The quantum experiment that could help find evidence of the multiverse

FOR an experiment designed to help us find evidence of other universes, it looks surprisingly modest. As Zoran Hadzibabic walks me into the lab, it feels more like a classroom, complete with linoleum floors, fluorescent lighting and a whiteboard with scribbled equations. And yet it is here, in amongst the tangle of stainless-steel chambers and brightly coloured wires set on a raised platform, that researchers are trying to replicate the primordial quantum bubbling that may have created our universe in a vast multiverse.

The idea that our universe is just one of many is among the most captivating in physics, and the logic seems sound enough, in the sense that the idea is itself an outgrowth of widely accepted theories about how the cosmos came to be what we see today. But there also happens to be zero empirical evidence for its existence – which is where Hadzibabic’s experiment at the University of Cambridge comes in.

The researchers are betting that if we can cool and manipulate potassium atoms to extremely low temperatures, when tiny bubbles should form spontaneously, we will have a proxy for the otherwise unobservable processes thought to have sired new universes. By studying those bubbles, we could glean fresh clues as to how any past collisions between our universe and others would leave a mark that we might plausibly hunt down in astronomical data.

“The absolute dream would be that there’s something in the sky that we observed which confirms what we predicted in this experiment,” says Matt Johnson, a theoretical physicist at the Perimeter Institute in Canada.

What is the multiverse?

To be clear, what we are talking about here is the inflationary multiverse. This is not to be confused with the quantum multiverse, predicted by the “many worlds” interpretation of quantum theory, which says that every time we observe a quantum object, and so collapse a cloud of probabilities regarding its properties into something definite, all the possible outcomes persist in parallel universes.

The inflationary multiverse is different. The idea took shape in the 1980s, when physicists Andrei Linde and Alan Guth sought to make sense of observations showing that the afterglow of the big bang, known as the cosmic microwave background (CMB), was inexplicably uniform. They proposed that the universe expanded exponentially in its first split second, a period known as inflation. But as they explored the idea further, they realised that inflation is unlikely to have happened just once and stopped. Instead, it could have stopped in our universe but continued happening elsewhere, creating an infinite number of “bubble” universes.

The inflating space between these bubble universes would quickly hurl them apart, so they had little chance of interacting. But if the baby universes formed sufficiently close together, the idea says, they could have collided before being separated – which suggests we could find evidence of these collisions, presumably as some sort of marks or “scars” left behind, in our own universe.

But how do you even go about looking for them? Cosmologists have pursued various ways to find evidence for this multiverse over the years. Some are even trying to do it without any observations (see “What are the odds?”, below). But most cosmologists agree that the best place to look is the CMB. In other words, you look to the sky.

In 2011, Johnson, along with Hiranya Peiris at University College London and her colleagues, showed that colliding bubble universes should leave circle-shaped scars in the CMB. They created an algorithm to comb previous images of the CMB for such imprints. What they found was promising: four patches of the sky were compatible with the shape of collision imprints. It was exciting, but not evidence.

“There were uncertainties in our tests,” says Peiris – namely the rate at which new bubble universes should form and the probability that they would collide. “We had to assume very wide ranges for those parameters,” she says, resulting in large theoretical uncertainty. To reduce that and to improve their predictions, Peiris and Johnson need a better grasp of the finer points of the underlying idea regarding the process of how universes are actually born.

The hope is that Hadzibabic’s experiment can help with that. To understand how, we first need to get to grips with the strange world of quantum theory, the laws that govern the behaviour of nature’s most fundamental components, and how it applies to the formation of universes.

Quantum mechanics

In quantum theory, the lowest possible energy state for space-time – the stage on which everything there is plays out, including our possible multiverse – is called a vacuum. But if the space between universes is constantly inflating, it can’t be a true vacuum. Instead, there must be some inherent energy driving the expansion. Quantum field theory, a mathematical framework combining quantum theory and Albert Einstein’s theory of special relativity, suggests more than one vacuum state exists, but that most are “false” – that is, not the lowest possible energy.

As nature always strives to reduce its energy, a false vacuum isn’t fully stable. It is said to be “metastable”. And in the quantum realm, things can mysteriously “tunnel” to a lower energy state – akin to a marble in one valley suddenly appearing in the neighbouring one without having gone over the hill between.

Cosmologists care about these quantum processes, known as false vacuum decay, because they could explain how the universe began, and how other universes may have begun too. Our observations of the start of our universe, including its early rapid expansion, are consistent with it starting off as a bubble. This would have involved the cosmos tunnelling to a lower energy state, a process physicists call a phase transition, before eventually reaching a true vacuum.

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The trouble is that we can’t know for sure. The best support we have for this hypothetical scenario comes from solving complex equations in quantum field theory, which require huge approximations. “Using our best mathematical tools, the bubble ends up nucleating [appearing] instantly – perfectly formed – at one point in space,” explains Peiris. “We do not have the ability to trace how it goes from the top of the mountain to the valley.” And unless we know the details of this process, she argues, we can’t fully trust our theories.

