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I just watched one of those videos that shows the scale of the universe, zooming out from one thing all the way out to the sphere that is thought to be the universe. However, they also showed other universes outside our own, and they made it seem like a universe is right next to others in space. I have often heard that our universe is expanding, so I just want to ask: What would happen if one universe expanded enough to collide with another, and what would this look like from inside one of the universes?

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    $\begingroup$ It will be great if you include the link to the video you've mentioned. There may be some factual errors, omissions, or ambiguities in it that need to be pointed out. It's not like there are multiple universes in space that can expand, touch, and overlap. If it was explained that way, it was probably a misleading picture. $\endgroup$
    – uhoh
    Oct 2, 2021 at 22:32
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    $\begingroup$ Short answer: no one knows. $\endgroup$
    – Mick
    Oct 2, 2021 at 22:41
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    $\begingroup$ youtube.com/watch?v=kLcrBXl37BQ $\endgroup$
    – Margorp13
    Oct 3, 2021 at 4:00
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    $\begingroup$ The universe is probably infinite. But even if it is infinite, there could still be other universes. $\endgroup$
    – PM 2Ring
    Oct 3, 2021 at 9:21
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    $\begingroup$ The spheres you have in mind are the visibile and the observable Universe. Actually beyond them there is still our Universe. According to current knowledge it goes far far away, perhaps it is infinite. There might be other universes but they should not be out there. They should be other universes in their own spaces. $\endgroup$
    – Alchimista
    Oct 4, 2021 at 9:38

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The Universe is space and time. If there are other universes they can't be "next to" ours, as "next to" is a statement about the relationship between things in space, and the Universe is not in space. It is space.

You can't have two universes colliding at a particular time, because time is a property of the universe.

Your mental image of universes as spherical bubbles floating in space is wrong. Unfortunately, there isn't a better visual image. The notion of being outside the Universe and looking at it seems to be fundamentally impossible, because "outside" is a property of space, and space only exists inside a universe.

The last part of the video does show "bubbles floating in space" but this is not intended to be a representation of the multiverse (if it exists) but a metaphorical symbolisation of it. It is something that is fundamentally unknown and unknowable. It is beyond our ability to visualise in a realistic way, so it must be visualised in an unrealistic way.

This is, at least, our best guess. In fact questions of "outside the universe" are unanswerable. If we could know about something outside the universe, then it wouldn't be outside, it would be part of our universe.

So ultimately, Mick is right: Nobody knows.

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    $\begingroup$ Universes (as plural of Universe) need to exist in a higher dimensional space, otherwise we'd talk about our own universe. If there was a higher dimensional space, we couldn't (yet) tell if we're close to another "3,5d-universe". So there is no physics based answer to this question to me. Either the question is wrong or your answer is wrong, it seems to me. But i'd love to be corrected :) $\endgroup$
    – Alex B
    Oct 3, 2021 at 20:04
  • $\begingroup$ Or to put it another way: suppose there are two 2-dimensional universes, but their x/y plane is tilted 45° in 3d space. They definitely could collide, but neither could understand from the 2d point of view what's happening. To me, this is the gist of the question. $\endgroup$
    – Alex B
    Oct 3, 2021 at 20:12
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    $\begingroup$ @AlexB There is really no reason to believe that multiple universes, if there is such a thing, should be subspaces of some common space of higher dimension. In fact in the opposite is true; the very notion of space does not have any meaning outside a universe. This is also at the core of this answer. $\endgroup$
    – user43989
    Oct 3, 2021 at 20:30
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    $\begingroup$ @AlexB That's not two 2d universes, it's one 3d universe with two 2d planes inside it. The point this answer makes is that space is a property of the universe. In your scenario you can't describe your two planes without using co-ordinates from your 3d space. $\endgroup$
    – JBentley
    Oct 3, 2021 at 22:25
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    $\begingroup$ No, because your language is spacial ("separated") and temporal ("from the start", "exist" (in the present tense)) Of course your language is spacial and temporal. All languages are! You can't talk about any of this, because human language is fundamentally incapable of not using spacial and temporal expressions. As an exercise, try to imagine a topological circle S¹ without imagining the space around it. It can be described mathematically, but we can't imagine it, nor can we visualise it. We can't describe what such a circle is like, our language is not up to the task. $\endgroup$
    – James K
    Oct 4, 2021 at 21:54
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Semantics: Some answers are saying "no" by definition since "universe" means everything, including multiple different parts of a larger existence. However, that's not how the term is currently used and not how the question means it. "Universe" is a domain with spacetime as we see it, and a different such domain would be a different universe, as in the video referenced.

