The open-access journal Living Reviews in Relativity has published a new review article on 27 September 2019:

Sumati Surya,
“The causal set approach to quantum gravity”,
Living Rev Relativ (2019) 22:5
https://doi.org/10.1007/s41114-019-0023-1

Abstract:
The causal set theory (CST) approach to quantum gravity postulates that at the most fundamental level, spacetime is discrete, with the spacetime continuum replaced by locally finite posets or “causal sets”. The partial order on a causal set represents a proto-causality relation while local finiteness encodes an intrinsic discreteness. In the continuum approximation the former corresponds to the spacetime causality relation and the latter to a fundamental spacetime atomicity, so that finite volume regions in the continuum contain only a finite number of causal set elements. CST is deeply rooted in the Lorentzian character of spacetime, where a primary role is played by the causal structure poset. Importantly, the assumption of a fundamental discreteness in CST does not violate local Lorentz invariance in the continuum approximation. On the other hand, the combination of discreteness and Lorentz invariance gives rise to a characteristic non-locality which distinguishes CST from most other approaches to quantum gravity. In this review we give a broad, semi-pedagogical introduction to CST, highlighting key results as well as some of the key open questions. This review is intended both for the beginner student in quantum gravity as well as more seasoned researchers in the field.

Please, visit frequently our relativity channel (https://www.springer.com/gp/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Relativity has published a new review article on 8 July 2019:

Vitor Cardoso and Paolo Pani,
“Testing the nature of dark compact objects: a status report”,
Living Rev Relativ (2019) 22:4
https://doi.org/10.1007/s41114-019-0020-4

Abstract:
Very compact objects probe extreme gravitational fields and may be the key to understand outstanding puzzles in fundamental physics. These include the nature of dark matter, the fate of spacetime singularities, or the loss of unitarity in Hawking evaporation. The standard astrophysical description of collapsing objects tells us that massive, dark and compact objects are black holes. Any observation suggesting otherwise would be an indication of beyond-the-standard-model physics. Null results strengthen and quantify the Kerr black hole paradigm. The advent of gravitational-wave astronomy and precise measurements with very long baseline interferometry allow one to finally probe into such foundational issues. We overview the physics of exotic dark compact objects and their observational status, including the observational evidence for black holes with current and future experiments.

Please, visit frequently our relativity channel (https://www.springer.com/gp/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Relativity has published a new review article on 3 June 2019:

Ettore Minguzzi,
“Lorentzian causality theory”
Living Rev Relativ (2019) 22:3.
https://doi.org/10.1007/s41114-019-0019-x

Abstract:
I review Lorentzian causality theory paying particular attention to the optimality and generality of the presented results. I include complete proofs of some foundational results that are otherwise difficult to find in the literature (e.g. equivalence of some Lorentzian length definitions, upper semi-continuity of the length functional, corner regularization, etc.). The paper is almost self-contained thanks to a systematic logical exposition of the many different topics that compose the theory. It contains new results on classical concepts such as maximizing curves, achronal sets, edges, horismos, domains of dependence, Lorentzian distance. The treatment of causally pathological spacetimes requires the development of some new versatile causality notions, among which I found particularly convenient to introduce: biviability, chronal equivalence, araying sets, and causal versions of horismos and trapped sets. Their usefulness becomes apparent in the treatment of the classical singularity theorems, which is here considerably expanded in the exploration of some variations and alternatives.

Please, visit frequently our relativity channel (https://www.springer.com/gp/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Relativity has published a new review article on 29 April 2019:

Stefan L. Danilishin, Farid Ya. Khalili, Haixing Miao,
“Advanced quantum techniques for future gravitational-wave detectors”,
Living Rev Relativ (2019) 22: 2.
https://doi.org/10.1007/s41114-019-0018-y

Abstract:
Quantum fluctuation of light limits the sensitivity of advanced laser interferometric gravitational-wave detectors. It is one of the principal obstacles on the way towards the next-generation gravitational-wave observatories. The envisioned significant improvement of the detector sensitivity requires using quantum non-demolition measurement and back-action evasion techniques, which allow us to circumvent the sensitivity limit imposed by the Heisenberg uncertainty principle. In our previous review article (Danilishin and Khalili in Living Rev Relativ 15:5, 2012), we laid down the basic principles of quantum measurement theory and provided the framework for analysing the quantum noise of interferometers. The scope of this paper is to review novel techniques for quantum noise suppression proposed in the recent years and put them in the same framework. Our delineation of interferometry schemes and topologies is intended as an aid in the process of selecting the design for the next-generation gravitational-wave observatories.

