On the Physical Finiteness of a Geometrically Flat Plenum: Field-Theoretic Boundaries and Postquantum Stochasticity

This conceptual addendum establishes a physical framework for a spatially finite, flat Universe bounded by its own gravitational field, providing a concrete origin for the intrinsic metric stochasticity required by the Postquantum Theory of Classical Gravity (PQG). By synthesizing the field-theoretic approach to gravity (Feynman, Weinberg) with boundary-driven quantum effects (Zeldovich, Hawking) and CPT-symmetric cosmology (Boyle, Turok), we demonstrate that apparent geometric flatness on cosmological scales does not necessitate spatial infinity. Instead, the Universe can be modeled as a finite physical Plenum whose boundaries—manifested locally as black hole horizons—act as non-linear resonators. The chaotic back-reaction of self-interacting gravitational modes within these boundaries naturally yields the diffusion term postulated in postquantum gravity without requiring an axiomatic imposition of noise.