Is the Universe a Bubble? – Physicists Work on the Multiverse Hypothesis

Perimeter Institute for Theoretical Physics Examines the Multiverse Hypothesis

Screenshot from a video of Matthew Johnson explaining the related concepts of inflation, eternal inflation, and the multiverse. Credit: Perimeter Institute for Theoretical Physics

In several new studies and a short video, researchers from the Perimeter Institute for Theoretical Physics detail their work on the multiverse hypothesis.

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.

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 (cross-appointed at York University) 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.”

Specifically, Johnson has been 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.”

Simulating the whole universe – or more than one – seems like a tall order, but apparently that’s not so.

“Simulating the universe is easy,” says Johnson. Simulations, he explains, are not accounting for every atom, every star, or every 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 favorite 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 the field of multiverse cosmology. By producing testable predictions, the multiverse model has crossed the line between appealing story and 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 instance, collisions of one bubble universe with another would leave what Johnson calls “a disk on the sky” – a circular bruise in the cosmic microwave background. That the search for such a disk has so far come up empty makes certain collision-filled models less likely.

Meanwhile, the team is at work figuring out what other kinds of evidence a bubble collision might leave behind. It’s the first time, the team writes in their paper, that anyone has produced a direct quantitative set of predictions for the 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 can be testable. In other words, if we are living in a bubble universe, we might actually be able to tell.

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6 Comments on "Is the Universe a Bubble? – Physicists Work on the Multiverse Hypothesis"

  1. Madanagopal.V.C. | July 22, 2014 at 4:49 am | Reply

    In the realm of physics, energy can neither created nor destroyed. If one nanometer of space expands to quarter billion light years in a trillion trillion trillionth second, we can understand the Godly force which sparked off from a single point to create our universe. This much of energy , if not created afresh according to the realm of physics, should be a spark of the otherwise available universal energy which can create millions of universes like ours as envisaged in multiverse theory. If we stick to the dogma of science this explanation should be completely acceptable. But then that immense energy which was pre-existing should have been the real universe. Galaxies and light matters are but a speck of this vast.. Please don’t bring God’s theory which then would put a full stop the urge of science. We have to uncover mysteries one by one even if it could lead to nowhere, since otherwise more and more discoveries would not be forthcoming and religious full stop will take us to the Plato’s age when earth was flat and all the stars were studded in concentric glass spheres around earth with sun, moon,planets and stars. Even the Celestial music were heard only by the aged scholars then, funny isn’t ? I think that it should have been caused by ‘Tinnitus’ of the ears of those oldies who vouched for a lie. Thank You.

  2. Nathan Coppedge | August 5, 2014 at 10:41 am | Reply

    I have several interesting theories on the subject of multiverses:

    (1) Fossil Realism: the further we look into the background, the further into history that we go. In other words, the more we look for proof, the more we find permanent structures, and the less we find subtle ones.

    (2) Fossil Realism can have the effect of implicating consciousness theories. That is, on the subtle level, the universe is the least descriptive thing. For example, it is possible to construct a thought experiment in which there are two worlds. In the first world, which has the same macro model as the second world, life is not worth living. In the second world, life IS worth living. Since the difference is apparently determined by a ‘CHOICE’, it is more future-determined than past-determined. In other words, it concerns values more than beliefs, and systems of logical prediction (qua meaning) more than observations.

    (3) In some ways, all causes are products of the most fundamental causes. But it is hard to determine what these are. For example, one could look into value theory or justice theory, all implicating theories of atemporality.

    (4) There also may be a difference between VISIBLE physics, and REAL physics. What if two different microwave backgrounds with the same physical properties according to our measurements, have DIFFERENT FEELINGS? How do we come about to detect what that difference means? We cannot just say, “it is subjective” and call it a day. Apparently, there is a way in which variation is more about similarity than it is about difference. In this sense there is a relative absoluteness to accurate claims, and a quantum arbitrariness to merely theoretical ones. Yet some people find that relative absoluteness does not describe anything at all. But if it speaks to our subjective experience, we have to say that it concerns the universal, even though we do not know that it is the SAME universal that someone else is referring to.

    (5) The project of objective data begins in polar opposition, between (1) what is TRUE TO US, and (2) THE BEST WAY OF PORTRAYING IT. Only as a second step do we reach something which could be called universal, which is the unique character of reality, which is both TRUE TO US, AND ACCURATELY PORTRAYED. There is a radical need for non-arbitrary data, which runs contrary to science.

    (6) It is understandable if science runs against mere imagination, but if imagination is one of the faculties which allows us to OBSERVE WHAT SOMETHING IS, then we have to accept that there is some overlap, in all relevant data.

    (7) The problem being, we do not know if we are observing different universes. So we have to begin with the arbitrary, proceed through what is true to us and what is the best portrayal, and end up in what is not arbitrary. My overall impression from all of this is that there will be a greater role for the semantic scientist in the future. For example, consider that someone may think that some or another large chunk of the universe is more important than another. In some sense, this is a DIFFERENT PHYSICS. By sheerly determining what is important, it is as though there is a different map, a different reality. The only reason we have not discovered this, is that we have never had TOO MUCH GENUINELY RELEVANT DATA. We have always had a stark picture, not a real picture, or not a desirable picture. In the ideal reality, multiverses are a semantic point, which takes variables like degree of investigation. Perhaps that is actually, somehow, the final word on the matter for now, whether or not anyone believes me.

  3. Nathan Coppedge | August 5, 2014 at 11:04 am | Reply

    I guess what I meant to say is, there’s no rule against material semantics.

  4. Nathan Coppedge | August 5, 2014 at 11:10 am | Reply

    Another thing to look into would be if bubble-verses are not, in fact, bubble-shaped.

    There is a possibility here that multiverses behave through some sort of sucking worm-hole type activity, or even something more complex.

    As a corollary to this, perhaps researchers should investigate complexity issues, such as ‘how much channeled flux occurs’ relative to complexity.

    My intuition on this is that there is a lot of participation in multiverse-type activity in terms of a single element, but less evidence of cross-identification.

    In the first place, a common element can undergo semantic ambiguation, an active process that results in some changes of properties. But it is not precisely categorical. In the second place, trans-elemental activities require some sort of logical reason for correspondence, and thus, semantic ambiguation is less likely. However, semantic ambiguation COULD occur through some variable medium (emphasis added) such as energy, relativity, or quantum background.

    That’s my genius statement on the subject.

  5. Genius? Mere speculation I presume.

  6. Multiverse (of Tegmark’s Type II) as the requirement of Linde theory’s (of the origin of universes) has been proven:
    1) experimentally from 10+ billion 1Hz gravity measurements taken by the (Canadian) superconducting gravimeter as the Earth’s most accurate instrument used also for studying G;
    2) mathematically, by expressing G (and thus g) via c on both quantum and mechanist scales, as first hinted by Einstein in 1930s:
    3) multi-physically (w/o units since no one knows what they would be in another universe i.e. they have no multiversal meaning) as Newton attached units to G only in order to close his own (our universe’s) physics.

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