Emergence of classicality for primordial fluctuations: Concepts and analogies

TL;DR

This paper explains how quantum primordial fluctuations during inflation become classical, emphasizing the roles of squeezing and decoherence, and uses analogies to clarify the conceptual transition from quantum to classical behavior.

Contribution

It provides a detailed conceptual analysis of the emergence of classicality in cosmological perturbations, highlighting the roles of squeezing and decoherence with illustrative analogies.

Findings

Large-scale fluctuations become effectively classical due to squeezing.

Decoherence selects the field amplitude basis as the stable pointer basis.

Analogies help clarify the quantum-to-classical transition in cosmology.

Abstract

We clarify the way in which cosmological perturbations of quantum origin, produced during inflation, assume classical properties. Two features play an important role in this process: First, the dynamics of fluctuations which are presently on large cosmological scales leads to a very peculiar state (highly squeezed) that is indistinguishable, in a precise sense, from a classical stochastic process. This holds for almost all initial quantum states. Second, the process of decoherence by interaction with the environment distinguishes the field amplitude basis as a robust pointer basis. We discuss in detail the interplay between these features and use simple analogies such as the free quantum particle to illustrate the main conceptual issues.