Now Vilenkin had a whole new question to consider: “Can we see signs of such a collision?” The search for evidence that we are living in the aftermath of a cosmic crash was quickly picked up by other researchers, including Aguirre and his colleagues Matthew Johnson and Assaf Shomer, also at Santa Cruz. “The realization that a mild collision could leave behind something that could potentially be seen anywhere in our universe got us interested,” Aguirre says.

According to standard cosmology, the universe should appear much the same whichever way you look; after all, the extreme uniformity of the universe was one of the reasons why the inflation hypothesis caught on in the first place. But a fender bender with another universe that partially infiltrated ours and then moved away would disturb that symmetry in a subtle but distinctive way, leaving a scar in the heavens, Aguirre says.

The place to look for such a scar is the cosmic microwave background—the all-pervasive radiation left over from the Big Bang. The best measurements of this radiation were made earlier in this decade by NASA’s Wilkinson Microwave Anisotropy Probe, or WMAP, which produced a detailed map of cold and hot spots in the early universe (thought to correspond to relatively dense and empty zones, respectively). Although the pattern of the spots largely matches the random distribution predicted by standard cosmology, the map does show some unexpected features. One anomaly streaks across the microwave sky, marking out a strange alignment of certain cold and hot spots. Because it flies in the face of the standard belief that there are no special directions in the universe, this anomaly was mischievously dubbed the “axis of evil” by its discoverers, Kate Land and João Magueijo of Imperial College London, in 2005.

“The axis of evil could be a remnant hanging over from something that happened before inflation took place,” Aguirre says. Although inflation should have erased most details of what the cosmos looked like before that point, it might not have eliminated everything. The axis of evil could therefore be a relic of something huge and powerful that disturbed the infant universe in the very brief moment before inflation kicked in. “A bubble collision that happened before inflation would be a compelling explanation,” Aguirre claims.

He is quick to admit that a collision with another bubble universe is not the only possible explanation for the strange patterns seen by WMAP. For instance, some cosmologists suggest that our universe did not inflate perfectly symmetrically but stretched more in one direction. Others propose that the entire universe may be rotating, which would show up as a distortion in the cosmic microwave background. Aguirre recognizes that he needs more evidence to convince his colleagues—and himself—that our universe was the victim of a multiverse hit-and-run.

Physicist Thomas Levi of New York University is helping to dig up that evidence, albeit from a very different perspective. Levi’s fascination with the multiverse grows from his background in string theory, a physics model positing that all of the elementary particles consist of extremely tiny, vibrating strings. At the end of the last decade, string theory was being touted as the best route to a master explanation for all the physical laws in the universe. But by 2002 proponents of string theory had begun to realize that their equations were a little too good at predicting the laws of physics. Instead of providing one solution that would explain the conditions in our universe, the equations offered up a staggering 10⁵⁰⁰ possible solutions. Each solution seems to describe a different universe in a “string landscape,” each with its own physical laws and each (in theory) equally likely to exist.

This failure to explain the unique laws of our universe initially seemed to spell disaster for string theory. But then physicists began to tie the string landscape to the notion of a multi­verse. Perhaps, they argued, every universe predicted by string theory really does exist—each one in its own bubble within the far greater multiverse. The problem with this interpretation was that it was doubly speculative. There was (and still is) no observational support for string theory, and it did not seem possible that we could find such support for the multiverse, either, since we are locked inside our bubble with no access beyond its walls.

Levi thinks he may have spotted a clue, however. He and two NYU colleagues note another strange anomaly embedded in the cosmic microwave background: In the southern hemisphere of the sky there is one cold spot that is much bigger than the rest. Levi’s calculations show that an ancient wallop from a neighboring universe could have created this spot. “It’s tough to explain with standard cosmology how such a cold spot could have come about,” he says.

While Levi seeks observational support for cosmic collisions, another string theorist—Laura Mersini-Houghton of the University of North Carolina at Chapel Hill—is attempting to study mathematically how neighboring universes would interact. Working with colleagues at Carnegie Mellon University in Pittsburgh and Saga University in Japan, she is modeling where and how bubble universes might be born in the string landscape. The crucial twist: These researchers hypothesize that every budding universe is inextricably intertwined with its siblings. This theory is based on a well-known quantum effect known as entanglement. On a cosmic scale it means that neighboring universes can retain a ghostly influence on each other long after they have drifted apart.

In 2006 Mersini-Houghton predicted that the entanglement between our universe and another could show up as an otherwise inexplicable force pulling on galaxies in one part of the sky. Last year she was elated to hear that a group of NASA astronomers had observed just such an effect: clusters of galaxies being yanked along at a velocity of about 600 miles per second even though it is not at all clear what is doing the yanking. Dubbed “dark flow” by its discoverers, this movement seems mouthwateringly close to her predictions. “It makes me believe that this bizarre mathematical thing I’ve been considering may, in fact, be real,” Mersini-Houghton says, eyes gleaming.

Dark flow is not her only prediction. Entangled universes provide another possible explanation for the axis-of-evil feature in the cosmic microwave background. On the other hand, a recent experience makes Mersini-Houghton wary of highly tentative evidence for interactions with other universes. In December 2006 she and her team predicted that cosmic entanglement would gouge out a giant void in space. Within a year a group led by Lawrence Rudnick of the University of Minnesota announced that the giant WMAP cold spot in the southern sky corresponds to just such a void, one that is far too large to be explained by conventional physics. (It would appear cold because light loses energy as it traverses a vast, rapidly expanding empty space.)

