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. Using the effective field theory of large-scale structure (LSS), 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) temperature and polarization anisotropy data from Planck, in search for evidence of neutrino self-interactions. In agreement with previous results, we find a bimodal posterior distribution for the effective strength of neutrino self-interaction, showing that a vanishingly small interaction and a relatively strong interaction are both consistent with cosmological data, providing fits of nearly equal quality. We find that strong self-interactions in the neutrino sector can alleviate the $H_0$ tension while maintaining a good fit to the LSS data. Our results may have implications for particle model-building and ongoing neutrino oscillation experiments, and motivate further exploration of particle interactions that can generate a delay in neutrino free-streaming. We discuss sensitivity of the upcoming galaxy surveys to ruling out neutrino self-interaction at the level consistent with the current data.