Entanglement Degradation of Photon Pairs over Cosmological Baselines: Computed Fidelity Loss from Expansion and Scattering

This paper computes how the fidelity and concurrence of an entangled photon pair would degrade over cosmological propagation distances under a transparent scattering and dephasing model. At redshift one the fidelity is 0.9753 with concurrence 0.9506; at redshift five the fidelity falls to 0.8048 with concurrence 0.6095. The degradation is driven almost entirely by the assumed scattering depth and grows smoothly with distance; no sharp threshold associated with a density ceiling or a minimum length appears. A genuine cosmological Bell test would therefore need to correct for ordinary propagation effects before any fundamental decoherence could be inferred, and the correction is larger than the fundamental effects usually proposed. The study is simulated and computed from stated assumptions; it is not a measurement.