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 →

Black holes in massive gravity

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Eugeny Babichev

Eugeny Babichev is a CNRS researcher at the Laboratory for Theoretical Physics at Orsay. His work focuses on modified gravity theories, including their theoretical and observational issues, as well as phenomenological aspects, such as cosmology and black holes.

One of the biggest puzzles of modern physics is the present-day accelerated expansion of the Universe. The acceleration is usually attributed to the presence of a mysterious dark energy, a yet unknown substance of the Universe. Although in the framework of conventional General Relativity (GR), a cosmological constant can be added to mimic dark energy, the fine tuning required to adjust its value makes this explanation unsatisfactory. We can then ask whether a modification of Continue reading →

Generating dynamical bosons from kinematical fermions

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Ümit Ertem is a postdoctoral researcher in Ankara University (http://en.ankara.edu.tr/) Department of Physics. He will be visiting The University of Edinburgh for the next six months. He also writes on his own blog (https://ucuiyagi.wordpress.com/) sometimes.

Ümit Ertem is a postdoctoral researcher in Ankara University Department of Physics. He will be visiting The University of Edinburgh for the next six months. He also writes on his own blog, sometimes.

Spinors are mathematical objects used in physics mainly for defining fermions. Fermions are particles/field excitations that have half-integer spins as opposed to bosons that have integer spins. While fermions correspond to elementary constituents of matter, bosons correspond to the fundamental interactions of matter. There is a distinguishing property of fermions that an even number of them can combine to exhibit bosonic behaviour in analogy with the defining algebraic properties of half-integers and integers.

So immediately one can easily grasp the fact that the product of two spinors represent the above mentioned bosonic Continue reading →

A quantum kinematics for asymptotically flat gravity

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Miguel and Madhavan

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

Isolated gravitating systems are modelled by asymptotically
flat space-times with the classical gravitational field subject to intricate and detailed asymptotic behaviour. The question we are interested in is: Is there a notion of an isolated quantum gravitating system? Specifically, can the classical
asymptotic conditions be suitably incorporated in quantum theory? Our work analyses this issue in the broad context of the Loop Quantum Gravity (LQG) approach.

At first it may seem this cannot be possible: The fundamental excitations in LQG are Continue reading →