From Ethan Siegel’s argument for the multiverse:
If cosmic inflation and quantum field theory are both correct, then the Multiverse arises as an inevitable consequence of the two, combined…
Those regions of space where inflation end and the hot Big Bang begins are each their own, independent Universe, and together, they make up a Multiverse. We may not be able to measure these other Universes, at least not just yet, but there’s every reason to expect that if inflation and quantum field theory are both correct, then the Multiverse inevitably exists.Ethan Siegel and Adam Frank, “Is the Multiverse real? Two astrophysicists debate” at Big Think (February 24, 2022)
From Adam Franks’ argument against the multiverse:
It is important, from my viewpoint, to understand what is happening with inflation theory — because it is not really a theory the way, say, electromagnetism or quantum mechanics is… Instead, it is a class of theories with lots of wiggle room for individual instantiations…
It is possible that the only way the inflation extrapolation works is to accept an infinite number of Universes that you may never ever be able to observe. But that is not good. And it is not like anything else that’s happened in the history of physics. Sure, we cannot observe what is inside a black hole; and yes, we have dark matter that we cannot see; and yes, there are the parts of our Universe beyond the light horizon. But in the case of dark matter (if it exists), then we can at least learn a lot about it in bulk based on the detailed influences it exerts on the luminous matter we can see. And as for the insides of event horizons, I am not forced to accept infinite numbers of Universes as the price for accepting General Relativity. Same goes for what lies beyond the observable Universe.
To summarize, I would argue that inflation has some attractive features, but it simply does not stand as the kind of scientific edifice (in terms of having many, many points of contact with observation) that should force us to accept the Multiverse.Ethan Siegel and Adam Frank, “Is the Multiverse real? Two astrophysicists debate” at Big Think (February 24, 2022)
Siegel was allowed a rebuttal:
Adam’s response contains some interesting food-for-thought, but there is a dubious logical gambit in there at the core of his argument, which can be paraphrased this way: We don’t know everything, therefore how can we trust anything? In any scientific endeavor, you absolutely must be careful about what assumptions you are making that go beyond the limit of what you can observe and/or verify, but you must also not ignore the very generic predictions that show up independently of the assumptions that you make…
In other words, yes, inflation gives you some wiggle room in many ways, but you cannot wiggle out of the Multiverse. The only way out, as Adam says, is to postulate a Rumsfeldian “unknown unknown” to save you. And while that is always possible in any endeavor, I think it is far preferable to draw your best conclusions based on what is known to the limits of our best knowledge at the time. To retort with a quote from the late “Macho Man” Randy Savage, “You may not like it, but accept it.”Ethan Siegel and Adam Frank, “Is the Multiverse real? Two astrophysicists debate” at Big Think (February 24, 2022)
Must we “accept it,” despite problems with cosmic inflation as a theory, as Siegel insists? Have a look at some other approaches:
Robert J. Marks: Is The Big Bang Theory’s nerd right about the multiverse? Sheldon Cooper insists that in no universe would he dance with Penny. Given countless universes, are there truly none in which Sheldon Cooper dances? Maybe. Math shows why there cannot be an infinite number of universes.
Eric Holloway: Here is a way we can be sure if we are living in a multiverse. An experiment can test the idea that there is an infinite number of universes. For our experiment, we need a quantum coin flipper, a disintegration gun, and observers who are sure that there is an infinite array of universes out there.
Michael Egnor: We don’t live in a multiverse because the concept makes no sense. Neurologist Steven Novella and philosopher Philip Goff, both atheists, agree that there are many universes besides the one we live in. Atheists use the multiverse concept to counter the fact that our universe appears fine-tuned to allow life like ours. But is it a valid concept?
Multiverse cosmology is not a good argument against God. Or against fine tuning of our universe. God could have created countless universes on various principles for a variety of reasons. The key argument against the multiverse is that there is no evidence for it; it takes us outside the realm of observable science — a choice with consequences.