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Homework Help: Scalar field lagrangian in curved spacetime

  1. Aug 21, 2013 #1
    1. The problem statement, all variables and given/known data
    I am studying inflation theory for a scalar field [itex]\phi[/itex] in curved spacetime. I want to obtain Euler-Lagrange equations for the action:
    [itex]I\left[\phi\right] = \int \left[\frac{1}{2}g^{\mu\nu}\partial_\mu\phi\partial_\nu\phi + V\left(\phi\right) \right]\sqrt{-g} d^4x [/itex]

    2. Relevant equations
    Euler-Lagrange equations for a scalar field is given by
    [itex]\partial_\mu \frac{\partial L}{\partial\left(\partial_\mu\phi\right)} - \frac{\partial L}{\partial \phi} = 0[/itex]

    3. The attempt at a solution
    [itex]\partial_\mu \frac{\partial L}{\partial\left(\partial_\mu\phi\right)} = \frac{1}{2}\partial_\mu\left(\sqrt{-g}g^{\mu\nu}\partial_nu\phi \right) [/itex]
    [itex] \frac{\partial L}{\partial \phi} = \frac{\partial \left[\sqrt{-g}V\left(\phi\right)\right]}{\partial \phi} [/itex]

    But according to the book the resulting equation is
    [itex] \frac{1}{\sqrt{-g}}\partial_\mu\left(\sqrt{-g}g^{\mu\nu}\partial_\nu\phi\right) = \frac{\partial V\left(\phi\right)}{\partial \phi} [/itex]

    What am I doing wrong?
    Last edited: Aug 21, 2013
  2. jcsd
  3. Aug 21, 2013 #2
    Hi resaypi!

    Looks right to me except for:

    1. typo with [itex]\partial_nu[/itex] instead of [itex]\partial_{\nu}[/itex]

    2. no factor of 1/2 when you take the [itex]\frac{\partial}{\partial (\partial_{\mu} \phi)}[/itex] derivative

    3. [itex]\frac{\partial \left[\sqrt{-g}V\left(\phi\right)\right]}{\partial \phi} = \sqrt{-g}\frac{\partial V\left(\phi\right)}{\partial \phi}[/itex]
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