Geometric Resistance of Expanding Space-Time (GREST): A Phenomenological Metric-Response Framework for Galactic Dynamics

We present GREST (Geometric Resistance of Expanding Space-Time), a phenomenological, non-relativistic metric-response framework for galactic dynamics. The framework postulates a global inertial acceleration floor, a_0, derived from the cosmic expansion rate, which couples to local Newtonian gravity through a quadratic composition. Unlike Modified Inertia approaches such as MOND, GREST preserves the Weak Equivalence Principle in the particle sector by modifying the effective gravitational potential rather than particle inertia. A density-dependent saturation ansatz suppresses the modification in high-density environments, restoring Newtonian behavior in galaxy clusters and the early universe. The framework reproduces flat galaxy rotation curves, the baryonic Tully-Fisher relation, and the dynamics of ultra-diffuse galaxies (e.g., NGC 1052-DF2) without invoking non-baryonic dark matter. GREST is explicitly falsifiable through its prediction that the characteristic acceleration scale evolves with redshift, proportional to the Hubble parameter H(z).