An effective field theory during inflation: reduced density matrix and its quantum master equation

TL;DR

This paper develops a non-perturbative quantum framework using a reduced density matrix and master equation to analyze super-Hubble inflaton fluctuations during inflation, revealing decay of the power spectrum and scale invariance violation.

Contribution

It introduces a quantum master equation approach to study inflaton fluctuations, capturing decay and entanglement effects beyond perturbation theory during inflation.

Findings

Power spectrum decay upon horizon crossing.

Violation of scale invariance due to secular effects.

Non-perturbative resummation of loop corrections.

Abstract

We study the power spectrum of super-Hubble fluctuations of an inflaton-like scalar field, the "system", coupled to another scalar field, the "environment" during de Sitter inflation. We obtain the reduced density matrix for the inflaton fluctuations by integrating out the environmental degrees of freedom. These are considered to be massless and conformally coupled to gravity as a \emph{proxy} to describe degrees of freedom that remain sub-Hubble all throughout inflation. The time evolution of the density matrix is described by a quantum master equation, which describes the decay of the vacuum state, the production of particles and correlated pairs and quantum entanglement between super and sub-Hubble degrees of freedom. The quantum master equation provides a non-perturbative resummation of secular terms from self-energy (loop) corrections to the inflaton fluctuations. In the case studied here these are Sudakov-type double logarithms which result in the \emph{decay} of the power spectrum of inflaton fluctuations upon horizon crossing with a concomitant violation of scale invariance. The reduced density matrix and its quantum master equation furnish a powerful non-perturbative framework to study the effective field theory of long wavelength fluctuations by tracing short wavelength degrees of freedom.