A Vacuum-Driven Spherical Boundary Cosmology: Junction Conditions, Effective Dynamics, and a Heuristic Boundary Hypothesis for the Cosmic Microwave Background

We construct a mathematically rigorous cosmological model in which the observable Universe is modeled as a finite, spherically symmetric fluid region bounded by a timelike hypersurface \( \Sigma \), embedded in an exterior Schwarzschild--de Sitter vacuum. The boundary dynamics are derived from the Darmois--Israel junction conditions, yielding an effective equation of motion \( \ddot{R} = \alpha/R^{2} + \mathcal{O}(R^{-3}) \), where \( \alpha \) is an effective parameter encoding the surface stress-energy of the boundary layer and the exterior cosmological constant. We derive the shell conservation law for a general equation of state \( p = w\sigma \), decompose \( \alpha \) into its physical constituents (\( \alpha_{\rm surface} \), \( \alpha_{\rm mass} \), \( \alpha_\Lambda \)), and present the exact implicit solution. A complete linear stability analysis proves radial perturbations are stable if and only if \( \alpha > 0 \). We derive the luminosity-distance relation and demonstrate qualitative consistency with Type Ia supernova observations.We examine the \emph{heuristic possibility} that the Cosmic Microwave Background (CMB) might be reinterpreted through a boundary-layer framework, stressing that this is not a derived consequence of the junction formalism and remains entirely conjectural. All quantitative predictions beyond the background dynamics---including those pertaining to the Hubble tension, baryon acoustic oscillations, and early galaxy formation---are illustrative and require future validation through modified Boltzmann codes, N-body simulations, and rigorous Markov Chain Monte Carlo analysis. The model is presented as a mathematically consistent toy model for exploring boundary effects in general relativistic cosmology, with the exterior cosmological constant \( \Lambda \) remaining an essential ingredient.