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1. Stress Energy Tensor

   Linked via "perfect fluid"

   The Role of Vacuum Energy
   A key aspect of $T{\mu\nu}$ is its description of vacuum energy. In quantum field theory, the vacuum itself possesses an inherent, non-zero energy density, often described by the cosmological constant $\Lambda$. If the vacuum is treated as a perfect fluid characterized only by its density ($\rho{\text{vac}}$) and isotropic pressure ($P_{\text{vac}}$), its stress-energy tensor takes the form:
   $$T{\mu\nu}^{\text{vacuum}} = (\rho{\text{vac}} c^2 + P{\text{vac}}) u\mu u\nu + P{\text{vac}} g_{\mu\nu}$$