Scalar-tensor theories of gravity, neutrino physics, and the $H_0$ tension

Abstract: We use $Planck$ 2018 data to constrain the simplest models of scalar-tensor theories characterized by a coupling to the Ricci scalar of the type $F(\sigma) R$ with $F(\sigma) = N_{pl}^2 + \xi \sigma^2$. We update our results with previous $Planck$ and BAO data releases obtaining the tightest constraints to date on the coupling parameters, that is $\xi < 5.5 \times 10^{-4}$ for $N_{pl}=0$ (induced gravity or equivalently extended Jordan-Brans-Dicke) and $(N_{pl} \sqrt{8 \pi G})-1 < 1.8 \times 10^{-5}$ for $\xi = -1/6$ (conformal coupling), both at 95% CL. Because of a modified expansion history after radiation-matter equality compared to the $\Lambda$CDM model, all these dynamical models accommodate a higher value for $H_0$ and therefore alleviate the tension between $Planck$/BAO and distance-ladder measurement from SNe Ia data from $4.4\sigma$ at best to $2.3\sigma$. We show that all these results are robust to changes in the neutrino physics. In comparison to the $\Lambda$CDM model, partial degeneracies between neutrino physics and the coupling to the Ricci scalar allow for smaller values $N_{\rm eff} \sim 2.8$, $1\sigma$ lower compared to the standard $N_{\rm eff} = 3.046$, and relax the upper limit on the neutrino mass up to 40%.