Effective Action for Cosmological Scalar Fields at Finite Temperature

TL;DR

This paper derives the finite temperature effective action for cosmological scalar fields, revealing new damping behaviors and providing analytic tools for modeling their evolution in the early universe.

Contribution

It introduces a complex effective potential framework for scalar fields at finite temperature, improving understanding of their dynamics during cosmic evolution.

Findings

Effective potential becomes complex-valued at finite temperature.

Damping coefficients differ from previous literature.

Analytic approximations for scalar models provided.

Abstract

Scalar fields appear in many theories beyond the Standard Model of particle physics. In the early universe, they are exposed to extreme conditions, including high temperature and rapid cosmic expansion. Understanding their behavior in this environment is crucial to understand the implications for cosmology. We calculate the finite temperature effective action for the field expectation value in two particularly important cases, for damped oscillations near the ground state and for scalar fields with a flat potential. We find that the behavior in both cases can in good approximation be described by a complex valued effective potential that yields Markovian equations of motion. Near the potential minimum, we recover the solution to the well-known Langevin equation. For large field values we find a very different behavior, and our result for the damping coefficient differs from the expressions frequently used in the literature. We illustrate our results in a simple scalar model, for which we give analytic approximations for the effective potential and damping coefficient. We also provide various expressions for loop integrals at finite temperature that are useful for future calculations in other models.