The Spherical-Boundary Universe: A Vacuum-Driven Cosmology Where the CMB Defines the Photonic Surface of a Finite Expanding Cosmos

We propose a novel cosmological framework in which the Universe is a finite, spherically bounded luminous region expanding into an external vacuum devoid of matter, radiation, and spacetime curvature. In this model, the Cosmic Microwave Background (CMB) is reinterpreted not as relic radiation from a hot early phase, but as the photonic surface of the Universe itself: a boundary layer where radiation accumulates as the cosmos expands outward under the influence of an external vacuum-driven force. The expansion is governed by a radially dependent repulsive interaction producing an accelerated growth of the cosmic radius without invoking dark energy or inflation. We derive the full dynamical equations of motion, analyze the stability of the boundary, and compare the resulting expansion history with ΛCDM. A full sensitivity analysis and numerical integration using Python are provided. The model yields distinct falsifiable predictions regarding the anisotropy spectrum of the CMB, the luminosity-distance relation at high redshift, and the maximum observable radius of the cosmos. This work establishes the mathematical foundation for a boundary-limited cosmology and outlines observational tests capable of confirming or refuting the framework. The model is fully consistent with general relativity through the Darmois--Israel junction conditions.