Thermal Friction as a Solution to the Hubble Tension

Abstract

A new component added to the standard model of cosmology that behaves like a cosmological constant at early times and then dilutes away as radiation or faster can resolve the Hubble tension. We show that a rolling axion coupled to a non-Abelian gauge group exhibits the behavior of such an extra component at the background level and can present a natural particle-physics model solution to the Hubble tension. We compare the contribution of this bottom-up model to the phenomenological fluid approximation and determine that CMB observables sensitive only to the background evolution of the Universe are expected to be similar in both cases, strengthening the case for this model to provide a viable solution to the Hubble tension.

Comments: 5 pages, 2 figures. v2: We added clarifications, additional references, and condensed the paper to fit journal guidelines. v2 matches the published version