Ultra-light axions and the S ₈ tension: joint constraints from the cosmic microwave background and galaxy clustering

Abstract We search for ultra-light axions as dark matter (DM) and dark energy particle candidates, for axion masses 10 -32 eV ≤ m a ≤ 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 S 8 . We give legacy constraints from Planck 2018 CMB data, improving 2015 limits on the axion density Ω a h 2 by up to a factor of three; CMB data from the Atacama Cosmology Telescope and the South Pole Telescope marginally weaken Planck bounds at m a = 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 m a ≤ 10 -26 eV and < 1% for 10 -30 eV ≤ m a ≤ 10 -28 eV. BOSS data strengthen limits, in particular at higher m a by probing high-wavenumber modes ( k < 0.4 h Mpc -1 ). BOSS alone finds a preference for axions at 2.7 σ , for m a = 10 -26 eV, but Planck disfavours this result. Nonetheless, axions in a window 10 -28 eV ≤ m a ≤ 10 -25 eV can improve consistency between CMB and galaxy clustering data, e.g., reducing the S 8 discrepancy from 2.7 σ to 1.6 σ , since these axions suppress structure growth at the 8 h -1 Mpc scales to which S 8 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.