Massive neutrino self-interactions with a light mediator in cosmology

Understand

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 $N_{\rm eff}$, they cannot be totally canceled by these last.
• We harness cosmological data that includes Cosmic Microwave Background from Plank 2018, BAO measurements, local $H_0$, Ly-$\alpha$ 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 $g_{\rm eff}< 1.94 \times 10^{-7}$ for only Planck 2018 data and $1.97\times10^{-7}$ when Planck + BAO are considered.