Λψ-Retention: A Minimal Equation for Structural Saturation and the Emergence of a Post-Dynamical Regime

We propose a minimal formal framework describing the transition of the universe from a dynamical regime of structure formation to a regime of structural stability. Recent observational indications — including a persistent growth plateau (γ ≈ 0.64, as indicated by recent analyses), lensing–velocity decoupling, and the emergence of sharp void boundaries — suggest that late-time cosmology may not be governed solely by continuous gravitational clustering. We introduce a retentional constant Λψ and formulate a phenomenological second-order dynamical equation governing structural nodes Ξ: (d^2 Xi / dt^2) + R(psi)(dXi/dt) + Lambda_psi Xi = 0 This equation represents a minimal retentive extension of structural dynamics. It naturally produces a saturation regime in which growth ceases to accelerate and approaches a stable equilibrium. Within this framework, the observed growth plateau can be interpreted as an approximately stationary solution arising from the balance between retentional resistance and structural stabilization. The model suggests a transition to a post-dynamical regime characterized by structural inertia, non-diffusive void boundaries (“retentive shells”), and a decoupling between gravitational curvature and matter motion. These features remain empirically testable through upcoming Euclid and DESI observations.