My understanding is that there are two types of multiverse discussed in cosmology: one where universes can form or break off from existing universes such that our universe sprouted from another. Each new universe could have different initial conditions and with an almost infinite number of universes some like ours are suitable for complex life to evolve. Seems a reasonable idea although requires some evidence to support it. The other type of multiverse I understand comes from quantum mechanics with the idea that every time you have a quantum state with more than one possibility then every possibility is actually played out by the creating of a new universe. That seems to give an ever expanding "infinity" of universes because even a piece of "empty" space has virtual particles popping in and out of existence (based on quantum probabilities) so generating new universes. The latter multiverse sounds absurd to me. That doesn't mean it is wrong as lots of physics is counter intuitive! Is the second form of multiverse a mainstream belief in cosmology?