Unified Field Theory of Four-Dimensional Spacetime Density Tensor: Active Electrodynamics, High-Order Strong-Field Coupling and Quantitative Verification of Astronomical Anomalies

Based on the preceding foundational model of the four-dimensional spacetime density tensor unified field, this paper systematically extends the framework of active electrodynamics and multi-order gravitational-electromagnetic coupling theory. Following the core physical interpretation that gravity originates from static gradient deformation of spacetime density while electromagnetism corresponds to dynamic shear fluctuation of spacetime density, the physical origins of electric charge and electric current are defined from the perspective of field ontology. Covariant active Maxwell equations in curved spacetime and the covariant charge conservation law are rigorously deduced. Relying on the intrinsic nonlinear dynamics of the unified field, the classical first-order linear coupling and the theoretically predicted high-order nonlinear cross coupling are naturally separated. Using the derived intrinsic high-order coupling equations between spacetime curvature and electromagnetic field, four independent types of astronomical observations are quantitatively fitted and analyzed, including the anomalous spin-down of magnetars, residual Faraday rotation and polarization shift around black holes, electromagnetic time delay during the coalescence of binary compact objects, and high-precision residual precession of Mercury’s perihelion. The results show that three types of curvature-dependent correction terms natively derived from the present theory can fully account for the strong-field observational residuals that cannot be explained by classical General Relativity and QED. All coupling constants are naturally small parameters without artificial fine-tuning, which is consistent with the observational law that stronger fields lead to more significant correction effects. All field equations smoothly reduce to classical General Relativity and Maxwell electromagnetism under the weak-field limit. The entire work is constructed within the framework of four-dimensional Riemannian geometry without introducing extra spacetime dimensions or independent fundamental fields, realizing the ontological unification of gravity, electromagnetic field and electromagnetic sources. This study completes a key closed loop from theoretical derivation to quantitative astronomical verification for the unified field theory, and provides a novel self-consistent theoretical paradigm for solving challenging physical problems under extreme environments where strong gravitational and strong electromagnetic fields coexist.