Category Archives: Author Insights

Author Insights are short informal articles written by authors of high quality CQG papers for the broad community served by the journal.

Gravity Probe B, the simplest data analysis ever

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John Conklin

John W. Conklin is assistant professor in the University of Florida’s Department of Mechanical and Aerospace Engineering. Photo: Aurora M Agüero

What could be simpler than fitting a straight line to a set of data? The slope of the line is the science result for NASA’s Gravity Probe B experiment, a landmark satellite test of two predictions of general relativity. Prior to launch, the data analysis for GP-B was thought to be straight forward. After precise calibration of the measured data (not as easy as fitting a straight line), the resulting signal should change at a constant rate over time. Our goal was to measure this rate of change, hopefully with an accuracy of 1%.

In the fall of 2005, as the year-long science mission was concluding, the GP-B science team began producing plots of the science data collected from the Earth-orbiting probe.  The figure shows one such plot. The slope of the best-fit straight line to these data is the magnitude of the elusive frame dragging effect, the main science goal of Gravity Probe B. Fear began to set it in. Where was the straight line? Continue reading →

Classifying noise transients in advanced gravitational-wave detectors

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Read the full article in Classical and Quantum Gravity (Open Access):
Classification methods for noise transients in advanced gravitational-wave detectors   Jade Powell, Daniele Trifirò, Elena Cuoco, Ik Siong Heng and Marco Cavaglià 2015 Class. Quantum Grav. 32 215012
arXiv:1505.01299

Jade Powell

Jade Powell is a PhD student at the University of Glasgow

A careful analysis of detector noise is necessary to determine whether a real gravitational-wave signal exists in the data of Advanced LIGO and Virgo. Instrumental and environmental disturbances can produce non-astrophysical triggers in science data, so called “glitches”. These glitches may reduce the duty cycle of the interferometers, and they could lead to a false detection if they occur simultaneously in multiple detectors. In the initial science runs of LIGO and Virgo a glitch could be classified by looking at an image of its time series waveform or spectrogram. This proved to be a slow and inefficient method for characterising a large number of glitches. To solve this problem the detector characterization team proposed a challenge for the fast automatic classification of Continue reading →

The spin limit for cosmological black holes

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Read the full article in Classical and Quantum Gravity (Open Access):
The area-angular momentum inequality for black holes in cosmological spacetimes
María Eugenia Gabach Clément, Martín Reiris and Walter Simon 2015 Class. Quantum Grav. 32 145006
arXiv:1501.07243

In colloquial terms, the main achievement of our recent CQG article is simple to state: We have proven that the angular momentum J of an axially symmetric black hole (the Noether current) with surface area A satisfies the bound.equation Walter CQG+ post

Here \Lambda is the cosmological constant –  a standard ingredient in Einstein’s Continue reading →

Understanding blobs of spacetime

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Read the full article for free* in Classical and Quantum Gravity:
Spacetime condensation in (2+1)-dimensional CDT from a Hořava-Lifshitz minisuperspace model
Dario Benedetti and Joe Henson 2015 Class. Quantum Grav. 32 215007
arXiv:1410.0845
*until 25/11/15

Can we explain the condensation of spacetime seen in numerical simulations of Causal Dynamical Triangulations?

Dario Benedetti

Dario Benedetti is a CNRS researcher at the Laboratoire de Physique Theorique at Orsay, France. His work focuses on various approaches to quantum gravity including CDT and asymptotic safety.

In the search of a quantum theory of gravity, it is not often that we are faced with the challenge of explaining some novel physical phenomenon: experiments are notoriously lacking, and theoretical questions usually involve clarifying the features of the different approaches, or the paradoxes of established theories. One of the most exciting aspects of Causal Dynamical Triangulations (CDT) is that numerical studies can produce unexpected results, which must then be explained, much like in mainstream statistical mechanics research.

Our paper, published in Classical and Quantum Gravity, is concerned with providing such an explanation Continue reading →

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 →

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 →

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 →