ethan@utastro.UUCP (Ethan Vishniac) (05/18/84)
[Do not write in this space.] I mentioned in earlier submission that the observed primordial abundances provide a striking confirmation of a major prediction of the standard cosmology. D. Gwyn writes: >I should state that I have not been following the "first N seconds >after the big bang" developments, so I do not know to what extent >the current elemental abundances depend on the details of that >theory of their formation. I do know that there are so many >theoretical objections to a "big bang" that I would reserve it for >a theory of last resort. The prediction of elemental abundances is based on the idea that if the universe started out very hot, then at some point in its declining temperature conditions would be appropriate for nucleosynthesis. The sequence of events is summarized nicely in Weinberg's book Gravitation and Cosmology section 15.7. The idea is that as the universe cools the rate of weak reactions drops below that necessary to keep the populations of neutrons and protons in thermal equilibrium. The neutrons then decay away (half life of a little over 10 minutes). Long before the neutrons have disappeared temperatures become favorable for building nucleii. Almost all the neutrons quickly combine with protons into deuterium. Almost all the deuterium then proceeds to combine up to helium 4. A very small amount of lithium is made, but the usual difficulties in making heavier elements prevent any significant amount of heavier atoms from being made. This process is inherently a nonequilibrium process. Therefore the abundances that result will depend on the rate at which the temperature drops (which depends on the expansion rate of the universe) and on the neutron half life (about which some uncertainty remains). The cross sections used in the reaction rates are all well known and do not depend in any significant way on ideas about quarks or the nature of unified field theory. This calculation was first performed by Peebles (1966) and Wagoner, Fowler, and Hoyle (1967). The results have not changed significantly since. One of the nice points that result is that since the results depend on the expansion rate, which depends on the total local energy density, the presence of weakly interacting massless particles (which would have been produced with number densities comparable to the photons at still higher temperatures) makes a detectable difference. The detailed agreement obtained between the observed abundances and our calculations makes it possible to limit the number of separate massless (or low mass) neutrino species to equal to or less than four. This limit is the strictest limit set on the number of neutrino species (currently expected to be three). A striking test of this is now possible using the decay width of the neutral weak vector boson (the Z particle) which is now being detected in experiments at CERN. Within a few years the limit set from the Z particle decay should be comparable to the limit set from cosmology (or else some interesting results will emerge!). Regardless of the exact details of cosmological nucleosynthesis, it remains true that the large amount of helium present in the interstellar medium (compared to the small amount of heavier atoms) is inexplicable in terms of any model which tries to explain the composition of the interstellar medium in terms of nucleosynthesis in stars. All of which brings me to my last point. Doug Gwyn states that there exist so many objections to the big bang theory as to make it the theory of last resort. Just what are these objections? I haven't heard any from you that don't boil down to preferring ad hoc explanations based on nonexistent physics purely because of your ideas about the kinds of symmetries the universe should have. It may be tempting to ascribe to the universe, as to God, the greatest degree of perfection we can imagine, but I prefer a dialogue with the observations rather than a diatribe at them. "Just another Cosmic Cowboy" Ethan Vishniac {ut-sally,ut-ngp,kpno}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712