Abstract
THE observed interstellar abundance of 2H has been used1,2 to estimate the mean baryon density (ρb) 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 ρb. If one then assumes that all 2H was formed in a big bang, the observed abundance2 of this nuclide requires a value of ρb 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 ρb and could be used to estimate ρb 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 ρb, 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 ρb (for ρb>10−31) so that an upper limit is obtained by attributing all of the observed 7Li to the big bang.
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AUSTIN, S., KING, C. A constraint on the universal baryon density from the abundance of 7Li. Nature 269, 782 (1977). https://doi.org/10.1038/269782a0
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DOI: https://doi.org/10.1038/269782a0
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