The Expansion of the Universe During Big-Bang Nucleosynthesis

In conventional general relativity, the expansion rate H of a Robertson-Walker universe is related to the energy density by the Friedmann equation. Aside from the present day, the only epoch at which we can constrain the expansion history in a model-independent way is during Big-Bang Nucleosynthesis (BBN). We consider a simple two-parameter characterization of the behavior of H during BBN and derive constraints on this parameter space, finding that the allowed region of parameter space is essentially one-dimensional. We also study the effects of a large neutrino asymmetry within this framework. Our results provide a simple way to compare an alternative cosmology to the observational requirement of matching the primordial abundances of the light elements.