Imagine that the Universe we observe, from end-to-end, is just a drop in the cosmic ocean. That beyond what we can see, there’s more space, more stars, more galaxies, and more everything, for perhaps countless billions of light years farther than we’ll ever be able to access. And that as large as the unobservable Universe is, that there are again innumerably more
Universes just like it — some larger and older, some smaller and younger — dotted throughout an even larger spacetime. As rapidly and inevitably as these Universes expand, the spacetime containing them expands even more quickly, driving them apart from one another, and ensuring that no two Universes will ever meet. It sounds like a fantasy picture: the scientific idea of a Multiverse. But if the science we accept today is correct, it’s not only a valid idea, it’s an unavoidable consequence of our fundamental laws.
The idea of the Multiverse has its roots in the physics required to describe the Universe that we see and inhabit today. Everywhere we look in the sky, we see stars and galaxies, clustered together in a great cosmic web. But the farther away in space we look, the farther back in time we look as well.
The more distant galaxies are younger, and hence less evolved. Their stars have fewer heavy elements in them, they appear smaller as fewer mergers have happened, there are more spirals and fewer ellipticals (which take time to form from mergers), and so on. If we go all the way to the limits of what we can see, we find the very earliest stars in the Universe, and then a region of darkness beyond that, where the only light is the leftover glow from the Big Bang.
But the Big Bang itself — occurring everywhere at once some 13.8 billion years ago — wasn’t the start of space and time, but rather the start of our observable Universe. Before that, there was an epoch known as cosmic inflation, where space itself expanded exponentially, full of energy inherent to the fabric of spacetime. Cosmic inflation is itself an example of a theory that came along and superseded the one that came before it, in that it:
1Was consistent with all the successes of the Big Bang and encompassed all of modern cosmology.
2Explained a number of problems that the Big Bang couldn’t address, including why the Universe was the same temperature everywhere, why it was so spatially flat, and why there were no leftover high-energy relics like magnetic monopoles.
3And it made many distinct new predictions that could be tested observationally, most of which have been confirmed.
There’s also, however, one consequence that inflation predicts that we do not know whether we can confirm or not: the Multiverse.
The way inflation works is by causing space to expand at an exponential rate. This takes whatever existed before the hot Big Bang and made it much, much, much larger than it was previously. So far, so good: this explains how we get such a uniform, large Universe. When inflation ends, that Universe gets filled with matter and radiation, which is what we see as the hot Big Bang. But here’s where it gets weird.
In order for inflation to end, whatever quantum field is responsible for it has to roll from the high-energy, unstable state that drives inflation down into a low-energy, equilibrium state. That transition, and “rolling” down into the valley, is what causes inflation to come to an end, and create the hot Big Bang.