Massive neutrino self-interactions with a light mediator in cosmology

Nonstandard self-interactions can alter the evolution of cosmological neutrinos, mainly by damping free streaming, which should leave traces in cosmological observables. Although overall effects are opposite to those produced by neutrino mass and a larger Neff, they cannot be totally canceled by these last. We harness cosmological data that includes cosmic microwave background from Planck 2018, BAO measurements, local H0, Ly-??, and SNIa, to constrain massive neutrino self-interactions with a very light scalar mediator. We find that the effective coupling constant, at the 95% C.L., should be geff < 1.94 x 10???7 for only Planck 2018 data and 1.97 x 10???7 when Planck + BAO are considered. This bound relaxes to 2.27 x 10???7 (2.3 x 10???7) for H0 (H0 + SNe + Ly-??) data. Using the Planck + BAO dataset, the H0 tension lowers from 4.3?? (for ??CDM) to 3.2??. The Akaike information criterion penalizes the self-interacting model due to its larger parameter space for Planck or Planck + BAO data, but favors the interacting model when we use local H0 measurements. A somewhat larger value for H0 is preferred when we include the whole data pool, which comes accompanied with a larger value of Neff and a more constricted bound on ??m??.