The early Universe was hot and dense behaving as a cosmic nuclear
reactor during the first twenty minutes of its evolution. It was,
however, a ``defective'' nuclear reactor, expanding and cooling very
rapidly. As a result, only a handful of the lightest nuclides were
synthesized before the density and temperature dropped too low for
the nuclear reaction rates to compete with the universal expansion
rate. After hydrogen (1H
protons) the next most abundant
element to emerge from the Big Bang is helium (4He
alpha
particles). Isotopes of these nuclides (deuterium and helium-3) are
the next most abundant primordially. Then there is a large gap to
the much lower abundance of lithium-7. The relative abundances of
all other primordially-produced nuclei are very low, much smaller
than their locally observed (or, presently observable!) abundances.
After a brief description of the early evolution of the Universe
emphasizing those aspects most relevant to primordial, or ``Big Bang''
nucleosynthesis (BBN), the predicted abundances of the light nuclides
will be presented as a function of the one ``free'' parameter (in
the simplest, ``standard'' model: SBBN), the nucleon (or ``baryon'')
abundance. Then, each element will be considered in turn in a
confrontation between the predictions of SBBN and the observational
data. At present (Summer 1999) there is remarkable agreement between
the SBBN predictions of the abundances of four nuclides (D,
3He, 4H3, and 7Li) and their primordial
abundances inferred from
the observations. However, there are some hints that this concordance
of the hot big bang model may be imperfect, so we will also explore
some variations on the theme of the standard model with regard to
their modifications of the predicted primordial abundances of the
light elements.
In the simplest, standard, hot big bang model the currently observed
large-scale isotropy and homogeneity of the Universe is assumed to
apply during earlier epochs in its evolution. Given the currently
observed universal expansion and the matter and radiation (CBR:
``cosmic background radiation'', the 2.7K ``black body radiation'')
content, it is a straightforward application of classical physics
to extrapolate back to earlier epochs in the history of the Universe.
At a time of order 0.1 s after the expansion began the Universe was
filled with a hot, dense plasma of particles. The most abundant were
photons, electron-positron pairs, particle-antiparticle pairs of all
known ``flavors'' of neutrinos
(
e,
ยต, and
),
and trace amounts of neutrons and protons (``nucleons'' or ``baryons'').
At such early times the thermal energy of these particles was very high,
of order a few MeV. With the exception of the nucleons, it is known or
assumed that all the other particles present were extremely relativistic
at this time. Given their high energies (and velocities close to, or
exactly equal to the speed of light) and high densities, the electroweak
interactions among these particles were sufficiently rapid to have
established thermal equilibrium. As a result, the numbers and
distributions (of momentum and energy) of all these particles is
accurately predicted by well-known physics.