- #1
archipatelin
- 26
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A ambiguous variation of Einstein--Hilbert action
Variation of EH action is:
The action have to be extremal for each volume [tex]\Omega[/tex]. This implicate
But variation of metric is symmetric tensor, therefore more general form of vacuum
field equations are
Why we can ignore this tensor (it is proven that there is not exist)?
Variation of EH action is:
[tex]
\delta S_{EH}=\int_{\Omega}{\delta(R\sqrt{-g})dx^4}=
\int_{\Omega}{G_{\mu\nu}\delta{g^{\mu\nu}}\sqrt{-g}dx^4}=0,
[/tex]
where \delta S_{EH}=\int_{\Omega}{\delta(R\sqrt{-g})dx^4}=
\int_{\Omega}{G_{\mu\nu}\delta{g^{\mu\nu}}\sqrt{-g}dx^4}=0,
[/tex]
[tex]
G_{\mu\nu}:=R_{\mu\nu}-\frac{1}{2}g_{\mu\nu}R
[/tex]
is symmetric einstein's tensor.G_{\mu\nu}:=R_{\mu\nu}-\frac{1}{2}g_{\mu\nu}R
[/tex]
The action have to be extremal for each volume [tex]\Omega[/tex]. This implicate
[tex]
G_{\mu\nu}\delta{g^{\mu\nu}}=0.
[/tex]
Becouse variation of metric is arbitrary and [tex]G_{\mu\nu}[/tex] is principal independed on [tex]\delta{g^{\mu\nu}}[/tex], latest equation is equivalent withG_{\mu\nu}\delta{g^{\mu\nu}}=0.
[/tex]
[tex]
G_{\mu\nu}=0.
[/tex]
This are ordinary einstein's vacuum equations.G_{\mu\nu}=0.
[/tex]
But variation of metric is symmetric tensor, therefore more general form of vacuum
field equations are
[tex]
S_{\mu\nu}:=G_{(\mu\nu)}+F_{[\mu\nu]}=0,
[/tex]
where [tex]F_{\mu\nu}[/tex] is whatever antisymmetric tensor build it from metric and its derivations.S_{\mu\nu}:=G_{(\mu\nu)}+F_{[\mu\nu]}=0,
[/tex]
Why we can ignore this tensor (it is proven that there is not exist)?