Limits on Neutrino-Neutrino Scattering in the Early Universe

Abstract

In the standard model neutrinos are assumed to have streamed across the Universe since they last scattered when the standard-model plasma temperature was ∼MeV. 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 G_(eff) and put limits on new physics in the neutrino sector from the early Universe. Within the context of conventional Λ CDM parameters cosmological data are compatible with G_(eff)≲1/(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 z∼10^4 with a preferred interaction strength in a narrow region around G_(eff)≃1/(10  MeV)^2≃8.6×10^8G_F, where G_F 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.