Session Y5: Nucleosynthesis of the Lightest Nuclei

What's the matter with lithium?

Big-bang nucleosynthesis (BBN) has long provided a boundary between the possible and the impossible in particle physics and cosmology. Now that there are often stronger constraints from the laboratory and from the cosmic microwave background, BBN provides an important cross-check, a window into times long before recombination, and a set of initial conditions for the nuclear history of universe. However, some nagging issues remain for BBN, and probably none are as vexing as the Li/H ratios in low-metallicity turnoff stars of the Galactic halo. Since the early 1980's, the remarkably consistent Li/H ratios among these stars have been seen as evidence of the primordial Li/H, and the lack of star-to-star scatter has been matched by a lack of publication-to-publication scatter over time. The problem with this very consistent picture is that the measured Li/H is lower than the prediction of BBN by about a factor of 3, presenting us with a quandary: depletion of lithium after BBN by such a large factor without introducing scatter seems possible but not certain; stellar model atmospheres do not seem uncertain enough to provide a factor of 3; the uncertainties on the nuclear cross sections involved in BBN cannot accommodate a factor of 3; a few solutions with ``exotic'' early-universe physics beyond the standard model exist, but have weak motivation beyond BBN. An additional puzzle has cropped up in the last few years, as previous hints of a relatively constant $^6$Li/H ratio in halo stars have developed into a substantial data set. This is not predicted at all by standard BBN. I will review these developments, the several ways one might try to explain them, and the difficulties involved in establishing these explanations without contradicting known facts. Work supported by DOE contract DE-AC02-06CH11357.   

The Nucleosynthesis of Helium

The large cosmic abundance of Helium - second only to Hydrogen - is a testament to the importance of its formation in the cosmos. Both Helium-3 and Helium-4 emerge from Big Bang Nucleosynthesis in considerable quantities, the synthesis of the isotopes are links in the pp chain and other stellar nucleosynthesis processes, and they are also created during the initial stages of the r-process. The importance of Helium formation in these settings provides us with valuable information upon the environments in which it occurs. We survey the role of the synthesis of Helium in nuclear astrophysics, how its manufacture is affected by many diverse factors, and what we have learnt from observations of Helium abundances.   

The Primordial Deuterium Abundance: Current Status and Future Prospects

Measurements of the abundances of the light nuclei (H, D, $^3$He, $^4$He, and $^7 $Li) offer precise constraints on the cosmological parameters relevant to big bang nucleosynthesis (BBN). Deuterium is of particular interest, since, at the level of cosmological relevance, it is produced only during BBN. The advent of high resolution spectrographs on telescopes both on the ground and in space has enabled the measurement of the abundance of deuterium in a number of astrophysical environments, including those which give the primordial abundance, namely the absorption line systems seen toward distant quasars. In this talk, I will discuss the current state of the deuterium abundance, with a focus given to the primordial abundance, and will discuss the future roles deuterium can play in futher constraining physics during the epoch of BBN.