A search for ultra-light axions using precision cosmological data

TL;DR

This paper investigates ultra-light axions across a broad mass range using cosmological data, constraining their contribution to dark matter and dark energy, and finding they are consistent with current observations within certain mass and density limits.

Contribution

It provides the first comprehensive analysis of ULAs over the full mass range using multiple cosmological datasets, setting new constraints on their relic density and role in cosmic evolution.

Findings

ULAs with certain masses are constrained to be a small fraction of dark matter.

ULAs with very low masses can still significantly contribute to dark energy.

Data are consistent with ULAs being negligible or a major component depending on mass.

Abstract

Ultra-light axions (ULAs) with masses in the range 10^{-33} eV <m <10^{-20} eV are motivated by string theory and might contribute to either the dark-matter or dark-energy density of the Universe. ULAs could suppress the growth of structure on small scales, or lead to an enhanced integrated Sachs-Wolfe effect on large-scale cosmic microwave-background (CMB) anisotropies. In this work, cosmological observables over the full ULA mass range are computed, and then used to search for evidence of ULAs using CMB data from the Wilkinson Microwave Anisotropy Probe (WMAP), Planck satellite, Atacama Cosmology Telescope, and South Pole Telescope, as well as galaxy clustering data from the WiggleZ galaxy-redshift survey. In the mass range 10^{-32} eV < m <10^{-25.5} eV, the axion relic-density \Omega_{a} (relative to the total dark-matter relic density \Omega_{d}) must obey the constraints \Omega_{a}/\Omega_{d} < 0.05 and \Omega_{a}h^{2} < 0.006 at 95%-confidence. For m> 10^{-24} eV, ULAs are indistinguishable from standard cold dark matter on the length scales probed, and are thus allowed by these data. For m < 10^{-32} eV, ULAs are allowed to compose a significant fraction of the dark energy.