STAND by for a nightmare end to the Universe 鈥� a runaway expansion so violent that galaxies, planets and even atomic nuclei are literally ripped apart. The scenario could play out as soon as 22 billion years from now.
鈥淯ntil now we thought the Universe would either re-collapse to a big crunch or expand forever to a state of infinite dilution,鈥� says Robert Caldwell of Dartmouth College, New Hampshire. 鈥淣ow we鈥檝e come up with a third possibility 鈥� the 鈥榖ig rip鈥�.鈥�
Whether the big rip happens or not depends on the nature of the mysterious dark energy that is pulling the Universe apart. We know that the expansion of the Universe is speeding up, but most physicists assume the acceleration is likely to stay constant or get weaker over time. But Caldwell takes a different view. He thinks the dark energy causing the expansion could be growing more powerful. 鈥淲e call it phantom energy,鈥� he says. 鈥淚t鈥檚 pretty weird stuff.鈥�
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Under the influence of phantom energy, the runaway expansion of the Universe would become ever more violent, stretching more and more of the Universe further and further away until the light from the stars cannot reach us. 鈥淓very observer sees the visible Universe around them shrink ever faster, eventually down to a point,鈥� says Caldwell. For all practical purposes, the Universe will have ended.
The existence of phantom energy has always been a possibility 鈥� even if a pretty unlikely one. But astronomers have tried and failed to rule it out. In particular, detailed measurements released last month of background radiation left over from the early Universe (New 杏吧原创, 15 February, p 12) leave the door open.
Now, in a paper submitted to Physical Review (), Caldwell and his colleagues at the California Institute of Technology in Pasadena have calculated how phantom energy would bring the Universe to an end. They found that as the phantom energy grows, its repulsive force becomes strong enough to rip all bound systems apart, starting with galaxy clusters and rapidly moving down the scale to galaxies, stars, planets and atoms.
Caldwell says he was surprised by the violence of the Universe鈥檚 end 鈥� the received wisdom was that an ever-expanding Universe should end with a whimper. 鈥淚n the last moments, even atomic nuclei will be ripped apart,鈥� he says.
In the most extreme scenario, the big rip will happen 22 billion years from now, with the Milky Way destroyed 60 million years before the end and atoms torn to pieces in the final 10-19 seconds (see Graphic). 鈥淚f humanoids survive, they could observe all but the final millisecond,鈥� adds England鈥檚 Astronomer Royal Martin Rees, who has also considered the possibility of phantom energy. 鈥淭hat鈥檚 when the cosmic repulsion gets up to the tensile strength of our bodies and tears us apart. It鈥檚 unlikely, but it can鈥檛 be proved impossible.鈥�
Astronomers鈥� best bet for working out which fate is in store for the Universe is the Supernova/Acceleration Probe (SNAP), a satellite proposed for launch later this decade. SNAP will make detailed measurements of thousands of supernovae, to pin down exactly how fast they are moving away from us and hopefully work out how dark energy is changing over time.
Most physicists probably won鈥檛 be rooting for phantom energy. That鈥檚 because if it exists, it will cause them all kinds of theoretical headaches. For example, Einstein鈥檚 theory of gravity predicts the existence of minuscule wormholes 鈥� short cuts through space-time. Normally they snap shut so fast we never notice them. But phantom energy鈥檚 repulsive gravity would be powerful enough to hold wormholes open, and perhaps even push them wide enough apart for spacecraft to use them for faster-than-light travel. 鈥淭his raises the spectre of time machines and all their paradoxes, which physicists find very uncomfortable,鈥� says Caldwell.