Big Bang Nucleosynthesis with Stable ${}^8$Be and the Primordial Lithium Problem

Abstract

A change in the fundamental constants of nature or plasma effects in the early universe could stabilize ${}^8$Be against decay into two ${}^4$He nuclei. Coc et al. examined this effect on big bang nucleosynthesis as a function of $B_8$, the mass difference between two ${}^4$He nuclei and a single ${}^8$Be nucleus, and found no effects for $B_8\le 100$ keV. Here we examine stable ${}^8$Be with larger $B_8$ and also allow for a variation in the rate for ${}^4$He + ${}^4$He $⟶$ ${}^8$Be to determine the threshold for interesting effects. We find no change to standard big bang nucleosynthesis for $B_8<1$ MeV. For $B_8\gtrsim 1$ MeV and a sufficiently large reaction rate, a significant fraction of ${}^4$He is burned into ${}^8$Be, which fissions back into ${}^4$He when $B_8$ assumes its present-day value, leaving the primordial ${}^4$He abundance unchanged. However, this sequestration of ${}^4$He results in a decrease in the primordial ${}^7$Li abundance. Primordial abundances of ${}^7$Li consistent with observationally-inferred values can be obtained for reaction rates similar to those calculated for the present-day (unbound ${}^8$Be) case. Even for the largest binding energies and largest reaction rates examined here, only a small fraction of ${}^8$Be is burned into heavier elements, consistent with earlier studies. There is no change in the predicted deuterium abundance for any model we examined.

Comments: 7 pages, 2 figures, expanded discussion of 8Be binding energy, added reference, to appear in Phys. Rev. D