The observable universe. Does it mean the edge of space where nothing else is? Is it where the universe is currently at in its expansion after the big bang? Or is it just a barrier that our telescopes are yet to look beyond, and there are just more galaxies past it?
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Let’s say you’re on a sidewalk. The sidewalk has a crack in the concrete every 1 meter, dividing it up into 1-meter slabs.
Spread out along the sidewalk are a bunch of your friends. If you wanted to, you could walk to them, and say hi.
An evil wizard suddenly shows up and puts a spell on the sidewalk. Every ten seconds, each slab of the sidewalk will spontaneously multiply into two slabs.
There’s a friend on the sidewalk very close by, only a few slabs away, you can probably walk to them with no trouble. You walk faster than the sidewalk slabs are multiplying.
If you keep walking, you’ll find out that some of your friends on the sidewalk can’t be reached. The sidewalk is just multiplying in size too fast for you to keep up.
You could try to move faster. That would make you cover more ground, and meet more of your friends. But no matter how fast you run, at some point the sidewalk will be compounding so much multiplication that you simply cannot keep up. Even if you could run at literal light speed, the ultimate speed limit of the universe, there will be parts of the sidewalk you will never be able to keep up with. Any friends beyond that point might as well not exist, because no matter what you do, you can never reach them.
The universe, for whatever reason, seems to have this spell cast on it. Whether an evil wizard did it or not is not known. But bits of the “cosmic sidewalk”, per se, are multiplying in size in a very similar way. We usually phrase it as things “moving away” from us, but in some sense, nothing is actually moving anywhere. The literal space itself between things is just… multiplying. Somehow. We call this the “expansion” of the universe. And because nothing is actually moving because of this, the things that *look* like they’re “moving away” can do so faster than light speed.
And just like how there are friends on the expanding sidewalk that you can never reach, there are also things in our expanding universe that you can also never reach. And likewise, they can never reach you. This doesn’t just apply to going to visit one another. It also applies to, well, literally any method you can imagine for the two places to affect one another in any way. Sending messages? Rays of light? Gravity? None of those things can outpace the expansion. Since nothing past that point can in any way ever hope to affect you, we say those things aren’t “observable”. That is, there is no possible way you can ever know anything about them. By any method. Ever.
Thus, the “observable universe” is the bubble of things that we *can* observe. It’s all the things within that cutoff point that we could, by one method or another, hope to ever affect, or be affected by. It’s a very big bubble. Unfathomably large. But it’s not infinite. Even if the universe itself truly is infinite, it wouldn’t really matter, because anything outside the cutoff bubble is stuff we will never, *ever* see.
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