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 Is our universe just one of many? The “multiverse” has occupied the pages of theorists’ notebooks for decades. Now, astronomers are on the brink of testing this hypothesis as they begin the search for evidence of universes beyond our own.
 

 Though the
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2011/11/when-universes-collide/">
 first test
 </a>
 , using data from a satellite called WMAP, came up empty-handed, cosmologists are now turning their attention to fresh results from the European Space Agency’s
 <a href="http://www.esa.int/SPECIALS/Planck/index.html">
 Planck satellite
 </a>
 , which is mapping the cosmic microwave background radiation and creating an all-sky temperature map with three times greater resolution than its predecessor. If other universes exist, it is possible that they have collided with our own universe. Physicists believe that such a collision would leave an imprint on the background radiation in the form of a disc-shaped region of very-slightly different temperature than the surrounding background. Planck’s improved resolution will sharpen the edges of any collision-induced disc in the background radiation.
 

 Planck is also studying a property of the background radiation called polarization, which describes the angle at which the electromagnetic waves vibrate in relation to the direction they are traveling. (You encounter polarization every time you slip on polarized sunglasses; because sunlight reflected off the horizontal surface of a road or a body of water becomes polarized, these special lenses can selectively block it out, reducing glare.) Polarization is a sensitive probe of the conditions that prevailed when the photons were released. Though WMAP was not sensitive enough to see any patterns in the polarization of the photons, Planck, with three times more sensitivity, is expected to see such patterns, which just might contain the fingerprint of so-called “bubble collisions.”
 

 What would such a signal look like? Matthew Kleban of New York University and his colleagues have shown that bubble collisions should leave two highly-polarized rings surrounding the temperature disc.
 

 “According to our predictions for the probability of these bubble collisions, we are more likely to see larger discs than smaller discs,” says Kleban. “And it turns out that for larger discs, polarization is actually a very sensitive test. The signal is more distinctive, and it gets stronger as the disc gets bigger.”
 

 So, if we did see such signatures in the Planck data, what would it prove?
 

 “It would be conclusive of the fact that before our observable universe was formed, there was this precursor phase, and you could say with great certainty that that precursor phase still exists, somewhere, and that we are one small little pocket in a much, bigger multiverse,” says Matthew Johnson of the Perimeter Institute for Theoretical Physics. “It would be fairly direct evidence for the existence of a multiverse.”
 

 That would be revolutionary. It would also focus attention on an unorthodox view of quantum mechanics which is already growing in popularity: the
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2011/11/debating-the-meaning-of-quantum-mechanics/">
 many-worlds hypothesis
 </a>
 .
 

 Proposed in the 1950s by physicist Hugh Everett, the many-worlds hypothesis takes a radical view of what happens to the wave function—the equation that spells out the probability of finding a quantum system in a particular state—when a measurement is made. In Niels Bohr’s Copenhagen Interpretation, any time we make a measurement, the wave function “collapses,” giving us one outcome from an infinity of possibilities. Everett argued that the wave function never collapses. Rather, every possibility exists in a parallel universe. This suggests a staggeringly large number of other worlds.
 

 But are they the same “other worlds” predicted by eternal inflation? Recent work by Leonard Susskind of Stanford University and Raphael Bousso of the University of California, Berkeley, hints that the many worlds of quantum theory and the multiverse of eternal inflation might be
 <a href="http://arxiv.org/abs/1105.3796">
 two sides of the same coin
 </a>
 . By linking eternal inflation with Everett’s many worlds, Susskind and Bousso hope to establish the physical meaning of the probabilistic predictions that have confounded quantum physicists for decades.
 

 Yet even if bubble universes exist, the odds might be against spotting a collision. “Everyone thinks that we would have to be lucky,” says Susskind. “I would not try to estimate just how lucky, but at least somewhat lucky.” After all, our universe is much, much bigger than what we can see—so the collision may lie beyond our cosmic horizon.
 

 Henry David Thoreau once wrote, “The universe is wider than our views of it.” That is true, of course. But the quest to find evidence of universes beyond our own shows that our “view” of the universe is a window that widens just as far as technology, theory, and the laws of physics can stretch it.
 
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Seeking Signs of the Multiverse

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 ONE night in June 2007, I got to watch astronomer Sandra Faber put the 10-meter Keck II telescope through its paces. She was observing galaxies in a region of the sky called the Extended Groth Strip, in the direction of the constellation Ursa Major. We sat in the cozy confines of the telescope control room, far below the telescope’s perch near the 13,796-foot-high summit of the Mauna Kea volcano in Hawaii.
 

 Around midnight, Faber wrapped up her observations and we stepped out for a few minutes under the night sky. “I take comfort in the fact that it is a beautiful universe, and we belong here and that we fit,” Faber mused. “This is our home.”
 

