The concept of multiverses isn’t just in fiction. Physicists believe that a multiverse has to exist in our universe today, because of how the different clusters of stars and galaxies are placed, what they are made of, and how they move. Forbes details more reasons why scientists subscribe to this theory:
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While a variety of interpretations were initially suggested, they all fell away with more abundant evidence until only one remained: the Universe itself was undergoing cosmological expansion, like a loaf of leavening raisin bread, where bound objects like galaxies (e.g., raisins) were embedded in an expanding Universe (e.g., the dough).
If the Universe was expanding today, and the radiation within it was being shifted towards longer wavelengths and lower energies, then in the past, the Universe must have been smaller, denser, more uniform, and hotter. As long as any amount of matter and radiation are a part of this expanding Universe, the idea of the Big Bang yields three explicit and generic predictions:
a large-scale cosmic web whose galaxies grow, evolve, and cluster more richly over time,
a low-energy background of blackbody radiation, left over from when neutral atoms first formed in the hot, early Universe,
and a specific ratios of the lightest elements — hydrogen, helium, lithium, and their various isotopes — that exist even in regions that have never formed stars.
image via Forbes