Please, visit frequently our relativity channel (https://www.springer.com/gp/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Relativity has published a new review article on 18 December 2018:

Ishak, Mustapha,
“Testing general relativity in cosmology”,
Living Rev Relativ (2019) 22: 1.
https://doi.org/10.1007/s41114-018-0017-4

Abstract:
We review recent developments and results in testing general relativity (GR) at cosmological scales. The subject has witnessed rapid growth during the last two decades with the aim of addressing the question of cosmic acceleration and the dark energy associated with it. However, with the advent of precision cosmology, it has also become a well-motivated endeavor by itself to test gravitational physics at cosmic scales. We overview cosmological probes of gravity, formalisms and parameterizations for testing deviations from GR at cosmological scales, selected modified gravity (MG) theories, gravitational screening mechanisms, and computer codes developed for these tests. We then provide summaries of recent cosmological constraints on MG parameters and selected MG models. We supplement these cosmological constraints with a summary of implications from the recent binary neutron star merger event. Next, we summarize some results on MG parameter forecasts with and without astrophysical systematics that will dominate the uncertainties. The review aims at providing an overall picture of the subject and an entry point to students and researchers interested in joining the field. It can also serve as a quick reference to recent results and constraints on testing gravity at cosmological scales.

Please, visit frequently our relativity channel (https://www.springer.com/gp/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Relativity has published two new review articles in August 2018:

MacCallum, M.A.H.,
“Computer algebra in gravity research”,
Living Rev Relativ (2018) 21: 6.
https://doi.org/10.1007/s41114-018-0015-6

Schaefer, G. and Jaranowski, P.,
“Hamiltonian formulation of general relativity and post-Newtonian dynamics of compact binaries”,
Living Rev Relativ (2018) 21: 7.
https://doi.org/10.1007/s41114-018-0016-5

Please, visit frequently our relativity channel (https://www.springer.com/gp/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Relativity has published a new review article on “Relativistic dynamics and extreme mass ratio inspirals” by Pau Amaro-Seoane on 15 May 2018:

Amaro-Seoane, Pau,
“Relativistic dynamics and extreme mass ratio inspirals”,
Living Rev Relativ (2018) 21: 4.
https://doi.org/10.1007/s41114-018-0013-8

ABSTRACT:
It is now well-established that a dark, compact object, very likely a massive black hole (MBH) of around four million solar masses is lurking at the centre of the Milky Way. While a consensus is emerging about the origin and growth of supermassive black holes (with masses larger than a billion solar masses), MBHs with smaller masses, such as the one in our galactic centre, remain understudied and enigmatic. The key to understanding these holes—how some of them grow by orders of magnitude in mass—lies in understanding the dynamics of the stars in the galactic neighbourhood. Stars interact with the central MBH primarily through their gradual inspiral due to the emission of gravitational radiation. Also stars produce gases which will subsequently be accreted by the MBH through collisions and disruptions brought about by the strong central tidal field. Such processes can contribute significantly to the mass of the MBH and progress in understanding them requires theoretical work in preparation for future gravitational radiation millihertz missions and X-ray observatories. In particular, a unique probe of these regions is the gravitational radiation that is emitted by some compact stars very close to the black holes and which could be surveyed by a millihertz gravitational-wave interferometer scrutinizing the range of masses fundamental to understanding the origin and growth of supermassive black holes. By extracting the information carried by the gravitational radiation, we can determine the mass and spin of the central MBH with unprecedented precision and we can determine how the holes “eat” stars that happen to be near them.

Please, visit frequently our relativity channel (http://www.springer.com/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Relativity has published a new review article on “Tests of chameleon gravity” by Clare Burrage and Jeremy Sakstein on 16 March 2018:

Burrage, C. and Sakstein, J.,
“Tests of chameleon gravity”,
Living Rev Relativ (2018) 21: 1.
https://doi.org/10.1007/s41114-018-0011-x

