  In the standard model neutrinos are assumed to have streamed across the
Universe since they last scattered at the weak decoupling epoch when the
temperature of the standard-model plasma 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. Recent measurements of the CMB at large
multipoles made by the Planck satellite and high-l experiments are critical for
probing this physics. Within the context of conventional LambdaCDM 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 $G_{\rm eff} \simeq
1/({\rm 10 \, MeV})^{2} \simeq 8.6\times10^8 G_{\rm F}$, where $G_{\rm F}$ 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 in which a late onset of neutrino
free-streaming could occur, our constraints on the neutrino visibility function
are very general.
