A Unified Effective Mechanism for Galaxy Rotation Curves and the Hubble Tension

We investigate whether two long-standing observational anomalies---flat galaxy rotation curves and the Hubble constant tension---may originate from a common effective mechanism operating in low-density environments.

We introduce a minimal phenomenological framework in which departures from Newtonian expectations arise from a saturation of the effective gravitational response at the level of kinematic inference, without invoking dark matter particles or altering early-universe cosmology.
The framework reduces to standard Newtonian behavior in high-density regimes and introduces a smooth, bounded effective response in diffuse environments.

Applied to galactic dynamics, this approach 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 comparisons with representative galaxies from the SPARC sample show that the effective saturation scale naturally correlates with observed surface-density-dependent trends.

At cosmological scales, the same effective 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, while preserving the background cosmological evolution.
This provides a structural interpretation of the Hubble tension without introducing new dark components or modifying early-time physics.

We examine robustness, degeneracies with standard astrophysical effects, and limitations of the effective description, and we outline distinctive observational signatures, including environment-dependent redshift drift and weak lensing effects.
The results suggest that galaxy rotation curves and the Hubble tension may reflect a shared low-density phenomenology, testable with current and forthcoming observations.