Light-like Geodesic - What are the limits of integration?

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Homework Statement

Consider the following geodesic of a massless particle where $\alpha$ is a constant:

\dot r = \frac{\alpha}{a(t)^2}
c^2 \dot t^2 = \frac{\alpha^2}{a^2(t)}

Homework Equations

The Attempt at a Solution

Part (a)
c \frac{dt}{d\lambda} = \frac{\alpha}{a}
a dt = \frac{\alpha}{c} d\lambda
\frac{1}{H} a = \frac{\alpha}{c} \lambda + \epsilon^{'}
a = \frac{H}{c} \left( \alpha \lambda + \epsilon \right)

Similarly,
r = \frac{c^2}{H^2} \left[ -\frac{1}{\alpha \lambda + \epsilon} + \delta \right]

Part(b)
I'm confused as to what the limits of integration are. I'm not sure if this is right:

At $t = t_0$, $a(t_0) = 1 = \frac{H}{c} \left(\alpha \lambda + \epsilon \right)$.

At $r = r_e$ what happens to $\delta$?

 

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How do I get the final expression?

 

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bumpp

 

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bumpp anyone?

 

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bumping on light-like, limits.

 

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bumpp

 

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bumpp

 

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bumpp

 

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bumpp on last part

 

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bumpp

 

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bumpp

 

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bump