Unitarity and Page curve for evaporation of 2D AdS black holes

TL;DR

This paper analyzes the unitarity of black hole evaporation in 2D AdS spacetime, deriving the Page curve and showing that the end state is a regular, zero-entropy configuration, supporting the island proposal for information retrieval.

Contribution

It provides a detailed derivation of the Page curve for 2D AdS black holes using a simplified toy model and confirms the unitarity of the evaporation process.

Findings

The end-point of evaporation is a regular, zero-entropy state.

The entanglement entropy follows a Page curve consistent with unitarity.

The model supports the island proposal for resolving the information paradox.

Abstract

We explore the Hawking evaporation of two-dimensional anti-de Sitter (AdS$_2$), dilatonic black hole coupled with conformal matter, and derive the Page curve for the entanglement entropy of radiation. We first work in a semiclassical approximation with backreaction. We show that the end-point of the evaporation process is AdS$_2$ with a vanishing dilaton, i.e. a regular, singularity-free, zero-entropy state. We explicitly compute the entanglement entropies of the black hole and the radiation, as functions of the horizon radius, using the conformal field theory (CFT) dual to AdS$_2$ gravity. We use a simplified toy model, in which evaporation is described by the forming and growing of a negative mass configuration in the positive-mass black hole interior. This is similar to the ``islands'' proposal, recently put forward to explain the Page curve for evaporating black holes. The resulting Page curve for AdS$_2$ black holes is in agreement with unitary evolution. The entanglement entropy of the radiation initially grows, closely following a thermal behavior, reaches a maximum at half-way of the evaporation process, and then goes down to zero, following the Bekenstein-Hawking entropy of the black hole. Consistency of our simplified model requires a non-trivial identification of the central charge of the CFT describing AdS$_2$ gravity with the number of species of fields describing Hawking radiation.