Despite Conventional wisdom suggests that black holes are fully characterized by just two quantities, mass and spin, for several years physicists have been investigating whether black holes can have additional structures - in jargon "hair" - which would reveal the existence of new fundamental fields.
“In our work we have considered a wide class of extensions to Einstein’s theory of gravity that make interesting predictions in extreme regimes, such as the surroundings of black holes or neutron stars” says Alexandru Dima, Astrophysicist of SISSA and INFN – Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, first author of the paper Spin-induced black hole spontaneous scalarization, recently published in Physical Review Letters. “While previous studies have already provided examples of “hairy” black hole solutions, we have shown for the first time, thanks to numerical simulations, that black holes can spontaneously grow the simplest form of permanent hair (a scalar field) once they start spinning fast enough.” Together with Enrico Barausse (SISSA and IFPU - Institute for Fundamental Physics of the Universe), Nicola Franchini (SISSA and IFPU) and Thomas P. Sotiriou (University of Nottingham, UK), Dima also describes the way in which rotation controls the hair growth mechanism. In Einstein’s theory of gravity and many of its extensions, mathematical theorems ensure that black holes cannot sustain hair. They eventually shed it away through the emission of gravitational waves. However, in the theories considered by Dima et al., once the black hole starts rotating faster than a certain threshold it forces hair to grow, giving the black hole novel features.