Résumé
The baryon density of our Universe, Omega(b)h(2) = 0.0224 +/- 0.0009, as inferred from the WMAP first year of observations is used to predict the primordial abundances of light elements produced during Big Bang Nucleosynthesis (BBN). Such a baryon density, and a gravitation described by General Relativity (GR), lead to predictions for the abundances of He-4 and D in very good agreement with the observed ones; but they lead to a significant discrepancy between the calculated and the observed Li-7 abundances. Supposing that the standard non gravitational sector is not modified, we consider scalar-tensor theories of gravity, and study their impact on the Li-7 abundance. It is shown that solving the lithium problem requires a non trivial behaviour of the expansion rate of the Universe at the epoch of BBN, in order not to violate the constraints from He-4 and D.