The standard, hot, big bang cosmological model is simple (assuming
isotropy, homogeneity, Newtonian/Einsteinian gravity, standard particle
physics, etc.) and, likely, simplistic. In broad brush it offers
a remarkably successful framework for understanding observations of
the present and recent Universe. It may seem hubris to expect that
this model could provide a realistic description of the Universe during
the first 20 minutes or so in its evolution when the temperature and
density were enormously larger than today. According to the standard
model, during these first 20 minutes the entire Universe was a nuclear
reactor, turning neutrons and protons into the light nuclides. This
prediction presents the opportunity to use observations here and now
to test the theory there and then. As described in this article, SBBN
predicts observable primordial abundances for just four light nuclides
D, 3He, 4He, and 7Li, as a function of
only one adjustable parameter,
, the
nucleon-to-photon ratio, which is a measure of the universal
baryon abundance. Even though it is currently difficult to use the
extant observational data to bound the primordial abundance of 3He,
SBBN is still overconstrained, yielding three predicted abundances for
one free parameter. Furthermore, the baryon density inferred from SBBN
must be consistent with that derived from observations of the present
Universe. Given the many possibilities that SBBN could be falsified
by the empirical data, it is a remarkable success of the standard model
that there is consistency between theory and observations provided that
there are a few billion photons (most of them in the 2.7 K cosmic
background radiation) for every neutron or proton (nucleon) in the
Universe.
This success establishes primordial nucleosynthesis as one of the main pillars of our standard model of cosmology, providing the only probe of the physical Universe during its very early evolution. Alternate theories of gravity and/or particle physics must now be tested against the impressive success of SBBN.