# Compactified extra dimension and entanglement island as clues to quantum gravity

## TL;DR

This paper explores how compactified extra dimensions and entanglement islands can shed light on quantum gravity by resolving black hole singularities, explaining entropy, and demonstrating information preservation during black string evaporation.

## Contribution

It introduces a novel approach linking extra dimensions and island formation to address black hole information paradoxes and quantum gravity puzzles.

## Key findings

- Removal of curvature singularity via smooth bubble geometries.
- Emergence of islands outside the event horizon during evaporation.
- Finite entanglement entropy indicating information preservation.

## Abstract

We show that the compactified extra dimension and the emergence of the island can provide clues about quantum gravity because their combination can solve the deepest puzzles of black hole physics. Suppose that the time dimension and the extra dimension compactified on a circle are symmetric under \emph{double Wick rotation}, the curvature singularity would be removed due to the end of spacetime as a smooth bubble hidden behind the event horizon. The smooth bubble geometries can also be interpreted as microstates leading to the Bekenstein-Hawking entropy because the smooth bubble geometries live in the same region of mass and charge as the black string. In addition, by applying the quantum extremal surface prescription, we show the emergence of the island at late times of the black string evaporation where it is located slightly outside the event horizon. Due to the dominant contribution of the island configuration, the entanglement entropy of the radiation grows no longer linearly in time but it reaches a finite value that is twice the Bekenstein-Hawking entropy at the leading order. This transition shows the information preservation during the black string evaporation. Furthermore, we calculate the Page time which determines the moment of the transition between the linearly growing and constant behaviors of the entanglement entropy as well as the scrambling time corresponding to the information recovery time of the signal falling into the black string.