ABSTRACT:
Theories of modified gravity, where light scalars with non-trivial self-interactions and non-minimal couplings to matter—chameleon and symmetron theories—dynamically suppress deviations from general relativity in the solar system. On other scales, the environmental nature of the screening means that such scalars may be relevant. The highly-nonlinear nature of screening mechanisms means that they evade classical fifth-force searches, and there has been an intense effort towards designing new and novel tests to probe them, both in the laboratory and using astrophysical objects, and by reinterpreting existing datasets. The results of these searches are often presented using different parametrizations, which can make it difficult to compare constraints coming from different probes. The purpose of this review is to summarize the present state-of-the-art searches for screened scalars coupled to matter, and to translate the current bounds into a single parametrization to survey the state of the models. Presently, commonly studied chameleon models are well-constrained but less commonly studied models have large regions of parameter space that are still viable. Symmetron models are constrained well by astrophysical and laboratory tests, but there is a desert separating the two scales where the model is unconstrained. The coupling of chameleons to photons is tightly constrained but the symmetron coupling has yet to be explored. We also summarize the current bounds on f(R) models that exhibit the chameleon mechanism (Hu and Sawicki models). The simplest of these are well constrained by astrophysical probes, but there are currently few reported bounds for theories with higher powers of R. The review ends by discussing the future prospects for constraining screened modified gravity models further using upcoming and planned experiments.

Please, visit frequently our relativity channel (http://www.springer.com/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Computational Astrophysics has published a new review article on 11 December 2017:

Balsara, D.S., Higher-order accurate space-time schemes for computational astrophysics — Part I: finite volume methods, Living Rev Comput Astrophys (2017) 3: 2. https://doi.org/10.1007/s41115-017-0002-8

Abstract:
As computational astrophysics comes under pressure to become a precision science, there is an increasing need to move to high accuracy schemes for computational astrophysics. The algorithmic needs of computational astrophysics are indeed very special. The methods need to be robust and preserve the positivity of density and pressure. Relativistic flows should remain sub-luminal. These requirements place additional pressures on a computational astrophysics code, which are usually not felt by a traditional fluid dynamics code. Hence the need for a specialized review. The focus here is on weighted essentially non-oscillatory (WENO) schemes, discontinuous Galerkin (DG) schemes and PNPM schemes. WENO schemes are higher order extensions of traditional second order finite volume schemes. At third order, they are most similar to piecewise parabolic method schemes, which are also included. DG schemes evolve all the moments of the solution, with the result that they are more accurate than WENO schemes. PNPM schemes occupy a compromise position between WENO and DG schemes. They evolve an Nth order spatial polynomial, while reconstructing higher order terms up to Mth order. As a result, the timestep can be larger. Time-dependent astrophysical codes need to be accurate in space and time with the result that the spatial and temporal accuracies must be matched. This is realized with the help of strong stability preserving Runge–Kutta schemes and ADER (Arbitrary DERivative in space and time) schemes, both of which are also described. The emphasis of this review is on computer-implementable ideas, not necessarily on the underlying theory.

The open-access journal Living Reviews in Relativity has published three new review articles in November 2017:

Paschalidis, V. &; Stergioulas, N., “Rotating stars in relativity”, Living Rev Relativ (2017) 20: 7. https://doi.org/10.1007/s41114-017-0008-x

Frolov, V.P., Krtoua, P. &; Kubiznak, D., “Black holes, hidden symmetries, and complete integrability”, Living Rev Relativ (2017) 20: 6. https://doi.org/10.1007/s41114-017-0009-9

Liebling, S.L. &; Palenzuela, C., Dynamical boson stars, Living Rev Relativ (2017) 20: 5. https://doi.org/10.1007/s41114-017-0007-y

Please, visit frequently our relativity channel (http://www.springer.com/livingreviews/relativity) at http://livingreviews.org for other news.

The open-access journal Living Reviews in Relativity has recently published two new major updates of review articles:

“The Kerr/CFT correspondence and its extensions” by Geoffrey Compere (http://dx.doi.org/10.1007/s41114-017-0003-2) and “Interferometer techniques for gravitational-wave detection” by Charlotte Bond, Daniel Brown, Andreas Freise and Kenneth A. Strain (http://dx.doi.org/10.1007/s41114-016-0002-8).

Due to a technical error, the latter was published with a wrong article citation ID, which will be corrected as soon as possible. We would also like to apologize to the authors for the tremendous delays caused by workflow adjustments after the journal transfer to Springer.

The open-access journal Living Reviews in Relativity has published a new review article on “Extraction of gravitational waves in numerical relativity” by Nigel T. Bishop and Luciano Rezzolla on 4 October 2016 (metadata correction 10 November 2016):

Bishop, N.T. and Rezzolla, L.,
“Extraction of gravitational waves in numerical relativity”,
Living Rev Relativ (2016) 19: 2.
http://doi.org/10.1007/s41114-016-0001-9

ABSTRACT:
A numerical-relativity calculation yields in general a solution of the Einstein equations including also a radiative part, which is in practice computed in a region of finite extent. Since gravitational radiation is properly defined only at null infinity and in an appropriate coordinate system, the accurate estimation of the emitted gravitational waves represents an old and non-trivial problem in numerical relativity. A number of methods have been developed over the years to “extract” the radiative part of the solution from a numerical simulation and these include: quadrupole formulas, gauge-invariant metric perturbations, Weyl scalars, and characteristic extraction. We review and discuss each method, in terms of both its theoretical background as well as its implementation. Finally, we provide a brief comparison of the various methods in terms of their inherent advantages and disadvantages.

