Limits on neutrino-neutrino scattering in the early Universe

In the standard model neutrinos are assumed to have streamed across the Universe since they last scattered when the standard-model plasma temperature was. The shear stress of free-streaming neutrinos imprints itself gravitationally on the cosmic microwave background (CMB) and makes the CMB a sensitive probe of neutrino scattering. Yet, the presence of nonstandard physics in the neutrino sector may alter this standard chronology and delay neutrino free streaming until a much later epoch. We use observations of the CMB to constrain the strength of neutrino self interactions G₄₅₅ and put limits on new physics in the neutrino sector from the early Universe. Within the context of conventional parameters cosmological data are compatible with G₄₅₅1/ (56 MeV) ^2 and neutrino free streaming might be delayed until their temperature has cooled to as low as 25 eV. Intriguingly, we also find an alternative cosmology compatible with cosmological data in which neutrinos scatter off each other until z10^4 with a preferred interaction strength in a narrow region around G₄₅₅1/ (10 MeV) ^28. 610^8G₅, where G₅ is the Fermi constant. This distinct self-interacting neutrino cosmology is characterized by somewhat lower values of both the scalar spectral index and the amplitude of primordial fluctuations. While we phrase our discussion here in terms of a specific scenario, our constraints on the neutrino visibility function are very general.