# Living Reviews in Relativity: “The causal set approach to quantum gravity”

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.

# GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Shamik Banerjee,
“Symmetries of free massless particles and soft theorems”,
Gen Relativ Gravit (2019) 51: 128.
https://doi.org/10.1007/s10714-019-2609-z

Pierre Martin-Dussaud,
“A primer of group theory for Loop Quantum Gravity and spin-foams”,
Gen Relativ Gravit (2019) 51: 110.
https://doi.org/10.1007/s10714-019-2583-5

Guillaume Bossard and Severin Luest,
“Microstate geometries at a generic point in moduli space”,
Gen Relativ Gravit (2019) 51: 112.
https://doi.org/10.1007/s10714-019-2584-4

Joan Garcia i Tormo and Marika Taylor,
“One point functions for black hole microstates”,
Gen Relativ Gravit (2019) 51: 89.
https://doi.org/10.1007/s10714-019-2566-6

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# Living Reviews in Relativity: “Testing the nature of dark compact objects: a status report”

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.

# GRG Editor’s Choice free-to-read for GR22/Amaldi13

On the occasion of the GR22/Amaldi13 conference, all recent GRG Editor’s Choice articles will be free-to-read during July 2019!

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. This recent article deserves special attention:

Alan A. Coley,
“Mathematical general relativity”,
Gen Relativ Gravit (2019) 51: 78.
https://doi.org/10.1007/s10714-019-2559-5

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# Living Reviews in Relativity: “Lorentzian causality theory”

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.

# Living Reviews in Relativity: “Advanced quantum techniques for future gravitational-wave detectors”

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.

# GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Kastytis Zubovas, Andrew R. King,
“The M–$\sigma$ relation between supermassive black holes and their host galaxies”,
Gen Relativ Gravit (2019) 51: 65.
https://doi.org/10.1007/s10714-019-2549-7

Sudipta Sarkar,
“Black hole thermodynamics: general relativity and beyond”,
Gen Relativ Gravit (2019) 51: 63.
https://doi.org/10.1007/s10714-019-2545-y

Brian Allen,
“Sobolev stability of the Positive Mass Theorem and Riemannian Penrose Inequality using inverse mean curvature flow”,
Gen Relativ Gravit (2019) 51: 59.
https://doi.org/10.1007/s10714-019-2542-1

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# GRG Golden Oldie: The theoretical significance of experimental relativity by R.H. Dicke (1964)

The Golden Oldies series of the journal General Relativity and Gravitation reprints important papers in general relativity theory that were published 30 or more years ago and are either hard to get hold of, or were originally printed in a language other than English.

The latest work has been republished on 30 April 2019:

Dicke, R.H., Republication of: The theoretical significance of experimental relativity, Gen Relativ Gravit (2018) 51: 57. https://doi.org/10.1007/s10714-019-2509-2​​​​​​​

An editorial note by P.J.E. Peebles and a a brief biography by M.A.H. MacCallum are freely available at https://doi.org/10.1007/s10714-019-2508-3

# GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Eric Addison, Miguel Gracia-Linares, Pablo Laguna, Shane L. Larson,
“Busting up binaries: encounters between compact binaries and a supermassive black hole”,
Gen Relativ Gravit (2019) 51: 38.
https://doi.org/10.1007/s10714-019-2523-4

Eva Hackmann, Arnab Dhani,
“The propagation delay in the timing of a pulsar orbiting a supermassive black hole”,
Gen Relativ Gravit (2019) 51: 37.
https://doi.org/10.1007/s10714-019-2517-2

Thomas P. Kling, Eric Grotzke, Kevin Roebuck, Harry Waite,
“Examining the Kerr metric through wave fronts of null geodesics”,
Gen Relativ Gravit (2019) 51: 32.
https://doi.org/10.1007/s10714-019-2518-1

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserves special attention:

Stone, N.C., Kesden, M., Cheng, R.M. et al.,
“Stellar tidal disruption events in general relativity”,
Gen Relativ Gravit (2019) 51: 30.
https://doi.org/10.1007/s10714-019-2510-9

Osorio Morales, J. and Santillan, O.P.,
“The existence of smooth solutions in q-models”
Gen Relativ Gravit (2019) 51: 29.
https://doi.org/10.1007/s10714-019-2507-4

Chakraborty, S. and Parattu, K.,
“Null boundary terms for Lanczos-Lovelock gravity”,
Gen Relativ Gravit (2019) 51: 23.
https://doi.org/10.1007/s10714-019-2502-9

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# GRG Editor’s Choice: recent highlight article

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. This recent article deserves special attention:

Harte, Abraham I.,
“Gravitational lensing beyond geometric optics: I. Formalism and observables”,
Gen Relativ Gravit (2019) 51: 14.
https://doi.org/10.1007/s10714-018-2494-x

Abstract:
The laws of geometric optics and their corrections are derived for scalar, electromagnetic, and gravitational waves propagating in generic curved spacetimes. Local peeling-type results are obtained, where different components of high-frequency fields are shown to scale with different powers of their frequencies. Additionally, finite-frequency corrections are identified for a number of conservation laws and observables. Among these observables are a field’s energy and momentum densities, as well as several candidates for its corrected “propagation directions”.

