The Many Faces of the Multiverse

It's looking likely our universe is just one of many,. But what kind of universe do we live in asks Robert Adler

From New Scientist - The Collections: 21 Great Mysteries of the Universe 

WHETHER we are searching the cosmos or probing the subatomic realm, our most successful theories lead to the inescapable conclusion that our universe is just a speck in a vast sea of universes.

Until recently many physicists were reluctant to accept the idea of this so-called multiverse. Recent progress in cosmology, string theory and quantum mechanics, though, has brought about a change of heart. “The multiverse is not some kind of optional thing, like can you supersize or not,” says Raphael Bousso, a theoretical physicist at the University of California, Berkeley. Our own cosmological history, he says, tells us that “it’s there and we need to deal with it”.

These days researchers like Bousso are treating multiverses as real, investigating them, testing them and seeing what they say about our universe. One of their main motivations is the need to explain why the physical laws underlying our universe seem so finely tuned as to allow galaxies, stars, planets, complex chemistry, life – and us – to exist. Rather than appealing to God or blind luck, some argue that our existence sets parameters that reliably pluck universes like ours from the bottomless grab-bag of the multiverse.

Yet there is a problem. Different theories spin off very different kinds of multiverses. Our current standard theory of how the universe came to be, for example, predicts an infinite expanse of other universes, including an infinite number in which duplicates of you are reading this sentence and wondering if those other versions of you really exist. Meanwhile, string theory, which hoped to derive the particles, forces and constants of our universe from fundamental principles, instead discovered a wilderness of 10⁵⁰⁰ universes fundamentally different from ours. Even quantum mechanics implies that our universe is a single snowflake in a blizzard of parallel universes (see diagrams, “The multiverse hierarchy”).

We are now faced with trying to understand and relate these ideas to one another. Of late, we have made enormous strides in making theoretical sense of concepts of the multiverse. At the same time, several groups claim to have made astronomical observations in support of the idea. It seems that we are finally beginning to find our place within the universe of universes.

We can trace the idea of the multiverse back to the early 1980s, when physicists realised that the big bang created an equally big problem. When astronomers measured its afterglow, radiation known as the cosmic microwave background (CMB), they found that it was unfathomably uniform – even at opposite ends of the visible universe. Finding a temperature match between such widely separated regions to within 1/10,000th of a degree, as we now know it to be, was as surprising as finding that their inhabitants, if any, spoke the same language, says Brian Greene, a physicist at Columbia University in New York City.

The problem was solved brilliantly by cosmologists Alan Guth at the Massachusetts Institute of Technology and Andrei Linde at Stanford University, California, among others. Their insight was that in the first 10^-35 seconds of the universe’s existence, space itself expanded by a factor of roughly 10³⁰. This stupendous stretching, known as inflation, accounts for the uniform temperature of the CMB and resolves another conundrum: why space appears flat, like a three-dimensional version of an infinite table top. Inflation has become an incredibly successful theory, precisely predicting the subtle ripples since measured in the CMB, which are echoes of quantum perturbations thought to have seeded galaxies and stars.

Guth and Linde’s attempt to explain our universe also led directly to a multiverse. That’s because inflation didn’t conveniently stop at the farthest regions from which light can travel to us today. Depending on how inflation unfolded, Guth says, the universe could be 10¹⁰ times, 10²⁰ times, or even infinitely larger than the region we see. Inflation implies an expansion faster than the speed of light, meaning that beyond the horizon of our observable universe lie other parts of the universe that are effectively separated from ours. No influence can travel between these regions, essentially creating an infinite number of other worlds.

What would they look like? Max Tegmark, also a cosmologist at MIT, points out that although inflation predicts an abundance of universes, they all feature the same particles, forces and physical laws we find in our cosmic patch. But while in our universe elementary particles come together to make stars and galaxies in a particular way, the universe next door will contain a different arrangement of stars and galaxies, and so will our neighbours’ neighbours. Still, Tegmark has shown that even if a universe like ours were completely filled with elementary particles, they can only be arranged in a finite number of ways. It’s a huge number, 2 to the power of 10¹¹⁸, but since there’s no sign that space is finite, there’s room for every arrangement to repeat.

