Decoherence due to the Horizon after Inflation

TL;DR

This paper demonstrates that entanglement entropy between observable and unobservable regions after inflation causes decoherence of primordial fluctuations, with entropy growth proportional to the volume inside the horizon and the number of e-folds.

Contribution

It shows that horizon-induced entanglement entropy can account for decoherence of inflationary fluctuations, highlighting a volume-dependent growth rather than area dependence.

Findings

Entanglement entropy grows with volume inside the horizon.

Entropy is proportional to the number of e-folds since Hubble crossing.

Decoherence can be explained by horizon entanglement without new physics.

Abstract

The fluctuations in the inflaton field at the end of inflation which seed the density perturbations are prepared in a pure quantum state. It is generally assumed that some physics causes this pure state to decohere so that it should be treated probabilistically. We show that the entanglement entropy between the universe inside our observable horizon and that outside our horizon is sufficient to do this. For the modes which are super-Hubble at the end of inflation, this entanglement entropy grows with volume inside the horizon, rather than with the horizon's area, and is proportional to the number of e-folds since Hubble crossing.