CRASH COURSE
Microwave radiation traveling across the universe appears mostly uniform — as shown in the larger sphere — except for tiny temperature variations (red and blue) that indicate the locations of the seeds of future galaxies and other structures. Some cosmologists now say that if, in the formation of the early universe, another “bubble” universe — the smaller sphere — collided with our own, the collision might have left its mark on the cosmic microwave background radiation. A. Aguirre
Science fiction movies and books are full of parallel universes.
In a typical scenario, as in the movie Sliding Doors, something happens in one universe—like a woman misses a train—but in a parallel universe, the same woman catches it, setting in motion diverging life paths.
Or, as in Isaac Asimov’s imaginative novel The Gods Themselves, alien inhabitants of a parallel universe with different physical laws exchange energy with our universe and send coded messages to Earth.
Even without any real-life alien messages to decipher, though, many cosmologists believe that there really are other universes out there. It’s just that their existence has long seemed more of a philosophical speculation than a testable hypothesis. Seeing such universes directly would require exceeding the speed of light—a violation of the laws of physics better left to the science fiction writers.
Now, however, some cosmologists suggest that even though we can’t communicate with another universe, there might be a way to discern a sign of its existence if it collided with ours. Such a cosmic impact might leave its mark in the cosmic microwave background—our earliest snapshot of the origin of the universe.
“I think that’s a really, really fascinating idea,” says MIT cosmologist Max Tegmark. “Although the evidence has been mounting that parallel universes might exist, I think there’s been a feeling of resignation that they might remain just parallel universes that you could never touch, never see directly. And now suddenly comes this idea in from left field, suggesting not only that it might be possible to see them, but even that we might already have seen them, imprinted in the cosmic microwave background.”
Two groups, one led by Anthony Aguirre of the University of California, Santa Cruz, and the other by Matthew Kleban of New York University, posted papers online in December proposing the possibility of observing those ultra-cosmic collisions. Each group suggests that the collisions might be visible, although the details of what the signature might look like have yet to be worked out.
Inflating, eternally
Preposterous as parallel universes might seem, their existence may be an inevitable consequence of the physics behind the Big Bang birth of the universe. Cosmologists believe that shortly after our universe began, a brief period of rapid expansion, called “inflation,” enlarged it by many orders of magnitude, like blowing a tiny bubble of foam up to the size of a hot-air balloon.
Most of inflation’s testable theoretical predictions have been confirmed. “There’s a sense in which inflation is a key and somewhat indispensable part of our current understanding of how our universe began and evolved,” Aguirre says.
But the theory, proposed by MIT cosmologist Alan Guth in 1981, also implies that inflation didn’t just happen once, in our part of the cosmos, but rather keeps happening, inflating other patches of space like bubbles forming in a pint of beer.
This “eternal inflation” creates other “bubble universes” that likely have different properties from our own universe in terms of cosmological quantities and even physical laws. In some bubbles, the electromagnetic force might be so weak that it can’t hold atoms together, or the expansion rate might be so fast that galaxies can’t form.
The multiple universe consequence of inflation is hard even for scientists to wrap their minds around.
“Most people ignored this carefully for 20 years or so because they didn’t want to think about it,” Aguirre says. “They like to think of inflation as this little interlude. So most people ignored this idea. Although most of the people who invented inflation kept thinking about it because they saw that it was important.”
Cosmologists haven’t been alone in postulating multiple universes. String theory suggests that our universe contains extra dimensions, most of which are curled up and so tiny we can’t see them, but that underlie our particular set of physical laws. Many scientists wondered why our way of curling up the extra dimensions should be the only possible way.
In string theory, another universe could have as many as 10 dimensions of space, with seven curled up. The types and masses of fundamental particles, as well as the varieties and strengths of fundamental forces, could vary in an uncountable number of ways.
String theory predicts just the sort of eternal inflation that “will create lots of different universes,” says Aguirre, “or regions where inflation stops, but they’ll correspond, each of them, to different ways that those extra dimensions are curled up, and have different properties for the universe that they form.”
From the inside, each of these universes appears infinite. So the woman who missed the train in her universe can never know about the woman who caught it in the other—in a sense, she’s trapped in her bubble. “So it seems like the hypothesis that there are other bubbles out there with different properties is just speculation or metaphysics or fantasy,” Aguirre says.
When bubbles meet
However, kind of like the foam in a sink full of soapy water, these bubbles could run into each other. For a long time scientists thought that these collisions almost certainly happen but that the chances of seeing one would be rare. Or if you did see one, it would be fatal. “The bubble running into our bubble would come into our bubble and destroy us,” Aguirre says. “And since that obviously hasn’t happened, there’s some reason.”
With those things in mind, Guth, Jaume Garriga and Alex Vilenkin calculated how likely it would be to see a bubble collision in our observable region of the universe—which is just a tiny part of one bubble. They posted their paper online in December 2006 at arXiv.org.
“We concluded that most observers in bubble universes live very far away from the collision regions and will not see any signatures of the collisions,” wrote Vilenkin, of Tufts University in Medford, Mass., in an e-mail interview. He said they assumed that new bubbles form at a slow rate, so “bubble collisions are not very frequent.”
