# The first heat: production of entanglement entropy in the early universe

## TL;DR

This paper investigates how entanglement entropy evolves in the early universe, showing rapid growth and saturation after transitioning from de Sitter to flat space, with implications for cosmological decoherence.

## Contribution

It numerically demonstrates the transition of entanglement entropy from area law to volume law after the universe's transition, revealing a partially thermalized state.

## Key findings

- Entanglement entropy grows rapidly after the de Sitter phase ends.
- The entropy saturates at volume law scaling.
- The final state resembles a generalized Gibbs ensemble.

## Abstract

Entanglement entropy (EE) of a spatial region quantifies correlations between the region and its surroundings. For a free scalar in the adiabatic vacuum in de Sitter space the EE is known to remain low, scaling as the surface area of the region. Here, we study the evolution of entanglement after the universe transitions from de Sitter to flat space. We concentrate on the case of a massless minimally coupled scalar. We find numerically that, after the de Sitter stage ends, the EE and the R\'enyi entropy rapidly grow and saturate at values obeying the volume law. The final state of the subsystem (region) is a partially thermalized state reminiscent of a generalized Gibbs ensemble. We comment on application of our results to the question of when and how cosmological perturbations decohere.