Thermal Damping of Mass-Modulating Scalars

TL;DR

This paper derives general thermal damping rates for scalar fields coupled to particles in the early universe, highlighting their potential dominance over Hubble damping and implications for cosmological models.

Contribution

It extends previous work by providing approximate thermal damping rates for oscillating scalar backgrounds in various particle coupling scenarios.

Findings

Thermal damping can dominate over Hubble damping in certain models.

Derived damping rates for scalars coupled to neutrinos, gluons, and WIMPs.

Implications for QCD axion models and early universe cosmology.

Abstract

The cosmological evolution of a scalar field is shaped by Hubble damping. Any non-gravitational couplings of the scalar with the primordial thermal bath generically contribute additional damping. Although rarely considered, such thermal damping could be the dominant dissipative effect. We derive approximate but highly general thermal-damping rates of scalar fields that modulate the masses of thermally populated particles. We extend previous results to cover cases of particular phenomenological interest where the scalar background oscillates sinusoidally but not necessarily slowly compared to the thermalization rates of the primordial bath. Based on these results, we estimate the thermal damping of scalars coupled to neutrinos linearly, to gluons quadratically, and to WIMPs linearly, and demonstrate its importance in certain parameter space of these models. We also estimate the thermal damping rates in models of QCD axion.