Gravitational Black Hole Hair from Event Horizon Supertranslations

TL;DR

This paper explores how supertranslations at the black hole horizon create gapless excitations that serve as gravitational hair, potentially explaining black hole entropy through quantum criticality effects.

Contribution

It introduces a quotient algebra of supertranslations at the horizon, linking horizon excitations to black hole entropy and quantum criticality, extending previous BMS symmetry analyses.

Findings

Horizon supertranslations generate physical gapless modes.

These modes account for black hole gravitational hair.

Quantum criticality renders the infinite classical modes finite.

Abstract

We discuss BMS supertranslations both at null-infinity and on the horizon for the case of the Schwarzschild black hole. We show that both kinds of supertranslations lead to infinetly many gapless physical excitations. On this basis we construct a quotient algebra using suited superpositions of both kinds of transformations which cannot be compensated by an ordinary BMS-supertranslation and therefore are intrinsically due to the presence of an event horizon. We show that these quotient transformations are physical and generate gapless excitations on the horizon that can account for the gravitational hair as well as for the black hole entropy. We identify the physics of these modes as associated with Bogolioubov-Goldstone modes due to quantum criticality. Classically the number of these gapless modes is infinite. However, we show that due to quantum criticality the actual amount of information-carriers becomes finite and consistent with Bekenstein entropy. Although we only consider the case of Schwarzschild geometry, the arguments are extendable to arbitrary space-times containing event horizons.