Dissipation of oscillating scalar backgrounds in an FLRW universe

TL;DR

This paper investigates how oscillating scalar fields dissipate in an expanding universe using non-equilibrium quantum field theory, revealing conditions for complete decay linked to particle production.

Contribution

It provides analytical approximations for scalar background evolution in an FLRW universe and connects dissipation to the imaginary parts of self-energy and vertex functions.

Findings

Complete decay depends on the imaginary part of the retarded self-energy.

Analytical expressions for scalar evolution are derived using multi-scale analysis.

Dissipation mechanisms are interpreted through particle production processes.

Abstract

We study the dissipation of oscillating scalar backgrounds in a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker universe using non-equilibrium quantum field theory. To be concrete, a $Z_2$-symmetric two-scalar model with quartic interactions is used. For quasi-harmonic oscillations, we adopt the multi-scale analysis to obtain analytical approximate expressions for the evolution of the scalar background in terms of the retarded self-energy and retarded proper four-vertex function. Different from the case in flat spacetime, we find that in an expanding universe the condensate decay in this model can be complete only if the imaginary part of the retarded self-energy is not negligibly small. The microphysical interpretation of the imaginary parts of the retarded self-energy and retarded proper four-vertex function in terms of particle production is also discussed.