Particle creation, classicality and related issues in quantum field theory: II. Examples from field theory

TL;DR

This paper explores how quantum fields evolve in time-dependent backgrounds, examining the emergence of classicality and particle production through phase space analysis in different spacetime scenarios.

Contribution

It extends the formalism from Paper I to analyze classicality and particle creation in quantum field theory within external backgrounds like electric fields and FRW spacetimes, highlighting key differences and conceptual insights.

Findings

Classicality is multifaceted and not solely indicated by phase space peaking.

Strong particle production correlates with highly classical phase space states.

Differences between Schwinger effect and de Sitter evolution are clarified.

Abstract

We adopt the general formalism, which was developed in Paper I (arXiv:0708.1233) to analyze the evolution of a quantized time-dependent oscillator, to address several questions in the context of quantum field theory in time dependent external backgrounds. In particular, we study the question of emergence of classicality in terms of the phase space evolution and its relation to particle production, and clarify some conceptual issues. We consider a quantized scalar field evolving in a constant electric field and in FRW spacetimes which illustrate the two extreme cases of late time adiabatic and highly non-adiabatic evolution. Using the time-dependent generalizations of various quantities like particle number density, effective Lagrangian etc. introduced in Paper I, we contrast the evolution in these two limits bringing out key differences between the Schwinger effect and evolution in the de Sitter background. Further, our examples suggest that the notion of classicality is multifaceted and any one single criterion may not have universal applicability. For example, the peaking of the phase space Wigner distribution on the classical trajectory \emph{alone} does not imply transition to classical behavior. An analysis of the behavior of the \emph{classicality parameter}, which was introduced in Paper I, leads to the conclusion that strong particle production is necessary for the quantum state to become highly correlated in phase space at late times.