2. Baryogenesis All the relevant physical processes take place essentially in the range from a few MeV ∼ 0 . 1 sec down to 60 − 70 KeV ∼ 10 3 sec. During this period only photons, e ± pairs, and the three neutrino flavors contribute significantly to the energy density. Any additional energy density may be parameterized in terms of the effective number of light neutrino species N$_{ν}$ , so that BBN constraints the number of light neutrino species by N$_{ν}$ ≤ 4 [28, 30]. The four LEP experiments combined give the best fit as N$_{ν}$ = 2 . 994 ± 0 . 12 [319]. The likelihood analysis which includes all the three elements (D , $^{4}$He, and $^{7}$Li ) yields the baryon to photon ratio [320] Despite the uncertainties there appears to be a general concordance between theoretical BBN predictions and observations, which is now being bolstered by the CMB data Ω$_{b}$ h 2 = 0 . 02229 ± 0 . 00073 [258]. The baryon asymmetry of the universe (BAU) parameterized as η$_{B}$ ≡ ( n$_{B}$ − n ¯ $_{B}$) /s ≈ η is determined to be 0 . 9 × 10 − 10 by the recent analysis of WMAP data [13]. As pointed out by Sakharov [321], baryogenesis requires three ingredients: (1) baryon number non- conservation, (2) C and CP violation, and (3) out-of-equilibrium condition. All these three conditions are believed to be met in the very early universe. Baryogenesis during the electroweak phase transition [322] has been studied widely, see [323]. Another mechanism known as Affleck-Dine baryogenesis, which happens due to the non-trivial dy- namics of a light scalar condensate is a natural outcome of inflation [324–326]. It is also possible to convert leptonic asymmetry into baryonic asymmetry, B = a ( B − L ) [327–330], for a review see [331], where a = 28 / 79 in the case of SM and a = 8 / 23 for the MSSM [332]. A lepton asymmetry can be generated from the out-of-equilibrium decay of the right handed (RH) neutrinos into Higgs bosons and light leptons, provided CP − violating phases exist in the neutrino Yukawa couplings. The created lepton asymmetry will be converted into a baryonic asymmetry via sphaleron processes. This scenario works most comfortably 39 $^{g}$∗ = 10 . 75 + 7 4 ( N$_{ν}$ − 3) . (78) 4 . 7 × $_{10}$− 10 < η < 6 . 2 × $_{10}$− $^{10}$, 0 . 017 < Ω$_{b}$ h 2 < 0 . 023 . (79)