In the standard model neutrinos are assumed to have streamed across the Universe since they last scattered when the standard-model plasma temperature was \sim 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 \Lambda CDM parameters cosmological data are compatible with G _ { eff } \lesssim 1 / ( { 56 MeV } ) ^ { 2 } and neutrino free-streaming might be delayed until their temperature has cooled to as low as \sim 25 eV .
Intriguingly , we also find an alternative cosmology compatible with cosmological data in which neutrinos scatter off each other until z \sim 10 ^ { 4 } with a preferred interaction strength in a narrow region around G _ { eff } \simeq 1 / ( { 10 MeV } ) ^ { 2 } \simeq 8.6 \times 10 ^ { 8 } G _ { 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 .