Cosmology of self-interacting ultralight dark matter: simultaneous shifts in the Hubble constant and structure growth

Abstract

We study the cosmology of self-interacting ultralight dark matter. The scalar field evolves from an early Hubble-frozen phase to a quartic-dominated radiation-like phase and finally to a quadratic matter-like phase. The intermediate phase increases the pre-recombination expansion rate and reduces the sound horizon, while the late phase allows the same field to contribute to dark matter. We implement the background and linear perturbations of a mixed cold-dark-matter and ultralight-dark-matter cosmology in CLASS. For $m=10^{-22}\,{\rm eV}$ and $\tilde m\simeq1\,{\rm eV}$, increasing the ultralight-dark-matter fraction raises the fixed-$θ_*$ value of $H_0$, lowers the late-time clustering amplitude, strengthens the scale-dependent matter-power suppression, and produces a smooth CMB-temperature residual reaching about $9\%$ for the largest fraction considered. The model therefore generates simultaneous background and growth shifts in the directions relevant to the Hubble and structure-growth tensions. Establishing whether these shifts improve a joint fit requires a likelihood-level observational analysis.