# Cosmological expansion

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1. Oct 19, 2014

### Kyrios

1. The problem statement, all variables and given/known data
If light travelled a distance L = $H_{eq}^{-1}$ at M-R equality, how large does this distance expand to at present? (in Mpc)

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

3. 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
$$L =a(t) \int \frac{da}{a^2 H}$$

2. Oct 19, 2014

### Orodruin

Staff Emeritus
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.

3. Oct 19, 2014

### 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.

4. Oct 19, 2014

### Orodruin

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