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 $10^{-32}\mathrm{eV}\le m_{\mathrm{a}}\le 10^{-24}\mathrm{eV}$, 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 ${\Omega }_{\mathrm{a}}{h}^2$ by up to a factor of three; CMB data from the Atacama Cosmology Telescope and the South Pole Telescope marginally weaken Planck bounds at $m_{\mathrm{a}}=10^{-25}\mathrm{eV}$, 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 $m_{\mathrm{a}}\le 10^{-26}\mathrm{eV}$ and $<1\%$ for $10^{-30}\mathrm{eV}\le m_{\mathrm{a}}\le 10^{-28}\mathrm{eV}$. BOSS data strengthen limits, in particular at higher $m_{\mathrm{a}}$ by probing high-wavenumber modes ($k<0.4h{\mathrm{Mpc}}^{-1}$). BOSS alone finds a preference for axions at $2.7\sigma$, for $m_{\mathrm{a}}=10^{-26}\mathrm{eV}$, but Planck disfavours this result. Nonetheless, axions in a window $10^{-28}\mathrm{eV}\le m_{\mathrm{a}}\le 10^{-25}\mathrm{eV}$ can improve consistency between CMB and galaxy clustering data, e.g., reducing the $S_8$ discrepancy from $2.7\sigma$ to $1.6\sigma$, since these axions suppress structure growth at the $8h^{-1}\mathrm{Mpc}$ 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.

Comments: 53 pages, 22 figures. Minor changes to match version accepted for publication in JCAP