Unravelling Cosmological Perturbations

TL;DR

This paper demonstrates how quantum cosmological perturbations naturally transition to classical stochastic behavior during inflation through standard quantum mechanics, emphasizing the role of environment-induced localization.

Contribution

It provides a detailed, quantum-mechanics-based explanation for the quantum-to-classical transition of cosmological perturbations during inflation.

Findings

Perturbations become localized in field space due to environment interactions.

The conditioned state evolves into a classical stochastic process.

The process is described by a Langevin equation derived from the master equation.

Abstract

We explain in detail the quantum-to-classical transition for the cosmological perturbations using only the standard rules of quantum mechanics: the Schrodinger equation and Born's rule applied to a subsystem. We show that the conditioned, i.e. intrinsic, pure state of the perturbations, is driven by the interactions with a generic environment, to become increasingly localized in field space as a mode exists the horizon during inflation. With a favourable coupling to the environment, the conditioned state of the perturbations becomes highly localized in field space due to the expansion of spacetime by a factor of roughly exp(-c N), where N~50 and c is a model dependent number of order 1. Effectively the state rapidly becomes specified completely by a point in phase space and an effective, classical, stochastic process emerges described by a classical Langevin equation. The statistics of the stochastic process is described by the solution of the master equation that describes the perturbations coupled to the environment.