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Big Bang Nucleosynthesis

BBN describes the formation of light elements (hydrogen, helium, lithium) in the early universe, providing a testable prediction of the Big Bang Theory.
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The statement of the theorem

Big Bang Nucleosynthesis (BBN) predicts the primordial mass fraction of light elements. Let η\eta be the baryon-to-photon ratio (η=nb/nγ\eta = n_b / n_{\gamma}). The predicted mass fraction of Helium-4 (YpY_p) and Deuterium (D/HD/H) are functions of η\eta and the number of relativistic species NeffN_{eff}. For a given η\eta, the predicted Helium-4 mass fraction is approximately:\n\nYp≈0.24+0.012ln⁡(η10−10)Y_p \approx 0.24 + 0.012 \ln\left(\frac{\eta}{10^{-10}}\right) \n\nThis relationship provides a quantitative test of the early universe conditions.