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 descriptors are taken to be ontologically prior to any effective interior description. Motivated by structural features 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 an ordered structure N=(∂N,C,R,D). Domain identity is fixed by the null boundary, the induced causal accessibility relations, and the admissible boundary‑defined relational content, while any bulk‑style interior enters only as a derivative reconstruction. The paper distinguishes this boundary‑first ontology from holographic, causal‑diamond, observer‑centric reconstruction, and standard bulk‑first approaches, arguing that its distinctness lies in the non‑equivalence of primitive individuation rather than in any claim of dynamical completeness. The result is a disciplined ontology‑level framework for causally delimited regions in which boundary‑first ordering provides a coherent alternative to primitive interior‑first description.