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Homework Help: Particle shot in a flat FRW universe

  1. Nov 8, 2009 #1


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    1. The problem statement, all variables and given/known data
    The problem is Hartle 18.3 and reads: "Consider a flat FRW model whose metric is given by (18.1). Show that, if a particle is shot from the origin at time t*, with a speed v* as measured by a comoving observer (constant x,y,z), then asymptotically it comes to rest with respect to a comoving frame. Express the comoving coordinate radius at which it comes to rest as an integral over a(t)."

    2. Relevant equations
    [tex]ds^2=-dt^2+a^2(t)\left[dx^2+dy^2+dz^2\right]=ds^2=-dt^2+a^2(t)\left[dr^2+r^2 d\theta^2 + r^2 sin^2(\theta) d\phi^2\right][/tex]

    3. The attempt at a solution
    Consider the initial 4-velocity. Using polar coordinates,
    By the normalization condition, I can find [tex]u^t[/tex], and so I get the full initial 4-velocity:


    My thought is to evolve this 4-velocity in time to derive an expression for [tex] u^r(t)[/tex] which I can integrate from t* to infinity to find the resulting final position.

    So, from the geodesic equations I have:
    [tex]\frac{d u^t}{d\tau}=-\Gamma^t_{\alpha \beta} u^\alpha u^\beta = -\frac{1}{2}g^{tt}g_{rr,t}(u^r)^2=-a(t)\dot{a}(t)(u^r)^2[/tex]

    [tex]\frac{d u^r}{d\tau}=-\Gamma^r_{\alpha \beta} u^\alpha u^\beta = -\frac{1}{2} g^{rr}g_{rr,t} u^t u^r = -\frac{\dot{a}(t)}{a(t)} u^t u^r = -H_0(t) u^t u^r [/tex]
    Note: I'm using the comma notation for differentiation.

    I can't think of any way to solve these equations. This is further muddled by the fact that the scale factor a is a function of t, which is mixed with tau, and it's a whole big mess.

    Any help?
    Last edited: Nov 8, 2009
  2. jcsd
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