This means that if you travel far enough, you will eventually encounter a universe containing an identical copy of you. Tegmark has calculated that your nearest copy lives about 10 to the power of 10²⁹ metres away. Carry on and you will find our universe’s twin lies 10 to the power of 10¹¹⁸ metres from here (arxiv.org/abs/0905.1283v1). Since an infinite universe hosts an infinite number of variations, somewhere you have just won the Olympic 100 metres. Congratulations!

Abundant as these universes are, there is nothing exotic about them. Tegmark classes the universes implied by simple inflation or an infinite expanse of space as the first level of a four-tier hierarchy that gets much, much stranger.

Take the second type of multiverse. Soon after inflation was discovered, Linde realised that it could be an ongoing or eternal process. When the enormous energy of empty space creates an inflating baby universe, the space around it, still crackling with energy, continues to expand even faster. That space could then sprout more universes that themselves inflate, and so on and on. “Practically all models of inflation that have been discussed predict eternal inflation,” says Alexander Vilenkin at Tufts University in Boston, who pioneered the idea in the 1980s. Guth dubs it the ultimate free lunch.

The eternal-inflation smorgasbord includes an infinite number of level 1 universes, but many other varieties as well. Each universe turns out in a different way, so features once thought universal, such as the masses of elementary particles and the strength of fundamental forces, differ. The bubble universes from eternal inflation include every permutation the laws of physics allow, leading Linde to quip that it’s not just a free lunch but “the only one at which all possible dishes are available”.

Those universes are part of the second level of Tegmark’s multiverse hierarchy. It also includes some 10⁵⁰⁰ sorts of universe implied by string theory, the leading contender for a “theory of everything” that would explain all the particles and forces of nature.

Today’s standard model of particle physics includes a score of parameters whose values physicists can measure but can’t explain, such as the mass of an electron. String theorists hoped their theory would explain why those parameters have the values they do, and so why our universe is the way it is.

They were sorely disappointed. Rather than producing one perfect snowflake – the particles, forces and interactions underpinning our universe – string theory loosed an avalanche of universes, a daunting expanse that Leonard Susskind, a theoretical physicist at Stanford University, dubbed the string theory landscape.

What sets these universes apart is the nature of their space-time. In string theory, nature’s particles and forces come about from vibrations of tiny strings in 10 dimensions. The reason we only experience four dimensions is because the rest are “compactified” or knotted into intricate structures too small for us to experience. The physics that plays out in any given universe depends on how many dimensions are scrunched up, and their structure. Researchers have identified an enormous number of possible shapes that interact with string-theory fields to define a vast number of universes, most with unfamiliar physical laws and radically different forces and particles.

Eternal inflation provided a convincing mechanism for populating every point in the string theory landscape with an infinite number of real universes. “Originally, string theorists did not like the idea of the multiverse and wanted to have just one solution, but instead they found 10⁵⁰⁰,” says Linde. “Now we must learn how to live in a universe which offers us an enormous multitude of choices.”

Finding out why our universe is as it is, when there is such a vast number of alternatives, remains one of cosmology’s biggest challenges. Our universe seems inexplicably finely tuned to produce the conditions necessary for life. If gravity were a bit stronger, the big bang would have been a squib. A bit weaker and it couldn’t have made galaxies and stars. If the electron’s charge differed by a few per cent, stars could not have created the heavy elements that make Earth-like planets. If the strong force varied by half a per cent, carbon wouldn’t exist, so neither would life as we know it. Topping the fine-tuning list is our universe’s small cosmological constant – the tiny dose of dark energy that is the source of the accelerating expansion of the universe.

The discovery in the late 1990s that the universe’s expansion is accelerating shocked most cosmologists. Quantum theory predicts a level of dark energy roughly 10¹²⁰ times larger than what was found. Since that would blast the universe apart, most researchers had assumed that some undiscovered symmetry would cancel that huge number, leaving a cosmological constant of zero. Nobody predicted that it would not be zero – except one person.

A decade earlier, Steven Weinberg, a Nobel-prize-winning physicist at the University of Texas, Austin, had predicted a small positive cosmological constant. That coup came from applying anthropic reasoning to the multiverse, an approach that is still hotly contested. Weinberg reasoned that for a universe to generate galaxies – and so stars, planets and observers – the amount of dark energy had to fall within a certain range for us to be here to measure it. This amounts to homing in on a subset of universes within the multiverse that have those properties. This probabilistic approach allowed Weinberg to predict the value of the cosmological constant with remarkable accuracy.

