Focus issue: Relativistic effects in cosmology

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Kazuya Koyama

Kazuya Koyama is a Reader in Cosmology in the Institute of Cosmology and Gravitation, at the University of Portsmouth,

I am very pleased to bring you this focus issue summarising the recent developments in computing relativistic effects in cosmology.

The issue consists of two parts, with the first part examining general relativistic formulations of the observed over-density of galaxies.

The second part discusses relativistic effects on the formation of large scale structures.

The authors pioneered the development of our understanding of general relativistic effects in cosmological observations and we hope that this focus issue will be a basis for further advancement of the field. Continue reading →

Movie Review of Interstellar, by Richard Price

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See more Interstellar posters

Image copyright Warner Bros and Paramount Pictures

CQG has never published a movie review before. It is therefore with appropriate humility that I offer a review of Interstellar.  This scifi epic wins the historic honor because it lists a physicist, Kip Thorne, as an executive producer, and is advertised as based on his theories. Indeed, the movie plot and graphics do involve ideas of relativity in very important ways.

In the spirit of disclosure I state, right up front, that I am not a fan of science fiction, but am a fan of Kip Thorne; like many of his former students I have remained a friend.  My fan/antifan biases should cancel and leave me to do the objective job that a scientist is expected to do.

This is not, of course, a review for the general public. CQG is seldom found in the waiting room of dentists. If you Continue reading →

Holographic entanglement obeys strong subadditivity

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Aron Wall

Aron Wall is a member of the School of Natural Sciences at the Institute for Advanced Study. In his spare time he blogs at Undivided Looking. He was the 2013 recipient of the Bergmann-Wheeler thesis prize, which is sponsored by Classical and Quantum Gravity.

Gauge-gravity duality allows us to calculate properties of certain quantum field theories (QFT) from classical general relativity. One famous piece of this conjecture, due to Ryu and Takayanagi, relates the entanglement entropy in a QFT region to the area of a surface in the gravitational theory. In addition to being a clue about quantum gravity, this proposal is one of the few tools which allow us to calculate entanglement entropy analytically. Since the entanglement entropy is of increasing interest for field theory and condensed matter applications, it is important to check if the conjecture is true.

One important property of the entropy is strong subadditivity (SSA). This quantum inequality says that the sum of the entropies in two regions is always greater than the sum of the entropies of their union and intersection. My article uses proof Continue reading →

Black holes as beads on cosmic strings

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Amjad Ashoorioon and Robert Mann

Amjad Ashoorioon (left) is a Senior Research Associate at the physics department of Lancaster University in the United Kingdom. Robert B. Mann (right) is a Professor of Physics and Applied Mathematics at the University of Waterloo, Ontario, Canada.

Cosmic strings have been a source of fascination in cosmology since Tom Kibble first proposed their existence 40 years ago. Like an imperfection in a solidifying crystal, a cosmic string is a thread of energy that might have formed in the early universe during a symmetry breaking phase transition. Twenty years ago Ruth Gregory pointed out that a black hole could have a cosmic string as a single “hair”.   Turning this idea around, in this article we have proposed that a Continue reading →

New focus issue: Advanced interferometric gravitational wave detectors

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Peter Shawhan and Marie-Anne Bizouard

Peter Shawhan is an Associate Professor of Physics at the University of Maryland, USA. His primary research area is in the analysis of data from gravitational wave detectors and connections with astrophysical events.
Marie-Anne Bizouard is a research fellow at CNRS, Laboratoire de l’Accélérateur Linéaire, Orsay, France. She is an experimental physicist working on gravitational wave searches with ground based interferometric detectors.

The quest to detect gravitational waves directly has seen great advances over the past five decades, with the earlier resonant “bar” detectors being surpassed in sensitivity by large laser interferometers in the last decade.  The first generation of interferometric detectors proved the viability of the approach, progressively improving sensing and control techniques and running up against the fundamental limitations of their designs.  Along the way, many searches for gravitational wave signals were carried out and published, but none achieved the milestone of detecting a clear gravitational-wave signal.

All of that is about to change.  The lessons learned from the first full-scale interferometric detectors fed into the design of advanced detectors which are now being constructed and commissioned and will soon begin collecting data.  Higher laser power, Continue reading →

Memory at a distance

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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 →

New focus issue: Entanglement and quantum gravity

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Eugenio Bianchi and Carlo Rovelli

Eugenio Bianchi (left) is an assistant professor at Pennsylvania State University and Carlo Rovelli (right) is a professor at Aix-Marseille University at the Centre de Physique Theorique de Luminy

Quantum gravity alone is not the only major theoretical open problem in fundamental physics: gravity, quantum theory and thermodynamics form a triple, whose full interconnections we have definitely not yet understood. As soon as quantum effects appear in a curved spacetime, thermal aspects appear to be unavoidable. Combining thermodynamics and (full) gravity might turn out to be even more crucial than understanding the quantum aspects of the gravitational field alone. In recent years, it has become increasingly clear that entanglement entropy is a central ingredient for the synthesis we are seeking. Continue reading →

Cosmic magnification expanded

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Obinna Umeh

Dr Obinna Umeh is a postdoctoral researcher at the University of the Western Cape in Cape Town, supported by the Square Kilometre Array project in South Africa

The accurate determination of cosmological distances is the most important probe in cosmology. Observations of type Ia supernovae imply dark energy exists because we know the relation between the distance of an object and its redshift – this changes with the relative amount of matter to dark energy, for example. But intervening matter between the supernovae and us cause fluctuations in this relationship. To a first approximation this is just normal gravitational lensing, an integrated contribution from the wobbly path the light takes to us.

Is this an accurate enough picture? Maybe at the moment, but not Continue reading →

The new CQG Highlights of 2013-14

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Ben Sheard

Ben Sheard is the publishing editor of Classical and Quantum Gravity

It is my pleasure to present the CQG Highlights of 2013-14. The Highlights articles are chosen by the Editorial Board as a selection of some of the best work published in the journal, based on criteria of interest, significance and novelty.

The articles span the whole of CQG’s subject scope and include focus issue articles and topical reviews in addition to regular papers. All of the Highlights articles are free to download until the end of 2015.

As part of the promotion of the Highlights we produce an annual Highlights brochure which contains further information about journal activity including forthcoming special issues and prize Continue reading →

Towards quantum asymptotic flatness

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Miguel and Madhavan

Miguel, a postdoc at the Raman Research Institute (RRI), enjoying a traditional south Indian dish: masala dosa.
Madhavan (professor at RRI) not enjoying traditional South American drink: mate.

It is of great physical interest to construct a canonical quantization of asymptotically flat spacetimes. The classical phase space variables are subject to delicate boundary conditions at spatial infinity and the first challenge is to construct a quantum kinematics which carries an imprint of these boundary conditions.

This work is one of a series of papers which seeks to construct such a Continue reading →