Big Bang Nucleosynthesis with Stable 8Be and the Primordial Lithium Problem
Abstract
A change in the fundamental constants of nature or plasma effects in the early universe could stabilize 8Be against decay into two 4He nuclei. Coc et al. examined this effect on big bang nucleosynthesis as a function of B8, the mass difference between two 4He nuclei and a single 8Be nucleus, and found no effects for B8 100 keV. Here we examine stable 8Be with larger B8 and also allow for a variation in the rate for 4He + 4He 8Be to determine the threshold for interesting effects. We find no change to standard big bang nucleosynthesis for B8 < 1 MeV. For B8 1 MeV and a sufficiently large reaction rate, a significant fraction of 4He is burned into 8Be, which fissions back into 4He when B8 assumes its present-day value, leaving the primordial 4He abundance unchanged. However, this sequestration of 4He results in a decrease in the primordial 7Li abundance. Primordial abundances of 7Li consistent with observationally-inferred values can be obtained for reaction rates similar to those calculated for the present-day (unbound 8Be) case. Even for the largest binding energies and largest reaction rates examined here, only a small fraction of 8Be is burned into heavier elements, consistent with earlier studies. There is no change in the predicted deuterium abundance for any model we examined.
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