As we look at this week's evidence of gravitational waves and cosmic inflation, it's becoming increasingly hard to believe that the observable universe we live in is the only universe there is. Putting aside the hopelessly three-dimensional questions inflation raises, it's unlikely that the story of the entirety of the cosmos (memorably defined by Carl Sagan as "all that is or ever was or ever will be") happened to begin with the oldest event we can record, and happens to extend only to the furthest horizon we can see.
But what does the rest of the cosmos—or, as theorists are more likely to call it, the rest of the multiverse—look like? Minute Physics breaks multiverse theories down into three categories, namely bubble universes (based on the standard model), membranes (based on string theory), and the many-worlds hypothesis (based on a highly speculative form of quantum mechanics):
Of the three, the many worlds hypothesis—the idea that every possibility creates a new universe in which it is actualized—gets the most attention (probably because of its similarity to the idea of mirror universes or parallel dimensions), but it is by far the strangest and most complex. Bubble universes, in contrast, are so simple as to be almost self-evident—now that we know our universe inflated much like a balloon, the conclusion that it was inflated with something from somewhere seems obvious. But theoretical physics is not the study of obvious conclusions, simplicity is no substitute for evidence, and questions of this scope are difficult to answer. On the issue of the multiverse, and on so many other issues, we must learn to either commit to the unproven or make peace with ambiguity.