Black holes may have 'hair'

Black holes may have 'hair' to anchor them to the surrounding matter and probably to the entire universe, a new study suggests.
Black holes may be much "dirtier" as opposed to the hypothesis by Roy Kerr, who in 1963 proposed a "clean" black hole model, which is the current theoretical paradigm.
According to the traditional model, black holes are defined by only two quantities: mass and angular momentum (a black hole rotation velocity).
Once their progenitor has collapsed (a high mass star, for instance, that at the end of its life cycle implodes inwards) its memory is lost forever.
All that is left is a quiescent black hole, with almost no distinctive features: all black holes, mass and angular momentum aside, look almost the same.

According to a new study carried out by a group of scientists that includes Thomas Sotiriou, a physicist of the International School for Advanced Studies (SISSA) of Trieste, Italy, things may not have occurred this way.

"Black holes, according to our calculations, may have hair," said Sotiriou, referring to a well-known statement by physicist John Wheeler, who claimed that "black holes have no hair."
Wheeler meant that mass and angular momentum are all one needs to describe them.
"Although Kerr's 'bald' model is consistent with General Relativity, it might not be consistent with some well-known extensions of Einstein's theory, called tensor-scalar theories," said Sotiriou.

"This is why we have carried out a series of new calculations that enabled us to focus on the matter that normally surrounds realistic black holes, those observed by astrophysicists.
"This matter forces the pure and simple black hole hypothesised by Kerr to develop a new 'charge' (the hair, as we call it) which anchors it to the surrounding matter, and probably to the entire Universe," said Sotiriou.
The experimental confirmation of this new hypothesis may come from the observations carried out with the interferometers, instruments capable of recording the gravitational waves.
"According to our calculations, the growth of the black hole's hair is accompanied by the emission of distinctive gravitational waves. In the future, the recordings by the instrument may challenge Kerr's model and broaden our knowledge of the origins of gravity," said Sotiriou.