Cosmological Search for Ultra-Light Axions

Ultralight axions (ULAs) with masses in the range $10^{-33}~{\rm eV}\leq m_{a}\leq 10^{-18}~{\rm eV}$ (motivated by string theory) might contribute to the dark-matter or dark-energy density of the Universe. ULAs would suppress the growth of structure on small scales and change the shape of the cosmic microwave background (CMB) anisotropy power spectra. In this work, we compute cosmological observables over the full ULA mass range and then use them to search for evidence of ULAs using CMB temperature data from the \textit{Planck} satellite, large-scale CMB polarization data from Wilkinson Microwave Anisotropy Probe (WMAP), \and smaller-scale CMB experiments, as well as the WiggleZ galaxy-redshift survey. In the mass range $10^{-32}~{\rm eV}\leq m_{a}\leq 10^{-25.5}~{\rm eV}$, the ULA relic-density must obey the constraint $\Omega_{a}h^{2}\leq 0.006$ at $95\%$-confidence. For $m_{a}\gtrsim 10^{-24}~{\rm eV}$, ULAs are indistinguishable from standard cold dark matter on the length scales probed while for $m_{a}\lesssim 10^{-32}~{\rm eV}$, ULAs are allowed to compose a significant fraction of the dark energy. If primordial gravitational waves are detected, limits to the primordial isocurvature fraction will put severe constraints on ULA dark matter. In the future, weak-lensing measurements of the CMB will yield even more powerful probes of the ULA hypothesis