Fuzzballs and Microstate Geometries: Black-Hole Structure in String Theory

TL;DR

The paper addresses the black-hole information paradox and argues that string theory's fuzzball and microstate geometries provide a horizonless, unitary description of black-hole microstructure. It surveys the information paradox, the fuzzball principle, the spacetime CFT, current status of microstate geometries, world-sheet perspectives, and how these geometries replicate black-hole behavior and scrambling while avoiding information loss. Key results include the construction of numerous microstate geometries (multi-centered bubbling geometries, superstrata, microstrata), precise holographic matches, and stringy resolutions to near-horizon singularities, alongside open questions, non-BPS extensions, and potential observational signatures like shadows and echoes. The work emphasizes horizon-scale quantum structure as essential for resolving the paradox and outlines a roadmap for leveraging AdS/CFT, world-sheet techniques, and phase-space dynamics to advance our understanding of black-hole microstructure.

Abstract

The black-hole information paradox provides one of the sharpest foci for the conflict between quantum mechanics and general relativity and has become the proving-ground of would-be theories of quantum gravity. String theory has made significant progress in resolving this paradox, and has led to the fuzzball and microstate geometry programs. The core principle of these programs is that horizons and singularities only arise if one tries to describe gravity using a theory that has too few degrees of freedom to resolve the physics. String theory has sufficiently many degrees of freedom and this naturally leads to fuzzballs and microstate geometries: The reformation of black holes into objects with neither horizons nor singularities. This not only resolves the paradox but provides new insights into the microstructure of black holes. We summarize the current status of this approach and describe future prospects and additional insights that are now within reach. This paper is an expanded version of our Snowmass White Paper arXiv:2203.04981.