Science fiction movies and books are full of parallel universes. In a typical scenario, as in the movie Sliding Doors, something happens in one universe—like a woman misses a train—but in a parallel universe, the same woman catches it, setting in motion diverging life paths. Or, as in Isaac Asimov’s imaginative novel The Gods Themselves, alien inhabitants of a parallel universe with different physical laws exchange energy with our universe and send coded messages to Earth. Even without any real-life alien messages to decipher, though, many cosmologists believe that ther...