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So far, we know only about two types of universes: Infinite and finite universes (Unbounded edition).

In an infinite universe, there is no edge. Because well, an infinite space doesn't have an edge. It is as nonsensical to talk about the edge of an infinite universe, as it is to believe that the Earth is flat. Because, infinite spaces do not have any meaningfully defined "edge" to them.

In a truly infinite universe, if there are only a finite number of choices for structures to form, there can be a infinite amount of copies of an object all over the universe.

Indeed, about a few googol light-years away (stuff is really boring in our "observable universe"), an exact copy of you would be reading this exact question on (another form of) Astronomy Stackexchange, while sipping coffee, on an exact copy of Earth, orbiting an exact copy of the Sun, in an exact copy of the Milky- Ahh, I think I might be getting carried away with this.

Anyways, the point is, an infinite universe does seem exciting, but in reality it actually is boring. If a man from Earth took a warp drive (anything FTL that doesn't cause stuff like time-travelling and causality violating BS), and went just 15 gigaparsecs from here (technically the edge of the "observable" universe), he would find.... just more galaxies, stars and planets. Nothing really exciting at all, just the stuff we would see in our normal observable universe.

On the other hand, finite universes are kinda exciting (though just a bit more exciting than the infinite kind). So far, for some weird reason, the only kind of finite universes being discussed everywhere are just the unbounded kind.

I still don't fully understand the exact concept, but from what I heard, it's something about the universe being some kind of weird 4D sphere with no edge. That is, even if it's finite, it doesn't have a boundary. Assuming you have an unbounded universe that's a few hundred petaparsecs across (a petaparsec is about 3 quadrillion light-years across, probably something I made up because as far as I've heard, the largest term used for distance I've heard is a gigaparsec), even if you got your warp drive towards the said "edge" of the universe, you wouldn't feel anything, because there is no edge to be felt from the inside. You'd simply return back to where you came from. Kinda like that experiment where an ant crawls across the surface of an orange, only to return to the starting point, except it's 4D edition. Well, duh.

Back to the question

In this question, we're gonna be discussing about the finite and BOUNDED ones. Yes, ones that have an actual edge, not just a random 4D surface that's intangible from the inside.

For some reason, Wikipedia says:

Assuming a finite universe, the universe can either have an edge or no edge. Many finite mathematical spaces, e.g., a disc, have an edge or boundary. Spaces that have an edge are difficult to treat, both conceptually and mathematically. Namely, it is very difficult to state what would happen at the edge of such a universe. For this reason, spaces that have an edge are typically excluded from consideration.

Which implies there is not much data about the bounded types of universe. Yet, these kinds of universe interest me the most, because it really interests me to know what would happen at the edge of such a universe. I don't know what would there be beyond such an edge (though not as something as whimsical as God playing 10D-Minecraft on his Ultracomputer 4000, or breaking out of the simulation of a game inside a 5-yo kid's PC).

Supposing we again use our trusty warp drive, and for heaven's sake (and adding in the sake of simplicity), let's assume that the laws of physics are just the same inside that universe as it is inside ours. No mumbo jumbo weird tachyons, no weird atoms, no weird spacetime defects (I don't want SCP Foundation 2: Electric Boogaloo happening). Just normal good ol' physics.

Let's say, we have an indestructible observer in this warp drive, moving towards the edge of the bounded universe. Supposing he approached the edge, and happened to throw a piece of matter at it (Say a rock, a baseball, a passing asteroid, whatever). What would happen to the matter that "collided" with the edge of the universe?

Would it annihilate into pure energy? Vanish from existence? Bounce back like some sort of cosmic basketball? Or get frozen in time, as it got closer to it?

What happens at the edge of a finite, but bound universe?

Boundary conditions: I don't know much about these kind of physics, because I barely just stumbled across the concept of the bounded universes. Nearly all of the information on the internet is about the "unbounded" ones. But for simplicity's sake, assume that the conditions at the edge are just as same as that in intergalactic space. (I.e. a near-total vacuum, except for a few atoms of hydrogen gas per cubic meter).

Also, I don't know if this matters, but I don't think if it matters what's at the "centre" of such a universe. It could probably be a vacuum, a tiny galaxy, a floating rock, or whatever for that matter. I honestly have zero clues about how it could affect the conditions of the edge.

Also, the most mind-boggling concept I've encountered is what's "beyond" the edge. For our universe that's a non-issue, as our universe is assumed to be an infinite one, or a finite one that's unbounded. But the bound universe concept definitely doesn't have any exceptions regarding what's beyond the edge.

The thing is, since the universe is basically everything in existence put together (space, time, matter, energy etc etc), the concept of what lies beyond the boundary boggles my mind. So far, I cannot solve the problem, without running into exotic stuff like multiverses or parallel universes, or even sci-fi BS like the "inter-dimensional void" and whatnot. Since there is no space-time beyond the universe, one would definitely expect that if one were to stick their hand into the boundary, they would retract a bloody stump. Or if they were to punch it (don't try it unless you're an stupid astronaut addicted to crack), they would end up dislocating their shoulders or even breaking their arms, as it rebounded back.

Even then, my question stands. What I posited above were simply theories regarding my suspicions about the nature of the edge of a bounded universe.

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  • $\begingroup$ It could be kind of like the "particle in a box" model, in which the quantum fields of all particles are clamped to zero at the "edge". It's a singularity, so I suppose you can make up your own rules for the edge. $\endgroup$
    – James K
    Mar 29 at 15:27
  • $\begingroup$ You might consider a black hole to be a finite but bound universe, in that there is a boundary, and things inside the boundary can't escape. But inside the black-hole, you can't have a path that intersects the boundary. But I assume you are assuming a universe where there are possible paths that do intersect the boundary? $\endgroup$
    – antlersoft
    Mar 29 at 15:32
  • $\begingroup$ @antlersoft Now that you come to mention it... I honestly have no clue either. $\endgroup$
    – Alastor
    Mar 29 at 15:42
  • $\begingroup$ I thought a finite universe was one where you could start traveling in one direction and come back around to where you started. Or is that not strictly necessary for the concept of a finite universe? $\endgroup$ Mar 29 at 19:18
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    $\begingroup$ When treating a physics problem, we need boundary conditions. It's analogous to giving the initial conditions in classical mechanics: without that, you dont have enough information to solve the equations. Normally we pick periodic boundary conditions for simplicity, but that's essentially removing the boundary (ie, closing up a 2d plane into a torus). The only obvious alternative is to have all fields go to 0 at the edge, ie the edge is a vacuum. Beyond topological issues, weird things would happen to stuff moving close towards the edge, but otherwise local properties should be fairly normal $\endgroup$ Mar 30 at 0:04

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