Primordial nucleosynthesis as a test of the Friedmann equation in the early Universe

In the standard hot big bang model, the expansion of the early Universe is given by the Friedmann equation with an energy density dominated by relativistic particles. Since in a variety of models this equation is altered, we introduce modifications in the Friedmann equation and show that we can constrain them using big bang nucleosynthesis data. When there is no neutrino-antineutrino asymmetry these modifications are tightly bound but in the presence of an asymmetry the bounds become much looser. As an illustration, we apply our results to a model where the second and third families couple to gravity differently than the first family (nonuniversal gravity).