Abstract

This paper proposes spacetime-response gravity, a phenomenological framework in which baryonic matter excites a nonlinear scalar response that supplements the weak-field gravitational acceleration. Under static spherical symmetry, the chosen constitutive law yields an explicit transition function: it produces a square-root response at low acceleration and the asymptotic relation v_flat⁴ ≈ GMᵦaₛ, while approaching a nonzero residual acceleration aₛ in the high-acceleration limit. The model is presented as a testable reduced construction rather than a complete relativistic alternative to dark matter. Its viability therefore depends on a covariant, stable completion that consistently predicts galaxy dynamics, gravitational lensing, local-gravity behaviour, cluster phenomenology, and cosmology. The constant high-acceleration residual and the non-analytic infrared kinetic behaviour are identified as central theoretical and observational challenges.