There may be different definitions of multiverse. I am thinking of multiverse as outlined in Stephen Hawking’s “The Grand Design.” As I understand his concept, many universes spontaneously come into existence in some kind of larger space. The multiple universes are somehow separate from each other; I imagine they are embedded in the larger space like raisins in a muffin, but I am sure this is a gross oversimplification of the theory. Anyway, I think the concept is highly speculative, and I am not aware of any evidence.
I think it is an elaborate theory crafted to explain one of the great mysteries of physics, namely, how it happens that many properties and interactions of matter, and even the properties of space and time, seem to be fine-tuned to make our existence possible. Physicists imagine that if the universe had even slightly different fundamental properties, it would be incompatible with life. So the reasoning goes that if there are a vast number of universes, each with its own properties (its own set of fundamental physical constants), then it is more plausible that one of them could be “just right,” as our universe is.
