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 25% 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.
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.
Deuterium and the baryon density
Unlike helium, deuterium is a very fragile element. It
burns at a temperature of only 10^6 K, well below the temperature
in the solar core. A considerable fraction of any primordial deuterium
at the beginning of the galaxy would have been destroyed by the present
time. This is confirmed by observation: interstellar clouds contain
deuterium, as do protostars, stars which have not yet developed nuclear
burning cores, whereas evolved stars have essentially no deuterium.
Allowing for that destruction, one infers a pregalactic deuterium
abundance of 0.01 percent relative to hydrogen. Comparison with the
Big Bang prediction requires one to choose a larger density that
cannot exceed about a tenth of the critical density for closure of the
universe, otherwise too little primordial deuterium would have been
synthesized. There is no alternative to the Big Bang for
synthesizing deuterium: stars destroy it rather than produce it. The
significance of this result is that if the universe is at critical
density, ninety percent of the matter in the universe must be non
baryons, consisting of weakly interacting neutral particles that
did not participate in the nuclear reactions that led to deuterium
production.
Joe Silk