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, N-eff. We show that the cosmological data we considered are clearly suggesting the presence of an extra relativistic component with N-eff = 4.08(-0.68)(+0.71) at 95% C.L.. Performing an analysis on dark radiation sound speed c(eff) and viscosity c(vis) parameters, we found c(eff)(2) = 0.312 +/- 0.026 and c(vis)(2) = 0.29(-0.16)(+0.21) at 95% C.L., consistent with the expectations of a relativistic free streaming component (c(eff)(2) = c(vis)(2) = 1/3). Assuming the presence of 3 relativistic neutrinos, we constrain the extra relativistic component with N-v(S) = 1.10-0.72+0.79 and c(eff)(2) = 0.24(-0.13)(+0.08) at 95% C. L. while c(vis)(2) results as unconstrained. Assuming a massive neutrino component, we obtain further indications for dark radiation with N-v(S) = 1.12(-0.74)(+0.86) at 95% C.L..