What does the universe expand into?

[JaceDeanLove]

I know that no one has a definitive answer to such a difficult question, but I am curious about other ideas. My personal belief, looking at physics and my (very limited)understanding of it, the universe is expanding into nonexistence. The universe, unless there's a multiverse, which is a different topic, is all that exists. In this case, all that exists is simply expanding (stretching) into nonexistence. It's hard to wrap my mind around, but it makes sense in its own way. And, disregard what I said earlier. If you think there's a multiverse, explain your belief.

[Alex K]

There is a pretty definitive answer in a sense. If you look at the theory (of general relativity and of the friedmann lemaître robertson walker scenario of cosmology in particular) it becomes somewhat clear why the notion of a space into which the universe expands is not necessary.

I don't even want to go into the discussion of whether the universe is finite at all, and it may well be infinite, or whether it has boundaries (it can be finite and without boundaries, if it leads back onto itself like a three dimensional torus or sphere).

If you look at the maths, you see that the expansion comes from the dynamics of a field called the metric, which is the mathematical object which is used to calculate distances in spacetime. The universal expansion occurs because this field gets larger and larger values, such that the distance between objects at two different fixed coordinates is measured larger and larger as time proceeds. It's really more like a shrinking of the univeral measuring rod, and less like something expanding into something. There does not need to be an embedding of our universe into a bigger space for this to occur, merely our local metric field changes its value and becomes larger. That's how it works in general relativity.

[Napoléon]

I know that no one has a definitive answer to such a difficult question, but I am curious about other ideas. My personal belief, looking at physics and my (very limited)understanding of it, the universe is expanding into nonexistence. The universe, unless there's a multiverse, which is a different topic, is all that exists. In this case, all that exists is simply expanding (stretching) into nonexistence. It's hard to wrap my mind around, but it makes sense in its own way. And, disregard what I said earlier. If you think there's a multiverse, explain your belief.

Question:

Why is thinking a multiverse exists any more a belief than thinking it doesn't?

[JaceDeanLove]

I know that no one has a definitive answer to such a difficult question, but I am curious about other ideas. My personal belief, looking at physics and my (very limited)understanding of it, the universe is expanding into nonexistence. The universe, unless there's a multiverse, which is a different topic, is all that exists. In this case, all that exists is simply expanding (stretching) into nonexistence. It's hard to wrap my mind around, but it makes sense in its own way. And, disregard what I said earlier. If you think there's a multiverse, explain your belief.

Question:

Why is thinking a multiverse exists any more a belief than thinking it doesn't?

I'm sorry. I didn't mean it like that. I worded it wrong. They are both equally valid