Updated constraints on non-standard neutrino interactions from Planck

We provide updated bounds on non-standard neutrino interactions based on data from the Planck satellite as well as auxiliary cosmological measurements. Two types of models are studied - A Fermi-like 4-point interaction and an interaction mediated by a light pseudoscalar - and we show that these two models are representative of models in which neutrinos either decouple or recouple in the early Universe. Current cosmological data constrain the effective 4-point coupling to be GX ≤ (0.1 \, GeV)⁻², corresponding to GX ≤ 1.0 × 10⁷ GF. For non-standard pseudoscalar interactions we set a limit on the diagonal elements of the dimensionless coupling matrix, gᵢⱼ, of gᵢᵢ ≤ 1.2 × 10⁻⁷. For the off-diagonal elements which induce neutrino decay the bound is significantly stronger, corresponding to gᵢⱼ ≤ 2.4 × 10⁻¹¹(m/0.05 \, eV)⁻², or a lifetime constraint of τ ≥ 0.8 × 10¹⁰ \, s \, (m/0.05 \, eV)³ \,. This is currently the strongest known bound on this particular type of neutrino decay. We finally note that extremely strong neutrino self-interactions which completely suppress anisotropic stress are very highly disfavoured by current data (Δ χ² ~ 10⁴).