Do Black Holes Have Hair?
Friday, February 14, 2014
Science Update - A look at CfA discoveries from recent journals

Black holes with masses of millions or even billions of suns appear to reside at the nuclei of galaxies. In dramatic cases like quasars they are thought to be responsible for the spectacular phenomena like the ejection of narrow jets of particles at nearly the speed of light. Such outflows are thought to be driven by matter accreting onto a hot disk around the black hole. Much smaller black holes, closer in size to one solar mass, are thought to form as the result of the cataclysmic death of a star in a supernova.

A black hole in the traditional theory is characterized by having “no hair”; that is, it is so simple that it can be completely described by just three parameters, its mass, its spin, and its electric charge. Even though it may have formed out of a complex mix of matter and energy, all the specific details are lost when it collapses to a singular point. In the standard paradigm, the black hole is surrounded by a “horizon,” and once anything – matter or light (energy) – falls within that horizon, it cannot escape. Hence, the singularity appears black. Outside this horizon an accreting disk (if there is one) can radiate freely.

This picture may be appealing, but there is no direct proof as yet that any suspected galactic nuclei candidates are necessarily black holes and have horizons. The equations describing the collapse of matter in general relativity allow for other solutions, and do not require in every case that the final end-state must be a black hole. If, for example, matter were to condense very gradually instead of rapidly in a supernova, theory suggests that it is possible for the final point-like product to lack an event horizon. Such an object is called a “naked singularity” (“singularity” because, like a black hole, it has point-like dimensions, but naked because it lacks an event horizon and so light can escape its vicinity – it is not black).

CfA astronomer Ramesh Narayan and two colleagues have devised a way, at least in theory, for such a gradually collapsing process to form a naked singularity. Moreover, they then explore ways in which it might be observationally possible to distinguish one from a black hole by using the character of the radiation emitted from a disk around it. The results are not only fun to think about, they help astronomers probe how these bizarre objects may have formed in the first place.

Reference(s): 

"Distinguishing Black Holes from Naked Singularities Through Their Accretion Disc Properties,” Pankaj S. Joshi, Daniele Malafarina, and Ramesh Narayan, Classical and Quantum Gravity, 31, 1, 2014.