  A key pillar of modern cosmology, Big Bang Nucleosynthesis (BBN) offers a
probe of the particle content and expansion rate of the Universe a mere few
minutes after the beginning. When compared with the BBN predictions, the
observationally inferred primordial abundances of deuterium and helium-4
provide an excellent baryometer and chronometer respectively. Several hundred
thousand years later, when the Cosmic Background Radiation (CBR) photons began
progagating freely, the spectrum of temperature fluctuations imprinted on them
also encoded information about the baryon density and the expansion rate.
Comparing the constraints imposed by BBN with those from the CBR reveals a
consistent picture of the Universe at these two very widely separated epochs.
Combining these probes leads to new, tighter constraints on the baryon density
at present and on possible new physics beyond the standard model of particle
physics.
