As observations reveal, the present universe is filled with radiation and is expanding. According to the standard, hot big bang cosmological model the early universe was hot and dense and, during the first few minutes in its evolution, was a primordial nuclear reactor, synthesizing in significant abundances the light nuclides D, 3He, 4He, and 7Li. These relics from the Big Bang open a window on the early evolution of the universe and provide probes of the standard models of cosmology and of particle physics. Since the BBN-predicted abundances depend on the competition between the early universe expansion rate and the weak- and nuclear-interaction rates, they can be used to test the standard models as well as to constrain the universal abundances of baryons and neutrinos. This enterprise engages astronomers, astrophysicists, cosmologists, and particle physicists alike. A wealth of new observational data has reinvigorated this subject and stimulated much recent activity. Much has been learned, revealing many new avenues to be explored (a key message for the students at this school - and for young researchers everywhere). The current high level of activity ensures that many of the detailed, quantitative results presented in these lectures will need to be revised in the light of new data, new analyses of extant data, and new theoretical ideas. Nonetheless, the underlying physics and the approaches to confronting the theoretical predictions with the observational data presented in these lectures should provide a firm foundation for future progress.
Within the context of the standard models of cosmology and of particle physics (SBBN) the relic abundances of the light nuclides depend on only one free parameter, the baryon-to-photon ratio (or, equivalently, the present-universe baryon density parameter). With one adjustable parameter and three relic abundances (four if 3He is included), SBBN is an overdetermined theory, potentially falsifiable. The current status of the comparison between predictions and observations reviewed here illuminates the brilliant success of the standard models. Among the relic light nuclides, deuterium is the baryometer of choice. For N = 3, the SBBN-predicted deuterium abundance agrees with the primordial-D abundance derived from the current observational data for 10 = 5.6+0.6-1.2 (B = 0.020+0.002-0.004). This baryon abundance, from the first 20 minutes of the evolution of the universe, is in excellent agreement with independent determinations from the CMB (~ few hundred thousand years) and in the present universe (~ 10 Gyr).
It is premature, however, to draw the conclusion that the present status of the comparison between theory and data closes the door on further interesting theoretical and/or observational work. As discussed in these lectures, there is some tension between the SBBN-predicted abundances and the relic abundances derived from the observational data. For the deuterium-inferred SBBN baryon density, the expected relic abundances of 4He and 7Li are somewhat higher than those derived from current data. The "problems" may lie with the data (large enough data sets? underestimated errors?) or, with the path from the data to the relic abundances (systematic errors? evolutionary corrections?). For example, has an overlooked correction to the HII region-derived 4He abundances resulted in a value of YP which is systematically too small (e.g. underlying stellar absorption)? Are there systematic errors in the absolute level of the lithium abundance on the Spite Plateau or, has the correction for depletion/dilution been underestimated? In these lectures the possibility that the fault may lie with the cosmology was also explored. In one simple extension of SBBN, the early universe expansion rate is allowed to differ from that in the standard model. It was noted that to reconcile D, and 4He would require a slower than standard expansion rate, difficult to reconcile with simple particle physics extensions beyond the standard model. Furthermore, if this should be the resolution of the tension between D and 4He, it would exacerbate that between the predicted and observed lithium abundances. The three abundances could be reconciled in a further extension involving neutrino degeneracy (an asymmetry between electron neutrinos and their antiparticles). But, three adjustable parameters to account for three relic abundances is far from satisfying. Clearly, this active and exciting area of current research still has some surprises in store, waiting to be discovered by astronomers, astrophysicists, cosmologists and particle physicists. The message to the students at this school - and those everywhere - is that much interesting observational and theoretical work remains to be done. I therefore conclude these lectures with a personal list of questions I would like to see addressed.
Where (at what value of D/H) is the primordial deuterium plateau, and what is(are) the reason(s) for the currently observed spread among the high-z, low-Z QSOALS D-abundances?
Are there stellar observations which could offer complementary insights to those from HII regions on the question of the primordial 4He abundance, perhaps revealing unidentified or unquantified systematic errors in the latter approach? Is YP closer to 0.24 or 0.25?
What is the level of the Spite Plateau lithium abundance? Which observations can pin down the systematic corrections due to model stellar atmospheres and temperature scales and which may reveal evidence for, and quantify, early-Galaxy production as well as stellar depletion/destruction?
If further observational and associated theoretical work should confirm the current tension among the SBBN-predicted and observed primordial abundances of D, 4He, 7Li, what physics beyond the standard models of cosmology and particle physics has the potential to resolve the apparent conflicts? Are those models which modify the early, radiation-dominated universe expansion rate consistent with observations of the CMB temperature fluctuation spectrum? If neutrino degeneracy is invoked, is it consistent with the neutrino properties (masses and mixing angles) inferred from laboratory experiments as well as the solar and cosmic ray neutrino oscallation data?
To paraphrase Spock, work long and prosper!
It is with heartfelt sincerity that I thank the organizers (and their helpful, friendly, and efficient staff) for all their assistance and hospitality. Their thoughtful planning and cheerful attention to detail ensured the success of this school. It is with much fondness that I recall the many fruitful interactions with the students and with my fellow lecturers and I thank them too. I would be remiss should I fail to acknowledge the collaborations with my OSU colleagues, students, and postdocs whose work has contributed to the material presented in these lectures. The DOE is gratefully acknowledged for support under grant DE-FG02-91ER-40690.