The dark destroyer

A mysterious and powerful force - 'dark energy' - has made physicists change their view of the fate of the Universe. Now the race is on to find out more about it

The fate of the Universe is to end not with a bang but with a whimper, as TS Eliot famously wrote. This is the current view of cosmologists, who believe the Universe will expand forever, its constituent galaxies becoming ever more isolated islands of burnt-out stars in an endless sea of space. However, physicists in the US have now pointed out a radically different cosmic endgame. "We call it the Big Rip," says Robert Caldwell of Dartmouth College, New Hampshire. "It's a nightmare scenario in which all galaxies, stars and even atoms are violently torn apart, and it's still just compatible with the observations."

The key to understanding the Big Rip is the "dark energy", which burst on to the scene in 1998 when astrophysicists in the US discovered that distant "Type Ia" supernovae - a class of exploding stars believed to detonate with a standard intrinsic brightness - were fainter than they ought to be, taking into account their distance from the Earth. Confounding all expectations, the Universe's expansion had speeded up since the stars exploded, pushing them further away than predicted and making them appear fainter.

This was a bombshell dropped into the world of cosmology. The Universe's constituent galaxies are flying apart like pieces of cosmic shrapnel in the aftermath of the Big Bang, the titanic explosion in which everything was born 13.7 billion years ago. The only force thought to be affecting their motion was the gravitational pull of each galaxy tugging on every other. It should be braking the expansion, not speeding it up. Evidently, the Universe was being controlled by something else - something which nobody had anticipated.

It was dubbed "dark energy" by cosmologists. Invisible and mysterious, it lurks in the blackness of space and is exerting a cosmic repulsion on the matter of the Universe, counteracting gravity and driving the galaxies remorselessly apart. It can be thought of as "springy space". As galaxies move apart from each other, ever more springy space is created between them, which has the effect of pushing the galaxies apart more forcibly.

There are several possible candidates for the dark energy, distinguishable by the way they behave as the Universe expands. Caldwell and his colleagues, Marc Kamionkowski and Nevin Weinberg of the California Institute of Technology in Pasadena, have investigated a type which they have christened "phantom energy". Its central characteristic is that the energy contained in a fixed volume of space increases with time.

This has a double-whammy effect. Not only does the repulsive effect on galaxies go up because springiness of space increases, it goes up because more springy space is created between the galaxies as they fly apart. "The result is a runaway expansion of the Universe which gets ever more violent," says Caldwell. So fast is the expansion that more and more of the Universe is stretched so far away from any observer that its light cannot reach them. "Consequently, every observer sees the visible Universe around them shrink ever faster, eventually down to a single point."

But the most dramatic consequence is that, as the phantom energy grows, its repulsive effect becomes strong enough to rip apart all systems held together with familiar forces, starting with galaxy clusters and rapidly going down the scale to galaxies, stars and planets. "At the end of time, as the visible Universe around every point shrinks to nothing, even atoms will be ripped apart," says Caldwell.

In a paper submitted to the journal Physical Review, Caldwell's team calculates one possible timeline for a universe dominated by phantom energy. The Universe reaches the Big Rip in 22 billion years from now, with the Milky Way destroyed 60 million years before the end, planets exploded half an hour before and atoms torn to pieces in the final billion billionth of a second.

Caldwell admits to surprise that such a violent end is possible in an ever-expanding universe when the received wisdom was that such a universe would end with a whimper. Even more surprising is that this doomsday scenario is still compatible with cosmic observations, including those of the fleeing supernovae and cosmic background radiation, the "afterglow" of the Big Bang which still lingers throughout space. "It's unlikely but it can't be proved impossible," says Martin Rees of the University of Cambridge, who has carried out calculations similar to Caldwell. "If humanoids survive, they could observe all but the final millisecond - that's when the cosmic repulsion gets up to the tensile strength of our bodies and tears us apart."

Despite the unpleasant end to the Universe, phantom energy may answer a nagging question about the dark energy. It had an extremely small effect in the Big Bang but it has been growing ever since. Only comparatively recently has it begun to dominate the Universe. Why now? According to Caldwell, the answer is simple. The Universe began in uniformity - the smeared-out hot gas of the Big Bang - and it will end in uniformity, with all matter ripped apart and spread evenly throughout space. There is only a brief period in between when structures such as galaxies and planets are possible - later than the time gravity has pulled matter together and before the time the dark energy is strong enough to blast it apart again. "Since it's the only time we can exist, we should not be surprised to find ourselves living at such a time!" says Caldwell.

The scenario is not without problems, though. Phantom energy's repulsive effect could be used to hold open "wormholes", short-cuts through space and time which are permitted by Einstein's theory of gravity but should snap shut the instant they form. This raises the spectre of time machines. Physicists find these very uncomfortable since in principle someone could go back in time and kill their grandfather, with obvious paradoxical consequences.

The strong possibility remains that the dark energy is in a form less extreme than phantom energy. The two frontrunners are a "cosmological constant", proposed by Einstein and later dismissed by him as his "biggest mistake", and "quintessence".

In the case of a cosmological constant, the energy contained in a fixed volume of space drops with time. But this is exactly compensated for by the extra space created as the Universe expands, so the cosmic repulsion never dies away but ensures the Universe expands forever. In the case of quintessence, the energy contained in a fixed volume of space drops less quickly. In an extreme case, it actually stays constant. (If banknotes behaved like this, if you held a stack between your hands and pulled your hands twice as far apart, you would double the number of bank notes.) Quintessence therefore boosts the expansion of the Universe far more than a cosmological constant, though not of course as much as phantom energy. If banknotes behaved like phantom energy, they would fountain out of control from between your hands.

What physicists need is a way of distinguishing between the three different incarnations of dark energy. They may get it before the end of the decade. Astronomers are proposing a spaceprobe called the SuperNova Acceleration Probe (Snap). It will boast the largest camera ever put in space - 30 times bigger than the camera on Nasa's Hubble Space Telescope. Its task will be to make measurements of more than 2,000 supernovae as they detonate in far away galaxies. "It's one of the most important scientific projects of the next 10 years," says cosmologist Max Tegmark, of the University of Pennsylvania.

Snap is one of the US Department of Energy's highest scientific priorities. "How the dark energy changes with distance, or 'look-back time' is the key observation that will allow us to distinguish between the different types of dark energy," says one of Snap's originators, Greg Aldering of Lawrence Berkeley Laboratory in California. The plan is to launch Snap in 2008. Finally, we might get a handle on the nature of dark energy. Unless, of course, nature has another surprise up its sleeve.

Marcus Chown is the author of 'The Universe Next Door' (Headline, £7.99)