A search for ultra-light axions using precision cosmological data
  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.
