On the dark radiation role in the Hubble constant tension · arXivDesk
2306.15067Jun 26, 2023Invited chapter for the edited book Hubble Constant Tension (Editors E. Di Valentino and D. Brout, Springer Singapore, expected in 2024)
On the dark radiation role in the Hubble constant tension
Dark radiation, parameterized in terms of Neff, has been considered many times in the literature as a possible remedy in alleviating the Hubble constant (H0) tension. We review here the effect of such an extra dark radiation component in the different cosmological observables, focusing mostly on H0
Nearby in the stack
. While a larger value of
Neff
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
H0
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.