A question about multiverse I have a question that I would like to ask here since I'm not an expert or any near to that. 
Actually I just came up with this question when I was watching The fabric of cosmos.
I couldn't find where I can ask this kind of question. So hopefully this is the right place.


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*What is the "background" of multiverse?


Let me elaborate a little more what that means clearly. As far as I understand, multiverse is a cluster of many universe and it seems like (well from what I saw in documentary) that bubbles  in a cup of soda. 
What I am curious is that then what is the mechanism of the background or surrounding, I should say, of the multiverse?
Since if multiverse "exists" somewhere, then that means the surrounding has to have some kind of characteristics, such as space-time.
I hope that I explained comprehensibly, and somebody can give some reasonable explanation. Even better, any book recommendation would be appreciated!
Even though I studied physics for 1 year and half, I forgot many of them except curiosity. ;)
 A: If you've watched any of the popular programs on the universe you've probably seen spacetime modelled by a rubber sheet, and the effect of gravity modelled by the dents made in the rubber sheet by masses moving around on it. Like all analogies this one can be misleading (sometimes dangerously misleading!) but lets stick with it for the moment.
Unlike a rubber sheet, spacetime is infinitely stretchable. In fact at the singularity in the centre of a black hole spacetime does get infinitely stretched (though quantum gravity probably steps in before the curvature becomes infinite). That means you could have some areas of your rubber sheet that are reasonably flat, while there are others that are stretched into all sorts of strange shapes.
No matter how contorted your rubber sheet gets, it is still possible to imaging taking a marker pen and drawing a line from any point on the sheet to any other point. But in the real universe we can't move faster than light, so while we can indeed imagine a line connecting any bit of the universe to any other, you may need to travel faster than light to trace out such a path. This is exactly what happens in a black hole. The black hole hasn't been disconnected from the rest of the universe, and you can imagine lines that lead out of the black hole, but you'd need to move faster than the speed of light to follow these lines. That's why you can't get out of a black hole, because you can't travel faster than light.
This is basically the situation with the multiverse. There are various ideas about how multiverses might form, but the most popular is probably based on eternal inflation. We probably need to abandon the rubber sheet analogy at this point, but you can think of eternal inflation as rapidly stretching spacetime. In fact it's being stretched much much faster than anything we see in our universe at the moment. However some regions of spacetime stop stretching and form regions of approximately flat spacetime, and that's where we live.
In eternal inflation spacetime hasn't been torn or ruptured in any way, and in principle there are continuous lines that lead out of our region of flat spacetime through the raipdly stretch areas and into another flat region i.e. another "universe". However the limit of the speed of light means that there is no way that anything, not even light, could ever get out of our "universe" and into one of these other "universes". That's why you'll hear them described as separate universes.
So forget any notion of inter-universe portals that you might have read in science fiction books. It's all just one universe really and the multiverse is really just separate regions of flat spacetime.
A: To expand on John Rennie's mention of several different (possible) types of multiverse, another one is Hugh Everett's "Many Worlds" interpretation of quantum mechanics.
The idea is based on the maths behind a "wave function" - if you take the famous Double Slit experiment and look at the maths you have a wave function which describes a particle - now we can say "the particle was created in place A at time B" and "the particle ended up in place X at time Y" (within the limits of the uncertainty principle), but we can't say much about what happened in between.  The wave function is a description of what happened in between, but in this particular interpretation it's essentially describing every theoretically possible route from A to X.
Some of these are unlikely, some more likely, and if you add all the possibilities up you get "1" (the principle of unitarity) - in other words, it tells you the particle had to take some route but doesn't tell you any more than that, other than probabilities.
So Everett's explanation is that all of these "possible routes" do actually exist.  Just because we see only one result it doesn't reduce the "reality" of the other ones, they're still required because otherwise there's a possibility that the particle doesn't exist, which is impossible for something that has actually been observed to exist.
So to answer your question, in this context you're asking what's outside the wave function - in other words, "where do things that don't exist, exist?"
It's really easy to say that "what's outside the universe" is a nonsensical question ("where do you go if you walk to the north pole and keep going north?" is a common analogy), but I appreciate it's actually very difficult to give a simple and clear explanation of why it's a nonsensical question.
