$\star$ Neutrinos

There is the same a diffuse bottom cosmological neutrino which corresponds to the freezing of the interactions proton-neutrons by weak interaction ( $T_\nu = 1.94 K$. It is weaker bus the decoupling of the neutrinos of the remainder of the matter was carried out earlier (a few minutes after the Big-bang). This temperature corresponds to a density of $112 ~ { \rm neutrinos}.cm^{-3}$, that is to say a reduced density:

$$\Omega \nu = 1.15 h^{-2} .10^{-5}$$ (1.26)

where one defines as $H = H_0/100$.