 Faber, a professor at the University of California, Santa Cruz, was referring to the idea that there is something uncannily perfect about our universe. The laws of physics and the values of physical constants seem, as Goldilocks said, “just right.” If even one of a host of physical properties of the universe had been different, stars, planets, and galaxies would never have formed. Life would have been all but impossible.
 

 Take, for instance, the neutron. It is 1.00137841870 times heavier than the proton, which is what allows it to decay into a proton, electron and neutrino—a process that determined the relative abundances of hydrogen and helium after the big bang and gave us a universe dominated by hydrogen. If the neutron-to-proton mass ratio were even slightly different, we would be living in a very different universe: one, perhaps, with far too much helium, in which stars would have burned out too quickly for life to evolve, or one in which protons decayed into neutrons rather than the other way around, leaving the universe without atoms. So, in fact, we wouldn’t be living here at all—we wouldn’t exist.
 

 Examples of such “fine-tuning” abound. Tweak the charge on an electron, for instance, or change the strength of the gravitational force or the strong nuclear force just a smidgen, and the universe would look very different, and likely be lifeless. The challenge for physicists is explaining why such physical parameters are what they are.
 

 This challenge became even tougher in the late 1990s when astronomers discovered dark energy, the little-understood energy thought to be driving the accelerating expansion of our universe. All attempts to use known laws of physics to calculate the expected value of this energy lead to answers that are 10
 
 120
 
 times too high, causing some to label it the worst prediction in physics.
 

 “The great mystery is not why there is dark energy. The great mystery is why there is so little of it,” said Leonard Susskind of Stanford University, at a 2007 meeting of the American Association for the Advancement of Science. “The fact that we are just on the knife edge of existence, [that] if dark energy were very much bigger we wouldn’t be here, that’s the mystery.” Even a slightly larger value of dark energy would have caused spacetime to expand so fast that galaxies wouldn’t have formed.
 

 That night in Hawaii, Faber declared that there were only two possible explanations for fine-tuning. “One is that there is a God and that God made it that way,” she said. But for Faber, an atheist, divine intervention is not the answer.
 

 “The only other approach that makes any sense is to argue that there really is an infinite, or a very big, ensemble of universes out there and we are in one,” she said.
 

 This ensemble would be the multiverse. In a multiverse, the laws of physics and the values of physical parameters like dark energy would be different in each universe, each the outcome of some random pull on the cosmic slot machine. We just happened to luck into a universe that is conducive to life. After all, if our corner of the multiverse were hostile to life, Faber and I wouldn’t be around to ponder these questions under stars.
 

 This “anthropic principle” infuriates many physicists, for it implies that we cannot really explain our universe from first principles. “It’s an argument that sometimes I find distasteful, from a personal perspective,” says Lawrence Krauss of Arizona State University in Tempe, Arizona, author of
 
 A Universe From Nothing
 
 . “I’d like to be able to understand why the universe is the way it is, without resorting to this randomness.”
 

 And he’s not the only one who feels this way. Nobel laureate Steven Weinberg of the University of Texas at Austin once told me, “I would, and most physicists would, prefer not to have to rely on anything like the anthropic principle, but actually to be able to calculate things.”
 

 Nonetheless, there is growing and grudging acceptance of the multiverse, especially because it is predicted by a theory that was developed to solve one of the most frustrating of fine-tuning problems of all—the flatness of our universe.
 

 Spacetime today is flat, not curved—meaning that two rays of light that start out parallel stay parallel, neither converging nor diverging. This has been confirmed to exquisite precision by measurements of the cosmic microwave background, the radiation left over from the big bang. That means that a cosmological parameter called Omega, which dials in the curvature of spacetime, is very close to one. But for today’s universe to have an Omega anywhere near one, its value just one second after the big bang had to be exactly one to precision of about fourteen decimal places. This smacked of fine-tuning.
 

 But in 1979, the physicist Alan Guth, now of MIT, discovered a way to get that value of Omega without fine-tuning. Guth showed that in the instants after the big bang, the universe would have undergone a period of exponential expansion. This sudden expansion, which Guth called “inflation,” would have rendered our observable universe flat regardless of the value of Omega before inflation began.
 

 Imagine starting with a small balloon whose surface is curved and blowing it up some forty orders of magnitude. Any small piece of the balloon’s surface will now look flat. In the inflationary view, that’s what happened to our universe—our local patch of spacetime looks flat regardless of the curvature of spacetime before inflation began.
 

 Some physicists believe that inflation continues today in distant pockets of spacetime, generating one new universe after another, each with different physical properties. Inflation, therefore, walks both sides of the fine-tuning line: It lends credence to the anthropic principle by predicting a multiverse, but it also reminds us that parameters we once thought were fine-tuned, like Omega, can be explained by a more fundamental theory. “The history of physics has had that a lot,” says Krauss. “Certain quantities have seemed inexplicable and fine-tuned, and once we understand them, they don’t seem to so fine-tuned. We have to have some historical perspective.”
 

