Black Hole Horizon Fluffs: Near Horizon Soft Hairs as Microstates of Three Dimensional Black Holes

TL;DR

The paper presents an explicit construction of BTZ black hole microstates as horizon fluff, a finite set of zero-energy near-horizon soft hairs organized into orbits of the asymptotic Virasoro algebra. By relating near-horizon and asymptotic algebras, the authors impose a spectrum cutoff and count microstates with Hardy-Ramanujan partitions, reproducing the Bekenstein-Hawking entropy to leading order. This approach provides a concrete realization of black hole microstates without relying on supersymmetry or stringy embeddings and clarifies how soft hair can account for thermodynamic entropy. The work also outlines a path to generalize the horizon-fluff construction to Kerr black holes, suggesting a universal role for near-horizon symmetries in black hole microphysics.

Abstract

We provide the first explicit proposal for all microstates of generic black holes in three dimensions (of Banados-Teitelboim-Zanelli-type): black hole microstates, termed "horizon fluffs", are a particular class of near horizon soft hairs which have zero energy as measured by the horizon observer and cannot be distinguished by observers at finite distance from the horizon. These states are arranged in orbits of the two-dimensional conformal algebra associated with the asymptotic black hole geometry. We count these microstates using the Hardy-Ramanujan formula for the number of partitions of a given integer into non-negative integers, recovering the Bekenstein-Hawking entropy. We discuss possible extensions of our black hole microstate construction to astrophysical Kerr-type black holes.