The answer, and whether the questions is even meaningful at all, depends on the model of cosmology you are using.

The question may be meaningless if different universes are totally separate and are not part of a larger structure that itself has space-like properties where things can move around, grow, and take up room.

One example of this is Cosmological Natural Selection. Each daughter universe is in a different spacetime than the parent's, disconnected from it. They are just different, not locations in a larger space that has coordinates.

Different Big Bang universes are not separated by ordinary space; the BB takes up all the space. However, the Eternal Inflation model would have it that there is a different state of vacuum (space-time) that is inflating at a much faster rate, and when it decays at some point to our vacuum then that point grows outward at a slower rate. Now the "outside" containing these bubble universes has a different interpretation of what is space and time dimensions than the bubble's spacetime inside a bubble. But they are connected in a meaningful way.

These bubbles are growing at a much slower rate than the space between them, so they could never collide.

A Braneworld theory is that our 4D spacetime is structure that exists in higher dimensions, and other such "branes" also exist and they are floating around and might possibly come into contact.

The Cyclic Ekpyrotic Universe model relies on that, where two parallel branes (one of them being our universe) collide and bounce off each other, explaining the Big Bang (the collision) and Dark Energy (attraction between the two branes).

Other models would have other answers, and different meanings for what such a collision actually represents.

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Roger Penrose has posited that hot spots in the CMB are relics of an "earlier" Universe. Not widely accepted, perhaps, but he's an eccentric genius, not a quack. https://physicsworld.com/a/new-evidence-for-cyclic-universe-claimed-by-roger-penrose-and-colleagues/

It also bears mentioning that Brane Theory is a prominent feature of String Theory. In (highly-simplified) Brane Theory our universe is resident upon, or is, a membrane (Brane) "floating" around in a higher geometry, along with other, perhaps infinitely other, Branes. (As James K. says, "floating" is the least-worst visualization). Collisions between Branes can, in Brane Theory, be responsible for Inflation, the perceived weakness of gravity and the creation of entire new universes. https://www.nature.com/articles/news.2007.399

Some question whether String and Brane Theories are theories at all, because they appear to be untestable. Others believe that Inflation, for instance, will cause String-scale effects to be inflated to universe-scale effects, and so detected.

There are others here, fortunately, who can give a better take on these issues than I can, but there are, certainly, open lines of inquiry that are trying to address your question.

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This is difficult because we can only easily visualise 3D and space - the known universe we live in. Mathematically there could be any number - or an infinity - of universes, but they aren't parallel in an everyday sense. Maybe "coexisting" is a better word.

Cosmologists use the expression "the universe" to mean all of time and space, and all they contain. So by definition there isn't any space outside space, nor any time outside time.

So these parallel universes don't coexist like ink spreading out on blotting paper, that can overlap and collide.

The image you want is that they somehow coexist, if there is more than one, in multiple dimensions, each the only time and space in that dimension, each containing all the time and space in that dimension. They don't "collide", or at least don't collide in any sense we usually use the word.

This is quite possible, but extremely hard to imagine.

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The question depends on whether the void with universes is static? Well the answer is no . The universe is dynamic as showed by hubble constant. We dont mind properties of other universes because if: (1)Multiverse exists and (2)Our universe is expanding Then at a point of time the unknown universe may collide with our universe

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    $\begingroup$ This is another post which doesn't seem to make any sense. $\endgroup$
    – Rory Alsop
    Nov 8, 2021 at 12:53

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