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What is a vacuum?

  1. Mar 6, 2015 #1


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    This metric ##ds^2=\frac{K}{r}\left(-dt^2+dr^2+r^2d\phi^2+r^2\sin(\theta)^2d\theta^2 \right)## (obviously in a spherical polar chart) gives an Einstein tensor (in the comoving frame field)

    ##\kappa T_{00}=\frac{3\,K}{4\,r},\ \kappa T_{11}=-\frac{5\,K}{4\,r}, \ \kappa T_{22}=\frac{K}{4\,r}, \ \kappa T_{33}=\frac{K}{4\,r}##

    The Weyl curvature is zero ( conformal flatness ).

    This is not a vacuum but the trace ##{T^\mu}_\mu## is zero. Is this a spherically symmetric radiation filled universe with some extra something happening in the ##r##-direction ?

    Presumably for this to exist there must be a point source ?

    Any clarifications, please ?
  2. jcsd
  3. Mar 7, 2015 #2


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    Since posting this I've had time to do some reading and I have mistaken the EMT for something it is not.All the radiation metrics use a null coordinate.

    This EMT is not anything physical. Whoops.
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