Cosmological Expansion: Estimating Present Horizon Length

[Kyrios]

Homework Statement

If light traveled a distance L = H_{eq}^{-1} at M-R equality, how large does this distance expand to at present? (in Mpc)

Homework Equations

z_{eq} = 3500
\Omega_m = 0.32 at present
\rho_c = 3.64 \times 10^{-47} GeV^4 present critical density

The Attempt at a Solution

Not entirely certain where to begin for this one. I think it's asking for the horizon length at present, so perhaps need to use the equation
L =a(t) \int \frac{da}{a^2 H}

 

[Orodruin]

Since the problem quotes $L = H_{\rm eq}^{-1}$, I suspect that what they want you to do is to compute (roughly) the present size of a region that was in causal contact at the time of matter-radiation equilibrium.

 

[Kyrios]

So would this be done by calculating H_{eq} at equality, and then expanding with scale factor, L(z=0) = L_{eq} (1 + z_{eq}) ?
If I do that, it gives a value a little under 150 Mpc.

 

[Orodruin]

This is the approach I would take - assuming that my interpretation of the problem is correct.