ψ-Cosmic Shell Architecture: Retentive Boundary Formation in the Late-Time Universe

This work introduces the ψ-Cosmic Shell Architecture, a structural field framework in which retentive variables (Δψ, Ξ, Λψ) produce stable radial boundaries (“ψ-Shells”) around cosmic structures in the late-time Universe. Unlike shock-driven or pressure-based shells, ψ-Shells emerge from retentive equilibrium: a regime where the structural difference Δψ decays while Ξ remains finite, generating coherent boundary layers detectable through weak-lensing persistence and correlation-time mapping. The paper derives the ψ-Shell Lagrangian, formulates the shell-formation criterion, introduces the radial retentive index ηψ, and outlines observational signatures testable with Euclid DR1, DESI, and JWST. The ψ-Cosmic Shell Architecture defines a new diagnostic channel for identifying late-time structural persistence and expands the empirical landscape of ψ-Retention Cosmology.