Abstract In this work we investigate the impact of two phenomenological Beyond the Standard Model (BSM) scenarios concerning the role of neutrinos in the early universe: non-standard neutrino interactions (NSI) and non-unitary three-neutrino mixing. We evaluate the impact of these frameworks on two key cosmological observables: the effective number of relativistic neutrino species ( N eff ), related to neutrino decoupling, and the abundances of light elements produced at Big Bang Nucleosynthesis (BBN). For the first time, neutrino CC-NSI with quarks and non-unitary three-neutrino mixing are studied in the context of BBN, and the constraints on such interactions are found to be remarkably restrictive. In particular, the BBN limits are competitive with the ones derived from terrestrial experiments for the non-diagonal CC-NSI parameter ε udV eα , with α ≠ e and for the non-unitarity parameter α 22 . In the case of non-unitarity, the combination between neutrino decoupling and BBN imposes stringent constraints that can either mildly favour the existence of New Physics (NP), or reinforce the SM, depending on the choice of the experimental nuclear rates involved in the BBN calculation. These results stress the already noted need for further nuclear rates measurements in order to obtain more robust BBN theoretical predictions.
Barenboim et al. (2025) studied this question.