>>13650879>>13650272>>13650057There's different types of "multiverse"
From an astronomical observation, we have theories like eternal inflation. This gets you the "observable" multiverse.
From our understanding of time, we can picture the entire timeline of our universe(from birth to ending) as a piece of bread and each "current time" as a thin slice of that bread. Each slice of that timeline can be considered an entirety of "current" universe and with every slice having a "copy" of the universe making it another multiverse candidate.
From a quantum mechanics side of things, we get another different multiverse from a certain understanding of wave equation. This multiverse incorporates another variety of timeline splitting multiverse as well but in the affect of different branches of timeline.
Yet another multiverse can be asserted from our understanding of modal realism from logic. When we use modal logic to make arguments, we implicitly assume all characteristics of a real universe in an application to a modal logic.
There maybe other types of valid multiverse as well. String theory has layers of multiverse as well.
Finally, the actual multiverse can be a combination of all these if not more. For example, time bread slices and time divergent of QM's Many Worlds could be combined for a larger multiverse.