“It’s hard to build models of inflation that don’t lead to a multiverse. It’s not impossible, so I think there’s still certainly research that needs to be done. But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking [it] seriously.” -Alan Guth
It sounds like an unprovable fantasy: the idea that our Universe is just one of countless others, dotted across an eternally expanding empty space separating them. That’s generally how we picture the Multiverse, with each Universe having its own hot Big Bang distinct from every other Universe. But this isn’t simply pure speculation, but the result of a few simple facts combined: our Universe is quantum in nature, inflation gave rise to the Big Bang, and quantum fields spread out in value over time.
Inflation causes space to expand exponentially, which can very quickly result in any pre-existing curved space appearing flat. Image credit: E. Siegel (L); Ned Wright’s cosmology tutorial (R).
Put those pieces together, and you’ll find that no matter how small of a region inflation starts off in, so long as you demand you get enough inflation to stretch our Universe flat, ...