h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If the Universe has a boundary, what is past that boundary or edge?

This is a hard question to answer, because it can either be meaningless or confusing. I think what when most people wonder about is what is "outside" the universe, they forget the definition of a universe is something that includes all, including what is "outside" it. But that is just semantics, what is our spacetime accelerating into is what people wonder about. There are a couple of possibilities. It could be expanding into a higher-dimension, so what exists outside the universe is a higher-dimension of space. Another explanation is that there is nothing, not even spacetime. The universe contains all of space, and as it is expanding, it creates a larger space. What lies beyond the boundary is outside our comprehension, for if we were to travel out to it, we would eventually get back to where we started, like the bug who walks on the surface of a ball would return to his starting place. Hope that helps.

I try to picture it like this: Space doesn't have a size, as it is just defined as being empty. The objects in the universe are just getting farther apart, not the boundaries of the universe itself. Of course, boundaries are created, like the De Sitter ones mentioned in the main article.

A theory I've read about is the multiverse theory, that we are but a bubble, and there are an infinite amount of other bubbles, or universes, coexisting with our own. I like Hawking's analogy, of bubbles spontaneously forming in a pot of boiling water.