Living Reviews in Relativity has published a new review article on “Terrestrial Gravity Fluctuations” by Jan Harms on 2 December 2015.

Living Reviews in Computational Astrophysics has published a new review article on “Grid-based Methods in Relativistic Hydrodynamics and Magnetohydrodynamics” by Jose Maria Marti and Ewald Mueller on 22 December 2015.

Please find the abstracts and further details below.

—————-

PUB.NO. lrca-2015-3
Marti, Jose Maria and Mueller, Ewald
“Grid-based Methods in Relativistic Hydrodynamics and Magnetohydrodynamics”

ACCEPTED: 2015-12-01
PUBLISHED: 2015-12-22

FULL ARTICLE AT:
http://www.livingreviews.org/lrca-2015-3

ABSTRACT:
An overview of grid-based numerical methods used in relativistic hydrodynamics (RHD) and magnetohydrodynamics (RMHD) is presented. Special emphasis is put on a comprehensive review of the application of high-resolution shock-capturing methods. Results of a set of demanding test bench simulations obtained with different numerical methods are compared in an attempt to assess the present capabilities and limits of the various numerical strategies. Applications to three astrophysical phenomena are briefly discussed to motivate the need for and to demonstrate the success of RHD and RMHD simulations in their understanding. The review further provides FORTRAN programs to compute the exact solution of the Riemann problem in RMHD, and to simulate 1D RMHD flows in Cartesian coordinates.

UPCOMING ARTICLES AT:
http://computastrophys.livingreviews.org/Articles/upcoming.html

—————-

PUB.NO. lrr-2015-3
Harms, Jan
“Terrestrial Gravity Fluctuations”

ACCEPTED: 2015-11-16
PUBLISHED: 2015-12-02

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2015-3

ABSTRACT:
Different forms of fluctuations of the terrestrial gravity field are observed by gravity experiments. For example, atmospheric pressure fluctuations generate a gravity-noise foreground in measurements with super-conducting gravimeters. Gravity changes caused by high-magnitude earthquakes have been detected with the satellite gravity experiment GRACE, and we expect high-frequency terrestrial gravity fluctuations produced by ambient seismic fields to limit the sensitivity of ground-based gravitational-wave (GW) detectors. Accordingly, terrestrial gravity fluctuations are considered noise and signal depending on the experiment. Here, we will focus on ground-based gravimetry. This field is rapidly progressing through the development of GW detectors. The technology is pushed to its current limits in the advanced generation of the LIGO and Virgo detectors, targeting gravity strain sensitivities better than 10^-23 Hz^-1/2 above a few tens of a Hz. Alternative designs for GW detectors evolving from traditional gravity gradiometers such as torsion bars, atom interferometers, and superconducting gradiometers are currently being developed to extend the detection band to frequencies below 1 Hz. The goal of this article is to provide the analytical framework to describe terrestrial gravity perturbations in these experiments. Models of terrestrial gravity perturbations related to seismic fields, atmospheric disturbances, and vibrating, rotating or moving objects, are derived and analyzed. The models are then used to evaluate passive and active gravity noise mitigation strategies in GW detectors, or alternatively, to describe their potential use in geophysics. The article reviews the current state of the field, and also presents new analyses especially with respect to the impact of seismic scattering on gravity perturbations, active gravity noise cancellation, and time-domain models of gravity perturbations from atmospheric and seismic point sources. Our understanding of terrestrial gravity fluctuations will have great impact on the future development of GW detectors and high-precision gravimetry in general, and many open questions need to be answered still as emphasized in this article.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity had been recently acquired by Springer and has now resumed publication with two new review articles: “Exploring New Physics Frontiers Through Numerical Relativity” by Vitor Cardoso et al. and “The Hubble Constant” (major update) by Neal Jackson.

Please find the abstracts and further details below.

