Primordial nucleosynthesis: theory and observations

We review the cosmology and physics underlying Primordial Nucleosynthesis a nd survey current observational data in order to compare the predictions of Big Bang Nucleosynthesis with the inferred primordial abundances. From thi s comparison we report on the status of the consistency of the standard hot big bang model, we constrain the universal density of baryons (nucleons), and we set limits to the numbers and/or effective interactions of hypotheti cal new "light" particles (equivalent massless neutrinos). (C) 2000 Elsevie r Science B.V. All rights reserved.