On the dark radiation role in the Hubble constant tension

Understand

Dark radiation, parameterized in terms of $N_{\rm eff}$, has been considered many times in the literature as a possible remedy in alleviating the Hubble constant ($H_0$) tension.

• We review here the effect of such an extra dark radiation component in the different cosmological observables, focusing mostly on $H_0$.
• While a larger value of $N_{\rm eff}$ automatically implies a larger value of the Hubble constant, and one would naively expect that such a simple scenario provides a decent solution, more elaborated models are required.
• Light sterile neutrinos or neutrino asymmetries are among the first-order corrections to the most economical (tree-level) massless dark radiation scenario.