——————
PUB.NO. lrr-2015-1
Vitor Cardoso, Leonardo Gualtieri, Carlos A. R. Herdeiro and Ulrich Sperhake,
“Exploring New Physics Frontiers Through Numerical Relativity”

PUBLISHED: 2015-09-21

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2015-1

ABSTRACT:
The demand to obtain answers to highly complex problems within strong-field gravity has been met with significant progress in the numerical solution of Einstein’s equations – along with some spectacular results – in various setups. We review techniques for solving Einstein’s equations in generic spacetimes, focusing on fully nonlinear evolutions but also on how to benchmark those results with perturbative approaches. The results address problems in high-energy physics, holography, mathematical physics, fundamental physics, astrophysics and cosmology.

——————
PUB.NO. lrr-2015-2
Neal Jackson
“The Hubble Constant” (major update)

PUBLISHED: 2015-09-24

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2015-2

ABSTRACT:
I review the current state of determinations of the Hubble constant, which gives the lengthscale of the Universe by relating the expansion velocity of objects to their distance. There are two broad categories of measurements. The first uses individual astrophysical objects which have some property that allows their intrinsic luminosity or size to be determined, or allows the determination of their distance by geometric means. The second category comprises the use of all-sky cosmic microwave background, or correlations between large samples of galaxies, to determine information about the geometry of the Universe and hence the Hubble constant, typically in a combination with other cosmological parameters. Many, but not all, object-based measurements give H_0 values of around 72 – 74 km s^–1 Mpc^–1, with typical errors of 2 – 3 km s^–1 Mpc^–1. This is in mild discrepancy with CMB-based measurements, in particular those from the Planck satellite, which give values of 67 – 68 km s^–1 Mpc^–1 and typical errors of 1 – 2 km s^–1 Mpc^–1. The size of the remaining systematics indicate that accuracy rather than precision is the remaining problem in a good determination of the Hubble constant. Whether a discrepancy exists, and whether new physics is needed to resolve it, depends on details of the systematics of the object-based methods, and also on the assumptions about other cosmological parameters and which datasets are combined in the case of the all-sky methods.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity recently published two new articles: a major update of the review on “Time-Delay Interferometry” by Massimo Tinto and Sanjeev V. Dhurandhar and a new article on “Massive Gravity” by Claudia de Rham.

Please find the abstracts and further details below.

——————
PUB.NO. lrr-2014-7
de Rham, Claudia
“Massive Gravity”

ACCEPTED: 2014-07-18
PUBLISHED: 2014-08-25

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2014-7

ABSTRACT:
We review recent progress in massive gravity. We start by showing how different theories of massive gravity emerge from a higher-dimensional theory of general relativity, leading to the Dvali-Gabadadze-Porrati model (DGP), cascading gravity and ghost-free massive gravity. We then explore their theoretical and phenomenological consistency, proving the absence of Boulware-Deser ghosts and reviewing the Vainshtein mechanism and the cosmological solutions in these models. Finally, we present alternative and related models of massive gravity such as new massive gravity, Lorentz-violating massive gravity and non-local massive gravity.

——————
PUB.NO. lrr-2014-6
Tinto, Massimo and Dhurandhar, Sanjeev V.
“Time-Delay Interferometry”

ACCEPTED: 2014-07-28
PUBLISHED: 2014-08-05

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2014-6

ABSTRACT:
Equal-arm detectors of gravitational radiation allow phase measurements many orders of magnitude below the intrinsic phase stability of the laser injecting light into their arms. This is because the noise in the laser light is common to both arms, experiencing exactly the same delay, and thus cancels when it is differenced at the photo detector. In this situation, much lower level secondary noises then set the overall performance. If, however, the two arms have different lengths (as will necessarily be the case with space-borne interferometers), the laser noise experiences different delays in the two arms and will hence not directly cancel at the detector. In order to solve this problem, a technique involving heterodyne interferometry with unequal arm lengths and independent phase-difference readouts has been proposed. It relies on properly time-shifting and linearly combining independent Doppler measurements, and for this reason it has been called time-delay interferometry (TDI). This article provides an overview of the theory, mathematical foundations, and experimental aspects associated with the implementation of TDI. Although emphasis on the application of TDI to the Laser Interferometer Space Antenna (LISA) mission appears throughout this article, TDI can be incorporated into the design of any future space-based mission aiming to search for gravitational waves via interferometric measurements. We have purposely left out all theoretical aspects that data analysts will need to account for when analyzing the TDI data combinations.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity has published a new review article on “On the History of Unified Field Theories. Part II (ca. 1930 – ca. 1965)” by Hubert F. M. Goenner on 23 June 2014.

Please find the abstract and further details below.

