Mathematics and physics are two different things, what's true in the world of math is not always true in the physical world. I've read many books on quantum and classical physics, and I've never seen a proof for the existence of multiple universes, although I do believe in them, things like uncertainty and the anthropic principle don't suffice as formal proofs, and with the absence of most yet unknown quantum theorems, I don't see how they could prove anything with real facts yet, it's easy to prove things like Winston Churchill was a carrot, and 1=2, when you aren't using the facts correctly. If you have a copy of one of these proofs, I would love to see it.