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

• Received 16 September 2011

DOI:https://doi.org/10.1103/PhysRevD.84.123008