“The discovery of the cosmological constant was one of the most unexpected discoveries of the last century, and it was predicted by the multiverse,” says Vilenkin. “So it’s indirect evidence that we are living in a multiverse.”

Since then, other researchers have used anthropic reasoning to constrain the amount of dark energy, the ratio of dark matter to ordinary matter, and the mass of elementary particles such as neutrinos and quarks. Using anthropic reasoning to winnow out our kind of universe from the multiverse, it seemed, might explain that mysterious fine-tuning. It might, in fact, be the only way. “If the laws and physical constants are different in other places,” says Martin Rees, an astronomer at the University of Cambridge, “there’s no avoiding it.”

Unfortunately there’s a catch in using this approach to elucidate our universe’s place in the multiverse: the usual rules of probability may not apply, making it impossible to estimate the likelihood of universes like ours. “You have infinitely many places where you win the lottery, and infinitely many where you don’t. So what’s your basis for saying that winning the lottery is unlikely?” says Bousso. “It pulls the rug out from under us to prove a theory right or wrong.”

This “measure problem” may have been solved by Bousso and his student I-Sheng Yang, now at Columbia University. They got rid of the troublesome infinities by deriving probabilities for a universe’s parameters from local “causal patches” – everything an observer can ever interact with. The probabilities they found matched those from an alternative approach pioneered by Vilenkin and Jaume Garriga, at the University of Barcelona in Spain, putting predictions from the multiverse on what could be a solid footing for the first time (New Scientist, 6 March 2010, p 28).

While theorists have made great strides in understanding cosmology’s multiverses, another two levels of Tegmark’s hierarchy remain. Level 3 has its origins in quantum theory. Physicists accept that quantum mechanics works brilliantly. It can be used, for example, to calculate a value for the magnetic moment of the electron which matches measurements to 1 part in a billion. However, they have yet to agree on what it means. In the quantum realm, particles don’t exist as discrete entities that you can pin down, but as “probability waves”. It’s the evolution of those waves that lets quantum physicists predict how electrons cloak an atom, how quarks and gluons interact, and even how objects as large as buckyballs can interfere like light waves (New Scientist, 8 May 2010, p 36).

The pivotal question is what happens to an object’s probability wave – its wave function – when someone measures it. Niels Bohr, a founder of quantum mechanics, declared that observing the wave function caused it to collapse and a particle to appear at a particular time and place. That, he said, explains why we see just one outcome out of the infinite possibilities embodied in the wave function.

Yet Bohr’s interpretation has long been criticised because it suggests that nothing becomes a reality until someone observes it. In the 1950s, such arguments led Hugh Everett, then a graduate student at Princeton University, to explore what would happen if he jettisoned Bohr’s argument. When Everett pictured the wave function rolling majestically on, as the maths of quantum theory said it did, he arrived at a still astonishing and controversial conclusion. Known as the many-worlds interpretation, it calls for the existence of a vast swarm of universes paralleling ours, in which all the possibilities can play out.

How real are those parallel worlds? As real as dinosaurs, says David Deutsch, a quantum physicist at the University of Oxford. “We’ve only ever seen fossils, and dinosaurs are the only rational explanation for them,” he says. “Many worlds is the only rational explanation for the quantum phenomena we see. I think this is as good as dinosaurs.”

The parallel worlds of quantum theory and the multiple universes created by eternal inflation could not seem more different. However, theorists have started to explore the idea that the quantum landscape and the inflationary landscape are one and the same (New Scientist, 4 June, p 8). Bousso and Susskind argue that they produce the same collection of parallel universes. “We both thought for a long time that the multiverse idea and the many-worlds idea were redundant descriptions of the same thing,” Susskind says. Earlier this year, though, he and Bousso developed a consistent way of applying quantum rules to the multiverse. The inflationary multiverse, they conclude, is simply the collection of all the bubble universes that quantum mechanics allows. “The virtual realities of quantum mechanics become genuine realities in the multiverse,” Susskind says.