——————
PUB.NO. lrr-2014-5
Goenner, Hubert F. M.
“On the History of Unified Field Theories. Part II (ca. 1930 – ca. 1965)”

ACCEPTED: 2014-05-13
PUBLISHED: 2014-06-23

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2014-5

ABSTRACT:
The present review intends to provide an overall picture of the research concerning classical unified field theory, worldwide, in the decades between the mid-1930 and mid-1960. Main themes are the conceptual and methodical development of the field, the interaction among the scientists working in it, their opinions and interpretations. Next to the most prominent players, A. Einstein and E. Schrodinger, V. Hlavaty and the French groups around A. Lichnerowicz, M.-A. Tonnelat, and Y. Thiry are presented. It is shown that they have given contributions of comparable importance. The review also includes a few sections on the fringes of the central topic like Born-Infeld electromagnetic theory or scalar-tensor theory. Some comments on the structure and organization of research-groups are also made.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity has published an update of the review “The Confrontation between General Relativity and Experiment” by Clifford M. Will on 11 June 2014.

Please find the abstract and further details below.

This new edition of one of our most read reviews also marks a new milestone the journal’s history: as of this day, Living Reviews has published 130 articles on 90 topics!

——————
PUB.NO. lrr-2014-4
Will, Clifford M.
“The Confrontation between General Relativity and Experiment”

ACCEPTED: 2014-06-06
PUBLISHED: 2014-06-11

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2014-4

ABSTRACT:
The status of experimental tests of general relativity and of theoretical frameworks for analyzing them are reviewed and updated. Einstein’s equivalence principle (EEP) is well supported by experiments such as the Eötvös experiment, tests of local Lorentz invariance and clock experiments. Ongoing tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Tests of general relativity at the post-Newtonian level have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, the Nordtvedt effect in lunar motion, and frame-dragging. Gravitational wave damping has been detected in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Current and future tests of relativity will center on strong gravity and gravitational waves.

UPDATE NOTE:
Major revision, updated and expanded. Added new Section 2.3.3 on the Pioneer anomaly; split former Section 3 into new 3 and 4, and extended Section 3.3 on competing theories of gravity; added new Sections 5.3 and 5.4 on compact binary systems; added a new Section 8 on astrophysical and cosmological tests. The number of references increased from 299 to 454. Added two figures (8, 9) and updated Figures 1, 3, 5, and 7.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity has published a major update of the review “The Evolution of Compact Binary Star Systems” by Konstantin A. Postnov and Lev R. Yungelson on 5 May 2014.

Please find the abstract and further details below.

——————
PUB.NO. lrr-2014-3
Postnov, Konstantin A. and Yungelson, Lev R.
“The Evolution of Compact Binary Star Systems”

ACCEPTED: 2014-03-17
PUBLISHED: 2014-05-05

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2014-3

ABSTRACT:
We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Mergings of compact-star binaries are expected to be the most important sources for forthcoming gravitational-wave (GW) astronomy. In the first part of the review, we discuss observational manifestations of close binaries with NS and/or BH components and their merger rate, crucial points in the formation and evolution of compact stars in binary systems, including the treatment of the natal kicks, which NSs and BHs acquire during the core collapse of massive stars and the common envelope phase of binary evolution, which are most relevant to the merging rates of NS-NS, NS-BH and BH-BH binaries. The second part of the review is devoted mainly to the formation and evolution of binary WDs and their observational manifestations, including their role as progenitors of cosmologically-important thermonuclear SN Ia. We also consider AM CVn-stars, which are thought to be the best verification binary GW sources for future low-frequency GW space interferometers.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity recently published two new articles: a major update of the review on “Gravitational Radiation from Post-Newtonian Sources and Inspiralling Compact Binaries” by Luc Blanchet and a new article on “The Hole Argument and Some Physical and Philosophical Implications” by John Stachel.

Please find the abstracts and further details below.

——————
PUB.NO. lrr-2014-2
Blanchet, Luc
“Gravitational Radiation from Post-Newtonian Sources and Inspiralling Compact Binaries”

