nucleosynthesis

Nucleosynthesis was a major event in the radiation era. It consisted of the synthesis of atomic nuclei - the first elements - from existing baryons (protons and neutrons).

The temperature of the universe had cooled to about a billion degrees Kelvin. At such a temperature, deuterium, a heavy hydrogen nucleus (2H), was formed by nearly all neutrons and many protons (deuterium was stable only after the temperature dropped). This light element reacted with some of the remaining particles to form the helium nuclei, which consisted of two protons and two neutrons (4He). Some deuterium nuclei (about one in ten thousand) were not converted into helium nuclei, and similarly, about one in ten thousand of the helium nuclei remained as lighter isotope (3He). There was thus a ratio of about three protons to one neutron, as is observed currently.

During the event, the universe was comparable to one very large star (thousands of light years across), as it took the form of a plasma of freely moving particles. The universe remained fully ionised as it continued to expand, the heat too great for any atoms to exist normally. The heat did not change and the universe was not penetrated by radiation (radiation was absorbed by the freely moving electrons).

The abundances and proportions of deuterium, helium and lithium are successfully predicted by nucleosynthesis theory, in that it determines nuclear processes that eventually would have manifested these light elements. For example, it seems to accurately predict that about a quarter the universe is made of 4He. The theoretical calculations predicted by this theory are thus well supported by such contemporary stellar observations. The major implication of these calculations is that the total density of the universe is much less than the critical density and it will never stop expanding. This is generally accepted to be the case, refuting the Big Crunch theory. Nucleosynthesis accounts for the lighter elements, but heavier elements were created at a later stage.

Nucleosynthesis calculations also provide evidence for the inflationary model of the universe (which this analysis is focussing on, rather than strictly the Big Bang model). Such calculations were first perceived as demonstrating that the mass density of the universe was dominated by some unknown form of matter. This was a problem for cosmologists, but consideration of the early universe, particularly over the last decade, provided inflation as a sensible resolution.

- ASh