If we presume that:
● the hot Big Bang, which gave rise to the observable universe as we know it, was preceded by cosmic inflation
● all of the particles and fields within the universe are fundamentally quantum in nature
Then it follows that the existence of a multiverse is all but inevitable. It opens up a rich realm of physical possibilities that include not only parallel universes, but also an infinite number of them out there. If that’s the case, there could even be parallel universes identical to our own, where reality “forks” each time a quantum outcome occurs. Here’s what’s possible within a scientific consideration of the multiverse.Ethan Siegel, “Could there be a parallel universe identical to our own?” at BigThink (September 29, 2021)
That is where the naturalist project is just now.
You may also wish to read: Ethan Siegel makes another paper assault on the Big Bang. Is the Big Bang the least popular widely accepted science theory? Theoretical astrophysicist Ethan Siegel wishes it out of existence by positing a cosmic inflation that wipes out all possibility of knowledge.