A comment on Arthur's episode 'Parallel Universes'. A hundred years ago, HG Wells wrote a novel: Men Like Gods, and in chapter 4, Section3, he suggested a way of looking at parallel universes. Imagine the pages in a book, each page being a two dimensional universe in a three dimensional higher universe. Each universe is next to two other universes. The inhabitants of each universe know nothing about the other universes.
Now move up one dimension, so a stack of three dimensional universes are stacked next to each other in a four dimensional space, that is each three dimensional universe is a 'surface' in 4 space.
This ties in with the idea of branes, short for membranes, stacked next to each other.
Men Like Gods is not one of Wells' best novels. In the novel the hero, and a few other people, are diving along a road in southern England when the road suddenly changes, as does the surrounding countryside. They had driven through a gap between two universes.