A taxonomy of parallel universe theories

I have been working on a taxonomy of parallel universe theories. I think I can distinguish five primary types:

1) Physical Theories

Under this type I would include any theory of parallel universes which takes its inspiration from physics. I think the progenitor of this type must be the many worlds interpretation of quantum theory, as developed by Hugh Everett III and his various successors; but equally, other ensemble theories such as the string landscape, or the theory associated with cosmic inflation that different areas of the cosmos may represent different choices with respect to values for fundamental constants. I would also include in here the observation that, in an infinite non-deterministic universe, every possible event almost surely occurs.

2) Ultimate Ensemble theory

As per Max Tegmark. In other words, the universe of mathematics is identical to the universe of physics. I would say that this theory differs radically from the theories above, in that it no longer has a connection to any particular physical theory; it has definitely crossed over (far over) the boundary between physics and philosophy. And, it is an interesting question, how much it actually differs from Lewisian modal realism — certainly it does in inspiration: one, an attempt to explain the existence of the universe; the other, an attempt to explain the meaning of modal statements — but does it differ in substance?

3) Modal realism

As per David K. Lewis.

4) Simulation cases

As per Nick Bostrom’s Simulation Argument; parallel universes achieved through computer simulations.

5) Theistic theories

If one accepts the claims of classical theism, that there exists a deity who created the physical universe — might not that deity have created not merely one universe, but rather many? If the deity is omnipotent, surely it could do so if it wanted to; and even if it was not omnipotent, if it is powerful enough to create one universe, surely it is powerful enough to create many? And, this applies not only to classical theism, but equally to stronger (e.g. trinitarian theism) or weaker (e.g. deism or limited theism) positions. Now, unlike the previously mentioned viewpoints, I’m not aware of anyone who has ever actually defended this position, and yet, if one accepts the necessary theistic presumptions, which rightly or wrongly have been prevalent enough in history to be worth at least momentary consideration, it seems logically defensible. (Surah 1:2 is interesting in this context.)