Miércoles 16 de Mayo de 2007, Ip nº 191

Is the evidence for 'alien' universes all around us?
Por Zeeya Merali

IT HAS long been accepted, at least in theory, that other universes might exist and might even collide with ours. Yet the idea that we would ever be able to see the aftermath of such collisions, and so find evidence of other universes, has seemed beyond the scope of science. That is set to change.

Anthony Aguirre of the University of California, Santa Cruz, thinks the proof of cosmic collisions could be all around us, as imprints in the cosmic microwave background (CMB) left over from the big bang.

According to the standard model of cosmology, our universe underwent a phase of exponential expansion, known as inflation, just after the big bang. In theory, inflation could still be happening, with bubbles of space-time suddenly blowing up to create new pocket universes. The cosmological parameters, such as the rate of expansion, and the laws of physics could be different in each new universe, potentially giving rise to new types of matter.

The usual assumption is that these other universes are disconnected from us, and that we can't enter them and look around, or observe them in any way. "People often criticise discussions of multiple universes as meaningless because we can't detect whether they actually exist," says Aguirre. He doesn't accept this. Some of these assumptions may be wrong, he says, and other universes could leave behind telltale signs of their existence when they collide with ours. "Frustratingly, not much work had been done on the observable consequences of collisions," he says.

One reason for this neglect is the assumption that any such collision would be fatal for our universe, either because it would be so violent or because the matter and cosmological parameters from the colliding universe, all inhospitable to life, would bleed into ours. "The bubble wall of the other universe could start pushing into ours, destroying everything in its path," Aguirre says.

Yet when Alex Vilenkin of Tufts University in Medford, Massachusetts, and his colleagues recently calculated the probability of such lethal collisions, they found it to be very low. Aguirre has now considered the chances of benign collisions that leave our universe more or less intact, such as when another universe partially infiltrates ours. If the cosmological parameters in the second universe are not too different from our own, such a collision would not necessarily destroy life, Aguirre says. Another possibility is that the wall of our universe could expand into another, destroying its inhabitants but leaving our universe unscathed.

Aguirre and his colleagues Matthew Johnson and Assaf Shomer calculated the probability of such benign collisions and their observable repercussions (www.arxiv.org/abs/0704.3473). "What was surprising to me is not only that the signs would be there, but that they should be huge enough for us to observe," says Aguirre. What's more, such collisions could have occurred during the history of the universe, they found.

Signs would be visible as large-scale anomalies in the distribution of hot and cold spots in the CMB. "I probably shouldn't speculate, but observational cosmologists have spotted signs of a strange alignment in the CMB that could be compatible with this picture," says Aguirre. This large-scale pattern seems to be aligned along what is often called the "axis of evil", though the finding remains controversial (New Scientist, 13 April, p 10).

Carlo Contaldi, an expert on the CMB at Imperial College London, likes Aguirre's idea. "It's certainly one of the most exciting explanations of the odd CMB patterns," he says. "Although it may seem far-fetched, it is quite plausible that if these collisions did happen while our bubble was inflating, they could leave such a signature on the CMB." More detailed work needs to be done to put this idea to the test, he says.

Vilenkin is also impressed. "Until now we assumed that other universes are not observable," he says. "We could be living in the aftermath of such collisions, potentially allowing us to see evidence of other universes."

If universes really are crashing into us willy-nilly, should we be worrying about a fatal collision? "It's true, there is always a chance we will be hit by a lethal bubble, which would come without warning," says Vilenkin. "But since we'll just evaporate in an instant and there's nothing we can do to stop it, there's really no use in worrying."

  09/05/2007. New Scientist Magazine.