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 N_{eff} 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 found that while early dark energy does not alter the current
constraints on N_{eff}, a dark radiation component, such as that provided by
barotropic dark energy models, can substantially change current constraints on
N_{eff}, 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_p and the scalar perturbation index n_s. The ongoing Planck
satellite mission will be able to further discriminate between sterile
neutrinos and early dark energy.
