Inflation, you see:
Here’s the problem: if you mandate that you get enough inflation that our Universe can exist with the properties we see, then outside of the region where inflation ends, inflation will continue. If you ask, “what is the relative size of those regions,” you find that if you want the regions where inflation ends to be big enough to be consistent with observations, then the regions where it doesn’t end are exponentially larger, and the disparity gets worse as time goes on. Even if there are an infinite number of regions where inflation ends, there will be a larger infinity of regions where it persists. Moreover, the various regions where it ends — where hot Big Bangs occur — will all be causally disconnected, separated by more regions of inflating space.
Put simply, if each hot Big Bang occurs in a “bubble” Universe, then the bubbles simply don’t collide. What we wind up with is a larger and larger number of disconnected bubbles as time goes on, all separated by an eternally inflating space.Ethan Siegel, “Why Do Physicists Say A Multiverse Has To Exist?” at Forbes (February 25, 2021)
That, he claims, is the multiverse. He qualifies,
This doesn’t mean that different Universes have different rules or laws or fundamental constants, or that all the possible quantum outcomes you can imagine occur in some other pocket of the multiverse. It doesn’t even mean that the multiverse is real, as this is a prediction we cannot verify, validate, or falsify. But if the theory of inflation is a good one, and the data says it is, a multiverse is all but inevitable.Ethan Siegel, “Why Do Physicists Say A Multiverse Has To Exist?” at Forbes (February 25, 2021)
Our physics color commentator Rob Sheldon writes to offer in response,
The funny thing about this quote, is that “physics” never claimed to be an answer for everything. Even Aristotle, when he finished his “Physics”, wrote the next volume “MetaPhysics” about the stuff Physics couldn’t explain. For 1000 years, one obtained a “Doctor of Philosophy” or PhD and then studied the subtopic of “Natural History” which later became “Science”. The point is that Science or Physics was never understood historically as a overarching subject matter, but a tiny subset of Philosophy.
But as Einstein remarked, old physicists in their retirement think themselves philosophers, and pontificate on topics they have never studied. It is a failure of our educational system, that we have raised a generation of materialists with no appreciation of the long history of philosophy and theology that made the modern world possible.
Essentially, as physics shades into philosophy, the multiverse is a figment of the imaginations of those who need it. If cosmic inflation doesn’t give them a multiverse, not to worry, they will think up another theory that does. The wish is father to the thought.
See also: The multiverse is science’s assisted suicide