A Wave Function of the Universe Is Not a Theory of the Universe: The Domain Problem and the Interface Architecture of Quantum Mechanics

Despite quantum mechanics’ unrivalled empirical success, its unrestricted formal applicability does not by itself establish that it provides a physically self-sufficient account of an entirely quantum world. This paper argues that the common operational core of quantum mechanics is not a complete theory of purely quantum reality. It is an interface architecture: it connects quantum states and transformations to physical preparations, measurement contexts, stable records, and definite outcomes. The argument distinguishes formal universality, conditional predictive adequacy, and physical self-sufficiency. Quantizing the apparatus extends the chain of quantum correlations but does not derive a single outcome from unitary dynamics; decoherence explains interference suppression, pointer stability, and record proliferation, but does not by itself convert an improper mixture into the actuality of one alternative. A closed universe provides the decisive test because preparation, subsystem decomposition, time, probability, and records can no longer be assigned to an external domain. Decoherent histories, Everettian quantum mechanics, Bohmian mechanics, objective-collapse theories, relational quantum mechanics, and contextual approaches are accordingly analysed as distinct strategies for closing or redefining the interface. The diagnosis extends to quantum field theory and quantum gravity: quantizing matter, apparatuses, reference frames, or geometry does not alone close the record interface. A fundamental theory must derive, constrain, or replace the conditions under which determinate empirical reality becomes possible. The measurement problem is therefore a local manifestation of a broader problem concerning the physical domain of quantum mechanics.