Yearly Archives: 2015

The gravitational Hamiltonian, first order action, Poincaré charges and surface terms

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Read the full article for free* in Classical and Quantum Gravity:
The gravitational Hamiltonian, first order action, Poincaré charges and surface terms
Alejandro Corichi and Juan D Reyes 2015 Class. Quantum Grav. 32 195024
arXiv:1505.01518
*until 18/11/15

Ever since Einstein and Hilbert were racing to complete the general theory of relativity, almost 100 years ago, having a variational principle for it was at the forefront of the theoretical efforts. An action and the variational principle accompanying it are the preferred ways to describe a physical theory. At the classical level, all the information one can possibly ask about a physical system is conveniently codified into a single scalar function S. Additionally, in covariant approaches to quantum mechanics, the action S provides, through the path integral, a fundamental link between the classical and quantum descriptions. Ideally, the Hamiltonian structure of the theory itself -the starting point for canonical quantization- may too be extracted from the same action. Continue reading →

CQG Letters – open for submission

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Clifford Will

Clifford Will is the Editor-in-Chief of Classical and Quantum Gravity

I am very pleased to announce the launch of CQG’s new Letters section.

In a recent survey of CQG’s authors and readers, we identified a strong desire among gravitational physicists for CQG to launch this section.

The Letter article type is now available and I invite you to submit your next Letter to the journal.

The benefits

CQG’s peer review will ensure the best possible consideration for your Letter – certifying, not just Continue reading →

Gravity and scalar fields: live long and prosper?

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Read the full article for free* in Classical and Quantum Gravity:
Tensor-multi-scalar theories: relativistic stars and 3 + 1 decomposition
Michael Horbatsch, Hector O Silva, Davide Gerosa, Paolo Pani, Emanuele Berti, Leonardo Gualtieri and Ulrich Sperhake 2015 Class. Quantum Grav. 32 204001
arXiv:1505.07462
*until 16/12/15

Ulrich Sperhake et al

Large panel: Ulrich Sperhake and Emanuele Berti under Isaac Newton’s famous apple tree at Woolsthorpe Manor that (allegedly) started it all.
Clockwise in the small panels: Davide Gerosa, Hector O. Silva, Paolo Pani, Leonardo Gualtieri and Michael Horbatsch.

Newton’s theory of gravity was a spectacular achievement: for about two centuries, a simple law based on empirical observation was a perfect explanation for the behavior of gravity throughout the Solar System. Some cracks in this perfect edifice emerged around 1840, when François Arago, the director of the Paris Observatory, suggested to the French mathematician Urbain Le Verrier to study the details of Mercury’s orbital motion around the Sun using Newton’s gravity. Predictions from Le Verrier’s theory famously failed to match the observations. Mercury’s perihelion advances each time it orbits around the Sun. Most of the perihelion precession could be explained as due to the Continue reading →

4 questions about CQG Letters

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Adam Day

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

At the 2015 CQG Editorial Board meeting in London, it was agreed that a subset of the gravitational physics community should be surveyed to find out if there was interest in launching a Letters section for CQG.

In this survey, a number of gravitational physicists were asked to answer a set of questions about Letters. Just to make things interesting, invitees were split into 2 groups: one group was asked to think as authors and the other as readers.

Here are the results. Continue reading →

The curvature on a black hole boundary

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Read the full article for free* in Classical and Quantum Gravity:
On the Bartnik mass of apparent horizons
Christos Mantoulidis and Richard Schoen 2015 Class. Quantum Grav. 32 205002
arXiv:1412.0382
*until 04/11/15

Christos Mantoulidis

Christos Mantoulidis is a graduate student in Mathematics at Stanford University.

In our latest CQG paper we study the geometry (i.e. curvature) of apparent horizons and its relationship with ADM mass.

We were motivated by the following two foundational results in the theory of black holes in asymptotically flat initial data sets (slices of spacetime) satisfying the dominant energy condition (DEC):

  1. Apparent horizons are topologically equivalent to (one or more) two-dimensional spheres.(1)
  2. When the initial data set is additionally time symmetric (totally geodesic in spacetime), the apparent horizon’s total area A is bounded from above by the slice’s ADM mass per A \leq 16\pi m^2. This is called the Penrose inequality.(2) Equality is only achieved on Schwarzschild data, whose apparent horizon is a single sphere with constant Gauss curvature.

One then naturally wonders: Continue reading →

No go on spacetime reconstruction inside horizons

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Read the full article for free* in Classical and Quantum Gravity:
Covariant constraints on hole-ograhpy
Netta Engelhardt and Sebastian Fischetti 2015 Class. Quantum Grav. 32 195021
arXiv:1507.00354
*until 28/10/15

Spacetime reconstruction in holography is limited in the presence of strong gravity.

