Classical equipartition dynamics between axions and non-Abelian gauge fields

TL;DR

This paper investigates the early energy transfer dynamics between axions and non-Abelian SU(2) gauge fields using lattice simulations, revealing equipartition and differences from Abelian cases relevant for cosmological models.

Contribution

It provides a detailed non-linear simulation study of axion-gauge field interactions, highlighting non-Abelian effects and energy equipartition mechanisms in the early universe.

Findings

Initial exponential growth of gauge modes

Damping of axion oscillations observed

Energy equipartition between axion and gauge fields

Abstract

Motivated by axion-like inflation and its warm embedding within the Standard Model, we study the early stages of the energy transfer between an axion condensate and an SU(2) gauge ensemble, by employing non-linear classical real-time lattice simulations. The discretized equations of motion are worked out, elaborating on Gauss constraints. A numerical solution is implemented on the CosmoLattice platform. Adopting a quadratic potential, and omitting universe expansion for the moment, we establish initial exponential growth of the low-momentum gauge modes; damping of axion oscillations after some delay; and subsequent energy equipartition between axion and gauge ensembles. A clear difference between the SU(2) and U(1) dynamics is observed, likely associated with non-Abelian self-interactions. We elaborate on what this implies for the possible thermalization of the SU(2) ensemble.