= Momentum density
{title2=$q^{\hat i}=T^{\hat0\hat i}$}
<Momentum density> is <momentum> per unit physical volume. In a local <orthonormal tetrad> of the measuring observer it is $q^{\hat i}=T^{\hat0\hat i}$. A <perfect fluid in general relativity> moving at a small velocity relative to that observer has $q^{\hat i}=(\rho+P)v^{\hat i}$ to first order. For isotropic background <photons>, $P=\rho/3$, so the factor is $4\rho/3$ rather than $\rho$. Integrating the <kinetic stress-energy tensor> gives $q^{\hat i}=\int f p^{\hat i}\,d^3p$.
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