If 'universe' refers to the totality of what exists in space-time, then there can be only one universe. Call that the ontological use of 'universe.' On that use, which accords with etymology and common sense, there cannot be multiple universes or parallel universes. But if 'universe' refers to the totality of what we can 'see' (empirically detect) with our best telescopes in all directions out to around 14 billion light-years -- call that the epistemological use of the term -- then it is a reasonable speculation that there are many such universes.
After all, it is epistemically possible -- possible for all we know -- that there are other self-contained spatio-temporal regions beyond our current ken. Let's run with the speculation.
These many universes would then make up the one actual universe (in the ontological sense) which we can now call the multiverse: one universe with many spatio-temporally disconnected regions each with its origin in its own big bang.
Some journalists succumb to the conflation of the MODAL notion of possible worlds with the COSMOLOGICAL notion of a multiplicity of universes. These need to be kept distinct.
If there are many physical universes, as some cosmologists speculate, they are parts of total physical reality, albeit spatio-temporally disconnected parts thereof, and therefore parts of the total way things are, using 'are' tenselessly. But the total way things are is just what we mean by the actual world. To invoke the Tractarian Wittgenstein, "The [actual] world is all that is the case." "The [actual] world is the totality of facts not of things." The actual world is the total (maximal) way things are, and merely possible worlds are total ways things could have been. Therefore, if there are many physical universes, they are all 'located within' the actual world in the sense that they are all parts of what is actually the case.
In other words, each universe in the multiverse is a huge chunk of actuality; universes other than ours are not merely possible. They are actually out there beyond our ken. So it is a mistake to refer to the universes in the multiverse as possible worlds. This should be obvious from the fact that there is a possible world in which there are no universes beyond the one we 'see.' Obviously, this possible world is not identical to a physical universe beyond the reach of our telescopes.
Now suppose we want an answer to the question, Why is there anything physical at all, and not rather nothing physical at all? Does the multiverse idea help with this question?
Not in the least.
First of all, we can ask the same question about the multiverse that we asked about the plain old universe prior to the popularity of the multiverse theory. We can ask: why does the multiverse exist? After all, it is just as modally contingent as 'our' universe, the one we 'see.' Even if there are infinitely many universes in the multiverse, there might not have been any. What then explains the existence of the multiverse?
If I want to know why 'our' universe exists, it does no good to say that it is one of the universes in the multiverse, for that simply invites the question: why does the multiverse exist?
You might say, "The multiverse contains every possible universe, and therefore, necessarily, it contains ours." This is not a good answer because the ensemble of universes -- the multiverse -- might not have existence at all. Surely there is a possible world in which nothing physical exists.
Let us also not forget that the multiplicity of universes comes into existence. So there is need of a multiverse-generating mechanism which will have to operate on some pre-given stuff according to laws of nature. Even if different universes have different laws, there is need for meta-laws to explain how the base-level laws come to be. According to Paul Davies, as paraphrased and quoted here,
. . .to get a multiverse, you need a universe-generating mechanism, "something that's going to make all those Big Bangs go bang. You're going to need some laws of physics. All theories of the multiverse assume quantum physics to provide the element of spontaneity, to make the bangs happen. They assume pre-existing space and time. They assume the normal notion of causality, a whole host of pre-existing conditions." Davies said there are about "10 different basic assumptions" of physical laws that are required "to get the multiverse theory to work."
Davies then made his deep point. "OK, where did those laws all come from? What about those meta-laws that generate all the universes in the first place? Where did they come from? Then what about the laws or meta-laws that impose diverse local laws upon each individual universe? How do they work? What is the distribution mechanism?" Davies argues that the only thing the multiverse theory does is shift the problem of existence up from the level of one universe to the level of multiple universes. "But you haven't explained it," Davies asserted.
Davies dismissed the idea that "any universe you like is out there somewhere. I think such an idea is just ridiculous and it explains nothing. Having all possible universes is not an explanation, because by invoking everything, you explain nothing."
Here Davies may be going too far. If you want to explain why the physical constants are so finely tuned as to allow the emergence of life and consciousness and the minds of physicists, then it does seem to be a good explanation to say that there are all the possible universes there might have been; it would then be no surprise that in our universe physics exists. It had to exist in at least one! One would not then need God to do the fine-tuning or to actualize a life-supporting universe. But this still leaves unexplained why there is the ensemble of universes in the first place.
Davies' critique of the multiverse goes deeper. To explain the universe, he rejects "outside explanations," he said.
"I suppose, for me, the main problem [with a multiverse] is that what we're trying to do is explain why the universe is as it is by appealing to something outside of it," Davies told me. "In this case, an infinite number of multiple universes outside of our universe is used as the explanation for our universe."
Then Davies makes his damning comparison. "To me, multiverse explanations are no better than traditional religion, which appeals to an unseen, unexplained God — a God that is outside of the universe — to explain the universe. In fact, I think both explanations — multiverse and God — are pretty much equivalent." To Davies, this equivalence is not a compliment.
I don't see the damnation nor the equivalency. The appeal to God is the appeal to a necessary being about which it make no sense to ask: But why does it exist? The crucial difference between appealing to God and appealing to the multiverse is that the former is a necessary being while the second is not.
I grant, though, that the idea of a necessary being is a very difficult one!
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