Do wormhole exits require matter? If not, would they change the edge of the universe?

Question

As a layperson who watches way too many theoretical physics and astronomy documentaries, I've seen the folded-piece-of-paper analogy of wormholes a bajillion times. Usually they explain that black holes might connect to other parts of the universe, but they don't really explain what would have to be on the other end for any kind of information to travel through it. When they do, they usually suggest white holes -- but I'm not entirely convinced.

I don't have too much trouble understanding how a black hole forms, and to some extent, how that could lead to a distortion of space-time sufficient to causing the "fold". But it's hard to imagine what would cause a white hole to form. Even if there were such thing as negative mass, there doesn't seem to be any way that an object can come to have a negative density sufficient to becoming a white hole, so there couldn't possibly be a process that causes them to form.

So if black holes could be wormholes at all, it doesn't seem to be the case that the exit would require matter to be present before it forms. If anything, a black hole wormhole would have to "perforate" a white hole or some other kind of exit into existence -- somewhere.

But there doesn't seem to be anything that determines where that exit would form, so one could exist outside the boundaries of the matter that defines the edges of the universe. That would mean that the universe is infinite in size, since white holes could (and probably would, if their position were completely random) appear very, very far away.

So then either black holes can't be wormholes, the universe is infinite in size, or I'm missing something. Neither of the former two conclusions have been reached to the best of my knowledge -- so what am I missing?

Answer 1

The sort of wormholes you'll have seen on documentaries are not related to black holes.

The Scharzschild metric that describes a black hole contains a wormhole. This type of wormhole is known as a Schwarzschild wormhole, or there is a variant called an Einstein-Rosen Bridge, but nothing can traverse these wormholes. Even if it could, we'd need to watch for an infinite time to see anything emerge from the other end.

The sort of wormholes you've probably seen on TV are traversable wormholes, and these do allow matter to pass through them. However they probably don't exist. It's not hard to find wormhole solutions to the Einstein equations. You start with the wormhole geometry and work backwards to find out what stress-energy tensor you need to create the wormhole. The problem is that the result is always that you need negative matter i.e. matter that gravitationally repels other matter. It would take a brave scientist to declare absolutely that negative matter doesn't exist, but we've never seen any trace of it.

Incidentally, the universe is probably infinite or at least it doesn't make any sense to consider it having an edge. It could be finite but unbounded, like a sphere, but if so the length scale must be vastly larger than the observable universe because the observable universe is flat to within experimental error.

Rotating and charged black holes can act as links between different regions of space, but these aren't wormholes in the sense that you could use them as a short cut. If you pass through a rotating or charged black hole into a different region of space there is no way back to where you started from, or at least not without travelling faster than light. Actually the same is true of the Schwartzschild/Einsten-Rosen bridge wormhole. It links different regions of spacetime, but doesn't link two points in our spacetime.

You might be interested to have a look at my answer to Entering a black hole, jumping into another universe---with questions. This describes in more details hole black holes can link different areas of spacetime.