Environment-Dependent Scalar–Tensor Modified Gravity: A Unified Framework for Screening, Galactic Dynamics, and Cosmic Acceleration

This paper introduces a unified modified gravity framework based on an environment-dependent scalar field inspired by chameleon screening. The scalar field interacts with matter through a conformal coupling and develops a density-dependent effective mass. Inside dense objects, screening suppresses deviations from General Relativity, while in low-density cosmic environments the scalar becomes light and influences large-scale gravitational dynamics. The model builds a conceptual and mathematical connection between: galaxy rotation curves, environmental screening, scalar-field cosmology, and the accelerated expansion of the universe. A full scalar–tensor action is derived, including the Klein–Gordon equation, effective potential, screening criteria, thin-shell configurations, cosmological Friedmann equations, and modified gravitational coupling for linear perturbations. The framework provides an alternative pathway to explain galactic dynamics and cosmic acceleration without invoking dark matter or a cosmological constant.