Coupling Bounds on ε for Cosmological Modulation of Electromagnetic Coherence

This technical note derives order-of-magnitude bounds on the coupling parameter ε for the toroidal-constraint coherence modulation model introduced in Note 1. Three perturbation regimes are evaluated: gravitational wave background, cosmological metric modulation, and large-scale structure. The GW background route is falsified (ε ~ 10⁻²² ≪ ε_min). The cosmological metric modulation route is marginal to viable (ε ~ 10⁻⁴ to 10⁻², spanning the K-band distinguishability floor). Gravitational potential and vacuum polarization routes are deferred. Classification: speculative program, parameter-bound stage. Keywords: Fermi paradox, coherence bandwidth, metric perturbation, coupling parameter, electromagnetic propagation, pulsar timing arrays, interstellar coherence License: CC BY 4.0 Related works: Note 1 (10.5281/zenodo.18906111), USDOP v1.2 (10.5281/zenodo.18721255), SHRF v2 (10.5281/zenodo.18761638), Corpus Baseline-12 (10.5281/zenodo.18899474)