The idea of a multi universe system is interesting and complex. The problem is that it is not falsifiable so all hypothesis or claims along these lines are purely speculative in nature.
There are many ideas on how a multi universe works and it is not restricted to cosmology or physics. The concept of parallel universes is as old as religion. Hinduism mentions them from its very inception and even Islam claims that god created many worlds.
The philosopher and psychologist William James coined the phrase parallel universes in 1895. Since then science fiction writers have been having a field day with the idea. Currently there is even a TV show called Fringe which is based on the idea that there is a parallel universe where all of us have doubles leading similar lives. The Star Trek series envisioned a mirror universe where the good guys from the “regular” universe were evil in the mirror world. Then there is the anti matter world. Again a mirror image but made of antimatter.
Other suggestions have included the idea that black holes are the gateway to other universes or that they are themselves other universes.
James’s idea of parallel universes was later given more credence by Hugh Everett's Many World’s interpretation of quantum mechanics. Many other scientists since have had their own interpretations including Stephan Hawking. Richard Feynman gave Hawking the impetus for his interpretations through Feynman's multiple histories hypothesis. M theory suggests multiple universes through its concept that we live in but 4 dimensions when in fact (it suggests) there are at least 11 and perhaps as many as, if not more than, 26.
Each interpretation is different. Though each postulates many worlds, none of them see those worlds in exactly the same way. Some tell us that our universe is but one in a multi universe. Others like Bell told us we live in a bifurcating universe. That means that each choice we make splits our universe into many. One for each choice we did not make. This theory would quickly see billions upon billions of universe made each day, and I assume that the other universe would split each time anything made a choice there as well. We can see how that idea is not without problems.
Feynman’s idea was based on his hypothesis that there is no wave particle duality and that in a double split experiment the particle takes every possible rout to get to the target. Hence making an interference pattern that looks like a wave. But both he and Hawking envision universes that do not have our physics. Unlike the parallel, mirror, or bifurcating universe, Feynman and Hawking’s interpretation is not so much about people as it is about all the possible types of properties, and therefore the different types of physics all these worlds must have. In fact, in this theory all possible worlds exist; even those which have physics that can’t produce or sustain life of any kind.
As you can see this is a complex idea with many possible variations. But what most of the more culturally accepted ones have in common is that all possibilities are played out. World War 2 happened here, but somewhere else it did not. Somewhere Germany won. Somewhere Hitler was killed before the end of the war. In some he would have never existed and in some he would have never risen to power.
These are theories of anything. Anything is not only possible but everything that can be thought of exists in its own world. Somewhere there are unicorns. Somewhere there are invisible pink squirrels and divine spaghetti monsters with meatball eyes.
Yes, it soon all starts to get absurd. But that can’t be helped if all possibilities must play out and all possible physics must exist. All your wildest fantasies are true. But just…. not here. What a shame.
The thing that strikes me most about all these ideas is the fact that is they are correct then the idea of free will or indeterminism is out the window. All choices are going to played out anyway regardless of how we feel about it or what we do about it. You may have saved your daughter/s life on this world but then in another you will have killed her yourself.
Of course the fact is you are not these other people any more than a clone of you is you. You are not consciously connected to these others. You don't think through their minds or they through yours. They are completely separate people in a separate universe. Not you.
While many of these ideas are meant to show a super-symmetry in the universe and infinite possibilities, if every possibility must become actual, then our choices are completely predetermined.
Of course there are other possibilities as well. There may be many universes with the same physics as ours where we all are doing exactly the same thing at the same time. In fact a super-symmetry would suggest that is the case as well. Again, all the interpretations of a multi universe must be correct and live side by side all others if all possibilities are going to be played out.
Even if we say that not everything is possible and try to get out of it that way, the concept of a super-symmetry suggests that everything is possible in a universe that makes it possible even if not in ours.
This has led new age mystics to think that everything we can think of is real and all things are possible. Our imagination creates reality, or our perception creates reality.
The trouble is, it doesn’t, and that is a demonstrable fact. I wrote my first essay for Hub Pages on topic so I won’t go into observer driven reality here.
The fact remains that there is no proof of a multi universe system. Feynman’s all possible histories theory is still a hypothesis, not a theory at all. Even though all the interpretations of the many world’s theory would explain certain observable quantum phenomenon, that’s the problem: they all explain it and they almost all contradict each other.
An interpretation is just a model. The model may filled with facts, but how the model puts those facts together and in particularly the conclusions it comes to about those facts are not in and of themselves factual. In other words, they are guesses based on certain facts.
None of these hypotheses are falsifiable yet, and may never be. The universe may be infinite. How in the world could we then send a probe outside it to find other universes and more importantly, is that even a reasonable question? It isn’t, of course.
But even then M theory and some other interpretations like those surrounding black holes will tell us that isn’t a problem because all universes are in different dimensions and exist in the same space we do simultaneously. The imagination is endless, and that’s all any of this is.
I’m not saying there are no other dimensions or universes. I don’t know and neither does anyone else. That’s the point. Our culture runs amok with these kinds of ideas. People believe all manner of things that are not to be believed because they are baseless speculation.
That’s not science. No scientist worth their salt would tell us anything else. All interpretations are models. They are not to be taken as facts until they are proven. It is really the media and culture and the imagination that tries to make models into fact or have people debating in cafes sure that many worlds is the just the way it is.
To me it is important that we remember that models are speculative. They are a “what if” and not the latest facts from science.