Axion as a cold dark matter candidate: low-mass case

TL;DR

This paper investigates the effects of ultra-light axion dark matter with very low mass on cosmological structures, proposing a model with evolving axion mass to address early universe coherence issues and analyzing observational signatures.

Contribution

It introduces a low-mass axion model with evolving mass to reconcile coherence in the early universe and examines its distinct observational effects on cosmological power spectra.

Findings

Significant damping in baryonic power spectrum for axion mass below 10^{-24} eV.

Presence of a small-scale cutoff in the baryon density power spectrum.

Distinct deviations from standard CDM and hot dark matter models in matter and CMB spectra.

Abstract

Axion as a coherently oscillating scalar field is known to behave as a cold dark matter in all cosmologically relevant scales. For conventional axion mass with 10^{-5} eV, the axion reveals a characteristic damping behavior in the evolution of density perturbations on scales smaller than the solar system size. The damping scale is inversely proportional to the square-root of the axion mass. We show that the axion mass smaller than 10^{-24} eV induces a significant damping in the baryonic density power spectrum in cosmologically relevant scales, thus deviating from the cold dark matter in the scale smaller than the axion Jeans scale. With such a small mass, however, our basic assumption about the coherently oscillating scalar field is broken in the early universe. This problem is shared by other dark matter models based on the Bose-Einstein condensate and the ultra-light scalar field. We introduce a simple model to avoid this problem by introducing evolving axion mass in the early universe, and present observational effects of present-day low-mass axion on the baryon density power spectrum, the cosmic microwave background radiation (CMB) temperature power spectrum, and the growth rate of baryon density perturbation. In our low-mass axion model we have a characteristic small-scale cutoff in the baryon density power spectrum below the axion Jeans scale. The small-scale deviations from the cold dark matter model in both matter and CMB power spectra clearly differ from the ones expected in the cold dark matter model mixed with the massive neutrinos as a hot dark matter component.