The universe offers more than enough mysteries for most people—but not for Matthew Kleban. The New York University physicist spends much of his time looking beyond the boundaries of the known, thinking about other universes and how we might detect them.
It might seem like a futile endeavor. Although many respected scientists have developed theories proposing that there are universes other than our own, nobody knows if they actually exist. Plus, our universe is so vast that most of it is hidden from view. If we can’t see to the edge of our own cosmos, how could we ever expect to view other ones?
Yet there may be a way—a loophole that’s not only big enough to drive a truck through, it may be big enough to drive an entire universe through. Even if we cannot look beyond our universe, we might still be able to detect signs of another universe if, sometime in the distant past, it came careering into ours, leaving behind vestiges of that crash for some wily observers to pick up on. In that case, the evidence of other universes might be right here at home.
That prospect excites a small cadre of researchers, perhaps none more so than Kleban, who has made it his mission to try to find the traces of an ancient cosmic collision. Like a forensic scientist sifting for clues before it is even clear whether a crime has been committed, Kleban and his colleagues are figuring out exactly what kinds of patterns, hidden within observable astronomical features, might expose the existence of universes beyond our own. These ideas are now starting to be put to the test. Amazingly soon—perhaps even within a year—we might know whether something as preposterous as a hit-and-run encounter with another universe has actually happened.
Big Bangs Galore The reason that scientists do not dismiss the notion of colliding universes as sheer nonsense can be summed up in a single word: inflation. Not the kind that eats away at your paycheck, mind you, but the kind that fantastically expands the fabric of space. In 1980 Stanford astrophysicist Alan Guth proposed that as our universe sparked into existence, it underwent an explosive burst of growth, ballooning by a factor of 1030 or more within a trillionth of a trillionth of a trillionth of a second. Then this runaway growth spurt abruptly ended; the universe continued to expand but not nearly at the rate of that fleeting surge.
Guth’s inflationary model quickly earned converts because it made sense of many of the present-day universe’s characteristics. For example, a sudden burst of expansion explains why matter is spread relatively evenly in every direction; it would have been stretched and smoothed during the spasm of rapid growth. Yet one aspect of inflation left scientists scratching their heads: Why did it end so quickly? In fact, Tufts University cosmologist Alex Vilenkin began to wonder if it ended at all. Guth’s model, Vilenkin realized, suggested that if inflation stopped in one place, it should continue somewhere else, setting off a cascade of expanding regions in space. In 1983 Vilenkin fleshed out that idea and arrived at a startling conclusion: Maybe inflation really did keep going, and our universe is just one of many that keep popping up as new episodes of inflation begin. In this view, the Big Bang that created our universe was not unique. It was just one of a countless stream of Big Bangs, each creating an expanding bubble of space with its own set of physical laws. Our universe could be just one bubble in a vast froth of other bubble universes.
Vilenkin put aside his bubble hypothesis for a while, but he could not resist revisiting it when observational evidence for inflation emerged in 2003. That year NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) produced a survey of the light released soon after the Big Bang. Just as Guth predicted, this radiation, known as the cosmic microwave background, was incredibly uniform. But within the overall uniformity was a very slight pattern of variation, and that pattern also closely matched the predictions of inflation.
Jostling Bubbles Vilenkin was ready to take his startling ideas even further. In 2007 he published a paper, together with Guth and University of Barcelona cosmologist Jaume Garriga, that explored whether our universe might ever run into one of the others. As our bubble expands, they argued, the odds of an encounter with an outlying bubble keep going up. “If we wait long enough, our bubble universe will eventually undergo an infinite number of collisions with other bubble universes,” Vilenkin says. Now the challenge was coming up with a way to prove that such collisions truly occurred.
The search for inter-universe collisions has been energetically taken up by Kleban. He started his career studying string theory, which posits that every fundamental particle and force is composed of vibrating strings some 10-33 centimeter in size. That is way too small to observe directly. But if string theory is correct, then the interaction of all those strings can lead to phenomena at very large scales, including multiple universes—a separate theoretical reason to think our universe is not the only one. That connection between the very small and the unthinkably large attracted Kleban to the hunt for bubble collisions.
Kleban, along with a few other physicists, figured that evidence of other universes might be etched into the cosmic microwave background, the farthest boundary of our universe that we can observe. If our bubble had collided with another in the distant past, the smashup would have injected a huge amount of energy into a portion of our universe. That jolt could have imparted an observable, localized disturbance in the otherwise homogeneous microwave backdrop. Along with NYU postdoctoral researchers Spencer Chang and Thomas Levi, Kleban thought about the features of such a disturbance. They noted that if two spherical bubbles came into contact, the impact zone would be in the shape of a circle. A pulse of energy from this circular zone would travel into our bubble like a shock wave, where it would presumably leave a disk-like imprint in the microwave background. The center of the disk would register as slightly warmer or slightly colder than its surroundings. Kleban, Chang, and Levi published their results in a 2008 paper, “Watching Worlds Collide.”
Embracing a Long Shot Following Kleban’s paper, cosmologists launched a search for such a disk. Matthew Johnson of the Perimeter Institute in Ontario conducted the most exhaustive survey last year, but his results have been equivocal at best. His team identified eight possible disk-shaped features in wmap images warranting follow-up analysis, but Johnson says they could be explained by other astronomical phenomena or attributed to random fluctuations.
Kleban is not discouraged. Data from another, higher-precision space telescope called Planck will be released next year. Planck is not only more sensitive than WMAP, but it also has novel capabilities. In addition to measuring the temperature of the cosmic microwave background, Planck can determine its polarization, the direction in which the waves of light vibrate as they move through space.
That capability is important because last year Kleban predicted that a bubble collision would produce a specific polarization feature: two concentric rings, each with light polarized in a particular direction, outlining the edge of the disk. No other known astrophysical phenomenon would yield that pattern. “This would be a true smoking-gun signal,” Kleban says.
Cosmologists are anxiously awaiting the Planck data release and the chance to test out Kleban’s predictions. “The discovery of a bubble collision would be a revolution whose importance could not be overstated,” says Stanford physicist Leonardo Senatore, who is analyzing WMAP data for the telltale signs and will soon do the same with Planck data. A positive detection would confirm the idea that there are at least two bubble universes—and probably more, since the inflationary process that spawned our universe can presumably churn them out indefinitely. “Once you go from one universe to two, going from two to many is a trivial step,” Senatore says.
Despite this excitement, Kleban warns that there are a number of reasons why we might not see any signs of universe collisions. First, our universe might truly be the only one. Or the signal from a collision may be too faint because the crash occurred too far away. Even if they are out there, finding signatures of other universes is a long shot, Kleban acknowledges. “The best reason for hope,” he says, “is that we can’t conclude it’s hopeless.” And that is saying something, when you are searching beyond the edge of everything we know.
Steve Nadis is a science writer based in Cambridge, Massachusetts. He is coauthor of The Shape of Inner Space.
