Dark Radiation candidates after Planck

Abstract

Recent Cosmic Microwave Background (CMB) results from the Planck satellite, combined with previous CMB data and Hubble constant measurements from the Hubble Space Telescope, provide a constraint on the effective number of relativistic degrees of freedom of Neff=3.62^{+0.50}_{-0.48} at 95% CL. These new measurements provide a unique opportunity to place limits on models containing relativistic species at the decoupling epoch. Here we review the bounds or the allowed parameter regions in sterile neutrino models, hadronic axion models as well as on extended dark sectors with additional light species based on the latest Planck CMB observations.