The Null-Bounded Cosmological Domain: Boundary-First Ontology for Causally Delimited Regions

This paper introduces a null-bounded cosmological model class in which null boundary structure, induced causal accessibility, and admissible relational content are taken to be ontologically prior to any effective interior description. Motivated by the structural properties of null propagation—including the absence of a rest frame, the absence of proper time, and the primacy of causal delimitation—the framework formalizes a null-bounded domain as the ordered structure N = (∂N, C, R, D), where ∂N denotes the null boundary, C the induced causal accessibility structure, R the admissible invariant and relational descriptors, and D any effective interior description reconstructed from the preceding elements. The contribution is ontological rather than dynamical. The framework introduces no new field equations, interaction laws, or reconstruction algorithms. Instead, it establishes a boundary-first criterion of primitive individuation, defines admissibility conditions for primary descriptors, develops the class of admissible interior reconstructions, and demonstrates that multiple interior descriptions may correspond to the same identity-bearing boundary-relational structure. The paper systematically distinguishes this framework from holography, causal-diamond and light-cone constructions, observer-centric reconstruction, characteristic null formulations, causal-set theory, and conventional bulk-first cosmology. Its distinctiveness lies not in proposing new dynamics, but in identifying a selective ontology-level model class in which boundary-relational structure is constitutive while interior description is derivative. The result is a disciplined conceptual framework for causally delimited regions that provides a coherent alternative criterion of primitive individuation while remaining fully compatible with established physical theories.