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, N₄₅₅. We show that the cosmological data we considered are clearly suggesting the presence of an extra relativistic component with N₄₅₅=4. 08-₀. ₆₈^+0. 71 at 95% C. L. . Performing an analysis on dark radiation sound speed c₄₅₅ and viscosity cₕ₈ₒ parameters, we found c₄₅₅^2=0. 3120. 026 and cₕ₈ₒ^2=0. 29-₀. ₁₆^+0. 21 at 95% C. L. , consistent with the expectations of a relativistic free streaming component (c₄₅₅^2=cₕ₈ₒ^2=1/3). Assuming the presence of 3 relativistic neutrinos, we constrain the extra relativistic component with N_^S=1. 10-₀. ₇₂^+0. 79 and c₄₅₅^2=0. 24-₀. ₁₃^+0. 08 at 95% C. L. while cₕ₈ₒ^2 results as unconstrained. Assuming a massive neutrino component, we obtain further indications for dark radiation with N_^S=1. 12-₀. ₇₄^+0. 86 at 95% C. L. .