IT IS the ultimate neighbour from hell: a rogue “bubble” universe that could rip into our world at any time and eat us and everything else in a flash.
Eduardo Guendelman at Ben Gurion University in Beer-Sheva, Israel and Nobuyuki Sakai at Yamagata University in Japan discovered that our universe might face this gruesome end as they were investigating how patches of space-time expand. Alternatively, our universe could be the one feasting on its neighbours right now.
According to the standard model of cosmology, our universe underwent a phase of rapid expansion known as inflation just after the big bang. In theory, inflation could still be happening to pockets of space-time, blowing them up to create new universes disconnected from ours. However, nobody knows exactly what would trigger this inflation, says Guendelman.
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He and Sakai wanted to see if bubbles of space-time could inflate into pocket universes without having to be kick-started by anything as dramatic as a big bang. They found that this is possible, provided the bubbles contain a weird form of repulsive “phantom energy”. Some physicists think phantom energy is similar to dark energy, and both are posited to explain the acceleration of the universe’s expansion. But phantom energy is much more powerful, and if it really is behind the acceleration, it will create runaway expansion that will eventually rip our universe apart (New Scientist, 8 March 2003, p 14).
Guendelman and Sakai’s calculations show that small bubbles of phantom energy would start to “breathe”, gently expanding and contracting as the phantom energy inside battles against the bubble’s wall, before spontaneously expanding into a full-blown universe. The problem is that the expansion can play out in two ways, depending on the resistance of the wall.
Ideally, the bubble would disconnect from its surroundings, says Guendelman. This “good” pocket universe would look like a black hole from the outside, but inside it would be creating its own space-time – effectively a new universe. In contrast, “rogue” bubbles would expand uncontrollably into the space-time around them, and we probably wouldn’t see one before it destroyed us because it would expand at the speed of light. The researchers have submitted their work to Physical Review D ().
“We probably wouldn’t see one of these rogue bubbles before it destroyed us because it would expand at the speed of light”
There are also hints that our universe is itself a rogue bubble. Using some speculative models of the early universe, physicists have calculated that its fundamental physical properties could have fluctuated as its temperature fell – the strength of the gravitational force among them. As that varied, it would have created a similar breathing bubble effect, which is the precursor to rapid expansion, says Guendelman. “But of course, we will never know, because we cannot measure the features at the edge of our universe,” he says.
Anthony Aguirre, an expert on bubble universes at the University of California, Santa Cruz, thinks the researchers have found a “neat solution” to explain inflation without the need for a big bang. But he points out that while phantom energy could exist, there is as yet no evidence for it, so we shouldn’t get too worried about the prospects of a rogue bubble chomping through our own universe. “There are scenarios where it could happen, but we just have to hope they don’t come true.”
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