Energy-momentum tensor for dust

289
2
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?
 

The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top