Axions, Time Varying CP Violation, and Baryogenesis

TL;DR

This paper explores how axion cosmology can lead to significant CP violation and departures from thermal equilibrium, potentially impacting baryogenesis regardless of axions being dark matter.

Contribution

It introduces two cosmological features of axions—large CP violation at low temperatures and a transient departure from equilibrium—that could influence baryogenesis.

Findings

Large CP violation persists down to ~2 GeV temperatures.

Axion field oscillations can cause brief non-equilibrium conditions.

The strong CP parameter can be order one during these periods.

Abstract

We derive two features of axion cosmology that may have cosmological implications, whether or not axions are dark matter: For the full range of allowed axion masses, the evolution of a cosmic axion background allows large CP violation until temperatures as low as $\sim$ 2 GeV, and once the axion field begins to oscillate, the cosmological axion field's relaxation to its ground state can briefly provide a new departure from thermal equilibrium, via time-varying CP violation. During both of these periods, the Strong CP violating parameter $\bar\theta$ can be as large as O(1).