However, in 2017, physicists in New Zealand and Australia published a game-changing paper. Their work showed that, under the right conditions, the equations describing false vacuum decay in the early universe are equivalent to those describing a quantum phase transition in a kind of exotic matter called a Bose-Einstein condensate – usually comprised of atoms at extremely low temperatures – in which bubbles akin to a true vacuum are created. By studying the formation and behaviour of such bubbles in the lab, they argued, we could learn something about how multiple universes might have formed, filling the gaps Peiris encountered when pondering potential evidence for a multiverse, such as the probabilities of bubble universes colliding.

Johnson and his colleagues were intrigued by the idea. But it was only after they had carefully studied the equations that they felt the concept was worth exploring experimentally. Then, they started collaborating with Silke Weinfurtner, an experimental physicist at the University of Nottingham, UK, with experience of investigating similar cold-atom systems. Now, Weinfurtner leads an international consortium of theoretical and experimental physicists exploring the condensate bubble idea.

The new experiment

Hadzibabic was initially sceptical about the possibility of creating a set-up sophisticated enough to be a direct analogue of cosmological false vacuum decay. The sample would have to be uniform to allow bubbles to form anywhere, for example, and cold enough to exhibit true quantum effects, undisturbed by thermal fluctuations. But after discussing it with his colleagues, and exploring the maths involved, he became increasingly optimistic that it could teach us something about the early universe. “It is sort of Occam’s razor [the idea that the simplest explanation is often the most accurate],” he says, “to the extent you can do Occam’s razor with the origin of the universe.”

The first stage, which already works, produces the Bose-Einstein condensate by making potassium atoms colder than anything in the universe. When a gas cloud on the scale of microns reaches such temperatures, it behaves like a single quantum particle. This is what makes Bose-Einstein condensates so useful, allowing physicists to study quantum processes more or less with their own eyes.

Next, Hadzibabic will prepare the condensate in a metastable vacuum state and wait for it to drop to the true vacuum state via quantum tunnelling, watching as expanding bubbles of true vacuum form. This stage will last seconds, after which the condensate will be destroyed and the whole process – cooling, tunnelling, bubble nucleation – will start again.

The tricky part will be determining whether the outcome is indeed an early universe analogue, says Johnson. “It’ll come down to lots and lots of detailed checks.” Ultimately, you will have to compare the results with approximate mathematical simulations and look for potential problems. You can then refine the experiment to try to account for the problems and compare again until, hopefully, experiment and simulation fit. If that doesn’t happen, we may need to revise our theory of the early universe – an equally exciting prospect for cosmologists.

Verifying an experimental analogue with theory, whilst also trying to verify the theory with the experiment, is incredibly difficult. “But this is pretty much how all of science is done, and the best we can do when our observational data of what happened in the early universe is so limited,” says Katie Mack, a cosmologist at the Perimeter Institute, who feels the experiment is an important one.

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And there is reason for optimism. A team including Gabriele Ferrari, a physicist at the University of Trento in Italy, recently completed a simpler version of the experiment – in one dimension, essentially an extremely thin tube – and actually saw “bubbles”, which appear as lines in this set-up. The conditions aren’t cold enough to represent a purely random quantum mechanical process; thermal fluctuations can kick-start tunnelling events. But Ferrari argues that this isn’t necessarily a problem. “Thermal fluctuations may well have ignited false vacuum transitions in the early universe,” he says. The team’s results, yet to be peer reviewed, also match theoretical models of false vacuum decay in one dimension, suggesting physicists are on the right track at least. “It is a really interesting first result,” says Weinfurtner, albeit not quite what cosmologists are looking for. For example, in addition to being one-dimensional, the gas doesn’t have uniform density, making the bubbles more likely to end up in the middle, where there is more gas. This makes it hard to glean insights into the distributions and interactions of bubbles in the multiverse.

Universes colliding

Hadzibabic’s experiment, on the other hand, will be two-dimensional and perfectly uniform, thanks to a “box trap” made of laser light holding the condensate in a perfect rectangle. The trap, which is crucial to achieve a good analogy with the universe, was invented by his team and is now used by several other researchers. When I visited the lab, most of the experiment, housed in two van-sized boxes, was almost ready to go. “We hope to see some bubbles next year,” says Hadzibabic.

It will be intriguing to see how those bubbles interact. Johnson and his colleagues have already shown theoretically that bubbles are likely to form in clusters, making collisions more likely. “If we verify their result, that will be really cool,” says Peiris.

The results of the experiment could help physicists re-evaluate unexplained patches in the CMB, such as the four Peiris and Johnson found. “This might also give something else to look for in the sky,” says Johnson. For example, while two bubble universes colliding head on wouldn’t produce ripples in space-time, known as gravitational waves, several clashing at once would. And we might be able to detect them with new gravitational wave observatories.

But even if it turns out that our universe hasn’t crashed into another, Hadzibabic’s experiment still promises to be revealing. There may have been other phase transitions, governed by the same maths that would create a multiverse, in the first moments of cosmos. So testing and improving our general theory for such transitions, as the team is planning, could ultimately help us to decipher what went on in our universe’s very earliest moments.

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Miriam Frankel is a freelance journalist based in London, UK