What Good Are Other Universes?

At The Speculist yesterday I wrote a piece that discusses the  possibility that irregularities discovered in the background radiation of the universe (see irregularly colored map of the entire universe, above) may be evidence of gravitational influences coming from other universes.

Surely the notion that there are other universes is about as profound an idea as we can entertain. There was a time, less than a century ago, when astronomers believed that the Milky Way -- our own little local group of some 300 billion stars -- was the one and only galaxy in the universe. Back in those days, the words "galaxy" and "universe" pretty much meant the same thing. Then an astronomer named Heber Curtis introduced a revolutionary and highly controversial idea: what if Andromeda and some of the other spiral nebula believed to be part of our galaxy were actually galaxies in their own right, much bigger and much farther away than we had ever before dreamed? When the debate was finally settled and astronomers agreed that our galaxy was just one of many (with "many" now understood to mean hundreds of billions of galaxies), the universe became vastly larger.

Now, if we accept the idea that there are multiple universes, the cosmos grows again. How big does it get?

Hold on to your hats.

Max Tegmark of the MIT Department of Physics has broken it all down for us with a four-level of multiverse (a multiverse is what you get when you realize that you don't just live in a universe):

Level I: Regions Beyond our Cosmic Horizon

In this model, space is infinite. We can only see so far because the expansion of our universe provides an outer bound to what is visible to us, but out beyond that bound there are lots of other universes all expanding in their own right. Each started from the same set of basic conditions that ours started from and are governed by the same set of laws.

As Tegmark explains it, two basic ideas drive this model: 1) the notion that space is infinite and 2) the idea that matter and energy should be uniformly distributed throughout. Those two factors work together to mean that our universe is one of an infinite number of such universes located throughout the infinity of space. So even at the lowest level of the hierarchy, we have an infinite number of universes.

Somewhere out in that sea of universes there are many instances of each of us running around leading highly similar (or extremely different) versions of our lives. There are also many exact duplicates of this universe.

Level II: Other Post-Inflation Bubbles

In this model, our infinite space containing an infinite number of universes is not the only such space. There would be many of them, and each would have the same basic laws of physics, but might have different constants, different particles, more dimensions.

According to Tegmark, the theory of Cosmological Inflation -- which has to do with the early rapid expansion of our universe -- is best explained by our infinite collection of universes being only one of an infinite number of such collections. So now we have an infinite number of infinite sets of universes.

Level III: The Many Worlds of Quantum Physics

In this model, the universe splits every time any quantum uncertainty is resolved. So anything that could go one way or the other actually goes both ways -- and you get an entire universe to account for each outcome. In the end, the many worlds hypothesis gives you the same infinite variations on our universe as we saw in level 1, but the universes are arranged differently. As Tegmark explains it:
In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional Hilbert space.
Any questions on that? None? Okay, moving on.

Level IV: Other Mathematical Structures

In this model, we have not only an infinite number of universes that share the same basic laws as our universe, we have an infinite number of universes to represent every possible set of laws that could govern universes. The idea here is that the universe we know is itself a mathematical structure. It exists as a set of math equations and as a universe representing those equations. There's no good reason to believe that our set of equations is privileged. Why shouldn't there be a universe -- or rather an infinite number of universes or, to be more precise, an infinite number of collections of infinite sets of universes -- to represent all possible mathematical structures?

(If that isn't enough universes for you, I'm sorry -- that's all we can come up with. You will just have to find a way to make do.)

And now let me say, having established all those universes that might be out there -- so what? Maybe there isn't anything all that profound about parallel universes after all. I mean, seriously, what good are they? There is no way we can visit any of them or even communicate with them. We're pretty much sealed off from them forever (that's what makes it appropriate to describe them as separate universes.)

I can think of one reason they are relevant.

So our universe is just one of an infinite number of possible combinations of atoms. But if every possible combination is really out there, that means every alternative version of the universe that you have ever imagined (assuming you imagined physically possible versions) really exists. Everything you ever hoped would happen. Everything you ever dreamed that you might do. Everything you always wished you did or said.

They are all really out there. And they are as real as the ground under your feet.

Moreover everything that you are still hoping will happen -- and even the things that you no longer bother to hope will happen -- they're all going to happen. They are bound to happen. There is no choice but that they will happen.

There are a whole bunch of universes -- perhaps an infinite number of universes -- in which they definitely will happen. And since they can happen there, in universes sharing the same laws and starting conditions as this universe, they can happen here. THIS can be one of the universes where those things happen.

That's not to say that it will be, but it can be.

And that, I think, is a pretty profound idea.

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