Melvin magnetic cosmologies

Magnetic fields are ubiquitous in the universe – observed on scales ranging from stellar, through galactic and beyond – and are key to the physics of dramatic astrophysical objects such as pulsars and active galactic nuclei. Meanwhile, the origin of large-scale magnetic fields is still a topic of great debate in the cosmological literature.

Our recent CQG article presents a new family of exact solutions to the Einstein-Maxwell equations for cosmological magnetic fields. These solutions are both inhomogeneous and anisotropic, with the magnetic field having nontrivial dependence on Continue reading

Probing the notion of gravitational entropy in inhomogeneous cosmologies

Roberto Sussman

Dr Roberto A Sussman is a senior researcher in Theoretical Cosmology at the Institute for Nuclear Sciences (ICN) of the National University of Mexico (UNAM).

One of the long standing open problems in General Relativity is to find a self-consistent theoretically robust definition of a classical “gravitational” entropy, which is distinct (though possibly connected) to the entropy of the field sources (hydrodynamical or non-collisional) and to holographic and black hole entropies. Current research has produced two main classical gravitational entropy proposals: one by Clifton, Ellis and Tavakol, based on an effective construction from the “free” gravitational field associated to the Bell-Robinson tensor (the CET proposal), the other, by Hosoya and Buchert, is based on the Kullback-Leibler functional of Information Theory (the HB proposal).

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High-order fully general-relativistic hydrodynamics: new approaches and tests

Pablo Laguna

Pablo Laguna is the Chair of the School of Physics at Georgia Tech

As we approach the era of gravitational-wave astrophysics driven by observations, it is imperative to have general-relativistic hydrodynamic codes capable of revealing in exquisite detail phenomena driven by strong dynamical gravity.

In this paper, Radice, Rezzolla and Galeazzi introduce a new approach to build a code, called WhiskyTHC, with the potential to help deliver that. The new approach borrows elements from the Whisky and Template Hydrodynamics codes. The Whisky code is widely used by the numerical relativity community, and the Continue reading

First occurrence of a double layer in a gravity theory found

José Senovilla

Jose M. M. Senovilla is a full Professor of Theoretical Physics at the University of the Basque Country (UPV/EHU) in Bilbao, Spain

Gravitational double layers turn out to be feasible in quadratic theories of gravity. New physics arises.

Double layers (DL) may be found in several disciplines: in biology separating two different forms of matter, in chemistry as interfaces between different phases (liquid and solid), or in physics when two laminar parallel shells with opposite electric charges are found next to each other. DL are especially important in plasma and cellular physics, representing abrupt drops in the electric potential by which the cell, or plasma, “protects” itself from the environment.

However, gravitational DL were nowhere to be found in gravitational physics, until now. Continue reading

Non-CMC solutions to the constraints on AE manifolds

Caleb Meier

Caleb Meier is a postdoctoral researcher in mathematics at the University of California, San Diego.

In the n+1 formalism of general relativity, the (n+1)-dimensional space-time is decomposed into n-dimensional space-like slices that are parametrized by a time function.  This is the basis for formulating Einstein’s equation as an initial value problem.  In an effort to understand which space-times are constructible, an important question is, “What is the admissible class of initial data for this problem?”  This question is addressed by analyzing the so-called Einstein constraint equations, which are an undetermined system of equations to be solved for a metric and an extrinsic curvature tensor.
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General-relativistic hydrodynamics: going beyond second-order convergence

High accuracy in numerical relativity simulations is essential: now it can also be achieved for non-vacuum spacetimes.

Merging binary neutron stars are among the most promising sources of gravitational waves (GWs) for the next generation of interferometric detectors. Such waves carry valuable information about the masses, radii, and deformability of the two stars. Even a single detection would set stringent constraints on the equation of state of nuclear matter, which is still poorly known. Gravitational-wave observations, in combination with electromagnetic/neutrino counterparts, would also help to unravel the mystery behind gamma-ray bursts. Continue reading

Non-CMC solutions of the Einstein constraint equations on asymptotically Euclidean manifolds

Niall O'Murchadha

Niall ‘O Murchadha is an Editorial Board Member for Classical and Quantum Gravity and a Professor of Physics at University College Cork, Ireland

This is a very nice article which deserves to be studied carefully by anyone interested in finding solutions to the constraints. In particular, they show how to construct a solution which is far from maximal, and, at the same time, is asymptotically flat. Readers should be aware that the first theorem, Theorem 1.1, covers a much broader range of data than the second theorem, Theorem 1.2. Further, they should be aware that the titles of the theorems ‘Far-from-CMC’ (Theorem 1.1), and ‘Near-CMC’ (Theorem 1.2), especially the second one, are not particularly illuminating.

