A comment on a fine-grained description of evaporating black holes with baby universes

TL;DR

This paper explores a detailed description of evaporating black holes by incorporating baby universes, providing insights into entropy calculations, holographic duals, and modifications to gravitational laws.

Contribution

It introduces a novel partial fine-grained framework for black hole evaporation using baby universes and connects it to holographic duality and gravitational operator dressing.

Findings

Purification of Hawking radiation entropy via baby universes.

Modification of semi-classical gravitational Gauss law.

Impact on gravitational dressing of operators behind the horizon.

Abstract

We study a partially fine-grained description of an evaporating black hole by introducing an open baby universe with a boundary. Since the Page's calculation of the entropy of Hawking radiation involves an ensemble average over a class of states, one can formally obtain a fine-grained state by purifying this setup. For AdS black holes with a holographic dual, this purification amounts to introducing an additional boundary (i.e., baby universe) and then connecting it to the original black hole through an Einstein-Rosen bridge. We uncover several details of this setup. As applications, we briefly discuss how this baby universe modifies the semi-classical gravitational Gauss law as well as the gravitational dressing of operators behind the horizon.