Remarkably, mathematical analysis also revealed—and here’s where the multiverse enters—that as space expands the cosmic fuel replenishes itself, and so efficiently that it is virtually impossible to use it all up. Which means that the big bang would likely not be a unique event. Instead, the fuel would not only power the bang giving rise to our expanding realm, but it would power countless other bangs, too, each yielding its own separate, expanding universe. Our universe would then be a single expanding bubble inhabiting a grand cosmic bubble bath of universes—a multiverse.

It’s a striking prospect. If correct, it would provide the capstone on a long series of cosmic reappraisals. We once thought our planet was the center of it all, only to realize that we’re one of many planets orbiting the sun, only then to learn that the sun, parked in a suburb of the Milky Way, is one of hundreds of billions of stars in our galaxy, only then to find that the Milky Way is one of hundreds of billions of galaxies inhabiting the universe. Now, inflationary cosmology was suggesting that our universe, filled with those billions of galaxies, stars, and planets, might merely be one of many occupying a vast multiverse.

Yet, when the multiverse was proposed back in the 1980s by pioneers Andrei Linde and Alexander Vilenkin, the community of physicists shrugged. The other universes, even if they existed, would stand outside what we can observe—we only have access to this universe. Apparently, then, they wouldn’t affect us and we wouldn’t affect them. So what role could other universes possibly play in science, a discipline devoted to explaining what we do see?

And that’s where things stood for about a decade, until an astounding astronomical observation suggested an answer.