Primordial nucleosynthesis: A critical comparison of theory and observation

Primordial nucleosynthesis is reexamined in the context of a detailed comparison of theory and observation. A new argument is presented to show how the observed abundances of D and 3He can be used to derive a lower bound to the nucleon density. In concert with the previously known upper bound from D alone, we define a conservative (''safe bet'') range for the nucleon-to-photon ratio: eta = (3-10) x 10-10. New observations of 7Li are consistent with the abundances of D and 3He and help us sto define a reasonable (''best bet'') range: eta = (4-7) x 10-10. In either of these ranges the predicted and observed abundances of D, 3He, and 7Li re all in concordance. The upper bounds correspond to Ω/sub N/< or =0.14-0.19, and we conclude that nucleons fail to close the universe by at least a factor of 5-7. We review the recent observational data on 4He and conclude that there is complete consistency between the predicted abundance of 4He and those of the other light elements. In particular, for the standard model (N/sub v/ = 3, 10.4< or =tau/sub 1/2/< or =10.8 minutes) we find that Y/sub p/< or =0.25 for < or approx. =7 x 10-10. We predict a lower bound to the 4He mass fraction of Y/sub p/> or =0.24 if N/sub v/> or =3 and tau/sub 1/2/> or =10.4 minutes