ψ-Cosmology: The Second Retentive Axis and the Transition to Structural Persistence Towards a Unified Post-ΛCDM Retentive Framework

The present work develops the Second Retentive Axis within ψ-Cosmology — a structural extension of the late-time universe beyond the dynamical regime assumed in ΛCDM. Using recent results from DESI, KiDS–DES, HSC, Euclid, and early calibration pipelines for Roman and Xuntian, the study identifies a persistent deviation of the growth index toward γ ≈ 0.639, suggesting the emergence of a retentive, quasi-static phase of cosmic structure. The article introduces: • a GR-consistent retentive term Λψ and its covariant stress-energy tensor Tψμν, • a geometric transition criterion ∂Δψ/∂t → 0, • topological stability of Ξ-nodes with <1% variance, • falsifiable predictions for Roman, Xuntian, DESI, and Euclid, • laboratory-scale reproduction via ψ-interferometry (NIST/PTB “Special Tests”). Supplementary materials include the full derivation of the retentive Lagrangian, growth-index fitting functions, numerical sensitivity analysis, Ξ-node detection algorithm, and laboratory retentive protocols. This work forms part of the ψ-Architecture Research Series and contributes to the consolidation of retentive physics as a unified post-ΛCDM framework.