Abstract

Recent cosmological data analyses hint at the presence of an extra relativistic energy component in the early universe. If further data confirms this, it could suggest new dark matter such as a sterile neutrino or a decaying particle, or nonstandard thermal history. Or it could indicate that dark energy does not fade away to the ∼10^−9 fraction of the energy density at CMB recombination that is predicted by the cosmological constant. Indeed, some proposed particle physics explanations for dark energy involve scaling fields with an early energy density which is a constant fraction of the energy density of the dominant component. Another possibility is that the evidence for the extra relativistic component may be signaling the presence of a “dark radiation” component in the early universe, as predicted by certain higher dimension braneworld scenarios. Any of these would be exciting extensions to the standard, concordance model. Uncovering new degrees of freedom would be of great importance, and distinguishing between the possible origins could give valuable insight into physics and cosmology. We examine the capabilities of current and future data to constrain and discriminate between these explanations, to detect the early dark energy density associated with them and to constrain neutrinos properties.