The Hubble Tension as a Hint of Leptogenesis and Neutrino Mass Generation

Understand

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 $\Delta N_{\rm eff} \sim 0.5$.
• 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 $\sim 10\%$ in the primordial majoron cosmology (to be compared with $\sim 0.1\%$ in the case of $\Lambda$CDM).