ACCEPTED: 2014-01-27
PUBLISHED: 2014-02-14

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2014-2

ABSTRACT:
To be observed and analyzed by the network of gravitational wave detectors on ground (LIGO, VIRGO, etc.) and by the future detectors in space (eLISA, etc.), inspiralling compact binaries — binary star systems composed of neutron stars and/or black holes in their late stage of evolution — require high-accuracy templates predicted by general relativity theory. The gravitational waves emitted by these very relativistic systems can be accurately modelled using a high-order post-Newtonian gravitational wave generation formalism. In this article, we present the current state of the art on post-Newtonian methods as applied to the dynamics and gravitational radiation of general matter sources (including the radiation reaction back onto the source) and inspiralling compact binaries. We describe the post-Newtonian equations of motion of compact binaries and the associated Lagrangian and Hamiltonian formalisms, paying attention to the self-field regularizations at work in the calculations. Several notions of innermost circular orbits are discussed. We estimate the accuracy of the post-Newtonian approximation and make a comparison with numerical computations of the gravitational self-force for compact binaries in the small mass ratio limit. The gravitational waveform and energy flux are obtained to high post-Newtonian order and the binary’s orbital phase evolution is deduced from an energy balance argument. Some landmark results are given in the case of eccentric compact binaries — moving on quasi-elliptical orbits with non-negligible eccentricity. The spins of the two black holes play an important role in the definition of the gravitational wave templates. We investigate their imprint on the equations of motion and gravitational wave phasing up to high post-Newtonian order (restricting to spin-orbit effects which are linear in spins), and analyze the post-Newtonian spin precession equations as well as the induced precession of the orbital plane.

——————
PUB.NO. lrr-2014-1
Stachel, John
“The Hole Argument and Some Physical and Philosophical Implications”

ACCEPTED: 2013-11-17
PUBLISHED: 2014-02-06

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2014-1

ABSTRACT:
This is a historical-critical study of the hole argument, concentrating on the interface between historical, philosophical and physical issues. Although it includes a review of its history, its primary aim is a discussion of the contemporary implications of the hole argument for physical theories based on dynamical, background-independent space-time structures.

The historical review includes Einstein’s formulations of the hole argument, Kretschmann’s critique, as well as Hilbert’s reformulation and Darmois’ formulation of the general-relativistic Cauchy problem. The 1970s saw a revival of interest in the hole argument, growing out of attempts to answer the question: Why did three years elapse between Einstein’s adoption of the metric tensor to represent the gravitational field and his adoption of the Einstein field equations?

The main part presents some modern mathematical versions of the hole argument, including both coordinate-dependent and coordinate-independent definitions of covariance and general covariance; and the fiber bundle formulation of both natural and gauge natural theories. By abstraction from continuity and differentiability, these formulations can be extended from differentiable manifolds to any set; and the concepts of permutability and general permutability applied to theories based on relations between the elements of a set, such as elementary particle theories.

We are closing with an overview of current discussions of philosophical and physical implications of the hole argument.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity has published a new review article on “Gravitational-Wave Tests of General Relativity with Ground-Based Detectors and Pulsar-Timing Arrays” by Nicolas Yunes and Xavier Siemens on 6 November 2013.

Please find the abstract and further details below.

This publication also introduces a redesigned HTML article layout, which allows better indexing by search engines and improves usability (easy linking to sections and figures, replacement for popups):

http://relativity.livingreviews.org/Articles/lrr-2013-9/fulltext.html

——————
PUB.NO. lrr-2013-9
Yunes, Nicolas and Siemens, Xavier
“Gravitational-Wave Tests of General Relativity with Ground-Based Detectors and Pulsar-Timing Arrays”

ACCEPTED: 2013-10-08
PUBLISHED: 2013-11-06

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2013-9

ABSTRACT:
This review is focused on tests of Einstein’s theory of general relativity with gravitational waves that are detectable by ground-based interferometers and pulsar-timing experiments. Einstein’s theory has been greatly constrained in the quasi-linear, quasi-stationary regime, where gravity is weak and velocities are small. Gravitational waves will allow us to probe a complimentary, yet previously unexplored regime: the non-linear and dynamical strong-field regime. Such a regime is, for example, applicable to compact binaries coalescing, where characteristic velocities can reach fifty percent the speed of light and gravitational fields are large and dynamical. This review begins with the theoretical basis and the predicted gravitational-wave observables of modified gravity theories. The review continues with a brief description of the detectors, including both gravitational-wave interferometers and pulsar-timing arrays, leading to a discussion of the data analysis formalism that is applicable for such tests. The review ends with a discussion of gravitational-wave tests for compact binary systems.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity has published three new review articles:

“Cosmology and Fundamental Physics with the Euclid Satellite” by Luca Amendola et al. (Euclid Theory Working Group),

“Testing General Relativity with Low-Frequency, Space-Based Gravitational-Wave Detectors” by Jonathan R. Gair et al.,

“Classification of Near-Horizon Geometries of Extremal Black Holes” by Hari K. Kunduri and James Lucietti.

Please find the abstract and further details below.

