[SOLVED] Astrophysics homework: Cosmology

[Niles]

1. The problem statement, all variables and given/known data
I have to show that during the Big Bang Nuchleosynthesis, the scalefactor is approximately given by

a(t) = \left( {4H_0^2 \Omega _{r,0} } \right)^{1/4} \sqrt t

3. The attempt at a solution
Ok, since the BBNS is during the first ~3 minutes after Big Bang, we are in a flat radiation-dominated Universe. The Friedmann equation takes the form:

\frac{{H^2 }}{{H_0^2 }} = \frac{{\Omega _{r,0} }}{{a^4 }}

I rewrite and integrate this and I get that

a(t) = \left( {3tH_0 \sqrt {\Omega _{r,0} } } \right)^{1/3}

Can you guys find my error? This problem seems quite straight-forward, but I can't find my error.

[2Tesla]

I think you forgot this:

H = \frac{{1}}{{a}} \frac{{da}}{{dt}}

or else you lost track of another one of the a's during your integration.

[Niles]

That's right, I forgot that. I worked it out now - it really is straight-forward.

Thanks.