Non-equilibrium dynamics of a scalar field with quantum backreaction

TL;DR

This paper investigates the non-equilibrium evolution and thermalization of a scalar field using the 2PI framework, highlighting non-perturbative processes like parametric resonance and spinodal instability during reheating.

Contribution

It introduces a renormalized 2PI approach with the Hartree approximation to study out-of-equilibrium scalar field dynamics, including backreaction and finite temperature effects.

Findings

Identification of parametric resonance and spinodal instability during reheating

Demonstration of the impact of spinodal modes on the evolution of the field

Development of a renormalized 2PI framework applicable to inflationary reheating

Abstract

We study the dynamical evolution of coupled one- and two-point functions of a scalar field in the 2PI framework at the Hartree approximation, including backreaction from out-of-equilibrium modes. We renormalize the 2PI equations of motion in an on-shell scheme in terms of physical parameters. We present the Hartree-resummed renormalized effective potential at finite temperature and critically discuss the role of the effective potential in a non-equilibrium system. We follow the decay and thermalization of a scalar field from an initial cold state with all energy stored in the potential, into a fully thermalized system with a finite temperature. We identify the non-perturbative processes of parametric resonance and spinodal instability taking place during the reheating stage. In particular we study the unstable modes in the region where the vacuum 1PI effective action becomes complex and show that such spinodal modes can have a dramatic effect on the evolution of the one-point function. Our methods can be easily adapted to simulate reheating at the end of inflation.