Some Key Questions in Early Universe Cosmology and Big Bang Nucleosynthesis

For all the interesting possible phenomena which might have occurred during the early universe, there are really only two observational cosmological probes. Of those two, the only probe during the relevant radiation dominated epoch is the yield of light elements during the epoch of big bang nucleosynthesis. The synthesis of light elements occurs in the temperature regime from 108 to 1010 K and times of about 1 to 104 sec into the big bang. The other probe is the spectrum of temperature fluctuations in the CMB which (among other things) contains information of the first quantum fluctuations in the universe, and the details of the distribution and evolution of dark matter, baryonic matter and photons up to the surface of photon last scattering. Here, we emphasize the role of primordial nucleosynthesis in answering some key questions of the big bang and early universe cosmology.