The MCU might have introduced the multiverse. Are we just one of many realities?

Last week's episode of WandaVision ended with a shocking twist that could have huge implications for the Marvel Cinematic Universe as we know it. We won't get into the details of what happens for the sake of anybody who is somehow still avoiding spoilers, but we are going to talk about the possible science behind the twist.

The MCU might just be part of a big and weird multiverse — are we part of one too?

The MCU's creators have hinted at a multiverse before. Marvel Comics are full of alternate universes, the time travel in Avengers: Endgame was basically multiverse-hopping, and the title of the upcoming Doctor Strange sequel Doctor Strange in the Multiverse of Madness seems pretty explicit. All signs point to a collection of related but disparate realities converging on one another. And while this provides ample opportunities for interesting stories (not to mention IP crossover) this staple of sci-fi storytelling has some roots in reality — maybe.

INFLATION

In order to talk about multiverses, we must first talk about the early moments of our own universe. The notion of the Big Bang and an expanding universe are taken as table stakes today, but prior to 1980, we didn't have a good explanation for why the universe is structured the way it is.

Observations indicate that our universe is moderately uniform in temperature. Evidence for this comes by way of the Cosmic Microwave Background. But an expanding universe wouldn't be able to transfer heat between distant locales, so what gives?

In 1980, physicist Alan Guth introduced the idea of rapid inflation in the early universe. This era outstrips our normal understanding of expansion. In the first 10^-32 seconds after the Big Bang, the universe expanded by a factor of 10^26. For those of you who are not mathematically inclined, that's, like, a lot.

Then things slowed down. The massive inflation ceased, and a slower (but increasing) expansion took over. This model of the universe's earliest moments explains why cosmic temperatures are uniform overall, and also explains the shape of the universe. It solves some useful questions about what we observe in nature, but it also has some interesting consequences.

Some of the math suggests that inflation might not have stopped in all places at the same time. This could mean that our universe is just a bubble of that inflation that decayed, while some larger portion of reality continued rapidly inflating. If that's true, and if the decay of inflation happens at different times in different space-time regions — much like the half-life of atoms — then it's possible that other universes exist at each point of inflation decay, giving rise to a multiverse.

THE POPULAR MULTIVERSE

In popular fiction, the notion of a multiverse generally refers to the Many Worlds Interpretation (or MWI), which suggests that each time a quantum interaction takes place, whether in the laboratory or in nature, all possible outcomes are realized. When we observe a quantum interaction here on Earth, there is only one outcome, but that is never the only possible outcome. MWI suggests that all possible outcomes occur, but in different realities. Because quantum interactions happen all the time, this necessitates an infinitely branching set of neighboring worlds, each only slightly changed from the last.

It's this idea that inspired shows like Quantum Leap and Sliders. It's also maybe the explanation for Endgame's time travel. We see the Avengers traveling to realities identical to their own before they change events by absconding with an infinity stone or two.

While visiting the events of the first Avengers film set in 2012, Bruce Banner meets with The Ancient One, and she tells him "The infinity stones create what you experience as the flow of time. Remove one of the stones, and that flow splits."

The Ancient One is, it seems, a proponent of the wave function view of reality which suggests that all of reality is a wave of probability. What we see in our reality is just one eventuality of that function, but the other functions don't disappear when one is realized. They all exist in an expanding tapestry of reality, of which we see only a single thread.

While this is the interpretation of the multiverse that gets the most screen time, it isn't the only one.

TYPES OF MULTIVERSE

Like the universes that they attempt to describe, there is not only one view of the multiverse. Max Tegmark, a cosmologist at MIT, breaks down four different types of multiverse that might exist. He calls them levels.

Level One is the simplest and least prone to controversy. It suggests that our universe is much larger than we suspect. There is a limit to how far we can see, constrained by the speed of expansion and the speed of light. Beyond that is a sort of light barrier. We'll never see past it. But just because we can't see it doesn't mean it isn't there.

Our universe could be orders of magnitude larger, or even infinite. If the universe is truly infinite then all things that can happen, will happen. Anything not forbidden by the laws of physics will occur. In fact, because infinity is... infinite, anything that can occur will occur infinite times and in infinite variations.

In such a universe, a copy of you would be out there somewhere, living your exact same life. Another copy will be living a life just like yours except for one slight difference. Or a large difference. Or two big differences and three small. Or... you get the picture.

Functionally, such a universe is no different from what we think of when we imagine the Many Worlds multiverse. The important difference is those copies don't exist in a separate universe; they exist in ours.

Level Two leans on the inflation of the early universe and suggests that inflation doesn't happen at the same rate in all places. Once inflation stopped in our universe, it continued elsewhere. Each time inflation decays, that "bubble" becomes its own universe with potentially different laws of physics.

These universes would not only be beyond our view, but forever beyond our grasp, existing in some far-flung region of space-time. Inflation happens so rapidly and at such incredible scales, that universes born even moments apart could never interact with one another. At least not with any of the knowledge or technology we now possess.

Level Three describes the now-familiar Many Worlds Interpretation we talked about in the previous section.

Level Four goes in a wholly different, and entirely speculative, direction. Tegmark makes the claim that math doesn't only describe the universe. It is the universe. But he takes this mode of thinking one step further. If some equations so elegantly describe what we see in nature, what of those equations that are internally consistent but have no analogue in the real world? Tegmark suggests they do, just not in our world.

Level Four states that all internally consistent mathematical models describe some universe, somewhere in the vastness of reality.

Some of these ideas are easier to swallow than others, but they all have some basis in science on which they build their foundations. Each of them offers an explanation for the available evidence.

We must be clear, however, that each of these also fails one of the guiding principles of science: testability. And in some cases, there are other explanations that fit our understanding of the universe without requiring an acceptance of other realities that we cannot and will maybe never be able to see.

Still, science requires the introduction of radical new ideas. It requires a wild sense of wonder and a willingness to imagine beyond the boundaries of our current knowledge. Every day is a chance to discover new worlds, far away or here at home. And if nothing else, it makes for good TV.