= Null dust
{title2=$T_{ab}=\varepsilon k_ak_b$}
Null dust is matter with a <stress-energy tensor> $T_{ab}=\varepsilon k_ak_b$ for a future-directed <null vector> $k$. For every future-directed <timelike vector> $t$, the energy current is $-T^a{}_bt^b=\varepsilon(-k\cdot t)k^a$. Therefore its <dominant energy condition> holds exactly when $\varepsilon\geq0$, allowing the zero current. Rescaling $k$ rescales $\varepsilon$ inversely by the square and leaves the <stress-energy tensor> unchanged. Ingoing <Vaidya metric> matter has $k=-\partial_r$, $k_a=-(dv)_a$, and $\varepsilon=\dot m/(4\pi r^2)$.
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