Limits on Dark Radiation, Early Dark Energy, and Relativistic Degrees of Freedom

TL;DR

This paper investigates how current and future cosmological data can distinguish between extra relativistic energy components like sterile neutrinos and dark energy models, analyzing their effects on N_{eff} and other cosmological parameters.

Contribution

It provides a comparative analysis of how different dark energy models impact constraints on N_{eff} and related cosmological parameters using current and upcoming data.

Findings

Dark energy models do not alter current N_{eff} constraints.

Barotropic dark energy can reconcile N_{eff} with theoretical predictions.

Planck data can discriminate between sterile neutrinos and dark energy explanations.

Abstract

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.