A constraint on the universal baryon density from the abundance of ⁷Li

Abstract

THE observed interstellar abundance of ²H has been used^1,2 to estimate the mean baryon density (ρ_b) of the Universe. This follows, because (1) there is no plausible source for ²H other than the primordial big bang and (2) the production of ²H in a standard big bang decreases rapidly with increasing ρ_b. If one then assumes that all ²H was formed in a big bang, the observed abundance² of this nuclide requires a value of ρ_b sufficiently low¹ that, for a cosmological constant Λ = 0, the present expansion of the Universe will continue forever and the Universe is open. A major weakness in this argument is that another source of ²H may be found. It has been suggested, for example, that ²H could be made in shock waves accompanying a supernova explosion; this now seems unlikely³, but other mechanisms will certainly be suggested, so that it is important to obtain confirmation of the above conclusion. The predicted production of ⁷Li in a big bang² varies rapidly with ρ_b and could be used to estimate ρ_b if the fraction of the observed ⁷Li made in the big bang were known. Unfortunately there are many possible sources⁴ of⁷ Li and such estimates must be regarded with scepticism. In this note we point out that ⁷Li can be used to place an upper limit on ρ_b, even if other production mechanisms are important, and that this limit also strongly favours an open universe. This possibility arises because the big bang production of ⁷Li increases with increasing ρ_b (for ρ_b>10⁻³¹) so that an upper limit is obtained by attributing all of the observed ⁷Li to the big bang.