Any fan of ongoing comic book storylines has heard the term “multiverse” before. When Batman is bested in battle by a foe and all of his allies attend a funeral, fans know Bruce Wayne isn’t about to stay out of the pages of his comic for long. Maybe someone else will be Batman for a few issues, maybe this universe’s Bruce really is gone. That’s all fine and dandy, of course, but it’s certain to be reversed when an alternate reality’s Bruce Wayne waltzes in and don’s the cape and cowl.
Most fans would assume that the idea of infinite realities, of not just a universe but a multiverse of ever-expanding, limitless expanses of everything, is just science fiction. After all, the observable universe is just that: the only part of the cosmos we can observe. And, don’t be fooled, it is immense: we can see not just other galaxies beyond this one, but other clusters of galaxies. Unimaginably vast expanses of cosmic nothing separate us from our nearest neighboring galaxy, and we can see thousands of galaxies beyond even that.
One of the funny things about the way the observable universe works, from our point of view, is that it appears to be expanding. Based on our observation of cosmic background radiation, the stuff that was shot out to the boundaries of observable space with the Big Bang, space doesn’t appear to be curved or round, but is instead flat at the edges. This is weird, to be sure, but it also lets us see something else: the singularity that became existence wasn’t all that hot when it suddenly started expanding.
Due to our understanding of quantum physics, we can draw some conclusions about this. The first one is that any stretch of observable universe could, theoretically, inflate to such a point that it would reach an upper limit. At such a limit, a process called “reheating” would take place in that pocket of existence, and the process we understand at the “hot Big Bang” would take place.
This is the brain-warping part: as some pockets of the universe enter reheating and undergo a process of creating a hot Big Bang, other parts of space, infinitely distant from these events, would still be inflating in all directions at a different rate. Picture bubbles on a pool that are always expanding away from one another. Under such a system, universes would give rise to other universes, the quantum laws that govern them pushing them apart at a speed faster than light.
This is the most likely real-world version of a multiverse. An infinite cosmos, so vast and so unknowable that other pockets of reality are pushing out into the void beyond all the time. It is likely that, should this explanation be true, our own universe was not the first, but is instead the byproduct of another universe, one that itself was likely spawn by another, going back along an infinite spacetime to an unknowable cosmic origin.