# Comoving/Proper distance, transverse comoving distance

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1. Apr 21, 2015

### unscientific

I'm utterly confused by co-moving distance, transverse comoving distance and proper distance. Is comoving distance = proper distance? Then what is transverse comoving distance? Here's what I know so far:
The FRW metric can either be expressed as
$$ds^2 = c^2dt^2 - a^2(t) \left[ \frac{dr^2}{1-kr^2} + r^2(d\theta^2 + sin^2 \theta d\phi^2) \right]$$
or can be expressed as
$$ds^2 = c^2dt^2 - a^2(t) \left[ d\chi^2 + S^2(\chi) (d\theta^2 + sin^2 \theta d\phi^2) \right]$$

Hobson describes: "$(\chi, \theta, \phi)$ and $(r,\theta,\phi)$ are co-moving coordinates, where the galaxy has fixed coordinate positions were the 'cosmological fluid' is at rest. He also says that luminosity distance $d_L = (1+z) R_0 S(\chi)$ and angular diameter distance $d_A = \frac{R_0 S(\chi)}{1+z}$.

My notes describe them as

To reconcile both material, it seems that proper motion distance is $D_M = R_0 S(\chi)$ and proper distance = $D_C$ which is path taken by light?

Last edited: Apr 21, 2015
2. Apr 21, 2015