General relativity describes gravity through spacetime geometry and causal structure, yet gravitational phenomena are often discussed using force-centered language. This paper introduces gravitational admissibility as a structural vocabulary for describing how spacetime geometry restricts future-directed reachability. The framework introduces no new dynamics and does not modify general relativity. Its central object is the escape-admissible subset E+O (p): the set of future-directed causal curves from an event p that reach a specified observational domain O. For asymptotically flat black-hole spacetimes, choosing O = I + recovers the standard causal distinction between exterior events and the black-hole region: outside the horizon, E+I + (p) is nonempty; inside, it is empty. The value of the framework is not this restatement alone, but the organizational role it provides. Event, apparent, trapping, isolated, and dynamical horizons can be interpreted as different reachability diagnostics rather than as competing definitions of one boundary. Likewise, cosmological horizons can be described by choosing O as the future causal domain of a specified observer congruence. The contribution is organizational rather than predictive. Gravitational admissibility does not generate new observables; it clarifies how known diagnostics such as photon capture, redshift, orbital cutoff, shadows, ringdown, and cosmological horizons probe changes in accessible future structure.
Francis Smith (Sun,) studied this question.