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 $H_0$ tension, however only if one introduces additional dark radiation at the level of $\mathrm{\Delta }{N}_{\mathrm{e}\mathrm{f}\mathrm{f}}\sim 0.5$. 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 $H_0$ observed by the SH${}_0$ES collaboration is expected to occur at the level of $\sim 10\mathrm{\%}$ in the primordial majoron cosmology (to be compared with $\sim 0.1%$inthecaseof$\Lambda$CDM). This insight provides an intriguing connection between the neutrino mass mechanism, the baryon asymmetry of the Universe, and the discrepant measurements of $H_0$.