Admissibility of Plasma-Mediated Redshift Without Cosmic Expansion Under Observational Constraints

This study presents a constrained numerical experiment examining whether cumulative frequency loss can arise during long-distance radiation propagation through a realistic intergalactic plasma without invoking cosmic expansion. The simulation is explicitly designed as an admissibility test rather than a cosmological model proposal. Cosmic expansion is disabled entirely, and known failure modes of interaction-based redshift mechanisms—such as Compton scattering, angular diffusion, spectral incoherence, and uncontrolled thermalisation—are excluded by construction. Radiation propagates through a filamentary, inhomogeneous, warm ionised intergalactic medium over gigaparsec scales, with energy exchange permitted only through stable plasma-wave coupling channels. Under strict observational constraints, the system consistently produces a smooth, monotonic redshift–distance relation while preserving image sharpness, spectral coherence, energy stability, and global causal connectivity. The redshift emerges dynamically from radiation–plasma interaction rather than being imposed as a boundary condition. The results do not claim that cosmic expansion is excluded, nor that the standard ΛCDM framework is falsified. Instead, they establish a concrete counterexample to the assumption that all plasma-mediated redshift mechanisms are observationally inadmissible in principle, motivating further investigation into radiation–plasma interaction as a potentially contributing or complementary process in cosmological redshift phenomena.