Spacetime as Nexus: Black Holes and the Cosmological Convergence of Universe and Anti-Universe _A Mathematical and Modal Extension of Temporal Fission Theory Into the Architecture of Spacetime_

The Existential Nexus framework (Boether, 2026) and Temporal Fission Theory (Boether, 2025) together advance the thesis that the phenomenological present emerges from the dynamic convergence of complementary states at a point of equilibrium. This treatise extends these frameworks from the phenomenological to the cosmological register, proposing that spacetime itself—the dimensional fabric within which all physical events occur—is the structural instantiation of this same convergence. Drawing on the Boyle–Finn–Turok CPT-symmetric cosmological model, Einstein–Rosen bridge physics, and Susskind’s principle of information conservation, the paper argues that what is experienced as spacetime is the nexus through which our universe (time-forward, past-trajectory) and its CPT-symmetric anti-universe (time-backward, future-trajectory) maintain continuous structural contact. The argument is developed in three formal registers. First, a mathematical formalization extends the ontological Lagrangian and Noether conservation theorem of Boether (2026) to the cosmological scale, yielding a Cosmological Nexus Equilibrium Theorem and a transcendental derivation of black hole necessity from macrocosmic conservation. Second, a modal-logical formalization establishes the necessity of the anti-universe and the necessity of black holes as mediators in possible-worlds semantics restricted to the cosmological constraint set. Third, the Cosmonaut Thought Experiment (Boether, 2026c) provides a phenomenological test case linking the formal apparatus to subject-level horizon crossing. Black holes, on this view, are not pathological anomalies of general relativity but the discrete physical loci at which the universe–anti-universe equilibrium is maintained—the structural necessity of the cosmological present. The paper introduces the Cosmological Temporal Fission Hypothesis as the formal statement of this thesis and considers its implications for the philosophy of time, the black hole information paradox, modal metaphysics, and the integration of phenomenological and cosmological conceptions of temporality.