Recent results from cosmology place sever upper bounds on the sum of neutrino masses, starting to be in conflict with lower bounds from neutrino oscillations. If this neutrino tension persists, it may indicate new physics in cosmology and/or neutrino physics. After a brief review of the present state, I discuss a new physics scenario able to explain this tension. It is based on an extended seesaw model for neutrino mass, including a number of massless neutrinos acting as dark radiation, as well as a keV state as a dark matter candidate. The dark sector is brought into equilibrium with neutrinos between BBN and recombination. This scenario predicts a slight increase in the effective number of neutrino species Neff at recombination, potentially detectable by future CMB missions.
References:
https://arxiv.org/abs/2410.23926
https://arxiv.org/abs/2211.01729
