Quasinormal modes of rotating black holes with ultralight scalar fields

We report the first accurate computation of the leading-order shifts in the quasinormal-mode frequencies of rapidly rotating black holes surrounded by massive scalar fields with nonminimal curvature coupling. For fields with reduced Compton wavelengths no shorter than the black-hole mass scale, the numerical errors in the frequency shifts are typically ${10}^{-8}-{10}^{-5}$ for dimensionless spins between 0 and 0.9, and at most $\mathcal{O}({10}^{-3})$ next to the scalar-mass threshold. Our results open up the possibility of searching for ultralight scalar particles, prime dark-matter candidates, using existing gravitational-wave signals.