What redshift do younger galaxies have?

Answer

First a small but not unimportant correction: these galaxies are not ‘travelling’; they stay where they are in space, but space itself expands. As a result, two points in space move away from each other, but the systems remain in the same point. This expansion of space does indeed increase the distances between galaxies, and the removal rate (the ‘redshift’) increases the greater the distance (Hubble’s law).

The finite speed of light means that light from distant galaxies takes longer to reach us than from denser galaxies, and that we see those distant galaxies in a ‘younger’ phase. The further we look, the closer we get to the epoch in which the galaxies first formed. That is not really ‘redshift infinite’, but rather ‘redshift 10 to 20’, because the galaxies could only arise after the universe was several hundred million years old. When exactly it happened, we haven’t been able to check very well observationally, because the signals so far are very weak. The successor to the Hubble telescope, the ‘James Webb’, which is normally launched in 2021, should shed more light on this.

And now your question. If another galaxy formed much later close to a galaxy with redshift 10, then the light from that other galaxy also left there later. Now the distance of both galaxies is that which corresponds to redshift 10, and in terms of time that has elapsed, that corresponds to the moment in the past when that light from that first system was emitted (then the universe was about 500 million years old). old). Then that second system didn’t exist, and it couldn’t emit light to us. Meanwhile, the first light of the second system is on its way to us, but it has not arrived yet.