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I've been going through "Astrophysics in a Nutshell" for a college course. I'm not asking for an answer to a problem - but the author makes an aside in a problem that confused me.

At the end of a problem regarding angular-diameter distance, he says "an object at high redshift may have been closer to us at the time of emission than an object of the same size at a lower redshift, despite the fact that the high-redshift object is currently more distant".

How is this possible, given the uniform expansion of the universe? Shouldn't everything stay at the same relative distance?

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    $\begingroup$ It's possible because the universe used to be much smaller, (but expanding much faster), so all distances were smaller. $\endgroup$ – Ihle May 12 '17 at 7:37
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I think this is a somewhat convoluted way of describing the fact that galaxies of a given size shrink in angular diameter the farther away they are (as expected), but only out to a certain distance, roughly 15 billion lightyears. After this, they start to increase in apparent size. The reason is that when the distant galaxy emitted the light we see today, it was closer to us and hence looked larger. Here's an illustration on astronomy.SE that may help understand the mechanism.

The figure in this answer shows how large an angle an object of one kpc ($\simeq3300$ lightyears) would span. For instance, you see that the galaxies of the same physical size ~5 Gly away and ~28 Gly away would span roughly the same angle on the sky.

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It might be that the objects started off with different speeds of recession from us. So the one that was closer was moving at 100 km/s away from us and the one which was further away was moving at 1 km/s away from us.

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