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Structure formation with strongly interacting neutrinos—implications for the cosmological neutrino mass bound

We investigate a model where neutrinos are strongly coupled to a new, light scalar field. In this model neutrinos annihilate as soon as they become non-relativistic in the early universe, and a non-zero neutrino mass has a marginal effect on the matter power spectrum. However, the angular power spectrum of the cosmic microwave background (CMB) is changed significantly because the strongly interacting fluid of neutrinos and scalars does not experience any anisotropic stress. Such models are strongly disfavoured by current observations. Interestingly, this leads to the conclusion that the relativistic energy density around the epoch of recombination must be in the form of very weakly interacting particles. This conclusion is independent of the specific interaction model.