The mathematics of this expansion leads to the prediction that at the edges of this inflation other universes would form completely isolated from ours, and that, indeed, there may be a near infinite number of such universes comprising what is called a multiverse.
The multiverse may exist or it may not, but the claim that it does, though informed and supported by science, is not itself a scientific claim. It's a metaphysical (philosophical) claim. A questioner at the science web site Ask Ethan inquires whether the multiverse idea is a scientific hypothesis or whether it's a metaphysical or philosophical theory and receives this answer:
Now there’s a whole lot we don’t know about those other universes, including:But if the multiverse is a metaphysical idea it's in the same class of ideas as other scientifically informed and supported ideas which cannot be tested. Among these are, well, the existence of a personal, transcendent Creator of the universe. This hypothesis is certainly scientifically supported and informed, as the second premise in the following brief video makes clear, and it's a far simpler explanation of why our universe exists with the properties it has than the idea that there are an infinity of universes so one like ours just has to exist.
The answer may be “no” or “yes” to any or all of these questions; the conservative assumption is that the answers are “yes” “yes” and “no” respectively, but this brings us to the main point of John’s question: is this a scientific theory?
- Do they have the same physical laws, particles, and fundamental constants as our own?
- Do they have similar densities, properties histories to our own?
- Are we in some way entangled, quantum mechanically, with these other Universes?
The thing is, the Multiverse is not a scientific theory on its own. Rather, it’s a theoretical consequence of the laws of physics as they’re best understood today. It’s perhaps even an inevitable consequence of those laws: if you have an inflationary Universe governed by quantum physics, this is something you’re pretty much bound to wind up with.
But the Multiverse isn’t necessary to explain anything about the Universe we live in. It solves none of the outstanding problems that we presently have. (And if you say things like “landscape,” “vacuum energy,” “anthropic principle” and “cosmological constant,” you don’t understand what “solve” means.) And worst off, it makes no concrete predictions for something we can necessarily observe.
So what does that mean, when we put it all together? It means that the Multiverse — assuming that our current picture of the Universe and its history is valid — is probably real. There probably is much more Universe out there beyond what’s observable to us, and there probably are other Universes that began with other Big Bang that will never interact with our own.
But it also means it’s beyond the realm of testability, even in principle. The only way I could conceive of doing it is catastrophic: to restore the inflationary state, entangle multiple observers that get stretched into different inflating regions, and see if inflation ends and gives rise to different things at different times, presuming you can learn something when you break the quantum entanglement. (And I’m not sure you can.)
This necessitates surviving a hot Big Bang, mind you, so good luck with that. Unless you can, I’m with John on the skeptical front: the Multiverse may be interesting and a seemingly inevitable theoretical consequence of physics. But until we can test it scientifically — and it may be that we never can — it is not quite good enough to be science. It’s a theoretical conjecture, one that makes sense, but it isn’t a scientific theory, and thanks to the limitations of the Universe, it may never be.
What is it, then? It may be a new class of topics that we’re coming to understand: the first physically motivated “metaphysics” we’ve ever encountered. For the first time, we’re understanding the limits of our Universe, the information in it and what that means for what we can learn about it. Beyond that? After that? Perhaps that’s truly where metaphysics begins, and perhaps that’s where the Multiverse will forever reside.
Exit question: If the claim that the universe is the product of a personal transcendent agent is considered religious because it's untestable and has to do with ultimate things and is therefore subject to restrictions in our public life, why is not the claim that there is a multiverse which is also untestable and has to do with ultimate things not also subject to similar restrictions in our public life?