At the intersection of general relativity, quantum field theory, and information theory, increasing work points to a common picture: all observable structures of the physical universe can be viewed as "shadows" of some higher-dimensional mathematical object under different projections. Descriptions such as causal partial orders, spacetime manifolds, scattering matrices, boundary algebras, and quantum cellular automata (QCA) are merely images of this high-dimensional object in different categories. In this paper, within the framework of unified time scale, boundary time geometry, and QCA universe, we introduce and characterize a "Universe Terminal Object" to provide a precise definition of the "highest-dimensional mathematical structure of the universe". Specifically, we construct three types of universe categories with physical constraints: Operator--Scattering Universe Category (OpUniv), Boundary Time Geometry Universe Category (GeoUniv), and QCA/Matrix Universe Category (QCAUniv). These encode, respectively, the scattering--spectral shift--Wigner--Smith delay structure under unified time scale, boundary time geometry with generalized entropy and quantum null energy conditions, and discrete QCA universe with finite information. For each category, we construct a causal shadow functor to the category of locally finite causal partial orders (Caus), thereby formalizing the statement that "causal partial order is merely a shadow of high-dimensional structure". Based on this, we define the "Causally Compatible Universe Triplet" category (Uni\subsetOpUniv\timesGeoUniv\timesQCAUniv), whose objects are triple universe objects with aligned causal shadows in Caus, and morphisms are structure-preserving maps along coarse-graining directions and covariant on the causal side. Assuming the Unified Time Scale Mother Ruler axiom, Finite Information Principle, and appropriate Chain Completeness axiom, we prove: there exists a unique (up to isomorphism) terminal object \mathfrak U_{\max} in Uni, called the "Universe Terminal Object". The three projections of this object give the Operator--Scattering Terminal Object (O_{\max}), Geometric--Boundary Terminal Object (G_{\max}), and QCA Terminal Object (Q_{\max}), which are pairwise equivalent under appropriate bridging functors. Furthermore, we present three structural results. First, any physically realizable universe model can be viewed as a unique projection of \mathfrak U_{\max} in some categorical dual, thus causal partial orders, small causal diamonds, and observer worldlines are merely shadows of \mathfrak U_{\max} at different projections and scales. Second, based on the discrete version of generalized entropy and quantum focusing conjecture, we prove the "Small Diamond Refinement Theorem": under finite information axioms, causal small diamonds at any scale can be filled by families of smaller diamonds in a nearly entropy-additive manner; the minimal physical structure is determined only by the information cell scale, not by some fixed geometric minimal diamond. Third, observers, memory, and multi-observer consensus geometries can be characterized in \mathfrak U_{\max} as filters on the causal shadow and consistent subobjects in the three representations, transforming "time delay equals memory" and "volume is phenomenon decoded from boundary data" into precise theorems of relative entropy and scale density. Appendices provide detailed proofs for the existence and uniqueness of the universe terminal object, the small diamond refinement theorem, and the equivalence of the three representations.