On the dark radiation role in the Hubble constant tension

TL;DR

This paper reviews the impact of dark radiation, especially models with self-interacting particles, on the Hubble constant tension, highlighting that simple models are insufficient and exploring more complex, interacting scenarios as promising solutions.

Contribution

It provides a comprehensive review of dark radiation models, emphasizing the importance of second-order corrections and interactions in addressing the Hubble tension.

Findings

Simple dark radiation models are insufficient to resolve the Hubble tension.

Interacting dark radiation scenarios, such as self-interacting sterile neutrinos, are highly promising.

Second-order corrections and non-free streaming effects improve the fit to cosmological data.

Abstract

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. However, they are not fully satisfactory in solving the $H_0$ issue. We devote here special attention to second-order corrections: some interacting scenarios, such as those with new dark radiation degrees of freedom that exhibit a non-free streaming nature are highly satisfactory alternative cosmologies where to solve the Hubble constant tension. Models with self-interacting sterile neutrinos and/or majorons, both well-motivated beyond the Standard Model particles, will be discussed along our assessment.