Dark Matter Axions Revisited

TL;DR

This paper investigates the specific conditions under which axions can constitute all cold dark matter, analyzing parameter regions in light of recent cosmological data and different symmetry-breaking scenarios.

Contribution

It provides updated constraints on axion parameters for dark matter, considering recent observational data and different Peccei-Quinn symmetry-breaking scenarios.

Findings

Axions with mass around 67 μeV can account for all dark matter if symmetry is restored after inflation.

If symmetry breaks during inflation, axions can be all dark matter for masses below 15 meV with specific initial conditions.

Large symmetry-breaking scales imply very small initial misalignment angles, affecting axion dark matter viability.

Abstract

We study for what specific values of the theoretical parameters the axion can form the totality of cold dark matter. We examine the allowed axion parameter region in the light of recent data collected by the WMAP5 mission plus baryon acoustic oscillations and supernovae, and assume an inflationary scenario and standard cosmology. If the Peccei-Quinn symmetry is restored after inflation, we recover the usual relation between axion mass and density, so that an axion mass $m_a =67\pm2{\rm \mu eV}$ makes the axion 100% of the cold dark matter. If the Peccei-Quinn symmetry is broken during inflation, the axion can instead be 100% of the cold dark matter for $m_a < 15{\rm meV}$ provided a specific value of the initial misalignment angle $\theta_i$ is chosen in correspondence to a given value of its mass $m_a$. Large values of the Peccei-Quinn symmetry breaking scale correspond to small, perhaps uncomfortably small, values of the initial misalignment angle $\theta_i$.