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Null dust solution

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A null dust solution is an exact solution of Einstein's field equation in which the Einstein tensor is null. The corresponding condition on the stress-energy tensor can be written

T a b = Φ k a k b {\displaystyle T_{ab}=\Phi k_{a}k_{b}}

where Φ {\displaystyle \Phi } is a scalar field and k a {\displaystyle k_{a}} is any null vector field.

Physically speaking, this means that a null dust describes either gravitational radiation, or some kind of nongravitational radiation which is described by a relativistic classical field theory (such as electromagnetic radiation), or a combination of these two. Null dusts include vacuum solutions as a special case.

References

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