DESI DR2 Constraints on Coupled Hu--Sawicki $f(R)$ Gravity with Interacting Neutrinos: Implications for the Hubble and \(S_8\) Tensions

Abstract

We investigate the cosmological implications of viable Hu--Sawicki \(f(R)\) gravity models and their extension through an effective interaction with the neutrino sector. Motivated by the persistent \(H_0\) and \(S_8\) tensions, we explore whether the combined effects of modified gravity, neutrino free-streaming, and dark-sector interaction can alleviate these discrepancies while remaining consistent with current cosmological observations. Starting from the metric formulation of \(f(R)\) gravity, we derive the modified background and perturbation equations and implement the resulting cosmological scenario within the {MontePython}/{hi\_class} framework. A comprehensive Bayesian MCMC analysis is performed using combinations of Planck 2018 CMB data, CMB lensing, DESI DR2 BAO measurements, Cosmic Chronometers, and Pantheon supernova observations. Our results show that the interacting Hu--Sawicki scenario provides a noticeable improvement over both the standard \(Λ\)CDM cosmology and the non-interacting \(f(R)\) framework. In particular, the interacting model shifts the inferred Hubble constant toward larger values, yielding \(H_0 = 70.21 \pm 1.60~{\rm km~s^{-1}Mpc^{-1}}\), while simultaneously lowering the clustering amplitude to \(S_8 = 0.796 \pm 0.009\). Consequently, the residual tensions with SH0ES R22 and DES-Y6 are reduced to nearly the \(1σ\) level. From the statistical perspective, the interacting model provides the best overall fit according to the Akaike Information Criterion, with \(Δχ^2 \simeq -45\) relative to \(Λ\)CDM. The obtained neutrino mass constraints remain fully consistent with current cosmological bounds, with \(\sum m_ν< 0.122~{\rm eV}\). These results suggest that interacting modified-gravity scenarios offer a viable route to moderately easing both late-time tensions at once.