——————
PUB.NO. lrr-2013-6
Luca Amendola et al. (Euclid Theory Working Group)
“Cosmology and Fundamental Physics with the Euclid Satellite”

ACCEPTED: 2013-06-13
PUBLISHED: 2013-09-02

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2013-6

ABSTRACT:
Euclid is a European Space Agency medium-class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the Universe. Euclid will explore the expansion history of the Universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid’s Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

——————
PUB.NO. lrr-2013-7
Jonathan R. Gair, Michele Vallisneri, Shane L. Larson and John G. Baker
“Testing General Relativity with Low-Frequency, Space-Based Gravitational-Wave Detectors”

ACCEPTED: 2013-08-19
PUBLISHED: 2013-09-12

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2013-7

ABSTRACT:
We review the tests of general relativity that will become possible with space-based gravitational-wave detectors operating in the $sim 10^5 – 1$ Hz low-frequency band. The fundamental aspects of gravitation that can be tested include the presence of additional gravitational fields other than the metric; the number and tensorial nature of gravitational-wave polarization states; the velocity of propagation of gravitational waves; the binding energy and gravitational-wave radiation of binaries, and therefore the time evolution of binary inspirals; the strength and shape of the waves emitted from binary mergers and ringdowns; the true nature of astrophysical black holes; and much more. The strength of this science alone calls for the swift implementation of a space-based detector; the remarkable richness of astrophysics, astronomy, and cosmology in the low-frequency gravitational-wave band make the case even stronger.

——————
PUB.NO. lrr-2013-8
Kunduri, Hari K. and Lucietti, James
“Classification of Near-Horizon Geometries of Extremal Black Holes”

ACCEPTED: 2013-09-11
PUBLISHED: 2013-09-25

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2013-8

ABSTRACT:
Any spacetime containing a degenerate Killing horizon, such as an extremal black hole, possesses a well-defined notion of a near-horizon geometry. We review such near-horizon geometry solutions in a variety of dimensions and theories in a unified manner. We discuss various general results including horizon topology and near-horizon symmetry enhancement. We also discuss the status of the classification of near-horizon geometries in theories ranging from vacuum gravity to Einstein–Maxwell theory and supergravity theories. Finally, we discuss applications to the classification of extremal black holes and various related topics. Several new results are presented and open problems are highlighted throughout.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity has published a new review article on “Quantum-Spacetime Phenomenology” by Giovanni Amelino-Camelia on 12 June 2013.

Please find the abstract and further details below.

——————
PUB.NO. lrr-2013-5
Amelino-Camelia, Giovanni
“Quantum-Spacetime Phenomenology”

ACCEPTED: 2013-05-18
PUBLISHED: 2013-06-12

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2013-5

ABSTRACT:
I review the current status of phenomenological programs inspired by quantum-spacetime research. I stress in particular the significance of results establishing that certain data analyses provide sensitivity to effects introduced genuinely at the Planck scale. My main focus is on phenomenological programs that managed to affect the directions taken by studies of quantum-spacetime theories.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html

Living Reviews in Relativity has published a major update of the review “Relativistic Binaries in Globular Clusters” by Matthew J. Benacquista and Jonathan M.B. Downing on 4 March 2013.

Please find the abstract and further details below.

——————
PUB.NO. lrr-2013-4
Benacquista, Matthew J. and Downing, Jonathan M.B.
“Relativistic Binaries in Globular Clusters”

ACCEPTED: 2012-11-29
PUBLISHED: 2013-03-04

FULL ARTICLE AT:
http://www.livingreviews.org/lrr-2013-4

ABSTRACT:
Galactic globular clusters are old, dense star systems typically containing 10^4 – 10^6 stars. As an old population of stars, globular clusters contain many collapsed and degenerate objects. As a dense population of stars, globular clusters are the scene of many interesting close dynamical interactions between stars. These dynamical interactions can alter the evolution of individual stars and can produce tight binary systems containing one or two compact objects. In this review, we discuss theoretical models of globular cluster evolution and binary evolution, techniques for simulating this evolution that leads to relativistic binaries, and current and possible future observational evidence for this population. Our discussion of globular cluster evolution will focus on the processes that boost the production of tight binary systems and the subsequent interaction of these binaries that can alter the properties of both bodies and can lead to exotic objects. Direct N-body integrations and Fokker–Planck simulations of the evolution of globular clusters that incorporate tidal interactions and lead to predictions of relativistic binary populations are also discussed. We discuss the current observational evidence for cataclysmic variables, millisecond pulsars, and low-mass X-ray binaries as well as possible future detection of relativistic binaries with gravitational radiation.

UPCOMING ARTICLES AT:
http://relativity.livingreviews.org/Articles/upcoming.html