 We’ll gain such perspective only after we have a fundamental theory of everything—or perhaps when we detect
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2011/12/seeking-signs-of-the-multiverse/">
 signs of other universes
 </a>
 . The urge to understand our universe from first principles and not ascribe it to some divine force compels us to seek scientific explanations for what seems to be an incredible stroke of luck.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 FQXi:
 <a href="http://www.fqxi.org/community/articles/display/163">
 The Patchwork Multiverse
 </a>
 
 Raphael Buosso examines links between string theory, dark energy, and the multiverse.
 

 FQXi:
 <a href="http://www.fqxi.org/community/articles/display/155">
 Testing the Multiverse
 </a>
 
 Hiranya Peiris looks for evidence of other universes in the cosmic microwave background radiation.
 

 Skeptical Inquirer:
 <a href="http://www.colorado.edu/philosophy/vstenger/Cosmo/anthro.skinq.html">
 Anthropic Design: Does the Cosmos Show Evidence of Purpose?
 
</a>
 Victor Stenger provides a critical analysis of the “so-called anthropic coincidences.”
 

 TED:
 <a href="http://www.youtube.com/watch?v=bf7BXwVeyWw">
 Why Is Our Universe Fine-Tuned for Life?
 </a>
 
 In this video, Brian Greene asks why our universe appears so exquisitely tuned for life.
 
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Is the Universe Fine-Tuned for Life?

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 The vast space that we call our universe might be just a small speck in a fabric dotted with other universes, maybe an infinite number of them. We could be living in what physicists call a
 <a href="http://video.pbs.org/video/2168433605/">
 multiverse
 </a>
 . It is a tantalizing prospect, but verifying it has remained in the realm of fantasy–until now. For the first time, the theorists have a testable prediction: Collisions with nearby universes could have left a mark in the cosmic microwave background, the fossil radiation of the big bang. Now, the hunt is on to detect the echoes of these collisions, which would provide the first experimental evidence for the existence of other universes.
 

 Let’s back up a bit and see
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2011/11/a-tour-of-the-multiverse/">
 why physicists think there may be universes beyond our own
 </a>
 in the first place. Fractions of a second after the big bang, our universe is thought to have expanded exponentially in a phase called inflation. Soon after inflation was proposed by the physicist Alan Guth, now of MIT, other physicists, including Andrei Linde of Stanford University and Alex Vilenkin of Tufts University, realized that once inflation got going, it should never end. According to this idea, now called “eternal inflation,” what we think of as the vacuum of space isn’t actually empty; it contains energy that makes it unstable and prone to form new bubble-like vacuums, much like bubbles of air emerging in boiling water. Each bubble inflates in turn, and new bubbles can form within it. In this view, our universe is just one of a huge and ever-increasing number of bubbles, each capable of giving rise to a new universe.
 

 But how does one turn such esoteric theory into experimentally verifiable fact? Our best shot, according to Matthew Johnson of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and his colleagues, is to seek out evidence that one of these bubble universes collided with our own. Such a collision would create perturbations in the fabric of spacetime which would leave an imprint on the cosmic microwave background radiation.
 

 The background radiation is the universe’s first light, made up of photons that were released throughout the universe a mere 370,000 years after the big bang. The photons contain information about the state of the universe at that instant, and that state would have been influenced by events close to the big bang—such as inflation itself or even a collision with other bubbles.
 

 Johnson and his colleagues calculated that a collision would leave a distinct disc-shaped imprint on the background radiation. The temperature inside the imprint would be ever-so-slightly different from the temperature outside the disc. Properties such as the size of the disc, which could range from a few fractions of a degree to half the sky, and the intensity of the temperature difference between the inside and the outside of the disc, would depend on the exact nature of the collision.
 

 The team then set about predicting how many collisions one should expect to find in the data collected by NASA’s WMAP satellite, a telescope designed specifically to probe the minute variations in the CMB, and what the traces of those collisions would look like to WMAP. Their result may be a bit of a downer for fans of the multiverse—they discovered that the data are most consistent with zero collisions–but that is not the end of the story. It is possible that the instrument simply isn’t sensitive enough to detect the traces of collisions with other universes. In fact, there are a few spots in the data that match the kind of signal expected from bubble collisions, but the result is not statistically significant. Random fluctuations in the background radiation could also create such patterns.
 

 Still, Johnson is pleased. “Before this analysis these ideas seemed untestable,” he says. “It seemed like science fiction, but it’s not. It’s very exciting that you can rigorously do science in this theory. You might be unlucky and not see anything, but that’s kind of beside the point. It’s testable.”
 


 Next week, discover what’s next in the search for signs of the multiverse.
 

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Anil Ananthaswamy

Articles

Seeking Signs of the Multiverse

Is the Universe Fine-Tuned for Life?

When Universes Collide