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# New book: The Physics and Astrophysics of Neutron Stars

We would like to announce the publication of a new book in Springer’s ASSL series:

Luciano Rezzolla, Pierre Pizzochero, David Ian Jones, Nanda Rea & Isaac Vidana (eds):
“The Physics and Astrophysics of Neutron Stars”.
Astrophysics and Space Science Library, vol 457.
Springer, Cham (2018)
https://doi.org/10.1007/978-3-319-97616-7
https://www.springer.com/gp/book/9783319976150

This book summarizes the recent progress in the physics and astrophysics of neutron stars and, most importantly, it identifies and develops effective strategies to explore, both theoretically and observationally, the many remaining open questions in the field.

Because of its significance in the solution of many fundamental questions in nuclear physics, astrophysics and gravitational physics, the study of neutron stars has seen enormous progress over the last years and has been very successful in improving our understanding in these fascinating compact objects.

The book addresses a wide spectrum of readers, from students to senior researchers. Thirteen chapters written by internationally renowned experts offer a thorough overview of the various facets of this interdisciplinary science, from neutron star formation in supernovae, pulsars, equations of state super dense matter, gravitational wave emission, to alternative theories of gravity.

The book was initiated by the European Cooperation in Science and Technology (COST) Action MP1304 “Exploring fundamental physics with compact stars” (NewCompStar).

# Living Reviews in Relativity: “Testing general relativity in cosmology”

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.

# GRG Editor’s Choice: recent highlight article

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. This recent article deserves special attention:

Chiang-Mei Chen, Jian-Liang Liu, James M. Nester,
“Quasi-local energy from a Minkowski reference”
https://doi.org/10.1007/s10714-018-2484-z

Abstract:
The specification of energy for gravitating systems has been an unsettled issue since Einstein proposed his pseudotensor. It is now understood that energy-momentum is quasi-local (associated with a closed 2-surface). Here we consider quasi-local proposals (including pseudotensors) in the Lagrangian–Noether–Hamiltonian formulations. There are two ambiguities: (1) there are many possible expressions, (2) they depend on some non-dynamical structure, e.g., a reference frame. The Hamiltonian approach gives a handle on both problems. The Hamiltonian perspective helped us to make a remarkable discovery: with an isometric Minkowski reference a large class of expressions—namely all those that agree with the Einstein pseudotensor’s Freud superpotential to linear order—give a common quasi-local energy value. Moreover, with a best-matched reference on the boundary this is the Wang–Yau mass value.

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# GRG Editor’s Choice: recent highlight article

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. This recent article deserves special attention:

Abhay Ashtekar, Miguel Campiglia and Alok Laddha,
“Null infinity, the BMS group and infrared issues”
https://doi.org/10.1007/s10714-018-2464-3

Abstract:
There has been a recent resurgence of interest in the structure of the gravitational field at null infinity, sparked by new results on soft charges and infrared issues related to the S matrix theory in perturbative quantum gravity. We summarize these developments and put them in the broader context of research in the relativity community that dates back to several decades. In keeping with intent of this series, this overview is addressed to gravitational scientists who are not experts in this specific area.

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# GRG Golden Oldies by Lichnerowicz / Doroshkevich and Novikov

The Golden Oldies series of the journal General Relativity and Gravitation reprints important papers in general relativity theory that were published 30 or more years ago and are either hard to get hold of, or were originally printed in a language other than English.

Two recent works have been republished in October 2018:

Lichnerowicz, A., Republication of: Propagators, commutators and anti-commutators in general relativity, Gen Relativ Gravit (2018) 50: 145. https://doi.org/10.1007/s10714-018-2433-x

Editorial note by Giampiero Esposito is freely available at https://doi.org/10.1007/s10714-018-2440-y

Doroshkevich, A.G. and Novikov, I.D., Republication of: Mean density of radiation in the Metagalaxy and certain problems in relativistic cosmology, Gen Relativ Gravit (2018) 50: 138. https://doi.org/10.1007/s10714-018-2441-x

Editorial note by Ruth Durrer is freely available at https://doi.org/10.1007/s10714-018-2440-y

# Living Reviews in Relativity: New articles on “Computer algebra” and “Hamiltonian formalisms”

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.

# GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserve special attention:

Jolin, Shan W. & Rosquist, Kjell
“Analytic analysis of irregular discrete universes”
https://doi.org/10.1007/s10714-018-2436-7

Newman, Ezra T.
“Classical mechanics via general relativity and Maxwell’s theory: a bit of magic”
https://doi.org/10.1007/s10714-018-2424-y

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserve special attention:

Henric Krawczynski
“Difficulties of quantitative tests of the Kerr-hypothesis with X-ray observations of mass accreting black holes”
https://doi.org/10.1007/s10714-018-2419-8

Ivan Agullo
“Primordial power spectrum from the Dapor–Liegener model of loop quantum cosmology”
https://doi.org/10.1007/s10714-018-2413-1

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high-quality research that is of wide interest within the community. These recent articles deserve special attention:

Jeremy D. Schnittman
“The collisional Penrose process”
https://doi.org/10.1007/s10714-018-2373-5

Claus Kiefer, David Wichmann
“Semiclassical approximation of the Wheeler–DeWitt equation: arbitrary orders and the question of unitarity”
https://doi.org/10.1007/s10714-018-2390-4

Hideyoshi Arakida
“Light deflection and Gauss–Bonnet theorem: definition of total deflection angle and its applications”
https://doi.org/10.1007/s10714-018-2368-2

Emanuele Berti, Kent Yagi, Nicolas Yunes
“Extreme gravity tests with gravitational waves from compact binary coalescences: (I) inspiral–merger”
https://doi.org/10.1007/s10714-018-2362-8

Please, browse all Editor’s Choice articles at:
https://www.springer.com/gp/livingreviews/relativity/grg-editors-choice

Frank Schulz
Publishing Editor GRG

# Living Reviews in Relativity: “Relativistic dynamics and extreme mass ratio inspirals”

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.

# Living Reviews in Relativity: Updated articles from Euclid TWG and KAGRA/LIGO/Virgo

The open-access journal Living Reviews in Relativity has published major updates of two review articles in April 2018:

Abbott, B.P. et al. (KAGRA Collaboration, LIGO Scientific Collaboration and Virgo Collaboration), “Prospects for observing and localizing gravitational-wave transients with Advanced LIGO, Advanced Virgo and KAGRA”, Living Rev Relativ (2018) 21: 3. https://doi.org/10.1007/s41114-018-0012-9​​​​​​​

Changes:
Major revision, updated and expanded. Several updates to the document have been made. The most significant changes are the inclusion of details regarding KAGRA, and results from O1 and O2, including GW170817, the first detection with an unambiguous multi-messenger counterpart. The key differences are outlined in an appendix.

Amendola, L. et al. (The Euclid Theory Working Group), “Cosmology and fundamental physics with the Euclid satellite”, Living Rev Relativ (2018) 21: 2. https://doi.org/10.1007/s41114-017-0010-3​​​​​​​

Changes:
Major revision, updated and expanded. Seven new figures were added. About 300 new references have been cited. Forecasts are not updated in this version, with respect to 2012.

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

# Living Reviews in Relativity: “Tests of chameleon gravity”

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.

# GRG Editor’s Choice: recent highlight articles

In each volume of the journal General Relativity and Gravitation (GRG), a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community.

Three recent articles deserve special attention (free-to-read access until April 15, 2018):

Giorgio Sarno, Simone Speziale and Gabriele V. Stagno, 2-vertex Lorentzian spin foam amplitudes for dipole transitions, Gen Relativ Gravit (2018) 50: 43. https://doi.org/10.1007/s10714-018-2360-x

Pedro V. P. Cunha and Carlos A. R. Herdeiro, Shadows and strong gravitational lensing: a brief review, Gen Relativ Gravit (2018) 50: 42. https://doi.org/10.1007/s10714-018-2361-9

Dario Brooks, Paul-Christopher Chavy-Waddy, Alan A. Coley, Adam Forget, Daniele Gregoris, Malcolm A. H. MacCallum and David D. McNutt, Cartan invariants and event horizon detection, Gen Relativ Gravit (2018) 50: 37. https://doi.org/10.1007/s10714-018-2358-4

Frank Schulz
Publishing Editor GRG

# Living Reviews in Computational Astrophysics: Higher-order accurate space-time schemes for computational astrophysics

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.

# GRG Editor’s Choice: Higher curvature gravities cannot be bootstrapped

Deser, S., Higher curvature gravities, unlike GR, cannot be bootstrapped from their (usual) linearizations, Gen Relativ Gravit (2017) 49: 149. https://doi.org/10.1007/s10714-017-2308-6

Editor’s Choice (Research Article)
First Online: 07 November 2017

“This paper extends the author’s original contribution from 1970 to a broad class of theories that modify general relativity. The result is a paper that provides a continuation of the 1970 story. The author’s style and commitment to exposing the structural elements of the procedure make for a very nice article that reads well and serves a useful and illuminating purpose in the ‘extensions of GR’ literature.”

