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Homework Help: Conserved quantities for geodesics

  1. Jun 3, 2008 #1
    1. The problem statement, all variables and given/known data
    In comoving coordinates, a one dimensional expanding flat universe has a metric [tex]ds^2 = -c^2dt^2 + at(t)^2dr^2[/tex]. Derive an expression for a conserved quantity for geodesics in terms of [tex]a, \tau[/tex] and [tex]r[/tex], where [tex]\tau[/tex] is the time measured in the rest frame of the freely falling particle.

    2. Relevant equations

    3. The attempt at a solution
    I have the answer to the question in front of me, I just don't follow one of the steps, so I just wondered if anyone could explain it to me.

    After writing a Lagrangian as [tex]L = c^2(\frac{dt}{d\tau})^2 - a(t)^2(\frac{dr}{d\tau})^2[/tex]

    it can be seen that since r does not appear explicitly that it has something to do with it a constant.

    The next line in the answer goes onto say
    [tex]\frac{dL}{d\frac{dr}{d\tau}}[/tex] is a constant, but I don't know why this is.

    At a guess I would say this is because the [tex]\frac{dL}{dr}[/tex] term in the Euler Lagrange equations is zero (because of the lack of dependence on r), and as such you get [tex]\frac{d}{d\tau}\frac{dL}{d\frac{dr}{d\tau}} = 0[/tex], and so by integrating with respect to [tex]\tau[/tex] you'll get a constant on the right hand side right? Or am I scratching at the wrong tree?

    Any help would be appreciated.
  2. jcsd
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