HAVE we hastened the demise of the universe by looking at it? That's the startling question posed by a pair of physicists, who suggest that we may have accidentally nudged the universe closer to its death by observing dark energy, which is thought to be speeding up cosmic expansion.
Lawrence Krauss of Case Western Reserve University in Cleveland, Ohio, and colleague James Dent suggest that by making this observation in 1998 we may have caused the universe to revert to a state similar to early in its history, when it was more likely to end. "Incredible as it seems, our detection of the dark energy may have reduced the life-expectancy of the universe," says Krauss.
The researchers came to their conclusion by calculating how the energy state of our universe might have evolved. Until recently, cosmologists thought that the big bang 13.7 billion years ago occurred after a bubble of weird high-energy "false vacuum" with repulsive gravity decayed into a zero-energy "ordinary" vacuum. The energy released during this transition could have made matter and heated it to a ferocious temperature, which essentially created the massive explosion of the big bang. The discovery of dark energy - and the realisation that the universe's expansion is accelerating - reveals that the vacuum may not have decayed to zero energy, but to another false vacuum state. In other words, some energy was retained in this vacuum, and this is accelerating the universe's expansion.
Like the decay of a radioactive atom, such shifts in energy state happen at random. "So it is entirely possible it could decay again, wiping the slate of our universe clean," says Krauss. If this transition did happen, everything in our universe would cease to exist.
The fact that we are still here means this can't have happened yet. But cosmologists have long puzzled over why this should be, particularly as the probability of the false vacuum of our universe having survived decreases exponentially over time.
Building on a discovery made in the 1950s, Krauss's calculations show that if the universe did happen to hold out past a certain threshold, its chance of staying stable are substantially increased. In 1958, Russian physicist L. Khalfin discovered that after an extremely long time, the probability of a quantum system having survived stops falling exponentially and switches to a slower rate of decline. This means that if the false vacuum of the universe survives to the switching point between the two rates, it will effectively become eternal. This is because the false vacuum is known to grow exponentially fast, and past the switching point it will be created faster than it can be eaten away by any decay, he says.
According to Krauss, the smaller the energy gap between the false vacuum and zero, the earlier the switching point between the two rates. And - surprise, surprise - we live in a universe where the vacuum energy is just above zero, so we could be well past the crucial switching point.
At first glance, this seems like good news for us because it would mean our universe is on track to survive forever. However, things may not be as good as they seem, Krauss says. At the quantum level, whenever we observe or measure something, we reset its clock and stop it decaying - something known as the quantum Zeno effect. Our measurement of the light from supernovae in 1998, which provided evidence of dark energy, may have reset the false vacuum's decay clock to zero - back to a point when the likelihood of its surviving was falling exponentially over time. "In short, we may have snatched away the possibility of long-term survival for our universe and made it more likely it will decay," says Krauss.
Krauss's claim is controversial. Max Tegmark of the Massachusetts Institute of Technology maintains that the quantum Zeno effect does not require humans to make observations of light. "Galaxies have 'observed' the dark energy long before we evolved," he says, as they were affected by it and were encoding information about it. "When we humans in turn observe the light from these galaxies, it changes nothing except our own knowledge."
Author: Marcus Chown
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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 24 NOV 2007.
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