What prevents gravitational collapse in string theory?

Abstract

It is conventionally believed that if a ball of matter of mass [Formula: see text] has a radius close to [Formula: see text][Formula: see text]GM then it must collapse to a black hole. But string theory microstates (fuzzballs) have no horizon or singularity, and they do not collapse. We consider two simple examples from classical gravity to illustrate how this violation of our intuition happens. In each case, the ‘matter’ arises from an extra compact dimension, but the topology of this extra dimension is not trivial. The pressure and density of this matter diverge at various points, but this is only an artifact of dimensional reduction; thus, we bypass results like Buchadahl’s theorem. Such microstates give the entropy of black holes, so these topologically nontrivial constructions dominate the state space of quantum gravity.