Probing Self-Interacting Neutrinos with Cosmological data

Several analyses have revealed that some cosmic microwave background (CMB) data allows a cosmological scenario in which neutrinos strongly interact among themselves, delaying their free-streaming until close to the epoch of matter-radiation equality. While such results could hint at physics beyond the standard paradigm in the early Universe, in the absence of further analyses, this preference for strongly self-interacting neutrinos could arise from an accidental feature in the CMB sky. To clarify this situation, we recently investigated, in a CMB-independent way, the compatibility of the self-interacting neutrinos with the Full Shape of the galaxy power spectrum. Our analyses revealed that the large-scale distribution of galaxies also allows for a cosmological scenario in which neutrinos strongly interact among themselves. We further investigate this non-trivial agreement between the large-scale structure (LSS) and CMB data by performing a joint analysis of both data sets. We find that the simplest cosmological representation of self-interacting neutrinos is largely disfavoured, yet not ruled out, by the combination of CMB and LSS data. These results motivate the exploration of particle models capable of reconciling all CMB, LSS, and laboratory data. We finally discuss that more complex representations of interactions in the neutrino sector might demand us to sharply improve our numerical tools if we aim to unveil physics beyond the standard paradigm.