Thermal History of Non-equilibrated Scalars

TL;DR

This paper explores the dynamics of scalar fields in the early Universe that are neither in equilibrium nor fully decoupled, revealing how their non-trivial evolution can influence cosmological phase transitions.

Contribution

It introduces a systematic analysis of non-equilibrated scalar fields affecting effective couplings and demonstrates a novel impact on phase transition strength in an Abelian Higgs model.

Findings

Scalar fields can have evolving couplings during cosmological history.

Out-of-equilibrium scalars can modify phase transition dynamics.

Enhanced phase transition strength due to scalar coupling evolution.

Abstract

Scalar fields in the early Universe are mostly discussed in two limits: either in equilibrium or completely decoupled. In this work we discuss scenarios where there are scalar fields that are not in equilibrium, but for which the coupling to thermal bath leads to interesting non-trivial dynamics. For example, in theories where scalar fields control the effective couplings of the theory, such out-of-equilibrium behavior can lead to cases where the couplings vary during cosmological evolution. We systematically examine the generic features governing the evolution of these couplings, and as an application we highlight a novel effect where the scalar quartic coupling of an Abelian Higgs model is modified, leading to stronger cosmological phase transitions than would be obtained for static non-evolving quartics.