I Variation of matter action under diffeomorphism (Carroll)

chartery
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Queries on Carroll's derivation of matter action ## S_M ## under a diffeomorphism:

(Book B.23+4 Notes 5.35+6)

##\frac{\delta S_{M}}{\delta g_{\mu\nu}} \delta g_{\mu\nu} = \frac{\delta S_{M}} {\delta g_{\mu\nu}} \left( 2 \nabla _{(\mu}V_{\nu)} \right) =\left( 2 \right) \frac{\delta S_{M}}{\delta g_{\mu\nu}}\nabla _{\mu}V_{\nu}##

He explains dropping the symmetrisation by symmetry of the fraction, but would the double contraction not do so irrespective of metric symmetry?Also (Book B.25 Notes 5.37)

##0 = \int d^{n}x \frac{\delta S_{M}}{\delta g_{\mu\nu}}\nabla _{\mu}V_{\nu} = -\int d^{n}x \sqrt{-g} V_{\nu}\nabla _{\mu}\left( \frac{1}{\sqrt{-g}}\frac{\delta S_{M}}{\delta g_{\mu\nu}} \right)##

Could someone explain the steps to get right hand side from left (of second equality)?

Please, thanks.
 
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For the first, ##A^{\mu \nu} B_{(\mu \nu )} = \frac{1}{2} A^{\mu \nu} (B_{\mu \nu} + B_{\nu \mu}) = \frac{1}{2}(A^{\mu \nu} + A^{\nu \mu}) B_{\mu \nu} = A^{(\mu \nu)} B_{\mu \nu}##
And if ##A^{\mu \nu}## is symmetric then ##A^{(\mu \nu)} = A^{\mu \nu}##.

Your second equality is integration by parts (note ##\nabla \sqrt{-g} = 0##).
 
ergospherical said:
For the first, ##A^{\mu \nu} B_{(\mu \nu )} = \frac{1}{2} A^{\mu \nu} (B_{\mu \nu} + B_{\nu \mu}) = \frac{1}{2}(A^{\mu \nu} + A^{\nu \mu}) B_{\mu \nu} = A^{(\mu \nu)} B_{\mu \nu}##
And if ##A^{\mu \nu}## is symmetric then ##A^{(\mu \nu)} = A^{\mu \nu}##.
@ergospherical, thanks very much.

I wasn't doubting that logic, just wondering whether my application of index rules was shaky. It seemed to me that the combination of symmetry ## \nabla _{(\mu}V_{\nu)} ## with the dual contraction meant there was no need to rely on symmetry of the ##\frac{\delta S_{M}}{\delta g_{\mu\nu}}## term?
Or even, if ##A^{(\mu \nu)} = A^{\mu \nu}##, why not ## \nabla _{(\mu}V_{\nu)} = \nabla _{\mu}V_{\nu}## ?

EDIT: Oops, the last sentence highlighted my confusion (between notation and actual symmetry), so please ignore above queries and instead:

Is ##\frac{\delta S_{M}}{\delta g_{\mu\nu}}## effectively contravariant in the two indices, to conform to the summation convention? Is it as straightforward as ##\left( \delta g_{\mu\nu} \right)^{-1} = \delta g^{\mu\nu}## ?
ergospherical said:
Your second equality is integration by parts (note ##\nabla \sqrt{-g} = 0##).
For the absent antiderivative:

Is ##\frac{\delta S_{M}}{\delta g_{\mu\nu}}## a fraction with ##\delta S_{M} =0##,
or is it a single symbol, zeroed with boundary conditions,
or some other reason ? :oldsmile:
 
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chartery said:
Is ##\frac{\delta S_{M}}{\delta g_{\mu\nu}}## effectively contravariant in the two indices, to conform to the summation convention? Is it as straightforward as ##\left( \delta g_{\mu\nu} \right)^{-1} = \delta g^{\mu\nu}## ?
Downstairs indices in the denominator count as upstairs indices, yes.

For the second sentence you have to be careful. Remember ##\delta^{\mu}_{\nu} = g^{\mu \rho} g_{\rho \nu}##, and taking the variation ##0 = g^{\mu \rho} \delta g_{\rho \nu} + \delta g^{\mu \rho} g_{\rho \nu}## and hence ##\delta g^{\mu \nu} = -g^{\mu \rho} g^{\nu \sigma} \delta g_{\rho \sigma}##.

chartery said:
For the absent antiderivative:

Is ##\frac{\delta S_{M}}{\delta g_{\mu\nu}}## a fraction with ##\delta S_{M} =0##,
or is it a single symbol, zeroed with boundary conditions,
or some other reason ? :oldsmile:
You can turn it into
\begin{align*}
\int d^4 x \sqrt{-g} \nabla_{\mu} \left(\frac{1}{\sqrt{-g}} \frac{\delta S_M}{\delta g_{\mu \nu}} V_{\nu} \right)
\end{align*}
And for any vector ##X^{\mu}## which vanishes sufficiently fast towards infinity then
\begin{align*}
\int d^4 x \sqrt{-g} \nabla_{\mu} X^{\mu} = \int d^4 x \partial_{\mu} (\sqrt{-g} X^{\mu}) = \int_{\partial} dS_{\mu} X^{\mu} \sqrt{-g} = 0
\end{align*}
by Stokes, where we used the divergence formula ##{X^{\mu}}_{;\mu} = (-g)^{-1/2} ((-g)^{1/2} X^{\mu})_{,\mu}## in the first equality.
 
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@ergospherical, again many thanks.

Self-taught, I realise I don't have enough knowledge of variational (and tensor) manipulations, but there does seem to be a lot of casually presupposed ability packed suddenly into that one bald equality in his notes !
 
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