Nucleosynthesis in the Early Universe and the test of Big Bang Model

标准大爆炸宇宙学为我们提供了一幅简单而自然的宇宙图景，因为它解释了宇宙微波背景的起源以及^4He、D、^3He和^7Li等轻元素的丰度。标准大爆炸核合成(SBBN)只有一个自由参数，即重子与光子数比Eta=n_B/n_t；将SBBN的预测与从观测数据推断的轻元素丰度进行比较，我们得到了宇宙的重子密度omega_b&lt；近似等于&gt；0.01。然而，最近有报道称，大爆炸核合成理论与观测结果之间存在差异(BBN危机)。我们发现，如果一个电子中微子由于轻子不对称而具有净化学势，BBN的预测与从观测数据中推断的轻元素的原始丰度^4He、D、^3He和^7Li是一致的。我们估计xi_&lt；nue&gt；=0.043^&lt；+0.040&gt；_&lt；040&gt；(95%C.L.)和omega_bh^2=0.015^&lt；+0.006；003&gt；(95%C.L.)。我们还详细地研究了非均匀宇宙模型中的核合成，证实了轻元素观测对重子与光子数比的约束基本上与标准模型相同，因为Li的观测给了非均匀宇宙模型密度对比度的严格限制。

Standard big bang cosmology gives us a simple and natural picture of our universe since it explains the origin of cosmic microwave background and abundances of light elements, ^4He, D,^3He and ^7Li. The standard big bang nucleosynthesis (SBBN) has a single free parameter, i.e. baryon to photon number ratio eta=n_B/n_<gamma> where n_B and n_<gamma> are the number densities of baryons and photons. Comparing SBBN predictions with abundances of light elements inferred from observational data, we obtain the baryon density of the universe, OMEGA_b<approximately equal>0.01.Recently, however, it is reported that there is the discrepancy between big bang nucleosynthesis theory and observations (BBN crisis). We show that BBN predictions agree with the primordial abundances of light elements, ^4He, D,^3He and ^7Li inferred from the observational data if an electron neutrino has a net chemical potential xi_<nue> due to lepton asymmetry. We estimated that xi_<nue>=0.043^<+0.040>_<040> (95% C.L.) and OMEGA_bh^2=0.015^<+0.006>_<003> (95% C.L.). We also investigated in detail nucleosynthesis in inhomogeneous universe model, and confirmed that essentially the constraint on the baryon to photon number ratio from light element observations is the same as that of standard model, since the observation of Li gives stringent limit to the density contrast of the inhomogeneity.