Quantum Squeezing and Late Time Classical Behavior of Massive Fields in Expanding Robertson-Walker Universe

TL;DR

This paper analyzes how quantum fluctuations in an expanding universe become classical over time, identifying mechanisms like squeezing and potential decoherence for both light and heavy fields during inflation.

Contribution

It introduces a Schroedinger picture approach to distinguish two decoherence mechanisms affecting vacuum fluctuations in cosmology, applicable to various Robertson-Walker models.

Findings

Turning of quantum oscillators causes decoherence of vacuum fluctuations.

Rapid squeezing leads to classical behavior of heavy fields.

Heavy fields with masses near the Hubble parameter undergo classical evolution.

Abstract

A Schroedinger picture analysis of time dependent quantum oscillators, in a manner of Guth and Pi, clearly identifies two physical mechanisms for possible decoherence of vacuum fluctuations in early universe: turning of quantum oscillators upside-down, and rapid squeezing of upside-right oscillators so that certain squeezing factor diverges. In inflationary cosmology the former mechanism explains the stochastic evolution of light inflatons and the classical nature of density perturbations in most of inflationary models, while the later one is responsible for the classical evolution of relatively heavy fields, with masses in a narrow range above the Hubble parameter: 2 < (m/H_0)^2 < 9/4. The same method may be applied to study of the decoherence of quantum fluctuations in any Robertson-Walker cosmology.