What prevents gravitational collapse in string theory?

TL;DR

This paper explores how string theory microstates, called fuzzballs, avoid gravitational collapse into black holes by having nontrivial topology and no horizon, challenging classical expectations.

Contribution

It demonstrates through classical gravity examples how string theory microstates evade collapse, highlighting the role of extra dimensions and topology.

Findings

Microstates lack horizons and singularities.

Topologically nontrivial extra dimensions prevent collapse.

These microstates account for black hole entropy.

Abstract

It is conventionally believed that if a ball of matter of mass $M$ has a radius close to $2GM$ then it must collapse to a black hole. But string theory microstates (fuzzballs) have no horizon or singularity, and they do {\it 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.