The Case for Dark Radiation

TL;DR

Recent cosmological data suggest the presence of Dark Radiation, an extra relativistic component, with parameters indicating it behaves like a free-streaming relativistic fluid, consistent with additional neutrino-like particles.

Contribution

This study provides new constraints on Dark Radiation parameters using cosmological data, including its effective degrees of freedom, sound speed, and viscosity, highlighting its potential neutrino-like nature.

Findings

Dark Radiation's effective number of relativistic degrees of freedom is around 4.08.

Sound speed and viscosity parameters are consistent with a relativistic free-streaming component.

Extra relativistic particles are suggested to be neutrino-like with N_eff approximately 1.1.

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, $\neff$. 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. .