The 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 for an extra relativistic component with $\neff=4.08_{-0.68}^{+0.71}$ at 95% c.l.. Performing an analysis on Dark Radiation sound speed $c_{\rm eff}$ and viscosity $c_{\rm vis}$ parameters, we found $\ceff=0.312\pm0.026$ and $\cvis=0.29_{-0.16}^{+0.21}$ at 95% c.l., consistent with the expectations of a relativistic free streaming component ($\ceff=\cvis$=1/3). Assuming the presence of 3 relativistic neutrinos we constrain the extra relativistic component with $\nnus=1.10_{-0.72}^{+0.79}$ and $\ceff=0.24_{-0.13}^{+0.08}$ at 95% c.l. while $\cvis$ results as unconstrained. Assuming a massive neutrino component we obtain further indications for Dark Radiation with $\nnus=1.12_{-0.74}^{+0.86}$ at 95% c.l. . 
Maria Archidiacono, Erminia Calabrese, Alessandro Melchiorri
astro-ph.CO gr-qc hep-ph