPERIMENTAL GRAVITy Focus issue: Tests of the weak equivalence principle Guest Editors: C C Speake and C M Will This focus issue reviews the current status of tests of the weak equivalence principle, or the universality of free fall, which underlies all metric theories of gravity.In addition to an article on the theoretical implications of WEP, the issue covers laboratory experiments, lunar-laser ranging tests, binary pulsar tests, proposed tests of the free fall of antimatter, and future possible tests in space.articles in CQG Volume 29 Issue 18 Reduced basis representations of multi-mode black hole ringdown gravitational waves Sarah Caudill, Scott E Field, Chad R Galley, Frank Herrmann and Manuel Tiglio 2012 Class.Quantum Grav.29 095016 Impact Factor has risen steadily for several years.The most recent Impact Factor of 3.320 puts the journal at an all-time high.The international Editorial Board oversees the scientific strategy of the journal and provides expert advice on the CQG peer review.The Editor-in-Chief is the final arbiter of the content and quality of the journal.desParticules, FranceThe CQG Advisory Panel (AP) includes some of the best referees in gravitational physics.Members are invited to the panel after showing excellent judgment as regular referees for CQG.The AP provides quick and authoritative advice, which adds enormous value to the CQG peer review process.
Cosmology George F R Ellis University of Cape Town, South Africa Andrew Pontzen University of Oxford, UK Linearization of homogeneous, nearly-isotropic cosmological models Andrew Pontzen and Anthony Challinor 2011 Class.Quantum Grav.28 185007 invite you to read the articles in CQG Volume 29 Issue 19 Classical and Quantum Gravity iopscience.org/cqg/highlights11-1219 The gravitational exclusion principle and null states in anti-de Sitter space Alejandra Castro, Thomas Hartman and Alexander Maloney 2011 Class.Quantum Grav.28 195012 Volume 29 Issue 22 Possibly the breakthrough for conquering mirror coating thermal noise to enhance the detection rate of gravitational waves.Comment from Editorial BoardA new point of view to widen the observational frequency band towards lower frequency in gravitational wave detection.Comment from Editorial Board