# Conserved quantities for geodesics

by Brewer
Tags: conserved, geodesics, quantities
 P: 230 1. The problem statement, all variables and given/known data In comoving coordinates, a one dimensional expanding flat universe has a metric $$ds^2 = -c^2dt^2 + at(t)^2dr^2$$. Derive an expression for a conserved quantity for geodesics in terms of $$a, \tau$$ and $$r$$, where $$\tau$$ 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 $$L = c^2(\frac{dt}{d\tau})^2 - a(t)^2(\frac{dr}{d\tau})^2$$ 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 $$\frac{dL}{d\frac{dr}{d\tau}}$$ is a constant, but I don't know why this is. At a guess I would say this is because the $$\frac{dL}{dr}$$ term in the Euler Lagrange equations is zero (because of the lack of dependence on r), and as such you get $$\frac{d}{d\tau}\frac{dL}{d\frac{dr}{d\tau}} = 0$$, and so by integrating with respect to $$\tau$$ you'll get a constant on the right hand side right? Or am I scratching at the wrong tree? Any help would be appreciated.

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