We investigate whether two long-standing observational anomalies---flat galaxy rotation curves and the Hubble constant tension---may originate from a single effective modification of gravitational response operating in low-density regimes.We introduce a minimal phenomenological model in which the effective gravitational response saturates below a characteristic acceleration scale, without invoking dark matter particles or altering early-universe cosmology.The model reduces to Newtonian gravity in high-density environments and introduces a smooth, bounded modification in diffuse regions.Applied to galactic dynamics, this framework reproduces the main features of observed rotation curves across a wide range of galaxy types using baryonic matter alone, with stable parameter values and no halo-by-halo tuning.Explicit numerical fits to representative galaxies from the SPARC sample demonstrate that the effective saturation scale correlates naturally with observed surface-density trends.At cosmological scales, the same mechanism predicts environment-dependent deviations in the locally inferred expansion rate.Cosmic voids emerge as maximal probes of the saturated regime, leading to a systematic offset between local and global measurements of the Hubble constant.This provides a structural explanation for the Hubble tension without introducing new dark components or modifying early-time physics.We discuss degeneracies, limitations, and robustness against baryonic uncertainties, and we outline distinctive observational signatures, including void-dependent redshift drift and lensing effects.The results suggest that galaxy rotation curves and the Hubble tension may reflect a common low-density phenomenology, testable with current and forthcoming observations.
Jérôme Beau (Mon,) studied this question.