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Abstract:

Big Bang nucleosynthesis (BBN) contributes the bulk to D, and good fractions to the 3He, 4He, and 7Li observed in the present Universe. It is examined, if BBN may have proceeded in its standard, simplest version, or if current observations may force us to have to amend standard BBN. In this context, the accurate observational determination of primordial light-element abundances is crucial. Systematic uncertainties of such determinations are discussed. Particular emphasis is given to the isotope of 7Li, the discrepancy between observationally inferred and within standard BBN predicted 7Li/H, and our incomplete understanding of the amount of stellar 7Li depletion. The connection to the 6Li isotope is established and new surprising 6Li/7Li observations in low-metallicity stars are presented. This new data may present a second Li-problem. BBN non-standard scenarios which may solve one, or both, of these Li problems are identified. It is discussed which future observations may either confirm standard BBN, or drive us to have to amend standard BBN.