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

Understand

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.