杏吧原创

TALK about a cure that鈥檚 worse than the disease. A proposed quantum field can account for the universe鈥檚 ever-quickening expansion, but it would also trigger the universe鈥檚 death in a catastrophic collapse.

In 1998, astronomers discovered that the universe has been ballooning at ever-faster rates for the past few billion years. They dubbed the mysterious entity responsible 鈥�dark energy鈥� and have been striving to identify it ever since.

The simplest explanation is that particles briefly bubbling into and out of existence imbue every cupful of space with the energy needed to accelerate the universe鈥檚 growth. But this quantum stew, known as vacuum energy, is no panacea.

Energy, like matter, causes space to curve, according to Einstein鈥檚 general theory of relativity. Calculations suggest that this vacuum energy is so strong that it would make the universe curve in on itself until it spans less than the distance from Earth to the moon 鈥� and clearly it鈥檚 bigger.

To get around this discrepancy, of the University of California, Davis, and of the University of Nottingham in the UK attempted to cancel out the curvature caused by the quantum instabilities by modifying the equations of general relativity on the largest scale possible: the whole of space-time.

Last year, they found a way to do this that , leaving just enough to explain the acceleration we observe (Physical Review Letters, ). But their method requires space-time to be finite, which implies that cosmic expansion must eventually stop and reverse, causing time to end when the universe collapses. 鈥淭he universe returns back to where it banged from,鈥� says Kaloper.

聯Cosmic expansion must eventually reverse鈥� the universe returns back to where it banged from聰

Now the researchers have proposed a trigger for that collapse: a new quantum field that permeates the universe. The field鈥檚 energy would drop slowly over time, eventually becoming negative and setting off the cosmos鈥檚 contraction. Before the contraction begins, however, the field would cause the universe鈥檚 initial expansion to accelerate, just as it is doing now. 鈥淚t鈥檚 as if the dark energy is the harbinger of doom,鈥� says Padilla (, ).

鈥淚t鈥檚 an interesting approach, but it鈥檚 still incomplete,鈥� says of the Perimeter Institute in Waterloo, Canada. 鈥淲hat is the underlying theory that tells you that this is how you should write down your equations?鈥�

The researchers are now working on a generalisation of their proposal that should help address these concerns, says Kaloper. They are also calculating when the universe鈥檚 contraction may begin, though they suspect it wouldn鈥檛 start for another few tens of billions of years, says Padilla.

鈥淲e鈥檙e pretty safe,鈥� says Arvanitaki. 鈥淚鈥檓 not going to lose any sleep over it.鈥�

This article appeared in print under the headline 鈥淒ark energy, the harbinger of doom鈥