IT CAN drag or it can race, but what if time stopped altogether? It now seems that time could disappear from our universe – and we may already have found evidence of its forthcoming demise.
When astronomers observed a decade ago that supernovae are apparently spreading apart faster as the universe ages, they assumed that something must be causing the expansion of the universe to speed up. But so far, nobody has been able to explain where the “dark energy” causing this acceleration comes from.
Now José Senovilla at the University of the Basque Country in Bilbao, Spain, and his colleagues have a radical answer – we are fooled into thinking that the expansion of the universe is accelerating, because time itself is slowing down.
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Senovilla’s work, which will appear in Physical Review D, is based on speculative string theory models in which our universe is confined to the surface of a membrane, or brane, floating in a higher-dimensional space known as the “bulk”. String theorists have used such models to explain why gravity seems to be so weak in comparison with other forces; in these scenarios, gravity is the only force that can move between branes and leak into the bulk.
Although our brane contains three spatial dimensions and one time dimension, there is no law saying that all branes floating in the bulk must have this same “signature”, says Senovilla. Other branes could have no time dimensions, or even two or more. What’s more, the branes could swing between different signatures, with time dimensions switching to space dimensions and vice versa.
Senovilla and his colleagues wanted to know how we would interpret such a change of signature – with time changing to space, say – if it ever happened on our brane. His calculations showed that such a transition would appear to be a smooth change, with “brane-time” gradually slowing down relative to “bulk-time” until it finally disappears and the brane gets its extra spatial dimension. This would have dramatic consequences for the brane and everything in it.
“If this is really happening, we would start by experiencing things that we just cannot explain,” says Senovilla. Time in the bulk will run at the same rate as always, but clocks in our brane will tick increasingly slowly relative to bulk time. Because we aren’t aware of this, when we calculate the speed at which things are moving, it would appear that they are gradually moving faster.
Although everyday events may quicken, the effects would be imperceptible. Senovilla thinks we’d get the first clear hint that something was amiss from measurements on cosmological scales, where astronomers have been gauging how the universe has changed over billions of years.
“Our calculations show that we would think that the expansion of the universe is accelerating,” says Senovilla. “[Any] observation of dark energy could be evidence that our brane is changing signature and that time is disappearing,” he adds.
In the 1990s, astronomers measured the relative movement of similar supernovae from different eras in the history of the universe. They inferred this motion by looking at the frequency of light emitted from the supernovae at various distances from the Earth. However, the measurement of frequency depends on our perception of time so their results may have been skewed, leading to the “false” view that things are moving faster, says Senovilla.
It may seem like an extravagant solution to the problem of dark energy but Senovilla argues that, unlike other attempts at solving the mystery, he does not need to invoke any exotic particles or new forms of energy.
If he is correct, things will seem to get faster and faster until time finally disappears. “Then everything will be frozen, like a snapshot of one instant, forever,” he says. The good news is that we needn’t worry that time will disappear in our lifetime. The team have calculated that even if it is already happening, time wouldn’t freeze for billions of years. “Our planet will be long gone by then,” says Senovilla. “It is only the poor brane that will feel the effects.”
“Things will get faster until time finally disappears. Then everything will be frozen, like a snapshot of one instant, forever”
Gary Gibbons, a cosmologist at the University of Cambridge, likes the idea. “We believe that time emerged during the big bang, and if time can emerge, it can also disappear – that’s just the reverse effect,” he says.
Gibbons says the team’s work gives us new insight into brane dynamics. “It’s intriguing to think that if you’re confined to the brane, you see things from a very different perspective, and we could mistake a change of signature for dark energy,” he says.
Whether this is a realistic explanation for dark energy is another question. Gibbons would like the team to make other, testable predictions to back up their idea. “Until then, we can’t say that it is happening, but we can at least say that it could be,” he says.
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