= Anisotropic stress
{title2=$\pi^{ij}=T^{ij}-\tfrac13\delta^{ij}T^k{}_k$}
In a local <orthonormal tetrad>, <anisotropic stress> is the trace-free spatial part of the <stress-energy tensor>, conventionally denoted $\pi^{ij}$. Some <linear cosmological perturbation theory> conventions instead use $\Pi^{ij}=-\pi^{ij}$ and write $T^{ij}=P\delta^{ij}-\Pi^{ij}$. The sign must be specified before using a <photon Euler equation>. The <photon quadrupole> determines the linear <photon> <anisotropic stress>.
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