The multiverse hypothesis: Is the universe a bubble?

Perimeter Associate Faculty member Matthew Johnson and his colleagues are working to bring the multiverse hypothesis, which to some sounds like a fanciful tale, firmly into the realm of testable science.

Never mind the big bang; in the beginning was the vacuum. The vacuum simmered with energy (variously called dark energy, vacuum energy, the inflation field, or the Higgs field). Like water in a pot, this high energy began to evaporate, bubbles formed.

Each bubble contained another vacuum, whose energy was lower, but still not nothing. This energy drove the bubbles to expand. Inevitably, some bubbles bumped into each other.

It's possible some produced secondary bubbles. Maybe the bubbles were rare and far apart; maybe they were packed close as foam.

But here's the thing: each of these bubbles was a universe. In this picture, our universe is one bubble in a frothy sea of bubble universes.

That's the multiverse hypothesis in a bubbly nutshell.

It's not a bad story. It is, as scientists say, physically motivated, not just made up, but rather arising from what we think we know about cosmic inflation.

Cosmic inflation isn't universally accepted, most cyclical models of the universe reject the idea. Nevertheless, inflation is a leading theory of the universe's very early development, and there is some observational evidence to support it.

Inflation holds that in the instant after the big bang, the universe expanded rapidly, so rapidly that an area of space once a nanometer square ended up more than a quarter-billion light years across in just a trillionth of a trillionth of a trillionth of a second.

It's an amazing idea, but it would explain some otherwise puzzling astrophysical observations.

This is a screenshot from a video of Matthew Johnson explaining the related concepts of inflation, eternal inflation, and the multiverse. 

Credit: Perimeter Institute

Inflation is thought to have been driven by an inflation field, which is vacuum energy by another name.

Once you postulate that the inflation field exists, it's hard to avoid an "in the beginning was the vacuum" kind of story. This is where the theory of inflation becomes controversial, when it starts to postulate multiple universes.

Proponents of the multiverse theory argue that it's the next logical step in the inflation story. Detractors argue that it is not physics, but metaphysics, that it is not science because it cannot be tested.

After all, physics lives or dies by data that can be gathered and predictions that can be checked. That's where Perimeter Associate Faculty member Matthew Johnson comes in.

Working with a small team that also includes Perimeter Faculty member Luis Lehner, Johnson is working to bring the multiverse hypothesis firmly into the realm of testable science.

"That's what this research program is all about," he says. "We're trying to find out what the testable predictions of this picture would be, and then going out and looking for them."

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