That means the woman on the train would never know if her bubble crashed into the bubble containing the woman who missed the train, because the first woman is too far away from the place where the bubbles hit. Even if the collision destroyed part of one of the bubbles, the unaffected region each woman sees would still look infinite from her standpoint.
On the other hand, a collision might just wipe out the other bubble altogether.
Aguirre and his collaborators took another view. “Why should one of those collisions necessarily be so devastating?” he wondered. And what if a gentle collision happened close enough that you could actually see it?
In a paper published online at arXiv.org in April 2007, they tried to imagine what the collision would look like. “It’s sort of a disk on the sky,” Aguirre says, which “might be infinitesimally tiny in some cases, or the whole sky in other cases.”
But a question remained about when bubble collisions would be fatal to the observer. “And when might they just ‘ping’ the bubble the observer is in, not really disrupting it too much, but leaving some kind of signature?” Aguirre says, an indicator that someone in the bubble could see and say, “Oh look, there’s another universe.”
This is the scenario postulated in the paper he and postdoctoral researcher Matthew Johnson of the California Institute of Technology in
They found in many instances a collision wouldn’t necessarily be fatal, but might be seen as a disturbance of the microwave background. What exactly it would look like, Aguirre says, isn’t something they’ve been able to calculate—for example, if the “ping” would cover a large or small area on the sky. But if it was just one bubble, or even if it was many bubbles, it would appear to come primarily from one direction.
Imagine just one bubble bumping up against another, the situation NYU cosmologist Kleban and his collaborators considered in their paper, published on arXiv.org just days before Aguirre’s second paper. “If you’re inside one of them,” says Kleban, “obviously there is one direction where one bubble came from, and you’ll see something special in that direction.”
Oddly, that anisotropy—a greater signal from one direction than the other—would be the same even if there were multiple bubbles. Only in the extremely unlikely scenario that Earth occupied the exact center of the cosmos would bubbles hit equally from every direction.
A cosmic ‘axis of evil’
Another way to think about it is that in this bubble model, the Big Bang started in a particular place in space and time, says
Here’s where the cosmic microwave background comes in. This radiation, left over from the Big Bang, has been cooled by the universe’s expansion to about 2.7 kelvins, or degrees Celsius above absolute zero. It looks infinitesimally hotter and colder in spots, corresponding to the fluctuations in density of the early universe that led to the clumping of matter into galaxies. Up to now, the pattern of spots has appeared the same in all directions. But if Aguirre’s and Kleban’s speculations are correct, the cosmic microwave background would look perhaps slightly colder in one direction than in the opposite direction.
Tantalizingly, the most precise measurements of the cosmic microwave background to date, made by the Wilkinson Microwave Anisotropy Probe, or WMAP, satellite appear to hint at exactly that. “There is a bit of an anisotropy,” Kleban says. “In particular, there is a big cold spot in one direction,” which makes it look like the sky is rotating around an axis.
This anisotropy was dubbed the “axis of evil” by researchers João Magueijo of Imperial College London and Kate Land, now at the
Spergel, one of the investigators on the WMAP team, is skeptical. “I think the ‘axis of evil’ in the CMB is much like George Bush’s ‘axis of evil,’ in that if you go into the data looking for something,” he says, “you’ll find something.”
But other people are looking anyway. Last August, astronomer Lawrence Rudnick of the
That’s about what you’d expect if the cold spot is real. Such anisotropy might indicate a bubble collision—or it might not.
Spergel contends that the hottest spot and the coldest spot on the sky in the cosmic microwave background lie within the plane of our galaxy, which, he says, “suggests that what we’re really seeing is large-scale variations in dust properties within our galaxy, not something cosmological.”
Kleban agrees that it’s difficult to separate out the effects of interferences from within the galaxy. “It’s almost like you try to tune your TV to static,” he says, “and you keep being interfered with by sitcoms.”
He adds that he doesn’t yet know if a bubble collision would produce exactly the cold spot that may exist in the cosmic microwave background. Still, “the possibility, if it’s right, is very exciting,” he says. “It would really change our view of our place in the universe.”
There’s another possibility: a collision with another bubble hasn’t happened—yet. If a devastating collision is in our future, says Kleban, “we’re just squashed like bugs, and that’s the end of us.”
If two bubbles collide, the bubble wall between them would tend to accelerate toward one or the other bubble. “And if it accelerates towards us, then light or any other signal from the collision arrives just a moment before the wall itself arrives, and in that case, we’re dead,” Kleban says. “But one comfort is that there isn’t very much warning.” Happily, because of some of the particular properties of our universe, in most cases, the wall would move away from us, rather than into us, he says.
The next step is to better understand what theoretical models actually predict for the cosmic microwave background signature. “Much work remains to be done to reach any reliable conclusions, but the first steps made in Aguirre’s and Kleban’s papers are very important and interesting,” wrote Vilenkin in an e-mail.
Tegmark is optimistic. “This is an example of something we’ve seen over and over again in science, where the borderline between science and science fiction shifts,” he says. Atoms and black holes might have forever remained in the realm of science fiction, but new technology expanded the frontier of science and allowed them to be detected. Parallel universes, Tegmark says, “could be yet another case of something we thought was beyond science and ends up being within science.”
Walter
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