Islands in the Fluid: Islands are Common in Cosmology

TL;DR

This paper explores the conditions under which entanglement islands can exist in cosmological spacetimes with perfect fluids, revealing their presence in flat, non-radiation universes and in non-singular models like the Simple Harmonic Universe.

Contribution

It generalizes the conditions for entanglement islands to a broad class of cosmological models with perfect fluids, including non-singular universes, extending previous singularity-dependent results.

Findings

Islands exist in flat universes with time-symmetric slices where the Hubble parameter vanishes.

Islands can occur in universes with fluids other than radiation ($w eq 1/3$).

Islands are present in the non-singular Simple Harmonic Universe model.

Abstract

We discuss the possibility of entanglement islands in cosmological spacetimes with a general perfect fluid with an equation of state $w$. We find that flat universes with time-symmetric slices where the Hubble parameter vanishes always have islands on that slice. We then move away from such slices, considering still universes with a general perfect fluid. Under the local thermal equilibrium assumption, the comoving entropy density $s_c$ is constant. As a result, the conditions for an island become an inequality between the energy density (or Hubble parameter) and the temperature at some time of normalization. The consequences are that islands can exist for practically all fluids that are not radiation, i.e. $w\neq 1/3$. We also discuss the ramifications of our results for universes with spatial curvature. Finally, we show that islands occur in the Simple Harmonic Universe model which has no classical singularity at the background level, in contrast to all previous examples where islands occurred only in space-times with singularities.