Causal Accessibility as a Unified Geometric Origin for Relativistic Limits and Galactic Dynamics

This preprint introduces the geometric framework of "Causal Accessibility" (D) as a unified origin for both classical relativistic limits (the light-speed barrier, event horizons) and galactic-scale kinematics. By treating these phenomena as regime-dependent manifestations of outward causal reach, we provide a parameter-free, first-principles derivation of the Baryonic Tully-Fisher Relation (BTFR). Crucially, the fundamental acceleration scale (a₀) is not inserted as an ad-hoc free parameter; rather, it emerges naturally from the geometric and thermodynamic constraints of the de Sitter horizon. Furthermore, through the mechanism of Observational Resolution Modulation (ORM), this framework naturally accounts for the External Field Effect (EFE) as empirically identified in the SPARC dataset via a falsifiable "causal screening" effect. Unlike traditional dark matter models or standard modified gravity theories, our approach does not modify the Einstein field equations or introduce new stress-energy components. Instead, it reinterprets galactic anomalies as inherent constraints of causal network connectivity. By eliminating the need for hypothetical dark matter halos or arbitrary curve-fitting parameters, this framework offers a streamlined, ontological reassessment of spacetime geometry and galactic dynamics. Note: This preprint is currently under peer-review at Foundations of Physics.