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

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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.

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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.

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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

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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.

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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.

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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