Abstract:
We show that higher curvature order gravities, in particular the propagating quadratic curvature models, cannot be derived by self-coupling from their linear, flat space, forms, except through an unphysical version of linearization; only GR can. Separately, we comment on an early version of the self-coupling bootstrap.

GRG Editor’s Choice:
In each volume of GRG, a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community.

# Living Reviews in Relativity: recent publications

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.

# GRG Editor’s Choice: Notable articles in General Relativity & Gravitation

In each volume of the journal General Relativity & Gravitation, a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community.

Tanatarov, I.V. & Zaslavskii, O.B., Collisional super-Penrose process and Wald inequalities, Gen Relativ Gravit (2017) 49: 119. https://doi.org/10.1007/s10714-017-2281-0

Abstract:
We consider collision of two massive particles in the equatorial plane of an axially symmetric stationary spacetime that produces two massless particles afterwards. It is implied that the horizon is absent but there is a naked singularity or another potential barrier that makes possible the head-on collision. The relationship between the energy in the center of mass frame Ec.m. and the Killing energy E measured at infinity is analyzed. It follows immediately from the Wald inequalities that unbounded E is possible for unbounded Ec.m. only. This can be realized if the spacetime is close to the threshold of the horizon formation. Different types of spacetimes (black holes, naked singularities, wormholes) correspond to different possible relations between Ec.m. and E. We develop a general approach that enables us to describe the collision process in the frames of the stationary observer and zero angular momentum observer. The escape cone and escape fraction are derived. A simple explanation of the existence of the bright spot is given. For the particular case of the Kerr metric, our results agree with the previous ones found in Patil et al. (Phys Rev D 93:104015, 2016).

Sakovich, A. & Sormani, C., Almost rigidity of the positive mass theorem for asymptotically hyperbolic manifolds with spherical symmetry, Gen Relativ Gravit (2017) 49: 125. https://doi.org/10.1007/s10714-017-2291-y

Abstract:
We use the notion of intrinsic flat distance to address the almost rigidity of the positive mass theorem for asymptotically hyperbolic manifolds. In particular, we prove that a sequence of spherically symmetric asymptotically hyperbolic manifolds satisfying the conditions of the positive mass theorem converges to hyperbolic space in the intrinsic flat sense, if the limit of the mass along the sequence is zero.

# GRG Editor’s Choice: Cosmological solutions with charged black holes

Bibi, R., Clifton, T. and Durk, J., Cosmological solutions with charged black holes, Gen Relativ Gravit (2017) 49: 98. doi:10.1007/s10714-017-2261-4

Open Access | Editor’s Choice (Research Article)
First Online: 30 June 2017

In each volume of the journal General Relativity & Gravitation, a few papers are marked as “Editor’s Choice”. The primary criteria is original, high quality research that is of wide interest within the community.

Abstract:
We consider the problem of constructing cosmological solutions of the Einstein–Maxwell equations that contain multiple charged black holes. By considering the field equations as a set of constraint and evolution equations, we construct exact initial data for N charged black holes on a hypersphere. This corresponds to the maximum of expansion of a cosmological solution, and provides sufficient information for a unique evolution. We then consider the specific example of a universe that contains eight charged black holes, and show that the existence of non-zero electric charge reduces the scale of the cosmological region of the space. These solutions generalize the Majumdar–Papapetrou solutions away from the extremal limit of charged black holes, and provide what we believe to be some of the first relativistic calculations of the effects of electric charge on cosmological backreaction.

The authors:
Jessie Durk is currently a PhD student, researching theoretical cosmology with Dr Timothy Clifton, lecturer at Queen Mary University London. Rashida Bibi is a faculty member of the International Islamic University, Islamabad.

# Living Reviews and CompAC: new open access articles

The open-access journals Living Reviews in Relativity and Computational Astrophysics and Cosmology have published new articles in May 2017:

Brian D. Metzger, “Kilonovae”, Living Rev. Relativ. (2017) 20:3
https://doi.org/10.1007/s41114-017-0006-z

Douglas Potter, Joachim Stadel and Romain Teyssier, “PKDGRAV3: beyond trillion particle cosmological simulations for the next era of galaxy surveys”, Comput. Astrophys. Cosmol. (2017) 4:2
https://doi.org/10.1186/s40668-017-0021-1

Oliver Porth, Hector Olivares, Yosuke Mizuno, Ziri Younsi, Luciano Rezzolla, Monika Moscibrodzka, Heino Falcke and Michael Kramer, “The black hole accretion code”, Comput. Astrophys. Cosmol. (2017) 4:1
https://doi.org/10.1186/s40668-017-0020-2

Living Reviews are open-access review journals that allow authors to regularly update their articles to include latest developments. Its companion research journals primarily publish original work.