Observations of Hawking radiation: the Page curve and baby universes

TL;DR

This paper uses replica wormholes to analyze black hole information, showing that asymptotic observers see black holes as unitary systems with the Bekenstein-Hawking entropy, but with superselection sectors from baby universes.

Contribution

It reformulates black hole information using operational entropy and replica wormholes, linking late-time spacetime structures to quantum unitarity and baby universes.

Findings

Black holes are consistent with unitarity from an observer perspective.

Density of states matches Bekenstein-Hawking formula.

Superselection sectors relate to baby universes.

Abstract

We reformulate recent insights into black hole information in a manner emphasizing operationally-defined notions of entropy, Lorentz-signature descriptions, and asymptotically flat spacetimes. With the help of replica wormholes, we find that experiments of asymptotic observers are consistent with black holes as unitary quantum systems, with density of states given by the Bekenstein-Hawking formula. However, this comes at the cost of superselection sectors associated with the state of baby universes. Spacetimes studied by Polchinski and Strominger in 1994 provide a simple illustration of the associated concepts and techniques, and we argue them to be a natural late-time extrapolation of replica wormholes. The work aims to be self-contained and, in particular, to be accessible to readers who have not yet mastered earlier formulations of the ideas above.