Limits on dark radiation, early dark energy, and relativistic degrees of freedom

Recent cosmological data analyses hint at the presence of an extra relativistic energy component in the early universe. This component is often parametrized as an excess of the effective neutrino number N₄₅₅ over the standard value of 3. 046. The excess relativistic energy could be an indication for an extra (sterile) neutrino, but early dark energy and barotropic dark energy also contribute to the relativistic degrees of freedom. We examine the capabilities of current and future data to constrain and discriminate between these explanations, and to detect the early dark energy density associated with them. We find that while early dark energy does not alter the current constraints on N₄₅₅, a dark radiation component, such as that provided by barotropic dark energy models, can substantially change current constraints on N₄₅₅, bringing its value back to agreement with the theoretical prediction. Both dark energy models also have implications for the primordial mass fraction of Helium Y₏ and the scalar perturbation index nₒ. The ongoing Planck satellite mission will be able to further discriminate between sterile neutrinos and early dark energy.