This paper establishes a constraint-level result in cosmology: once sustained redistribution across a causal horizon is admitted, cosmic expansion follows as a necessary interior kinematic response of spacetime. Standard cosmological models successfully describe expansion phenomenologically but treat its existence as contingent on initial conditions, inflationary dynamics, or a freely specified cosmological constant. These approaches do not provide a causal explanation for why expansion must occur rather than merely being allowed. Here, the analysis proceeds at a higher logical level, independent of specific field content or microscopic dynamics. A horizon is treated as a dynamical interface mediating sustained redistribution of energy and information. Introducing a horizon-coupled redistribution current, the paper shows that nonvanishing flux across a causal boundary necessarily induces a nonzero interior source term. In homogeneous and isotropic spacetimes, no kinematic channel exists through which such sustained sourcing can be expressed except through the expansion scalar. As a consequence, expansion is not a free cosmological input but an unavoidable kinematic requirement whenever sustained horizon flux is present. The result holds independently of the particular microphysical realisation of that flux. Any consistent microscopic model must therefore reproduce this behaviour at the level of spacetime kinematics. Within this framework, the cosmological constant is reinterpreted as an effective, coarse-grained parameter encoding steady-state redistribution pressure rather than a fundamental driver of expansion. The analysis also supplies a natural arrow of time aligned with horizon-directed redistribution. This work does not propose a new dynamical theory, specify microscopic mechanisms, or introduce an action principle. Its contribution is structural: it isolates a necessary causal relation between horizons, redistribution, and expansion that must be satisfied by any viable cosmological model.

A formal locked-core structural formulation of the redistribution framework implicit in this analysis is presented separately in Appendix A - Locked Core ψ₀–OCM (Osborne Cosmological Model) Structure (A2). Zenodo. https://doi.org/10.5281/zenodo.18052496