Monthly Archives: March 2017

Black holes without special relativity

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By Jishnu Bhattacharyya, Mattia Colombo and Thomas Sotiriou.

Black holes are perhaps the most fascinating predictions of General Relativity (GR). Yet, their very existence (conventionally) hinges on Special Relativity (SR), or more precisely on local Lorentz symmetry. This symmetry is the local manifestation of the causal structure of GR and it dictates that the speed of light is finite and the maximal speed attainable. Accepting also that light gravitates, one can then intuitively arrive at the conclusion that black holes should exist — as John Michell already did in 1783!

One can reverse the argument: does accepting that black holes exist, as astronomical observations and the recent gravitational wave direct detections strongly suggest, imply that Lorentz symmetry is an exact symmetry of nature? In other words, is this ground breaking prediction of GR the ultimate vindication of SR?

Jishnu Bhattacharyya, Mattia Colombo and Thomas Sotiriou from the School of Mathematical Sciences, University of Nottingham.

Jishnu Bhattacharyya, Mattia Colombo and Thomas Sotiriou from the School of Mathematical Sciences, University of Nottingham.

These questions might seem ill-posed if one sees GR simply as a generalisation of SR to non-inertial observers. On the same footing, one might consider questioning Lorentz symmetry as a step backwards altogether. Yet, there is an alternative perspective. GR taught us that our theories should be expressible in a covariant language and that there is a dynamical metric that is responsible for the gravitational interaction. Universality of free fall implies that Continue reading →

Advancing the Search for Gravitational Waves with Next-Generation Citizen Science

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by Michael Zevin.

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Michael Zevin is a third-year doctoral student in astrophysics at Northwestern University. He is a member of the LIGO Scientific Collaboration, and in addition to citizen science and LIGO detector characterization his research focuses on utilizing gravitational-wave detections to learn about binary stellar evolution and the environments in which compact binaries form.

With the first observations of gravitational waves and the discovery of binary black hole systems, LIGO has unveiled a new domain of the universe to explore. Though the recent signals persisted in LIGO’s sensitive band for a second or less, these last words of the binary that were spewed into the cosmos provided an unprecedented test of general relativity and insight into the progenitor stars that subsequently formed into the colliding black holes. However, the hunt is far from over. With LIGO’s second observing run underway, we can look forward to many more gravitational-wave signals, and as is true with any new mechanism for studying the cosmos, we can also expect to find the unexpected.

The extreme sensitivity required to make such detections was acquired through decades of developing methods and machinery to isolate the sensitive components of LIGO from non-gravitational-wave disturbances. Nonetheless, as a noise-dominated experiment, LIGO is still susceptible to Continue reading →

Tails from Eccentric Encounters

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by Nicholas Loutrel.

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Nicholas Loutrel is a Graduate Student in the eXtreme Gravity Institute at Montana State University.

A new method of computation aims to fill in the gaps in our knowledge of gravitational waves from eccentric binaries.

The modeling of gravitational waves (GWs) suitable for detection with ground-based detectors has been mostly focused on binary systems composed of compact objects, such as neutron stars (NSs) and black holes (BHs). Binaries that form with wide orbital separations are expected to have very small orbital eccentricity, typically less than 0.1, by the time their GW emission enters the detection band of these instruments. However, in dense stellar environments, unbound encounters between multiple compact objects can result in the formation of binaries with high orbital eccentricity (close to, but still less than unity) and whose GW emission is in band for ground-based detectors. Such systems are expected to be Continue reading →

Can astrophysical black holes have “hair”?

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by Carlos Herdeiro and Eugen Radu, Guest Editors of Focus Issue: Hairy Black Holes.

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Carlos A. R. Herdeiro (left) got his PhD from Cambridge University (U.K.) in 2002. He is currently an assistant professor at Aveiro University, Portugal, and an FCT principal researcher. He is also the founder and coordinator of the Gravitation group at Aveiro University (gravitation.web.ua.pt). Eugen Radu (right) got his PhD from Freiburg University (Germany) in 2002. He is currently an FCT principal researcher at Aveiro University (Portugal).

One of the most recognizable statements about black holes is that they have “no-hair”. Close inspection, however, shows that this is a belief rather than a mathematically proven theorem. Moreover, decades of research on this topic have shown that, depending on what one precisely means, this statement may be simply wrong. That is, as solutions of Einstein’s equations, in a generic context, black holes are not necessarily “bald”. Then, less ambitious, but perhaps more relevant questions are: “Can astrophysical black holes have hair?” and “Can we test the existence of black hole hair with present and future astrophysical observations?”.

This CQG focus issue brings together a set of papers describing models in which black holes do have “hair”, as well as observational efforts that have the potential to assess if this is (or not) the case for astrophysical black hole candidates. This collection of research papers is by no means a faithful and complete description of all possible alternatives to the Kerr paradigm in the literature. Rather, the selected papers focus on Continue reading →

Issue of the Beginning: Initial Conditions for Cosmological Perturbations

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by Abhay Ashtekar and Brajesh Gupt.

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Abhay Ashtekar holds the Eberly Chair in Physics and the Director of the Institute for Gravitation and the Cosmos at the Pennsylvania State University. Currently, he is a Visiting Professor at the CNRS Centre de Physique Théorique at Aix-Marseille Université.

Although our universe has an interesting and intricate large-scale structure now, observations show that it was extraordinarily simple at the surface of last scattering. From a theoretical perspective, this simplicity is surprising. Is there a principle to weed out the plethora of initial conditions which would have led to a much more complicated behavior also at early times?

In the late 1970s Penrose proposed such a principle through his Weyl curvature hypothesis (WCH) [1,2]: in spite of the strong curvature singularity, Big Bang is very special in that the Weyl curvature vanishes there. This hypothesis is attractive especially because it is purely geometric and completely general; it is not tied to a specific early universe scenario such as inflation.

However, the WCH is tied to general relativity and its Big Bang where classical physics comes to an abrupt halt. It is generally believed that quantum gravity effects would intervene and resolve the big bang singularity. The question then is Continue reading →