BBN has entered the age of precision cosmology. This transition has brought triumph in the spectacular agreement between high-redshift deuterium and the BBN+WMAP prediction, and in the WMAP confirmation of the longstanding BBN prediction of nonbaryonic dark matter. But new precision has raised new questions: the measured primordial 7Li abundance falls persistently and significantly below BBN+WMAP predictions. Moreover, there are controversial hints of a primordial 6Li abundance orders of magnitude above the standard prediction.
As we have seen, disparate explanations for the lithium problem(s) remain viable. Fortunately, most alternatives are testable in the near future.
Astronomical observations. Recent indications of lithium depletion in extremely metal-poor halo stars are tantalizing. In the coming Great Survey era, we may expect many more such stars to be identified, and the lithium trends explored in large statistical samples. These will require careful comparison with theory. Observations of 6Li remain challenging, and as yet it remains unclear what trends exist with metallicity.
Great insight would result from alternative measures of primordial lithium, e.g., in the interstellar medium of metal-poor galaxies nearby or at high redshift.
Nuclear experiments. The enormous effort of the nuclear community has empirically pinned down nearly all nuclear inputs to BBN. Remaining are a few known or proposed resonances which would amplify 7Be destruction. These are within reach of present facilities, so that the nuclear physics of standard BBN can and will be fully tested.
Collider and dark matter experiments. The LHC is operational and much of minimal supersymmetry lies within its reach. The discovery of supersymmetry would revolutionize particle physics and cosmology, and would transform decaying particle BBN scenarios into canonical early universe cosmology. Alternatively, if the LHC fails to find supersymmetry and/or finds surprises of some other kind, this will represent a paradigm shift for all of particle physics and particle cosmology, and BBN will lie at the heart of this transformation.
5. Disclosure Statement
The author is unaware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.
It is a pleasure to thank my collaborators in primordial nucleosynthesis and closely related areas: Nachiketa Chakraborty, Richard Cyburt, John Ellis, Feng Luo, Keith Olive, Tijana Prodanović, Evan Skillman, Vassilis Spanos, and Gary Steigman. I am particularly grateful to Keith Olive for comments on an earlier version of this paper.