Kinematic Constraints at Radiative Boundaries: A Geometric Normalization for the Dark Sector

The ~10^120 vacuum catastrophe, the necessity of dark energy, and the Hubble tension are widely treated as distinct crises requiring novel physics. It is proposed here that they are symptoms of a single dimensional category error: evaluating macroscopic expansion as an extensive three-dimensional (3D) bulk fluid rather than a constrained two-dimensional (2D) boundary tension. By establishing kinematic and thermodynamic equilibrium strictly at a radiative decoupling surface, Newtonian gravity acts as a geometric filter, stripping spatial dimensions to yield a mass-independent 2D invariant: π³/15 ≃ 2.0671. Projecting this boundary normalization to the cosmic horizon derives the empirical dark energy fraction (ΩΛ = π³/45 ≃ 0.6890) without tunable parameters. Furthermore, adopting this geometric partition as an ansatz analytically yields the exact observed onset of cosmic acceleration (z_acc ≃ 0.642, T_CMB ≃ 4.47 K) within standard FRW kinematics. The H0 tension is then recast as a topological defect fraction (1/12) native to 3D sphere packing upon passing this hypothesized crystallization threshold. Gravity is thus reinterpreted as an emergent macroscopic boundary constraint.