  Primordial nucleosynthesis remains as one of the pillars of modern cosmology.
It is the testing ground upon which many cosmological models must ultimately
rest. It is our only probe of the universe during the important
radiation-dominated epoch in the first few minutes of cosmic expansion. This
chapter reviews the basic equations of space-time, cosmology, and big bang
nucleosynthesis. We also summarize the current state of observational
constraints on primordial abundances along with the key nuclear reactions and
their uncertainties. We summarize which nuclear measurements are most crucial
during the big bang. We also review various cosmological models and their
constraints. In particular, we analyze the constraints that big bang
nucleosynthesis places upon the possible time variation of fundamental
constants, along with constraints on the nature and origin of dark matter and
dark energy, long-lived supersymmetric particles, gravity waves, and the
primordial magnetic field.