For a moment Mersini-Houghton appeared headed toward celebrity. A young, vivacious woman, she was already living well outside of academia’s ivory tower. She had met with the prime minister of her native Albania, Sali Berisha, to help launch a campaign to spark interest in science; she had also begun receiving Bibles in the mail from people worried about the possible religious implications of the multiverse. “We’re asking fundamental questions about the nature of reality, so it’s understandable,” Mersini-Houghton says.

Soon there came another twist. Follow-up calculations by two other astrophysicists suggested that Rudnick was mistaken and that there is not any great void after all. The news drove home to Mersini-Houghton just how challenging it is to go chasing after bubble universes. “It’s dangerous to hastily point at a cold spot the sky and claim that it’s a window into another universe,” says Hiranya Peiris of the University of Cambridge, who is dubious about all the highly theoretical multiverse discussions. She points out that many of the anomalies seen by WMAP could simply be glitches created by the complicated way in which the microwave background data are interpreted. “It’s easy to read too much into the map,” she says.

For now, tales of colliding universes still play better on the beaches of Grand Cayman than they do in the pages of Physical Review Letters. To convince the many skeptics, Aguirre and like-minded theorists will need to do a lot more work. Levi plans to look for more detailed signatures of cosmic collision in the new, improved measurements of the microwave background being made by the European Space Agency’s Planck satellite, which began a comprehensive new sky survey two months ago.

Vilenkin, meanwhile, is conducting a mathematical census of all the different bubbles that could show up in the multiverse, totting up which values for the physical constants are likely to be shared by the largest number of universes. “We should hopefully be able to make a prediction for the masses of neutrinos [ghostly particles that interact weakly with ordinary atoms] based on which masses are most commonly found in different bubbles,” he says. If future experiments confirm that neutrinos have these predicted masses, that would offer impressive support for the multiverse.

To Mersini-Houghton, the mere fact that serious scientists are having these discussions signals a major turn in physics. “Copernicus shocked the world by telling us that our planet isn’t at the universe’s center,” she says. “We may soon find that our whole universe isn’t even at the cosmic center.”

WINNERS AND LOSERS

In a clash of the cosmos, what are the chances that our universe would emerge victorious? The prospects of survival for any bubble universe come down to the amount of energy embedded in it, Thomas Levi of New York University says. In the late 1990s, astronomers noticed that the expansion of the universe is speeding up—a phenomenon they ascribed to a mysterious “dark energy” pushing our universe apart. That entity may take the form of a cosmological constant, energy that is spread out through all of space. “If our cosmological constant is less than the alien bubble’s, we’re safe,” Levi says. Flip those conditions and “you don’t want to be around.”

In the latter case, a wall forms between the two universes when they collide. If the wall’s tension is less than a certain value, it will rush into our universe, wiping out everything in its path at almost the speed of light. “Some large chunk of the universe is eaten, while all the people who hit the wall are crushed and killed,” Levi says. Luckily, our cosmological constant is vanishingly small, making us primed to win if we do get caught in a cosmic battle.

For some time physicists have been perplexed by the exact value of this constant. Its level seems to be finely tweaked to create just the right conditions for stars, planets, and life to form. Had it been even slightly greater, the universe would have blown apart and life never could have evolved. Levi’s findings hint at why our universe—or any universe—is likely to have a small cosmological constant. “We don’t know enough to say that such collisions happen at all, let alone happen often,” Levi cautions. “But if a typical bubble goes through a large number of collisions, it would point to a reason why we find ourselves in this kind of universe.”

“It does make sense that if the cosmological constant is low, the bubble is more likely to survive,” cosmologist Alan Guth of MIT says. But even if the results looked less encouraging for our universe, Levi would not be too concerned. “I’m more worried about crossing the street in New York and getting hit by a car than being hit by a bubble,” he says. “I won’t go out and buy bubble-collision insurance just yet.”Z. M.

MAKING THE BANG

While other theorists worry about the destructive power of a cosmic collision, two maverick physicists propose that such a titanic accident actually gave birth to our universe—and that what we call the Big Bang is just the latest incarnation in an infinite cycle of creation.

Paul Steinhardt of Princeton University and Neil Turok, now at the Perimeter Institute in Ontario, Canada, devised their controversial alternative to Big Bang cosmology in 2002. Their idea is based on a mathematical model in which our universe is a three-dimensional membrane, or “brane,” embedded in four-dimensional space. The Big Bang, they say, was caused when our brane crashed against a neighboring one. The violence of the collision would have flooded both universes with energy and matter. These collisions should repeat every trillion years, each time triggering a new Bang and a new universe.

Steinhardt and Turok’s model predicts a slight but specific pattern of hot and cold spots that should be detectable in the microwave radiation from the universe’s early days. In 2007 researchers saw hints of that pattern in preliminary measurements from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP). “The signature fits the predictions for the cyclic model,” Steinhardt says. “But it’s too early to call.” Indeed, when a WMAP team revised some probe data, researchers could not confirm that the “signature” was more than a chance blip—but neither could they rule it out. New microwave studies from the Planck satellite may help break the tie. Z. M.