Neutrinos in the cosmic microwave background epoch
The statistical properties of CMB temperature and polarization anisotropy maps encode very precise information on the history and composition of our universe. They depend primarily on the behaviour of inhomogeneities in the photon and baryon medium until photon decoupling, which feels all other species in two ways: through their impact on the cosmological background evolution, and via their contribution to the local gravitational forces. This is why neutrinos play an indirect yet important role in the physics of CMB anisotropies, and why present (and future) data on these observables give us quite remarkable pieces of information on neutrino properties.
To understand this point quantitatively, ...