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Big Bang Nucleosynthesis (BBN) is a pillar of modern
cosmology, providing
a probe of the particle content and expansion rate of the Universe in its
infancy, a mere few minutes after the beginning. The observationally
inferred
primordial abundances of Deuterium and Helium-4, when compared
to the BBN predictions, provide an excellent baryometer and chronometer,
respectively. Helium-4 is also sensitive to the neutrino content of the
Universe and is a window onto any asymmetry between antineutrinos and
neutrinos and, a probe of the early Universe expansion rate. In
contrast, the
spectrum of temperature fluctuations imprinted on the Cosmic Microwave
Background radiation (CMB) some 400
thousand years later, and the growth
of Large Scale Structure (LSS) some 10
billion years later, are sensitive
to the baryon density and to the expansion rate of the Universe in its
youth and middle age. The complementary constraints imposed by BBN and
the CMB/LSS
are reviewed, revealing a consistent picture of the Universe
at these very widely separated epochs, leading to new, tighter constraints
on the baryon density at present and on possible new physics beyond the
standard models of particle physics and cosmology.
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