Case for dark radiation

Combined analyses of recent cosmological data are showing interesting hints for the presence of an extra relativistic component, coined dark radiation. Here, we perform a new search for dark radiation, parametrizing it with an effective number of relativistic degrees of freedom parameter, Neff. We show that the cosmological data we considered are clearly suggesting the presence of an extra relativistic component with Neff=4.08_-0.68+0.71 at 95% C.L.. Performing an analysis on dark radiation sound speed ceff and viscosity cᵥᵢₛ parameters, we found ceff²=0.312±0.026 and cᵥᵢₛ²=0.29_-0.16+0.21 at 95% C.L., consistent with the expectations of a relativistic free streaming component (ceff²=cᵥᵢₛ²=1/3). Assuming the presence of 3 relativistic neutrinos, we constrain the extra relativistic component with N_νS=1.10_-0.72+0.79 and ceff²=0.24_-0.13+0.08 at 95% C.L. while cᵥᵢₛ² results as unconstrained. Assuming a massive neutrino component, we obtain further indications for dark radiation with N_νS=1.12_-0.74+0.86 at 95% C.L..