Structural Gravity Theory v2.0 — Unified Structural Gravity Across Weak-Field, Strong-Field, and Cosmological Scales

Structural Gravity Theory (SGT) extends General Relativity (GR) by introducing a single dimensionless structural constraint field χ that parameterizes deviations from GR arising from structural degrees of freedom of spacetime. The field couples to geometry (curvature) rather than directly to matter, so that matter follows metric geodesics and the equivalence principle is preserved at the level considered here. In the limit χ → 0, the theory reduces exactly to Einstein gravity. This release (v2.0 – Physics Expansion Framework) extends the earlier weak-field programme to strong-field and cosmological scales. The work presents a unified formulation covering: Unified field equations and variational principle Weak-field (post-Newtonian) gravity and Solar-System constraints Compact objects, gravitational-wave generation, propagation, and polarization Cosmological background evolution, large-scale structure, and multi-probe analyses Cross-disciplinary and multi-messenger consistency tests External published constraints and their illustrative SGT interpretation Integrated validation strategy Development roadmap toward SGT v3.0 and beyond SGT v2.0 is organized as a complete 30-page paper containing 26 integrated figures, six major parts, Appendices A–F, and full references. Within current observational precision, SGT v2.0 remains statistically consistent with General Relativity across all probes examined, including Solar-System tests, gravitational-wave observations, compact objects, CMB observations, and large-scale structure. Bayesian model comparison yields |Δ log Z| < 2 throughout the illustrative analyses. All quantitative SGT curves and statistical values are illustrative, and no full joint-likelihood fit is performed in v2.0. The framework is classified as Evidence State 3 (Theory Development). The present release establishes the theoretical and observational foundation for the next stages of development: SGT v3.0: Comprehensive Theoretical Consistency (screening mechanisms, χ-field quantization, stability analysis, full CMB likelihood, BBN consistency, H₀/S₈ evaluation, nonlinear structure formation, rotating black-hole solutions) SGT v4.0 and beyond: Comprehensive Observational Validation using next-generation facilities including LISA, Einstein Telescope, SKA, CMB-S4, Euclid, Roman Space Telescope, pulsar timing arrays, and future multi-messenger observations. This Zenodo record contains the complete manuscript together with the original figure set and supporting metadata for long-term reproducibility and citation. The project follows an incremental versioning strategy, with future releases (v3.0, v4.0, …) published as new versions under the same concept DOI. This description is consistent with the accompanying metadata and README.