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, we need to follow photon decoupling at a much more detailed level than in Section 2.4.1. This is the subject of Section 5.1, where we overview the main features of CMB physics, of cosmological perturbation equations, the different contributions to the spectrum of CMB temperature anisotropies, and the effect of each cosmological parameter on the CMB spectrum. Neutrinos will appear on stage in Section 5.2, where we focus on the evolution of their perturbations until photon decoupling, and in Section 5.3, where we infer the effect of neutrino abundance, masses and properties on CMB anisotropies. Finally, Section 5.4 is a brief summary of recent constraints on neutrino properties, exploiting CMB data alone.