Ultralight 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. Ultralight axions (ULAs) are a well-motivated dark-matter candidate. Here, the impact of ULA dark matter (with mass 10^-27 to 10^-23 eV) 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 Stage 4 and ongoing Dark Energy Spectroscopic Instrument galaxy surveys, the kSZ effect (along with primary anisotropies) will probe ULA fractions ηₐ=Ωₐₓᵢₒₙ/ΩDM as low as ~5% if mₐ10^-27 eV (at 95% C.L.), with sensitivity extending up to mₐ10^-25 eV. If reionization and the primary CMB can be adequately modeled, Ostriker-Vishniac measurements could probe values ηₐ10^-3 if 10^-27 eVmₐ10^-24 eV, or ηₐ1 if mₐ10^-22 eV, within the fuzzy dark matter window.