Structural Differentiation Cosmology (SDC) interprets cosmic evolution not as physical expansion driven by additional energy components, but as irreversible internal differentiation within a closed universe. In this framework, cosmic change is described as a reconfiguration of pre-existing internal structures rather than the creation of new entities or the introduction of external mechanisms. Observable phenomena such as cosmic expansion and acceleration are treated as projections of this irreversible internal differentiation process. This version provides the minimal closure of the framework by identifying a single decisive observational criterion that distinguishes SDC from standard cosmology. No new entities or mechanisms are introduced. With respect to its core claims, the framework is explicitly falsifiable and complete. This work does not modify mathematical operations or algebraic rules. All mathematical expressions are formulated within standard mathematical frameworks; the novelty lies in the interpretational structure applied to cosmological observations. This release is intended as a minimal, figure-free formulation to establish the conceptual foundation of SDC. Further extensions, visualizations, and observation-based refinements are intentionally left for subsequent versions.