Primordial nucleosynthesis
The lifetime of a free neutron to decay is about ten minutes. However most neutrons do not have time to decay. After only about three minutes have elapsed, something else occurs. Neutrons interact with protons to form nuclei of deuterium, or heavy hydrogen. The deuterium soon gains another neutron to form tritium, which in turn rapidly absorbs a proton to form a helium nucleus of mass 4, consisting of two protons and two neutrons. There is no stable element of mass 5, nor of mass 8, so additional nucleosynthesis via He + p or He + He is generally not possible although trace amounts of one or two heavier elements, most notably lithium (of mass 7) do form. One finds that practically every neutron ends up in a helium nucleus. The Big Bang therefore predicts that there should be one helium nucleus for every ten protons, created in the first three minutes of the expansion. Approximately 25 percent by mass of the matter in the universe is now in the form of helium nuclei: the rest consists of protons. For the Sun helium is about 30%, since some of the hydrogen has already been processed through stars (including the Sun itself!), ie the solar material is not "primordial".A Helium abundance of about 1/4 by mass turns out to be a robust prediction of the Big Bang theory, and depends only on the fact that the very early universe passed through a high temperature, high density phase, much like the center of a star. This abundance is in fact just what we observe when we look at material which we believe to be close to primordial. Other important predictions include small amounts of deuterium and lithium, although the final abundances of these elements, deuterium especially, depend on the precise value of Omega_b. If the density of ordinary matter (baryons) is high, the early nucleosynthesis is efficient, and one makes essentially no deuterium. If the baryon density is low, however, one makes an amount of deuterium that is comparable to what is observed by astronomers in the absorption spectra of distant quasars.
Helium
Helium is synthesized inside stars by thermonuclear fusion. However, most
stars, like the sun, are still burning hydrogen and so have made little helium,
and certainly dispersed none of it. The synthesized helium is deep inside the
stellar interior. Yet the universe indeed is observed to contain one helium
atom for every ten atoms of hydrogen: by mass, it is about 25 percent helium.
This is close to the case for the sun, it is as observed in solar cosmic rays,
for interstellar gas in HII regions, and for hot stars, where the helium
emission lines are excited. Moreover, when we compare stars which are
metal-rich with metal-poor stars, one finds essentially the same helium
abundance. There are metal-deficient galaxies which contain almost the same
helium abundance. This confirms that helium has mostly not been synthesized
along with the heavier elements, such as the metals, but was made prior to the
formation of the first stars. The coincidence between observation and
prediction of the helium abundance in the universe provides one of the major
pieces of evidence for the Big Bang theory.