Tegmark also equates the infinite variants of our universe in the level 1 multiverse to the infinity of quantum worlds. “The only difference between level 1 and level 3,” he says, “is where your doppelgängers reside.”

The multiverses mentioned so far demote our universe to the status of a pebble in a vast landscape, but at least they allow it to be real. Philosopher Nick Bostrom at the University of Oxford ups the ante by arguing that the universe we experience is just a simulation running on an advanced civilisation’s supercomputer.

His argument is simple. Civilisations that last will develop essentially unlimited computing power. Some of those will run “ancestor simulations”, reconstructions of their forebears or other beings. Just as millions of us play video games like The Sims, a trans-human civilisation would probably run multiple simulations, making it plausible that we are in one of them. Bostrom doubts that we will find evidence that we do or don’t live in a simulation. He argues that an advanced civilisation smart enough to create a simulation in the first place would prevent people inside from noticing a problem, or erase the evidence. Tegmark categorises this and several even more speculative forays as level 4 multiverses.

We might not be able to test or disprove a simulation, but what about other kinds of multiverses? Theorists point to several ways that other universes could have left signs we can see. For example, the bubble universes of eternal inflation can collide. One result could be annihilation. “If you’re in a bubble and the wall accelerates towards you, it’s bad news,” says Matthew Kleban, a theoretical physicist at New York University. But in some cases bubbles bounce apart, leaving a telltale smudge in the CMB. Kleban and his colleagues have calculated the details – a symmetrical spot within a particular range of sizes that stands out from its surroundings because it has a different temperature and polarisation (arxiv.org/abs/1109.3473).

Something similar has been seen in the CMB, but Kleban admits that the evidence for a collision is still weak. Still, he says, a collision could be confirmed by data from the Planck satellite, which is currently studying the CMB, or by future missions. Such a find would provoke a new Copernican revolution. “It would tell us that we’re inside a bubble that is embedded with a huge number of other bubbles with different laws of physics,” he says. “That would be a very, very important discovery.”

Quantum mechanics predicts that emerging universes start out entangled with each other. That early entanglement may leave long-lasting signs. “The moment you have a physical mechanism that shows how the universe is born, you end up with a whole series of predictions of how that universe should look at later times,” says Laura Mersini-Houghton, a cosmologist at the University of North Carolina in Chapel Hill. She and her colleagues used this approach to generate four predictions all of which, she says, have been confirmed.

One was the existence of a giant void in our universe. Data from NASA’s WMAP satellite and the Sloan Digital Sky Survey show that something similar lurks in the constellation Eridanus (New Scientist, 24 November 2007, p 34). They also predicted that the overall intensity of the CMB should be about 20 per cent less than predicted by inflation and, surprisingly, that everything in our universe is flowing in a particular direction. “That was so outrageous that nobody believed it,” Mersini-Houghton says, “but it has been confirmed” (New Scientist, 24 January 2009, p 50).

Her group’s most controversial prediction involves particle physics. Most physicists expect that collisions at the Large Hadron Collider near Geneva in Switzerland will reveal the first signs of a new symmetry of nature known as supersymmetry. Considered an essential ingredient of string theory, supersymmetry requires that every particle in the standard model has a hefty partner waiting to be discovered. Most researchers believe that the high energies at the LHC are sufficient to create these beasts.

However, Mersini-Houghton has predicted that 10,000 times more energy is needed. So far no signs of supersymmetry have appeared (New Scientist, 19 March, p 10). Although some researchers are intrigued, most are taking a wait-and-see attitude towards this ambitious application of quantum mechanics to the multiverse.

Theorists have started to explore other testable predictions that follow from various kinds of multiverses. These include whether space is flat or slightly curved and particles and forces other than those predicted by the standard model or supersymmetry (New Scientist, 19 July 2008, p 36).

Three decades after the concept was born, many researchers now agree that at least some kinds of multiverse stand on firm theoretical foundations, yield predictions that astronomers and particle physicists can test, and help explain why our universe is the way it is. Still, Bousso says, multiverses exist at the frontier of science. “You never know anything for sure when you’re working on the edge of knowledge,” he says. “But if I didn’t think it was by far the best bet, I wouldn’t waste my time on it.” Susskind agrees, but adds: “We’re nowhere near the end of questions. Surprises will happen.”