Scalar damping in cosmological phase transitions

TL;DR

This paper calculates the scalar damping rate during cosmological phase transitions using kinetic theory, assessing the validity of phenomenological friction models and their relation to bubble dynamics.

Contribution

It provides a detailed calculation of scalar damping from Standard Model particles and evaluates the accuracy of hydrodynamical friction approximations in cosmological phase transitions.

Findings

Scalar damping rate from top quarks and gauge bosons is computed.

Convergence issues depend on treatment of soft modes.

Phenomenological friction approximation is marginally justified for Standard Model content.

Abstract

We outline how to calculate the scalar damping term during a cosmological phase transition from kinetic theory. We determine the scalar damping rate from top quarks and weak gauge bosons in a Standard Model-like theory. We find that the convergence of the bosonic contributions hinges on how the soft modes are treated. We discuss the validity of the phenomenological friction term employed in hydrodynamical simulations. We find that for a Standard Model particle content, this approximation is (marginally) justified. We also test the hypothesis that the pressure from a runaway wall acts as an upper bound on the pressure from the local friction term. We find that next-to-leading order contributions in terms of velocity and mass are negative and that in the regime of validity, the local damping term indeed cannot surpass the pressure from runaway bubbles.