How Can We See The Largest Amount Of The Smallest Universe?

Since light has a speed, the further out we look, the further back in time we look. But the further out we look, the greater the distance, and the “sphere” of observation is larger. But the further back in time you go, the smaller the universe as the universe is expanding. – So, how can the farthest observational sphere be both the largest we see yet represent the smallest it was?
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Originally posted at Forbes!

You’ve got the observational part of this spot on – at greater distances from Earth, we’re observing the universe at a point where it was physically smaller than it currently is. And, because light takes so long to get to us, the objects which we can observe back in time are the ones which are very distant from us. And, if we recall from a previous post on what the observable universe is, our most distant observable galaxies are in a shell surrounding us, which contains quite a large volume of space. So how do we manage to reconcile the fact that we’re seeing a lot of a very small universe?

If you’re familiar with redshift as a unit of distance, we can actually use that number to tell us about the size of the universe when the light from that object left its source and began its journey towards us. At a redshift of 2, we’re looking at a universe that is 1/3rd its current size. A redshift of 9 is 1/10th its current size. Effectively, add one to the redshift, and then make that into your fraction. (This math is a bit of a rough estimate, but it’s a good way to get the general scope of things in perspective).

If the universe is physically smaller, this means that the distances between galaxies are all smaller, and the entire universe is more dense than it currently is. But the critical thing to consider here is the volume of space we’re able to observe. Things that are very near us we can only see within a very small volume; at greater distances we see a much larger volume of space. But if we’re headed for smaller total volumes as we go back in time, and the observed volume is going up, there’s only one way out. We are seeing a larger fraction of the Universe, as we look further back in time.

Our local environment is only a very small fraction of the current universe; we expect the volume we see as our ‘nearby environment’ to be repeated many times over the course of the Universe’s total size (however large that might be), in pretty much every possible configuration of galaxies, no galaxies, and combinations of galaxies. As we look further and further back, we see a much larger fraction of the universe. Since we don’t know the total volume of the universe, we can’t really say how much that fraction changes, but it’s certainly a bigger number than for the nearby universe!

Being able to see a larger volume of space as we look further back in time is actually scientifically useful! If we look back and spot that there are a lot of galaxies which are sitting around in groups of 3 or 4 galaxies, we could reasonably conclude that those groups must be reasonably common, as we have a pretty good sample size to work with. Very nearby galaxies give us a much smaller set of galaxies to work with, since we have a smaller volume of space, so it’s harder to say how rare our local group of galaxies is, for instance.

The volume of space we’re able to see only helps us so far, though – ultimately we’re limited by the fraction of the Universe that we can see. If we weren’t limited by this, our studies of the universe would be very different!


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