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.