Big-Bang nucleosynthesis (Particle Data Group mini-review)

A critical review is given of the current status of cosmological nucleosynthesis. In the framework of the Standard Model with 3 types of relativistic neutrinos, the baryon-to-photon ratio, $η$, corresponding to the inferred primordial abundances of deuterium and helium-4 is consistent with the independent determination of $η$ from WMAP observations of anisotropies in the cosmic microwave background. However the primordial abundance of lithium-7 inferred from observations is significantly below its expected value. Taking systematic uncertainties in the abundance estimates into account, there is overall concordance in the range $η= (4.7 - 6.5) x 10^{-10}$ @ 95% c.l. (corresponding to a cosmological baryon density $Ω_B h^2$ = 0.017 - 0.024). The D and He-4 abundances, together with the CMB determination of $η$, provide the bound $N_ν= 3.24 \pm 1.2$ @ 95% c.l. on the effective number of neutrino species. Other constraints on new physics are discussed briefly.

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