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In Eternal Inflation Theory, there exists the idea of multiple "bubble" Universes that grow and according to this theory, can "collide" with each other. The paper suggests that you can search for evidence that our Universe collided with other "bubble" Universes, by looking for an imprint on the CMB radiation.

However, the concept of a Universe existing as a bubble that moves relative to other universes makes no sense:

  • How can you have a space "outside" of ours that allows the Universes to move relative to each other?
  • In what dimension are the other "Universes" moving relative to ours?
  • If time started in our Universe at the Big Bang, then in what time dimension (outside of ours) do collisions between Universes occur?

PBS Spacetime looks at this idea here: https://www.youtube.com/watch?v=lrS0rxX-UhA

Edit: A winning answer will answer all three points above.

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If time started in our Universe at the Big Bang, then in what time dimension (outside of ours) do collisions between Universes occur?

In inflationary cosmologies, time did not start at the Big Bang. There are many models, but in at least some of them, there is an "inflating" universe which extends infinitely in past (and possibly future) time, constantly creating new space within itself. Within that inflating universe, at any point with some low probability per unit of time, a phase transition (like water freezing to ice) can start to happen, fundamentally changing the nature of space and time at one point. That effect propagates out (again like supercooled water freezing) creating a growing bubble of this changed space-time, which in an abuse of language, we call "a bubble universe".

In what dimension are the other "Universes" moving relative to ours?

Of course there is no reason why this needs to happen just once in the whole of the infinite inflating universe. Indeed, it will happen infinitely often. If it happens at two points in space and time which are close enough together, the two bubbles may grow into one another (before they are pulled apart by the inflationary expansion of the space between them. This is (again perhaps by an abuse of language) called a "collision". Nothing is really moving, except the edges of the bubbles.

If I can suggest, the real answer to your question is that, in this branch of cosmology, "universe" does not mean quite what you think it does. Instead of referring to the totality of existence, it refers to one of these non-inflaying "bubbles" in inflating space-time.

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  • $\begingroup$ Would this multiverse be best described with a 4D coordinate system with 3D bubble universes growing in it? That's the only way I can visualize it right now. $\endgroup$
    – SurpriseDog
    Jul 30 '19 at 15:15
  • $\begingroup$ So is the idea that the local expansion of the phase transition must be faster than the exponential growth of inflating space? $\endgroup$
    – Peter Erwin
    Jul 31 '19 at 20:29
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There is a bit of a semantic mess in multiverse talk. The eternal inflation theory claims that, on the largest scales, inflation (exceptionally rapid expansion) continues for a long time, perhaps forever. Which would mean, that what we have been calling "The Universe" would be a subregion of space-time in which inflation ended earlier, in the first few seconds, and is now just an insignificantly small piece of the whole. The whole of space-time would then be a multiverse because it contains many other non-inflating sections like ours. Perhaps, if this theory turns out to be correct, we should call these slower growing sections "island universes" to be clearer. The problem though is that the literature has historically called this one "The Universe".

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  • $\begingroup$ Okay, but how do these "Universes" (however the term is defined) move relative to each other? $\endgroup$
    – SurpriseDog
    Jul 26 '19 at 12:04
  • $\begingroup$ @Benjamin -- think of two ice crystals forming in a glass of supercooled water. The are each growing through the surrounding medium, although not moving. They collide when their growths reach the same place. Here the surrounding medium is expanding inflating space-time and the crystals are "universes" of normal space-time. $\endgroup$
    – Steve Linton
    Jul 28 '19 at 15:05

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