The current theoretical understanding of black holes was introduced in 1963, when mathematician Roy Kerr proposed his "clean" black hole model.
But new research carried out by a group including Thomas Sotiriou, a physicist of the International School for Advanced Studies (SISSA), suggests black holes may not be as tidy as once believed.
The current model holds that black holes -- formed by the collapse of a star -- are defined simply by their rotation velocity, or the two measures mass and angular momentum.
"Black holes, according to our calculations, may have hair", explained Sotiriou, referring to a well-known statement by physicist John Wheeler, who claimed that "black holes have no hair."
"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," Sotiriou said.
"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 hypothesized 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," Sotiriou said.
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," concludes Sotiriou "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."