The Radial Acceleration Relation is Not Universal: Two Scaling Regimes and a Unified Model Across 171 SPARC Galaxies

Version 9. 5 We show that the Radial Acceleration Relation (RAR) is not a universal law when galaxies are separated by dynamical regime. Using 171 SPARC galaxies, we find that dwarf systems (Phase I) lie systematically below mature spirals (Phase III) at fixed baryonic acceleration, with a slope difference of 0. 248 +/- 0. 072 (bootstrap, p = 5. 5 x 10^-17). This result is model-independent and can be verified directly from public SPARC data. We propose a phenomenological model that captures this structure through two scaling regimes connected by a smooth transition. The model generalises without overfitting (-2. 4% on held-out subsamples) and is competitive with MOND under identical baryonic priors, particularly in the dwarf regime (67/67 galaxies). The results suggest that the empirical RAR is an effective envelope of distinct dynamical regimes rather than a single universal relation. Unified verification code (Python) included.