And now for something completely different…

Nils Andersson

Nils Andersson is Head of the Southampton University Gravity group. He is mainly focussed on problems involving the modelling of neutron stars and understanding various related astrophysical phenomena, from pulsar glitches to magnetar giant flares. Away from the office, he writes science-inspired books for kids and occasionally blows his own trumpet in public.

Each and every trade has its favourite tools, some more powerful than others. A plumber would not get by without a good wrench, a carpenter needs a hammer, a mechanic a screwdriver and so on. Theoretical physicists prefer action principles.

This preference is natural given that many of the phenomena we are interested in are associated with deviations from some minimum energy equilibrium state. It is well known that, once you understand a problem from the variational point-of-view, you have a very powerful tool at your hands. However, it is also generally accepted that this approach is restricted to conservative systems.

Our recent paper in Classical and Quantum Gravity challenges this consensus view. Working in the framework of classical general relativity (no extra dimensions and fancy stuff here!), we develop Continue reading

When coupling to matter matters

Claudia De Rham

Claudia de Rham is an assistant professor at Case Western Reserve University working on cosmology and particle physics and is particularly interested in models of modified gravity and their embedding within consistent field theory frameworks.

How does matter couple in theories involving several metrics? We unveil the possibility for a new effective metric.

While the theory of general relativity will mark its 100 year anniversary next fall, the realization that the expansion of our universe may currently be accelerating has opened up the door for a series of investigations to understand the behavior of gravity at large distances – as large as the current observable Universe or about 1010 light years. Among the different possible modifications of gravity explored in the past decade, theories of gravity which involve several metrics have played a crucial role. The idea that gravity could be the outcome of several interacting metrics is of course not a new concept and such theories have been explored for more than 70 years, but their consistent realization has only been derived very recently in the past few years, and we are finally reaching a stage where we can understand more precisely how matter couples to gravity in such theories. Continue reading

Non-CMC solutions to the Einstein constraint equations on asymptotically Euclidean manifolds with apparent horizon boundaries

Juan A. Valiente Kroon

Juan A. Valiente Kroon works on various aspects of mathematical Relativity and, in particular, on applications of conformal methods to analyse the global properties of spacetimes.

The construction of physically realistic data for the Einstein field equations is one of the great challenges of the Cauchy problem in General Relativity. In this paper C. Meier and M. Holst show how to construct solutions to the constraint equations of General Relativity representing data which will evolve, assuming that a certain form of weak cosmic censorship holds, into a spacetime containing one or more black holes.

The most studied procedure for solving the constraint equations is the so-called conformal method. This approach can be traced back to the pioneering work of Continue reading

Focus issue: Astrophysics and general relativity of dense stellar systems

Clifford Will and Pau Amaro-Seoane

Guest Editors: Clifford Will and Pau Amaro-Seoane

We invite you to read the latest CQG focus issue on “Astrophysics and General Relativity of dense stellar systems“, which is available to read now.

Dense stellar systems such as galactic nuclei and stellar clusters are unique laboratories, not only for astrophysics, but also for general relativity. The complexity of these systems is such that, in spite of a huge theoretical, observational and numerical effort there are still a large number of open key questions. This focus issue on the “Astrophysics and General Relativity of Dense Stellar Systems” brings together an array of invited articles on important aspects of these questions.

We hope that you will enjoy reading the articles in the focus issue; all of which are free for a period of time following publication.

Black holes against the universe – particle and photon orbits in McVittie spacetimes

Brien Nolan

Brien Nolan is a Senior Lecturer in the School of Mathematical Sciences, Dublin City University

Black holes have a potential technological application that is frequently overlooked: they allow you to look at the back of your own head. This could be useful for checking that your tie is properly tucked into your shirt collar, or – perhaps more relevant for physicists – that your pony tail is straight. This technology relies on the fact that there exist circular photon orbits in all members of the Kerr-Newman-de Sitter family of spacetimes for which the parameters (mass, charge and cosmological constant) correspond to a black hole.

The question arises as to whether this characteristic feature of electro-vac Continue reading

Memory at a distance

Jeff Winicour

The author, overshadowed by nature in the Muir Woods on the California coast.
Jeff Winicour is a Professor of Physics and Astronomy at the University of Pittsburgh

Can the supertranslation symmetry of radiating spacetimes affect angular momentum loss?

That was the question on my mind when I went to a workshop at Berkeley, California last winter. I knew that the supertranslations were a global aspect of the gravitational memory effect, which produces a net displacement between particles after passage of a gravitational wave. What I didn’t know, and learned from David Garfinkle at Berkeley, was that there was an electromagnetic analog of radiation memory, which produces a momentum kick on test charges after passage of a wave. Surprisingly, this result has apparently gone unnoticed in Continue reading

Isolated systems are asymptotically… flat

Martin Reiris

Martin Reiris is Junior Scientist at the Max Planck Institute for Gravitational Physics (Golm-Germany) since 2009. He received the PHD in pure math in 2005 at SUNYSB, and held a Moore Instructor in Math at MIT from 2006 to 2009. His interests are mainly in geometry and the mathematical structure of general relativity

In the extraordinary manuscript The Foundation of the Generalised Theory of Relativity, printed in 1916 in Annalen der Physics, Einstein begins addressing what he calls an epistemological defect of classical mechanics, (as well as of special relativity), whose dignity he attributes to E Mach. He imagined two bodies, A and B, made of the same fluid material and sufficiently separated from each other that none of the properties of one could be attributed to the existence of the other. Observers at rest in one body, he continues, see the other body rotating at a constant angular velocity, yet these same observers measure a perfect round surface in one case and an ellipsoid of rotation in the other case. It is then asked: “Why is this difference between the two bodies?”. Continue reading

A spacetime route to positive mass

Brien Nolan

Brien Nolan is a Senior Lecturer in the School of Mathematical Sciences, Dublin City University

This paper provides an important, unexpected and very satisfying route to positivity of mass in General Relativity. It shows positivity of the Trautman-Bondi mass in a way that avoids both the heavy differential geometric machinery of the work of Schoen and Yau, and the Continue reading

Homogeneous cosmological model from a discrete matter distribution

Mikolaj Korzynski

Mikolaj Korzynski is an Assistant Professor at the Center of Theoretical Physics
of the Polish Academy of Sciences, Warsaw

How does a homogeneous FLRW metric arise from a cosmological model with black holes as the only source of gravitational field?

In astrophysical applications of general relativity we often need to apply the Einstein’s field equations to situations where the matter distribution, and consequently also the metric tensor, has a complicated form with relatively smooth large scale behavior and a  complicated structure on smaller scales. The problems of this kind are usually approached in the following way: instead of solving the equations directly we apply them to an idealized metric with the small-scale structure removed by 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).

Continue reading