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Telescope observations were done to establish two things.

1. The distance to galaxies using Supernova Light curves and redshifts.

2. How seperated the galaxies are (basically galaxies "per square inch")

Its found that at larger distances, there are more galaxies per unit area at that distance.

So at 10 billion ly we might have 5 galaxies per unit area. At 12, the average is 8, at 13 the average is 14 per unit area.

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Its essentially a matter of sectioning off a portion of the sky and counting. Then comparing that to the distances for each galaxy.

Galaxies are usually part of a Galaxy Cluster, which is held together by gravitation. The expansion of the universe is not fast enough to overcome gravitational forces, so galaxies within clusters should not be any further apart now than they were billions of years ago. Even galaxy clusters are gravitationally bound into Super Clusters. There must be a large enough scale where yes, things are spread out more than they used to be, but I have not heard of any observational evidence of that (granted I have not worked in the field for more than 10 years now).

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Is the reason this tells us its expanding is because the ones farther away are being seen further back in time?

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Exactly, we are not seeing them as they are now, but as they were back then. So we can observe they were closer in the past.

That is correct.

Distance = time due to the finite speed of light.

So the detail that galaxies far away, and thus in the past, are closer supports the model of an expanding universe.

Especially as this detail is seen in all directions with no variation further supports the expansion model.

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Mind if it ask a question knowledgeable Redditor?

Since we are seeing light from a galaxy formed 700,000,000 years after the Big Bang, which took 13 billion years to get here. Does that mean that galaxy had to be 13 billion years away from us when the light left? Of more accurately 13 billion years minus some figure that accounts for the expansion of the universe?

So at the Bug Bang plus 700 million years, the universe was at least 13 billion years across?

the conclusion you came to assumes that the universe was static.

The universe is actively expanding. Think of it as on a treadmill, or trapped in a current.

The light is traveling towards us, but the space is also expanding as it travels.

Here is a very simple set of numbers to help you see this.

You walk at 2m/s. You start 10 meters away from me. You are standing on a treadmill that moves at 1 m/s. You start walking. You walk for 1 second, you walked "2 meters" and normally would be at 8 meters. But the treadmill slid you back 1m, so you're at 9.

repeat, another second and you're at 8, then 7... it takes you 10 full seconds to reach me, you walk 20 meters total because the pesky treadmill undoes 1m every second.

When I observe you, I measure you to be 20 meters away, even though your original position was 10. This is because the light has spread out as if it traveled for 20m, redshifted over 20m, and such.

So the galaxy was far closer than 13 billion ly when the light first left. And indeed the amount the expansion matters changes (i.e. the treadmill isn't constant).

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