Hot axions and the H0 tension

Scattering and decay processes of thermal bath particles involving heavy leptons can dump hot axions in the primordial plasma around the QCD phase transition. We compute their relic density, parameterized by an effective number ∆Neff of additional neutrinos. For couplings allowed by current bounds, production via scattering yields ∆Neff . 0.6 and ∆Neff . 0.2 for the cases of muon and tau, respectively. Flavor violating tau decays to a lighter lepton plus an axion give ∆Neff . 0.3. Such values of ∆Neff can alleviate the tension between the direct local measurement of the Hubble constant H0 and the inferred value from observations of the Cosmic Microwave Background, assuming ΛCDM. We analyze present cosmological data from the Planck collaboration and baryon acoustic oscillations with priors given in terms of the axion-lepton couplings. For axions coupled to muons, the tension can be alleviated below the 3σ level. Future experiments will measure ∆Neff with higher precision, providing an axion discovery channel and probing the role of hot axions in the H0 tension.