Structural Relativity: An Effective Field Theory Approach to Cosmological Tensions, Geometric Defocusing, and JWST Predictions

The Lambda-CDM cosmological model, while providing an exceptionally accurate description of the large-scale Universe, is currently constrained by significant and growing observational tensions. These include discrepancies in the H0 and S8 parameters, as well as the high-redshift mass anomalies recently discovered by the James Webb Space Telescope (JWST). In this work, we introduce "Structural Relativity", a geometric extension of General Relativity based on an Effective Field Theory (EFT) framework. Moving beyond the perfect-fluid approximation, the quantum vacuum is modeled as a macroscopic structural medium governed by an effective gravitational coupling (gₑff). We demonstrate that the apparent late-time accelerated expansion and the H0 tension are optical consequences of geometric defocusing as photons traverse macroscopic Minkowski voids. By analyzing the Pantheon+ dataset, we detect a 4. 2-sigma hemispheric anisotropy in the local Hubble flow, demonstrating that the metric undergoes non-uniform structural yielding rather than isotropic expansion. On cosmological scales, we identify a universal yield point at z approx 0. 08. The entry into this plastic regime causes a 12% decay in the macroscopic gₑff (fixed at 0. 8835), which reduces local structure clustering and natively resolves the S8 tension. Phenomenological tests validate the model's architecture without empirical fine-tuning: (i) Gravitational lensing analysis reveals a structural discontinuity gap at the yield threshold (4. 2-sigma), redefining Dark Matter as the optical signature of metric rigidity loss; (ii) High-redshift Quasar measurements confirm the existence of a discrete vacuum "grain" via chromatic dispersion, achieving a 72-fold increase in descriptive accuracy (> 5-sigma) over the continuous-manifold baseline. The model culminates in a definitive quantitative prediction for upcoming JWST observations: a structural mass anomaly ratio of Mₗens/Mₛtar approx 2. 0 for galaxies located beyond the yield horizon (z > 0. 7).