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
He is coauthor of The Shape of Inner Space.