In the same way, parallel universes aren’t optional in eternal inflation. They come as part of the package, and if you don’t like them, then you have to find a different mathematical theory that solves the bang problem, the horizon problem and the flatness problem, that generates the cosmic seed fluctuations—and doesn’t predict parallel universes. This, too, has proven difficult, which is why more and more of my colleagues are—often grudgingly—beginning to take parallel universes seriously.
