Science News - June 7, 2008

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 Pasadena published on arXiv.org in December 2007.

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

Princeton University theoretical astrophysicist David Spergel. And “if we’re living to the north and right of that spot, in one direction of the sky we should see more collisions with other bubbles than in other directions.”

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 University of Oxford in England, in a 2005 Physical Review Letters paper.

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 Rud-nick of the University of Minnesota in Minneapolis announced that he and his team, combing through data from the Very Large Array radio telescope near Socorro, N.M., found a giant void, nearly 1 billion light-years across. The void, centered on the WMAP cold spot, appears to be largely empty of galaxies or dark matter.

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

“It’s almost
like you
try to tune your
TV to static,
and you
keep being
interfered with
by sitcoms.”
matthew kleban

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.” s