The standard Big Bang Nucleosynthesis SBBN

The agreement between the predictions of the SBBN model for the abundances of deuterium, helium-4, helium-3 and lithium-7 and the observations of the primordial abundances of these light elements is one of the successes and therefore one of the cornerstones of the Big Bang Cosmology. 

The model has only one parameter, the baryon-to-photon ratio, r/. These abundances, which depend only on t], span some nine orders of magnitude. The synthesis of these light elements strongly depends on the physical conditions of the early Universe, at T about 1 MeV, when t is around 1 second. 

let us only remark that for a precise description of the state of the thermal equilibrium and of the conditions of decoupling of the various particles of the early Universe see Signore Puy (1999) and more recently Melchiorri et al. (2003). 

When T 1 MeV, neutrinos, electrons, positrons were still in equilibrium through the (n-p) reactions  

The ratio of the neutron and proton densities is given approximately by the Saha relation  

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