Ultra-light axions and the kinetic Sunyaev-Zel'dovich Effect

Measurements of secondary cosmic microwave background (CMB) anisotropies, such as the Sunyaev-Zel’dovich (SZ) effect, will enable new tests of neutrino and dark sector properties. The kinetic SZ (kSZ) effect is produced by cosmological flows, probing structure growth. Ultra-light axions (ULAs) are a well-motivated dark-matter candidate. Here the impact of ULA dark matter (with mass ${10}^{-27}\mathrm{e}\mathrm{V}$ to ${10}^{-23}\mathrm{e}\mathrm{V}$) on kSZ observables is determined, applying new analytic expressions for pairwise cluster velocities and Ostriker-Vishniac signatures in structure-suppressing models. For the future CMB-S4 and ongoing DESI galaxy surveys, the kSZ effect (along with primary anisotropies) will probe ULA fractions $\eta_a = \Omega_{\rm{axion}}/\Omega_{\rm DM}$aslowas$\sim 5%$if$m_{a}\simeq 10^{-27}~{\rm eV}$(at95\mathrm{\%}C\mathrm{.}L\mathrm{.}),withsensitivityextendingupto$m_{a}\simeq 10^{-25}~{\rm eV}$. If reionization and the primary CMB can be adequately modeled, Ostriker-Vishniac measurements could probe values ${\eta }_a\simeq {10}^{-3}$ if $10^{-27}~{\rm eV}\lesssim m_{a}\lesssim 10^{-24}~{\rm eV}$,or$\eta_{a}\simeq 1$if$m_{a}\simeq 10^{-22}~{\rm eV}$, within the fuzzy dark matter window.