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

TL;DR

This study explores the effects of stable $^8$Be on big bang nucleosynthesis, finding that significant $^8$Be stability could reduce primordial lithium without affecting deuterium or helium abundances.

Contribution

It extends previous work by analyzing larger $^8$Be binding energies and reaction rates, identifying conditions under which primordial lithium abundance can be explained.

Findings

Stable $^8$Be can reduce $^7$Li abundance in the early universe.

No significant change in deuterium or $^4$He abundances across models.

Only a small fraction of $^8$Be converts into heavier elements.

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 $\longrightarrow$ $^8$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.