Netta Engelhardt and Sebastian Fischetti

Netta Engelhardt (left) and Sebastian Fischetti (right) practicing some of their less-developed skills at UCSB. Netta is a graduate student at UCSB. Sebastian was a graduate student at UCSB at the time of writing, and is now a postdoc at Imperial College London.

In recent years, it has become clear that there is a deep connection between quantum entanglement and geometry.  This mysterious connection has the potential to provide profound insights into the inner workings of a complete theory of quantum gravity.  Many concrete hints for how geometry and entanglement are related come from the so-called AdS/CFT duality conjectured by J.Maldacena, which relates certain types of quantum field theories (the “boundary”) to string theory on a negatively-curved spacetime called anti-de Sitter (AdS) space (the “bulk”) of one higher dimension.  In a certain limit, the string theory is Continue reading →

Spontaneous Scalarization: Dead or Alive?

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Read the full article for free* in Classical and Quantum Gravity:
Slowly rotating anisotropic neutron stars in general relativity and scalar-tensor theory
Hector O Silva, Caio F B Macedo, Emanuele Berti and Luís C B Crispino 2015 Class. Quantum Grav. 32 145008
arXiv:1411.6286
*until 21/10/15

Emanuele Berti and Hector Okada da Silva

Hector O. Silva (right) is a graduate student of Professor Emanuele Berti (left) in the gravity group at the University of Mississippi (USA).

This is a time for celebration for anyone with even a passing interest in gravity. Einstein’s general theory of relativity is turning 100, Advanced LIGO started the first observing run on September 18, and LISA Pathfinder is scheduled to launch in the Fall. While we celebrate the centenary of general relativity, we should also remember that there are many good reasons why the theory may well require modifications. Cosmological observations indicate that most of the Continue reading →

Local and gauge invariant observables in gravity

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Read the full article for free* in Classical and Quantum Gravity:
Local and gauge invariant observables in gravity
Igor Khavkine 2015 Class. Quantum Grav. 32 185019
arXiv:1503.03754
*until 14/10/15

Generalized locality leads to lots of observables in gravity

Igor Khavkine

Igor Khavkine is finishing up his term as a postdoctoral researcher at the University of Trento, Italy. His main interests are mathematical aspects of classical and quantum field theory, with an emphasis on gravity.

The problem of observables in general relativity is essentially as old as the theory itself. Einstein’s guiding principle of “general covariance”, that is, explicit tensorial transformation of basic physical fields and their equations under general coordinate transformations, leads to a formulation of the theory with “gauge” degrees of freedom. Those are degrees of freedom that, simply speaking, don’t contain any physical information and can be arbitrarily altered by the application of a coordinate transformation or, more abstractly, a diffeomorphism. Such a formulation is simple and Continue reading →

Book Reviews: 100 years after Einstein’s stay in Prague edited by Jiří Bičák and Tomáš Ledvinka

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Reviews of “General relativity, cosmology and astrophysics. Perspectives 100 years after Einstein’s stay in Prague” and “Relativity and gravitation. 100 years after Einstein in Prague”, edited by Jiří Bičák and Tomáš Ledvinka.

 

Lars Andersson

Lars Andersson is a Research Group Leader in the Geometric Analysis and Gravitation group at the Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut).

The two volumes under review document the conference held June 25-29, 2012 at Charles University in Prague to commemorate the 100th anniversary of the productive time Einstein spent in Prague, during which he arrived at the principle of equivalence and also formulated other physical principles, summarized in his 1912 paper [1]. This was a crucial step in the development of general relativity, and Einstein devoted the following years to developing its mathematical formulation, finally arriving at the 1915 theory of general relativity, the centennary of which is now celebrated through many events all over the world.

Coincidentally, the 2012 Prague conference also marked the 70th anniversary of the birth of Jiří Bičák, one of the leading European relativists. He was one of the organizers of the conference and is, together with Tomáš Ledvinka, one of the editors of the current volume. The taste and style, as well as the careful attention to detail of both editors, is Continue reading →

On the mass of compact rotating stars

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Amending the computation of the mass of compact rotating bodies with non-zero energy density at the surface.

Borja Reina

Borja Reina is a PhD fellow at the University of the Basque Country (UPV/EHU).

A proper understanding of rotating bodies in General Relativity (GR) is fundamental for many astrophysical situations. The original relativistic treatment of rotating compact stars in equilibrium is due to Hartle, back in 1967. It constitutes the basis of most of the analytical approaches and is widely used to construct numerical schemes in axial symmetry.

Hartle’s scheme depicts the equilibrium (stationary regime) Continue reading →