Cosmo MSW effect for mass varying neutrinos

We consider neutrinos with varying masses which arise in scenarios relating neutrino masses to the dark energy density in the universe. We point out that the neutrino mass variation can lead to level crossing and thus a cosmo MSW effect, having dramatic consequences for the flavor ratio of astrophysical neutrinos and the composition of the relic neutrino background. Two of the most mysterious puzzles in particle physics and cosmology concern the mechanism generating small neutrino masses and the origin of the dark energy in the universe. There exists, however, an amazing coincidence between the order of magnitude of neutrino masses, mν < 1 eV and the vacuum density being responsible for the dark energy, ρV ≈ (10 −3 eV). In ref. [1] this coincidence was explained by generating sterile neutrino masses of order 10 eV via interactions with a scalar field, the “acceleron”. Recently a more elaborated scenario [2] was published, where the sterile neutrino mass variation is transmitted to the active sector in a seesaw framework, when integrating out the singlets under the standard model gauge group. This scenario implies the sterile neutrino masses to decrease with the cosmic evolution, while the active neutrino masses vary like the inverse neutrino density, mν(z) = (1 + z) mν0 → (1 + z) −3mν0. (1) Here z is the cosmological redshift, w ≈ −1 defines the equation of state of the dark energy, and the neutrinos are assumed to propagate in a non-relativistic background. Several effects to test this hypothesis have been discussed in [2], including observation of sterile neutrino states being in conflict with big bang nucleosynthesis, conflicts of terrestrial and astronomical neutrino mass measurements, as well as the predicted relation of the dark