The theory I had in mind when I started this post has escaped my attention, but I submit for argument two known ways of travel between universes. 1. A series of stairways, up or down, built into the very fabric of reality, each stairway twisting into a mobius stripe between two separate universes. 2. A temporary portal. I'm not sure why it's temporary, but probably something to do with connections between universes being limited in some way to make the story more interesting.
Um . . . So there are a number of video games which use this concept, and maybe I stole it and maybe I didn't, but as a story device it could be a lot of fun to play with.