On the Entropy and the Density Matrix of Cosmological Perturbations

TL;DR

This paper investigates how cosmological perturbations transition to classical behavior during inflation, analyzing two decoherence mechanisms and clarifying previous discrepancies in entropy estimates.

Contribution

It identifies and compares two distinct decoherence mechanisms affecting cosmological perturbations, resolving previous inconsistencies in entropy calculations.

Findings

Two decoherence mechanisms: dynamical and dephasing.

Negligible entropy gain during dynamical decoherence.

Irreversible dephasing causes rapid off-diagonal decay.

Abstract

We look at the transition to the semiclassical behaviour and the decoherence process for the inhomogeneous perturbations in the inflationary universe. Two different decoherence mechanisms appear: one dynamical, accompanied with a negligible, if at all, entropy gain, and the other, effectively irreversible dephasing, due to a rapid variation in time of the off-diagonal density matrix elements in the post-inflationary epoch. We thus settle the discrepancies in the entropy content of perturbations evaluated by different authors.