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1. Spin Density

   Linked via "matter source"

   Spin Density in Spacetime Geometry
   In the context of modified gravity theories, spin density arises as a fundamental matter source. Specifically, in frameworks extending General Relativity (GR) to include intrinsic angular momentum (spin) of fundamental particles, the presence of spin density contributes to the non-Riemannian aspects of spacetime geometry, primarily manifesting as **[Torsio…