Look out at the Universe all you want, with arbitrarily powerful technology, and you’ll never find an edge. Space goes on as far as we can see, and everywhere we look we see the same things: matter and radiation. In all directions, we find the same telltale signs of an expanding Universe: the leftover radiation from a hot, dense state; galaxies that evolve in size, mass, and number; elements that change abundances as stars live and die.
But what lies beyond our observable Universe? Is there an abyss of nothingness beyond the light signals that could possibly reach us since the Big Bang? Is there just more Universe like our own, out there past our observational limits? Or is there a Multiverse, mysterious in nature and forever unable to be seen?
Unless there’s something seriously wrong with our understanding of the Universe, the Multiverse must be the answer. Here’s why.
The Multiverse is an extremely controversial idea, but at its core it’s a very simple concept. Just as the Earth doesn’t occupy a special position in the Universe, nor does the Sun, the Milky Way, or any other location, the Multiverse goes a step farther and claims that there’s nothing special about the entire visible Universe.
The Multiverse is the idea that our Universe, and all that’s contained within it, is just one small part of a larger structure. This larger entity encapsulates our observable Universe as a small part of a larger Universe that extends beyond the limits of our observations. That entire structure — the unobservable Universe — may itself be part of a larger spacetime that includes many other, disconnected Universes, which may or may not be similar to the Universe we inhabit.
If this is the idea of the Multiverse, I can understand your skepticism at the notion that we could somehow know whether it does or doesn’t exist. After all, physics and astronomy are sciences that rely on measurable, experimental, or otherwise observational confirmation. If we are looking for evidence of something that exists outside of our visible Universe and leaves no trace within it, it seems that the idea of a Multiverse is fundamentally untestable.
But there are all sorts of things that we cannot observe that we know must be true. Decades before we directly detected gravitational waves, we knew that they must exist, because we observed their effects. Binary pulsars — spinning neutron stars orbiting around one another — were observed to have their revolutionary periods shorten. Something must be carrying energy away, and that thing was consistent with the predictions of gravitational waves.