# Entanglement and Expansion

## TL;DR

This paper investigates how entanglement entropy of a quantum scalar field grows during inflation due to superhorizon mode squeezing, persists afterward, and could influence today's universe, demonstrated via a 1+1D toy model.

## Contribution

It introduces a model linking inflation duration to entanglement entropy growth, highlighting its potential cosmological significance.

## Key findings

- Entanglement entropy grows with inflation duration.
- Entropy remains after radiation and matter eras.
- Toy model confirms theoretical predictions.

## Abstract

We study the entanglement entropy resulting from tracing out local degrees of freedom of a quantum scalar field in an expanding universe. It is known that when field modes become superhorizon during inflation they evolve to increasingly squeezed states. We argue that this causes the entanglement entropy to grow continuously as successive modes cross the horizon. The resulting entropy is proportional to the total duration of inflation. It is preserved during a subsequent radiation or matter dominated era, and thus it may be relevant for today's universe. We demonstrate explicitly these features in a toy model of a scalar field in 1+1 dimensions.