Cosmological constraints on neutrino self-interactions with a light mediator

If active neutrinos undergo nonstandard (``secret'') interactions (NSνIs), the cosmological evolution of the neutrino fluid might be altered, leaving an imprint in cosmological observables. We use the latest publicly available CMB data from Planck to constrain NSνIs inducing ν-ν scattering, under the assumption that the mediator φ of the secret interaction is very light. We find that the effective coupling constant of the interaction, geff⁴≡⟨σv⟩T_ν², is constrained at <2.35×10^-27 (95% Credible Interval), which strengthens to geff⁴<1.64×10^-27 when Planck nonbaseline small-scale polarization is considered. Our findings imply that after decoupling at T1 MeV, cosmic neutrinos are free-streaming at redshifts $z>3800$, or $z>2300$ if small-scale polarization is included. These bounds are only marginally improved when data from geometrical expansion probes are included in the analysis to complement Planck. We also find that the tensions between CMB and low-redshift measurements of the expansion rate H₀ and the amplitude of matter fluctuations σ₈ are not significantly reduced. Our results are independent of the underlying particle physics model as long as φ is very light. Considering a model with Majorana neutrinos and a pseudoscalar mediator, we find that the coupling constant g of the secret interaction is constrained at 7×10^-7. By further assuming that the pseudoscalar interaction comes from a dynamical realization of the seesaw mechanism, as in Majoron models, we can bound the scale of lepton number breaking v_σ as (1.4×10⁶)m_ν.