The hubble tension as a hint of leptogenesis and neutrino mass generation

The majoron, a neutrinophilic pseudo-Goldstone boson conventionally arising in the context of neutrino mass models, can damp neutrino free-streaming and inject additional energy density into neutrinos prior to recombination. The combination of these effects for an eV-scale mass majoron has been shown to ameliorate the outstanding H tension, however only if one introduces additional dark radiation at the level of Δ N_eff∼ 0.5. We show here that models of low-scale leptogenesis can naturally source this dark radiation by generating a primordial population of majorons from the decays of GeV-scale sterile neutrinos in the early Universe. Using a posterior predictive distribution conditioned on Planck2018+BAO data, we show that the value of H₀ observed by the SH₀ES collaboration is expected to occur at the level of ∼ 10 % in the primordial majoron cosmology (to be compared with ∼ 0.1 % in the case of Λ CDM). This insight provides an intriguing connection between the neutrino mass mechanism, the baryon asymmetry of the Universe, and the discrepant measurements of H₀.