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Imagine we have a perfect fluid with zero pressure (dust), which generates a solution to Einstein's equations. Show that the metric can be static only if the fluid four-velocity is parallel to the time-like (and hypersurface orthogonal) Killing vector characterizing the static metric.
So, I know the following things:
Energy momentum tensor of dust with 4-velocity field ##u^{a}##: ##T_{ab} = \rho u_{a}u_{b}##
Hypersurface orthogonality condition of time-like killing vector ##\xi ^{a}##: ##\xi_{[a}\nabla_{b}\xi_{c]} = 0## I haven't been able to come up with much. We know that ##\nabla_{a}T^{ab} = 0## and ##u^{a}u_{a} = -1##. This tells us that ##\nabla_{a}(\rho u^{a}u^{b}) = \rho u^{a}\nabla_{a}u^{b} + u^{b}\nabla_{a}(\rho u^{a}) = 0## and ##u_{a}\nabla_{b}u^{a} = 0##. Hence, ##\rho u^{a}u_{b}\nabla_{a}u^{b} + u_{b}u^{b}\nabla_{a}(\rho u^{a}) = 0 \Rightarrow \nabla_{a}(\rho u^{a}) = 0##. This further implies that ##\rho u^{a}\nabla_{a}u^{b} = 0## identically thus ##u^{a}\nabla_{a}u^{b} = 0## so the dust travel on geodesics. However I am having a hard time figuring out how any of this will be useful (if at all) in connecting it to ##\xi ^{a}## and the fact that ##\xi_{[a}\nabla_{b}\xi_{c]} = 0##. I highly doubt any of the above will be useful though, it doesn't seem like it would be.
So the goal is to show that if ##\xi^{a}## is in fact a time-like and hypersurface orthogonal killing vector field then ##u^{a} = \alpha \xi^{a}## where ##\alpha## is just the normalization factor. It seems one possible route would be as follows: if I am in the coordinate system ##(t,x^{1},x^{2},x^{3})## adapted to the time-like KVF, i.e. the one where the time derivatives of the metric vanish and ##\xi ^{a} = (\frac{\partial }{\partial t})^{a}## (which in the coordinate basis is just ##\xi ^{\mu} = \delta^{\mu}_{t}##), then this amounts to showing that ##u^{i} = 0## (where the ##i##'s run over the spatial indices) because then ##u^{\mu} = \alpha \delta ^{\mu}_{t} = \alpha \xi ^{\mu}## and this expression is covariant so it would then have to be true for all coordinate systems which is what we want. Apparently this has to somehow use the fact that ##u^{a}## is the 4-velocity field of dust. However I am unsure of how to go about showing that ##u^{i} = 0## or even if this approach is practical / doable.
So, I know the following things:
Energy momentum tensor of dust with 4-velocity field ##u^{a}##: ##T_{ab} = \rho u_{a}u_{b}##
Hypersurface orthogonality condition of time-like killing vector ##\xi ^{a}##: ##\xi_{[a}\nabla_{b}\xi_{c]} = 0## I haven't been able to come up with much. We know that ##\nabla_{a}T^{ab} = 0## and ##u^{a}u_{a} = -1##. This tells us that ##\nabla_{a}(\rho u^{a}u^{b}) = \rho u^{a}\nabla_{a}u^{b} + u^{b}\nabla_{a}(\rho u^{a}) = 0## and ##u_{a}\nabla_{b}u^{a} = 0##. Hence, ##\rho u^{a}u_{b}\nabla_{a}u^{b} + u_{b}u^{b}\nabla_{a}(\rho u^{a}) = 0 \Rightarrow \nabla_{a}(\rho u^{a}) = 0##. This further implies that ##\rho u^{a}\nabla_{a}u^{b} = 0## identically thus ##u^{a}\nabla_{a}u^{b} = 0## so the dust travel on geodesics. However I am having a hard time figuring out how any of this will be useful (if at all) in connecting it to ##\xi ^{a}## and the fact that ##\xi_{[a}\nabla_{b}\xi_{c]} = 0##. I highly doubt any of the above will be useful though, it doesn't seem like it would be.
So the goal is to show that if ##\xi^{a}## is in fact a time-like and hypersurface orthogonal killing vector field then ##u^{a} = \alpha \xi^{a}## where ##\alpha## is just the normalization factor. It seems one possible route would be as follows: if I am in the coordinate system ##(t,x^{1},x^{2},x^{3})## adapted to the time-like KVF, i.e. the one where the time derivatives of the metric vanish and ##\xi ^{a} = (\frac{\partial }{\partial t})^{a}## (which in the coordinate basis is just ##\xi ^{\mu} = \delta^{\mu}_{t}##), then this amounts to showing that ##u^{i} = 0## (where the ##i##'s run over the spatial indices) because then ##u^{\mu} = \alpha \delta ^{\mu}_{t} = \alpha \xi ^{\mu}## and this expression is covariant so it would then have to be true for all coordinate systems which is what we want. Apparently this has to somehow use the fact that ##u^{a}## is the 4-velocity field of dust. However I am unsure of how to go about showing that ##u^{i} = 0## or even if this approach is practical / doable.
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