A search for ultralight axions using precision cosmological data

Abstract

Ultralight axions (ULAs) with masses in the range 1 0^-33 eV ≤m_a≤1 0^-20 eV are motivated by string theory and might contribute to either the dark-matter or dark-energy densities of the Universe. ULAs could suppress the growth of structure on small scales, lead to an altered integrated Sachs-Wolfe effect on cosmic microwave-background (CMB) anisotropies, and change the angular scale of the CMB acoustic peaks. 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 1 0^-32 eV ≤m_a≤1 0^-25.5 eV , the axion relic-density Ω_a (relative to the total dark-matter relic density Ω_d) must obey the constraints Ω_a/Ω_d≤0.05 and Ω_ah²≤0.006 at 95% confidence. For m_a≳1 0^-24 eV , ULAs are indistinguishable from standard cold dark matter on the length scales probed, and are thus allowed by these data. For m_a≲1 0^-32 eV , ULAs are allowed to compose a significant fraction of the dark energy.