Cosmology of the companion-axion model: dark matter, gravitational waves, and primordial black holes

TL;DR

The paper explores a two-axion cosmological model that addresses the strong-CP problem, predicts dark matter candidates, and proposes observable signatures like gravitational waves and primordial black holes.

Contribution

It introduces the companion-axion model with two free parameters, analyzing dark matter production, domain wall problem resolution, and potential cosmological signatures.

Findings

Identifies preferred axion mass windows for dark matter.

Shows the model naturally solves the domain wall problem.

Predicts detectable gravitational wave background and primordial black holes.

Abstract

The companion-axion model introduces a second QCD axion to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. As in single-axion models, the two axions predicted by the companion-axion model are attractive candidates for dark matter. The model is defined by two free parameters, the scales of the two axions' symmetry breaking, so we can classify production scenarios in terms of the relative sizes of these two scales with respect to the scale of inflation. We study the cosmological production of companion-axion dark matter in order to predict windows of preferred axion masses, and calculate the relative abundances of the two particles. Additionally, we show that the presence of a second axion solves the cosmological domain wall problem automatically in the scenarios in which one or both of the axions are post-inflationary. We also suggest unique cosmological signatures of the companion-axion model, such as the production of a $\sim$10 nHz gravitational wave background, and $\sim 100\,M_\odot$ primordial black holes.