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19th July 2014

Direct evidence of a multiverse?

Scientists are working to bring the multiverse hypothesis – which sounds like science fiction to some – firmly into the realm of testable science.

 

multiverse

 

The Perimeter Institute for Theoretical Physics in Canada is to begin a series of experiments that could demonstrate – for the first time – direct evidence of the so-called multiverse. This theory postulates that other universes may reside outside our own. The central idea is that a "vacuum" existed prior to what we know as the Big Bang. This vacuum simmered with energy (variously called dark energy, vacuum energy, the inflation field, and the Higgs field). Like water in a pot, its high energy began to evaporate – forming bubbles.

Each bubble contained another vacuum, whose energy was lower, but still greater than zero. This energy drove the bubbles to expand, causing some to collide with each other. It’s possible that some produced secondary bubbles. Maybe the bubbles were rare and far apart; maybe they were packed as close together as foam. But the point is: each of these bubbles was a universe. In this picture, our universe is one bubble in a frothy sea of bubbles, possible an infinite number of them.

This version of the multiverse hypothesis is based on what's currently known about cosmic inflation. Although cosmic inflation isn’t accepted by everyone – cyclical models of the universe tend to reject the idea – it is nevertheless 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 at a super-exponential rate – so rapidly that a cubic nanometre of space became 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.

 

big bang inflation

 

Inflation is thought to have been driven by an inflation field – which is vacuum energy by another name. Once you postulate that an 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. We’re trying to find out what the testable predictions of this picture would be, and then going out and looking for them,” says Johnson. Specifically, his research team is considering the rare cases in which our bubble universe might collide with another bubble universe. He lays out the steps: “We simulate the whole universe. We start with a multiverse that has two bubbles in it, we collide the bubbles on a computer to figure out what happens, and then we stick a virtual observer in various places and ask what that observer would see from there.”

 

multiverse computer model

 

Simulating the whole universe – and indeed more than one – might sound extremely difficult, but apparently that’s not so.

“Simulating the universe is easy,” says Johnson. Simulations don't have to include every atom, star, or galaxy – in fact, they account for none of them. “We’re simulating things only on the largest scales,” he says. “All I need is gravity and the stuff that makes these bubbles up. We’re now at the point where if you have a favourite model of the multiverse, I can stick it on a computer and tell you what you should see.”

That’s a small step for a computer simulation program, but a giant leap for multiverse cosmology. By creating testable predictions, the multiverse model has crossed the line from appealing story to real science. In fact, Johnson says, the program has reached the point where it can rule out certain models of the multiverse: “We’re now able to say that some models predict something that we should be able to see, and since we don’t in fact see it, we can rule those models out.”

For example, the collision from a neighbouring bubble universe might leave a mark or imprint – what Johnson calls “a disk on the sky” – in the form of a circular bruise in the cosmic microwave background. The fact that no such pattern has been found yet makes certain collision-filled models less likely. Meanwhile, his team is working to figure out what other kinds of evidence a bubble collision may leave behind. It’s the first time, they say in their paper, that anyone has produced a direct, quantitative set of predictions for observable signatures of bubble collisions. And though none of those signatures has so far been found, some of them are possible to look for.

The real significance of this work is as a proof of principle: it shows that the multiverse hypothesis can be testable. In other words, if we are living in a bubble universe, we might actually be able to tell.

And what might neighbouring alternative universes look like? Supposing there are infinite numbers of them – it may mean that anything that can happen, will happen in at least one of them. A universe may exist in which dinosaurs survived the asteroid impact. A universe may exist where you are President of the United States. A universe may exist where flowers have the ability to talk. Some universes could be subject to entirely new and different sets of physical laws, with bizarre arrangements of matter and energy. If we ever reach Type V status on the Kardashev scale, we may know for sure.

 

 

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