Self-Interacting Neutrinos in Light of Large-Scale Structure Data

We explore a self-interacting neutrino cosmology in which neutrinos experience a delayed onset of free-streaming. We use the effective field theory of large-scale structure (LSS) to model matter distribution on mildly non-linear scales within the self-interacting neutrino cosmology for the first time. We perform the first combined likelihood analysis of BOSS full-shape galaxy clustering, weak lensing, and Lyman-\(\alpha\) forest measurements, together with the cosmic microwave background (CMB) data from Planck. We find that the full data set strongly favors presence of a flavor-universal neutrino self-interaction, with a characteristic energy scale of order \(10\) MeV. The preference is at the \(\sim 5\sigma\) level and is primarily driven by the Lyman-\(\alpha\) forest measurements and, to a lesser extent, the weak lensing data from DES. The self-interacting neutrino model eases both the Hubble tension and the \(S_8\) tension between different cosmological data sets, but it does not resolve either. Finally, we note a preference for a non-zero sum of neutrino masses at the level of \(\sim 0.3\) eV under this model, consistent with previous bounds. These results call for further investigation in several directions, and may have significant implications for neutrino physics and for future new-physics searches with galaxy surveys.