As cosmology deals with an abundance of precision data, redundancy will be the key to distinguishing systematic errors from evidence for new physics. BBN and the CBR provide complementary probes of the Universe at two epochs widely separated from each other and from the present. For the standard model assumptions (N = 3, S = 1) the SBBN-inferred baryon density is in excellent agreement with that derived from the CBR (with or without the extra constraints imposed by large scale structure considerations and/or the Lyman alpha forest). For this baryon density (10 6.1, B 0.022), the SBBN-predicted abundances of D and 3He are in excellent agreement with the observational data. For 4He the predicted relic mass fraction is ~ 2 higher than the primordial abundance inferred from current data, hinting at either new physics or the presence of unidentified systematic errors. For 7Li too, the SBBN-predicted abundance is high compared to that derived from very metal-poor stars in the Galaxy. While the tension with 4He can be relieved by invoking new physics in the form of a nonstandard (slower than expected) early-Universe expansion rate, this choice will not reconcile the BBN-predicted and observed abundances of 7Li. When both the baryon density and the expansion rate factor are allowed to be free parameters, BBN (D, 3He, and 4He) and the CBR (WMAP) agree at 95% confidence for 5.5 10 6.8 and 1.65 N 3.03.
The engine powering the transformation of the study of cosmology from its youth to its current maturity has been the wealth of observational data accumulated in recent years. In this data-rich, precision era BBN, one of the pillars of modern cosmology, continues to play a key role. The spectacular agreement between the estimates of the baryon density derived from processes at widely separated epochs has confirmed the general assumptions of the standard models of cosmology and of particle physics. The tension with 4He (and with 7Li) provides a challenge, along with opportunities, to cosmology, to astrophysics, and to particle physics. Whether the resolution of these challenges is observational, theoretical or, a combination of both, the future is bright.
Acknowledgments I am grateful to all my collaborators and I am happy to thank them for their various contributions to the material reviewed here. Many of the quantitative results (and figures) presented here are from recent collaborations or discusions with V. Barger, J. P. Kneller, H.-S. Lee, J. Linsky, D. Marfatia, K. A. Olive, R. J. Scherrer, V. V. Smith, and T. P. Walker. My research is supported at OSU by the DOE through grant DE-FG02-91ER40690. This manuscript was prepared while I was visiting the Instituto Astronomico e Geofísico of the Universidade de Sao Paulo, and I thank them for their hospitality.