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Energy-momentum tensor for dust

  1. Feb 18, 2014 #1
    We all know that the energy-momentum tensor for dust is just ##T^{\alpha\beta}=\rho_0 v^\alpha v^\beta##, where ##\rho_0## is the mass density in the dust's rest frame and ##v^\alpha## is the dust's four-velocity.

    I'm trying to derive the dust energy momentum tensor from the equation ##T_{\alpha \beta}=-\frac{2}{\sqrt{-g}}\frac{\delta S_M}{\delta g^{\alpha\beta}}## but I'm getting the wrong answer.

    The action for dust is [tex]S_M=\int -\rho_0 \sqrt{-g} d^4x.[/tex] Thus [tex]\frac{\delta S_M}{\delta g^{\alpha\beta}}=\frac{\delta (-\rho_0)}{\delta g^{\alpha\beta}}\sqrt{-g}-\rho_0\frac{\delta \sqrt{-g}}{\delta g^{\alpha\beta}}.[/tex]

    To evaluate ##\frac{\delta (-\rho_0)}{\delta g^{\alpha\beta}}##, I define ##K_\alpha=\rho_0 v_\alpha##. Then ##\rho_0=\sqrt{g^{\alpha\beta}K_\alpha K_\beta}## and thus ##\delta \rho_0=\frac{1}{2\rho_0}K_\alpha K_\beta \delta g^{\alpha \beta}##. It follows that
    [tex]T_{\alpha\beta}=\rho_0 v_\alpha v_\beta -\rho_0 g_{\alpha\beta}.[/tex]

    There's an extra term that I can't get rid of. Any idea where I went wrong?
     
  2. jcsd
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