There are conditions which are surprising. No restriction is placed on Τ2 (other than the AF condition), but we are asked Continue reading

Welcome to CQG+

Clifford Will

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity, Distinguished Professor of Physics at the University of Florida, Chercheur Associé at the Institut d’Astrophysique de Paris, and James McDonnell Professor of Space Sciences Emeritus at Washington University in St. Louis. His research interests are the observational consequences of general relativity and alternative theories of gravity, dynamics of systems containing compact bodies, and gravitational radiation.

Who would ever have imagined that perusing papers on gravitational physics would be like  trying to get a sip from a fire hose?  When I was starting my career 45 years ago, we used to receive by snail mail three or four times a year some mimeographed pages from the GR society listing the 20 or so recent preprints or published papers in the field that had been received at the offices in Bern, Switzerland.  Today, the gr-qc branch of the arXiv alone features 70 – 100 articles per week, and that doesn’t even include related articles in particle physics, astrophysics or mathematics.  We all learn, of course, how to narrow our focus to those papers that directly impact our own specific research, but what if we want to read a paper in some different area?  How do we identify a really good paper in that subtopic?

Today we launch CQG+, which we hope will help.  With CQG+ we will notify you of papers of exceptional quality, as identified by our referees.  We will ask authors or referees of such papers to write short commentaries explaining what the paper is about in accessible terms.  The entries will also include information about the author(s).  We will announce focus issues, where we have invited leading researchers in a specific topic to contribute papers on their latest research.   Our CQG “highlights’’, in which our editorial board selects the best articles from the previous year, will be featured in CQG+.  There may be an occasional editorial or commentary on a topic of interest to the readers of CQG+.  In fact the content of CQG+ may evolve with time, and to that end, we invite your feedback.   What would you find most useful in CQG+?

All you have to do is enter your email address at the foot of this page, and you will receive email alerts when there is something new in CQG+.

On a personal note, I want to thank the CQG Editorial Board and the CQG publishing team, headed by Adam Day, for their support of the journal during the last five years.  That stalwart support went a long way toward convincing me to “re-up’’ for another five-year term as Editor.  I also want to thank our CQG authors and referees – no journal can thrive without first-rate authors publishing great research or good referees holding authors’ feet to the fire to ensure that their papers are of the highest quality.  Our only goal is to publish the best work in gravitational physics, and I encourage you all to submit your next great paper to CQG.  Continue reading

CQG is 30

Adam Day

Adam Day is the Publisher of Classical and Quantum Gravity and CQG+

Time contracts when you are enjoying yourself! It doesn’t seem like 5 whole years have passed since we were celebrating CQG’s quarter-century – and we certainly have had a lot to celebrate over that short time – including a substantial rise in the number of high quality papers published by CQG.

In recognition of this excellent research, we are launching this new companion website, CQG+. Readers of CQG+ can look forward to notifications of high quality papers published in CQG and unique insights into those papers from authors and referees. Additionally, CQG+ will feature the latest news from CQG including notifications of new focus issues, the famous CQG highlights and more. Sign up for email alerts using the box at the foot of this page. Continue reading

Black holes dual to exotic superconductors

 

Figure 2c

Each point under the blue curve corresponds to a superconducting p-wave black hole with a helical structure at temperature T and with helical pitch 2πk. The red line denotes the thermodynamically preferred black holes which have the smallest free energy at a given temperature. The black holes exhibit a reversal of the direction of the helical pitch at T~ 0.04.

The gauge-gravity correspondence provides a fascinating theoretical framework for investigating non-perturbative features of strongly coupled quantum systems using weakly coupled dual gravitational descriptions in one lower space-time dimension. In particular, the thermodynamic phase structure of the quantum system is obtained by finding the black hole solutions with the smallest free energy. Such studies have led to the discovery of fundamentally new classes of black hole solutions and it is hoped that these endeavours will lead to new insights into exotic materials which are observed in nature. Continue reading