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# Big-Bang Nucleosynthesis

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### Abstract

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, $$\eta$$, corresponding to the inferred primordial abundances of deuterium and helium-4 is consistent with the independent determination of $$\eta$$ from 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 $$\eta = (5.7-6.7)\times 10^{-10}$$ at 95% CL (corresponding to a cosmological baryon density $$\Omega_B h^2 = 0.021 - 0.025$$). The D and He-4 abundances, when combined with the CMB determination of $$\eta$$, provide the bound $$N_\nu=3.28 \pm 0.28$$ on the effective number of neutrino species. Other constraints on new physics are discussed briefly.

### Author and article information

###### Journal
03 December 2014
###### Article
10.1088/1674-1137/38/9/090001
1412.1408