ψ-Potential: A Retentional Transition Principle for Late-Time Cosmology

This work introduces the ψ-potential (Λψ) as a minimal structural term associated with the retention of already formed differences in late-time cosmology. Within this framework, cosmic evolution is interpreted as a transition from a dynamical regime of growth to a retentional regime in which structural variation is suppressed while expansion continues. The proposed formulation establishes a set of retentional theorems and a unified transition principle describing conditions under which growth saturation, timescale separation, and bounded evolution of the growth index emerge. A minimal data fit demonstrates consistency with observed values of γ ≈ 0.64 and the low-redshift behavior of the fσ₈ observable. The framework does not modify gravity or introduce new forces, but instead imposes a structural constraint on variation, providing a complementary perspective on late-time cosmological observations.