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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 centre of the cosmos would bubbles hit equally from all directions.
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. "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 kelvin, or a few degrees Celsius above absolute zero (-273°C). 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. Until now, the pattern of spots has appeared the same in all directions. But if Aguirre's and Kleban's speculations are correct, the CMB would look perhaps slightly colder in one direction than the other.
Tantalisingly, the most precise measurements of the CMB to date, made by the Wilkinson Microwave Anisotropy Probe, (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" in a 2005 Physical Review Letters paper authored by U.K.-based researchers João Magueijo of Imperial College in London and Kate Land, now at the University of Oxford. Spergel, one of the investigators on the WMAP team, is sceptical. "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. In August 2007, astronomer Lawrence Rudnick of the University of Minnesota in Minneapolis announced that he and his team, combing through data from the Very Large Array radio telescopes near Socorro, New Mexico, found a giant void, nearly one billion light-years across. The void, centred 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 were real. Such anisotropy might indicate a bubble collision – or it might not.
Spergel contends that the hottest and coldest spots on the sky in the CMB 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 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 CMB. 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 bubbles collide, the wall between them would tend to accelerate toward one of the bubbles. "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. 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 CMB 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," says Vilenkin.
Tegmark is optimistic, though. "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 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."
Diana Steele is a science writer in Ohio, USA.
Science News, 23 May 2008.
