The Case for Dark Radiation

Abstract

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

Comments: 6 pages, 8 figures minor revisions, accepted for publication on PRD