We present a forecast-ready effective framework in which late-time cosmic acceleration arises not from a fundamental dark-energy substance but from a geometric response with causal memory. The construction is formulated as a minimal scalar-tensor theory in Jordan frame, where a coarse-grained macrostate, phi, couples covariantly through F(phi)R and evolves in response to a history functional G(a) defined operationally from coarse-grained structure growth. This removes the main arbitrariness that usually affects phenomenological late-time extensions: the closure is not introduced as an unspecified function of redshift, but is anchored to a computable proxy of gravitational entropy production derived from the filtered matter power spectrum. The same framework is then mapped into the standard observational language of modified cosmology through the scalar-sector response functions mu(a,k) and Sigma(a,k), the slip parameter eta(a,k), and the tensor-sector luminosity-distance ratio dLgw/dLem parameterized by (Xi0,n). A stable causal relaxation law supplies the memory sector, while an explicit environmental screening prescription guarantees compatibility with local gravity tests and predicts a built-in critical experiment in low-density regions. The resulting theory is strongest when interpreted as a design-study and falsification program: it delivers a non-arbitrary route to effective dark energy, a conditional pathway toward partial dark-sector unification on cosmological and intermediate scales, and a clear kill-switch structure based on growth, lensing, standard sirens, and void-based differential tests. Rather than overclaiming a full all-scale replacement of the dark sector, we show that the proposal can now be stated in publication-grade form as a sharply testable effective framework with coherent scalar, tensor, and environmental signatures.
César Daniel Reyna Ugarriza (Thu,) studied this question.