Credit: MEHAU KULYK / SCIENCE PHOTO LIBRARY

In cosmology and astrophysics, it is the era of data. Measurements made using various telescopes and satellites are revealing the Universe and its history in a degree of detail few could have anticipated; the general theory of relativity that once seemed untestable is now under scrutiny.

All tests of general relativity so far have found no flaws in Einstein's logic. It is, at very least, a successful effective theory of gravity at the scales that can be probed at present. But what if it is only an effective theory, and there exists some more fundamental theory beyond it — how might that be detected? Writing in Physical Review Letters, Dimitrios Psaltis and colleagues consider what we might learn from those seemingly least informative of astrophysical objects — black holes (Phys. Rev. Lett. 100, 091101; 2008).

Of the four exact black-hole solutions to Einstein's field equations, the Kerr metric corresponds to a black hole that is rotating (as may be expected from its formation following the collapse of a rotating star) and uncharged. Whether the Kerr metric does describe the external spacetime of an astrophysical black hole could soon be testable, using, for example, data collected in searching for gravitational waves or in high-energy observations.

However, Psaltis et al. show that several types of extension to the general theory — adding scalar, vector or tensor degrees of freedom, such as might exist in a theory of quantum gravity — still result in the Kerr metric. The exciting experimental possibility of proving the Kerr metric would, in fact, offer little scope for discrimination between approaches to a fundamental theory.

The flipside, of course, is that should any deviation from the Kerr metric be detected, then there's definitely something else out there.