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[Serapion]

How do we know that there's only one universe?
Why couldn't there be other universes?

[$py]

It depends on what you mean by "universe."
If you mean "everything" then by definition there is only one universe.
If you mean that volume of stuff that is consistent with what we observe (i.e. everything consistent with our understanding of the cosmological principle) then you could admit the idea of other universes that have far different properties.
Our cosmological principle would then be true only for our universe within this greater metauniverse.
It is intriguing to speculate along these lines, and the point about the Copernican Principle is interesting.
However, we have no real way to know.