Within the framework of $\Lambda$CDM, the local determination of the Hubble constant disagrees -- at the 4.4$\sigma$ level -- with that inferred from the very accurate CMB observations by the Planck satellite. This clearly motivates the study of extensions of the standard cosmological model that could reduce such tension. Proposed extensions of $\Lambda$CDM that reduce this so-called Hubble tension require an additional component of the energy density in the Universe to contribute to radiation at a time close to recombination.
In this talk, I will show that pseudo-Goldstone bosons -- associated with the spontaneous breaking of global lepton number in type-I seesaw models -- lead to a non-standard early Universe evolution that can help to reduce the Hubble tension. I will show that current CMB observations can set a lower bound on the scale at which lepton number is broken as high as 1 TeV. Finally, I will argue that future CMB observations will test wide and relevant regions of parameter space of scenarios in which the spontaneous breaking of global lepton number is the mechanism behind the observed neutrino masses.