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SCIENCE FICTION MOVIES AND BOOKS are full of parallel universes. In a typical scenario, as in the 1998 movie Sliding Doors, something happens in one universe – such as a woman missing 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, many cosmologists believe there really are other universes. 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 there might be a way to discern the existence of another universe, if it happened to collide with ours. Such an impact might leave its mark in the cosmic microwave background (CMB) – our earliest snapshot of the origin of the universe.
"I think that's a really, really fascinating idea," says Max Tegmark, a cosmologist at the Massachusetts Institute of Technology (MIT) in Cambridge, USA. "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 fi eld, 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 rival groups, one led by Anthony Aguirre of the University of California in Santa Cruz, and the other by Matthew Kleban of New York University, both proposed the possibility of observing those ultra-cosmic collisions in December 2007. Each group suggests the collisions might be visible, although the details of what the signature might look like are unclear.
Surprisingly, the existence of parallel universes might be an inevitable consequence of the physics behind the birth of the universe. Cosmologists say that soon after the Big Bang, a brief period of rapid expansion called 'infl ation' enlarged the universe by many orders of magnitude, like blowing up a tiny bubble to the size of a hot-air balloon.
Most of inflation's testable theoretical predictions have been confirmed. "Inflation is a key and somewhat indispensable part of our current understanding of how our universe began and evolved," Aguirre says. But the theory of cosmic inflation, proposed by MIT cosmologist Alan Guth in 1981, implies that it 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 glass of beer.
This 'eternal inflation' creates other 'bubble universes' that likely have different properties from our own universe. 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 for even 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 ... the people who invented inflation kept thinking about it because they saw that it was important."
