Ultra-light axions and the S8 tension: joint constraints from the cosmic microwave background and galaxy clustering

We search for ultra-light axions as dark matter (DM) and dark energy particle candidates, for axion masses 10-32 eV < ma < 10-24 eV, by a joint analysis of cosmic microwave background (CMB) and galaxy clustering data - and consider if axions can resolve the tension in inferred values of the matter clustering parameter S8. We give legacy constraints from Planck 2018 CMB data, improving 2015 limits on the axion density Stah2 by up to a factor of three; CMB data from the Atacama Cosmology Telescope and the South Pole Telescope marginally weaken Planck bounds at ma = 10-25 eV, owing to lower (and theoretically-consistent) gravitational lensing signals. We jointly infer, from Planck CMB and full-shape galaxy power spectrum and bispectrum data from the Baryon Oscillation Spectroscopic Survey (BOSS), that axions are, today, < 10% of the DM for ma < 10-26 eV and < 1% for 10-30 eV < ma < 10-28 eV. BOSS data strengthen limits, in particular at higher ma by probing high-wavenumber modes (k < 0.4h Mpc-1). BOSS alone finds a preference for axions at 2.7u, for ma = 10-26 eV, but Planck disfavours this result. Nonetheless, axions in a window 10-28 eV < ma < 10-25 eV can improve consistency between CMB and galaxy clustering data, e.g., reducing the S8 discrepancy from 2.7a to 1.6u, since these axions suppress structure growth at the 8h-1 Mpc scales to which S8 is sensitive. We expect improved constraints with upcoming high-resolution CMB and galaxy lensing and future galaxy clustering data, where we will further assess if axions can restore cosmic concordance.