Question: What are the boundaries of the Universe?

Richard Fielder answered on 1 Jul 2020:

There might not be any. One of the big cosmological questions is how big the universe actually is and what happens at the edges. There are various arguments about whether it is infinite or not, and if not how big it might be. Based on observations we can be sure it goes on further than we can see, but obviously it’s tricky to know what’s happening beyond where we can see, because we can’t see it.

If the universe turns out to be infinite, then it can’t have any boundaries because that’s what infinite means – something that goes on forever can’t have an end. If it’s not inifinite, then things get a bit more interesting because there are lots of possibilities for what shape it might be. One of the more popular theories is that the universe is finite but unbounded. That means that even though it has a limited size, it doesn’t actually have any edges but is instead essentially wrapped around on itself. A common analogy is to imagine an ant walking around on a huge rubber ball – the ant only understands two dimensions and can’t see the curvature of the ball. All it knows is that it can walk forever in any direction and never hit a boundary or see an edge. The 2-dimensional surface of the ball is finite, but has no boundaries. However, people still don’t agree on exactly how many dimensions our universe might have, or whether it’s actually a ball, a donut, or some more complicate shape.

In a more practical sense, the boundary of the universe can be thought of as what we call the observable universe. This doesn’t quite mean what we can actually see, but rather how far light has travelled since the Big Bang. No matter how big you build a telescope or how clever your technology, it’s impossible to ever see, or interact in any way with, anything further than that. Due to the expansion of the universe, the observable universe is actually bigger than just multiplying its age by the speed of light would suggest – the universe is only 13.8 billion years old, but the edge of the observable universe is about 46 billion light years away. No matter what size and shape the universe turns out to be, we won’t ever be able to see more of it than that.