Parallel Worlds
The concept of a multiverse—universes like our own but also slightly different—comes to us from two places in theoretical physics. One is from quantum mechanics and our attempts to understand the many-branching possibilities found in subatomic experiments. The other is a little more concrete, coming from our theories of the extremely early universe.
But to get to the multiverse we have to start with just the universe. By definition, the universe is “all the things”—it is the sum total of complete physical existence. If it’s a thing, it’s in the universe. But even with that definition, we can start to crack the door to “other” universes. One way to do that is to recognize that the universe is only so old, and light can only travel at a finite speed. So there’s a limit to what we can observe in the universe (that limit is about 45 billion light years away).
However, that explanation gives you a very weak vision of the multiverse. Actually, there’s more to the universe than what we can observe, so there’s more “stuff” out there—more stars, more galaxies, maybe even more intelligent creatures—than we could ever make contact with. All of these things are in the universe, but are definitely not a part of our own world.
Those regions outside our observable bubble of the universe still look and act the same as inside it. It’s all the same physics at play, just with various other combinations. But that’s not where the story has to end.
✅ What is cosmic inflation?
After the Big Bang, space expanded at an exponential rate. This explosive expansion, or inflation, lasted from 10−36 seconds to between 10−33 and 10−32 seconds, then slowed down. However, space is still expanding at a rate of about 73 kilometers per second per megaparsec, as the Hubble Space Telescope estimated in 2022.
A Universe Is Born
We currently do not have a solid understanding of the earliest moments of the Big Bang. We know the general outline: our universe was once much smaller and hotter in the past; nowadays it’s not so small, and it’s a whole lot colder. We’ve tested this basic idea against a variety of experiments, too. But as we rewind the clock to the Big Bang, we reach a scale where our physics simply breaks down. When the universe was less than a second old, the conditions of the cosmos were so extreme that we have no theory of physics to guide us.
That said, we do suspect that in its earliest moments the universe underwent a radical transformational event, known as inflation. It appears from all available evidence that when our cosmos was only a fraction of a second old, it rapidly expanded to enormous proportions, growing by at least a factor of 1060.
This inflationary event set the stage of the remainder of the Big Bang, when our universe flooded with particles and radiation that would then grow up to become galaxies, stars, and planets.
Here’s where we get a multiverse: maybe inflation never ended. Maybe the entire universe is constantly undergoing this out-of-control rapid expansion, but pieces of it branch off and settle down into something more sedate. Thus, what we call “the universe” is just a tiny bubble of the true, ever-inflating, ever-expanding huge universe.
Making a Multiverse
In this view of inflation, the entire universe never stops inflating. It just keeps getting bigger at an accelerated pace (faster even than the speed of light). What triggers a patch to slow down and pinch off is merely a random quantum fluctuation. Our patch of the universe just happened to randomly stop inflating (compared to the larger universe), but the rest of the universe outside our bubble continues to do what it was doing before, and what it will always do.
Our patch is not alone. Different patches can also randomly settle down and become a normal, calmly expanding universe. To observers in any of those patches, they will see a Big Bang in their past (just like we do); they will have a cosmos filled with matter and radiation (just like we do); and they will have a limit to what they can observe (just like we do).
In this never-ending-inflation scenario, each of these patches (or bubbles, or pockets, or whatever metaphor makes the most sense to you) appears as its own universe, with each universe separated by a vast and ever-growing expanse of absolutely nothing. This is a physically motivated, and possibly very real, multiverse: a collection of independent, separate universes, filled with entities (stars, planets, people), each doing their own thing.
In the multiverse, our universe is not the first bubble to arise, but merely one of an infinite chain of universes. Imagine a giant foam, like the top of a bubble bath. The multiverse is the foam itself, always growing and always creating new bubbles, with each bubble acting as its own independent cosmos.
All of these bubble universes exist within the same framework of spacetime. If you point your finger in any random direction, somewhere out there, past some unfathomably huge distance, is another universe, and beyond that, another, and beyond that, another.
If this kind inflation truly never ends, then there are an infinite number of universes out there in the multiverse. Each one of those universes could have ended their local inflation in the same way, but it’s also possible that as each universe pinched off, it got a brand new set of physics to go along with it, with different collections of forces and particles.
Some of those universes would look incredibly similar to our own. Others may have failed, full of nothing but void. Some may be far stranger than we can possibly imagine.
Testing Reality
And some may be exact replicas of us. If—and this is a big if—there are only a finite number of ways to arrange all the particles in a given universe, then with an infinite number of universes you’re bound to get repeated copies. That means that not only is there another universe out in some random direction, but that if you follow that line far enough, you’ll encounter a duplicate of you doing the exact same thing, right now, in this present moment.
This is all pretty wild, but difficult to test. The problem is that all the bubbles of the multiverse are completely inaccessible from each other. They exist, but not in any connected way. So we can’t just get in a rocket and fly off to head to our nearest neighbor.
But there may have been some cosmic accidents in our ancient past. When our universe was younger, it had just broken off from the larger inflation-driven flow. If another bubble universe just happened to nucleate close to ours, then there’s a small chance that our universes may have briefly intersected before being permanently driven away from each other.
🚀 Could We Achieve Warp Speed?
There’s a Theoretical Limit to How Fast Warp Speed Really Could Be
The chances of that happening are incredibly small—but not zero—which provides a way to test the multiverse. Unfortunately, no observations of the larger cosmos have revealed any indications that we have suffered such a collision. While those experiments don’t rule out the multiverse idea, they don’t exactly help.
The only thing left we have to go on is our theoretical understanding of the early universe . . . which we don’t really understand. We have only a vague picture of what inflation is like; we do not know what powered it, why it had the energies that it did, or why it shut off in our cosmos. We don’t even know if inflation automatically leads to a multiverse, or if we’re misunderstanding our own math.
Still, while physicists continue to debate the idea, it does make for a good story.
Paul M. Sutter is a science educator and a theoretical cosmologist at the Institute for Advanced Computational Science at Stony Brook University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page.
