Keep in mind this is completely a theoretical question... In the end I know nothing about physics. I just have a question. If parallel universes exist, and two or more of them had the ability to travel time (If that were even possible at all. Doubtful.) could someone technically travel to this parallel universe and travel back to their universe at a past time and completely change things? What about the grandfather paradox? If you did go to a parallel universe, would it be possible to travel back to your universe? I know very little about this subject but I would like any input.