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Advances in Astronomy
Volume 2012 (2012), Article ID 486750, 9 pages
http://dx.doi.org/10.1155/2012/486750
Review Article

Testing the No-Hair Theorem with Sgr A*

Physics Department, University of Arizona, 1118 E. 4th Street, Tucson, AZ 85721, USA

Received 16 May 2011; Accepted 5 July 2011

Academic Editor: Francesco Shankar

Copyright © 2012 Tim Johannsen. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The no-hair theorem characterizes the fundamental nature of black holes in general relativity. This theorem can be tested observationally by measuring the mass and spin of a black hole as well as its quadrupole moment, which may deviate from the expected Kerr value. Sgr A*, the supermassive black hole at the center of the Milky Way, is a prime candidate for such tests thanks to its large angular size, high brightness, and rich population of nearby stars. In this paper, I discuss a new theoretical framework for a test of the no-hair theorem that is ideal for imaging observations of Sgr A* with very long baseline interferometry (VLBI). The approach is formulated in terms of a Kerr-like spacetime that depends on a free parameter and is regular everywhere outside of the event horizon. Together with the results from astrometric and timing observations, VLBI imaging of Sgr A* may lead to a secure test of the no-hair theorem.