Résumé
Combined with other cosmic microwave background experiments, the Wilkinson Microwave Anisotropy Probe (WMAP) survey provides an accurate estimate of the baryon density of the universe. In the framework of the standard big bang nucleosynthesis (BBN), such a baryon density leads to predictions for the primordial abundances of 4He and D in good agreement with observations. However, it also leads to a significant discrepancy between the predicted and observed primordial abundance of 7Li. Such a discrepancy is often termed ``the lithium problem.'' In this paper we analyze this problem in the framework of scalar tensor theories of gravity. It is shown that an expansion of the universe slightly slower than in general relativity before BBN, but faster during BBN, solves the lithium problem and leads to 4He and D primordial abundances consistent with the observational constraints. This kind of behavior is obtained in numerous scalar tensor models, both with and without a self-interaction potential for the scalar field. In models with a self-interacting scalar field, the convergence toward general relativity is ensured without any condition, thanks to an attraction mechanism that starts to work during the radiation-dominated epoch.