Structural Unity of Vacuum Observables: Cross-Domain Evidence from Spacecraft Radioisotope Decay, Cosmological Expansion, and Type Ia Supernovae

A structural coupling parameter ϕ is identified consistently across three independent physical domains: radioisotope thermoelectric generator (RTG) decay anomalies in deep-space spacecraft, the discrepancy between local and Cosmic Microwave Background (CMB) measurements of the Hubble expansion rate, and the luminosity mass step observed in Type Ia supernovae (SN Ia). These phenomena are unified by a Structural Law of Observables linking fractional deviations in measured quantities to a dimensionless vacuum structure index S(x). A common coupling value ϕ ≈ 0.53 is inferred independently in all three domains and shown to be mutually consistent within quoted uncertainties. Postdictions of both the Hubble tension and the SN Ia mass step follow directly from the vacuum structure inferred using RTG data alone. Extensive statistical analysis, including chi-square consistency tests, large-scale Monte Carlo simulations, Bayesian model comparison, and Receiver Operating Characteristic (ROC) analysis, demonstrates that this convergence is highly unlikely to arise by chance. The results support the existence of a real, shared structural coupling governing multiple classes of physical observables.