The entropy of finite gravitating regions

TL;DR

This paper introduces a formalism to compute the entanglement entropy of spatial regions in gravitating spacetimes, revealing a minimal surface area principle and proposing a new 'terrestrial holography' concept.

Contribution

It develops a path integral approach for entanglement entropy in gravitating regions, connecting it to minimal surface areas and introducing a novel holographic perspective.

Findings

Entanglement entropy equals the minimal surface area among enclosing regions.

Path integral over embeddings interpretable as sum over edge modes.

Proposes 'terrestrial holography' as an alternative to boundary holography.

Abstract

We develop a formalism for calculating the entanglement entropy of an arbitrary spatial region of a gravitating spacetime at a moment of time symmetry. The crucial ingredient is a path integral over embeddings of the region into the overall spacetime, interpretable as a sum over the edge modes associated with the region. We find that the entanglement entropy of a gravitating region equals the minimal surface area among all regions that enclose it. This suggests a notion of "terrestrial holography" where regions of space can encode larger ones, in contrast to the standard form of holography, in which degrees of freedom on the celestial sphere at the boundary of the universe encode the interior.