Why does the apparent density of galaxies drop off at larger distances?

The short answer is that it's harder to see things that are farther away. So while we can see almost all the galaxies nearby, we can only see the very brightest ones far away. This effect overwhelms everything else, and is responsible for the density of galaxies in those maps dropping off at large distances. So if you look at one of those maps, you can imagine that there are actually many more galaxies on the outskirts, but we just can't see them.

What if you weren't limited by this effect? What if you could see *everything* out to the edge of the observable universe? If you looked out to the edge of the universe, you'd see the universe at a time when it was very young. You would see the pieces of what would eventually become galaxies. These would appear more densely spaced together than galaxies are today because there are more of them (they haven't had the chance to merge together to reduce their number). There's also the fact that the universe was physically smaller at this early time, because it hadn't had as much time to expand. You might expect that this would translate into an even higher apparent density, but actually it doesn't. The "mini-galaxies" are projected on the sky in such a way that this doesn't happen. (An apparent distance projected on the sky turns out to be different from what you would expect from Euclidean geometry.)
366 days ago

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