Neutrinos and BBN (and the CMB)
  During Big Bang Nucleosynthesis (BBN), in the first 20 minutes of the evolution of the Universe, the light nuclides, D, 3He, 4He, and 7Li were synthesized in astrophysically interesting abundances. The Cosmic Microwave Background Radiation (CMB) observed at present was last scattered some 400 thousand years later. BBN and the CMB (supplemented by more recent Large Scale Structure data), provide complementary probes of the early evolution of the Universe and enable constraints on the high temperature/energy physical processes in it. In this overview the predictions and observations of two physical quantities, the baryon density parameter and the expansion rate parameter, are compared to see if there is agreement between theory and observation at these two widely separated epochs. After answering this question in the affirmative, the consequences of this concordance for physics beyond the standard models of particle physics and cosmology is discussed. 
Gary Steigman