Ethan Siegel says it follows naturally from inflation; Adam Frank says inflation is not that robust a theory:
Frank: 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.
Siegel: 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.”
News, “Astrophysicists lock horns over whether multiverse must exist” at Mind Matters News
Takehome: Inflation is only one factor; other sources weigh in on issues around math, testability, reality-based thinking, and, inevitably, what God would do.
Must we “accept it”? Here are 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?
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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.