麻豆传媒

THERE鈥橲 a lone ray of hope in one of the most calamitous and violent views of our cosmos鈥檚 future. Even after dark energy 鈥� the mysterious entity speeding the universe鈥檚 expansion 鈥� has ripped up galaxies, planets and atoms, there can be revival.

A new model for the future of the universe, known as the 鈥渜uasi rip鈥�, proposes that new structures can rise, phoenix-like, from the ashes, offering a form of cosmic reincarnation. 鈥淥ur universe has a chance to be rebuilt from the ashes after the terrible rip,鈥� says Hao Wei of the Beijing Institute of Technology in China, the leader of the team behind the quasi-rip idea. The first glimmer of a possible revival after cosmic ripping, the idea may comfort anyone spooked by the prospect of our universe ending forever.

鈥淭he idea may comfort anyone spooked by the prospect of our universe ending forever鈥�

Though no one knows exactly what dark energy is, the ultimate fate of the universe depends on its distribution throughout space, and whether this will change as the universe evolves. Most think the density will remain constant, with the expansion continuing to accelerate at the same rate as now. The implications of this for the far future would be a rather slow 鈥渉eat death鈥� in which galaxies drift from each other, and stars run out of fuel and burn out. Though space-time itself would remain, the universe would be cold, dark and featureless.

That鈥檚 not the only way dark energy might behave, though. Exploding stars, or supernovae, provide the main evidence for dark energy. Observations of these, along with other measurements of dark energy鈥檚 current properties, indicate that its density is very close to a constant but can鈥檛 say precisely whether it is changing or if it might change in the future.

鈥淩ight now it could be slowly increasing and we wouldn鈥檛 know it,鈥� says of Vanderbilt University in Nashville, Tennessee. There鈥檚 even evidence that the density started increasing about 2.5 billion years ago, and we are just now entering a phase of cosmic super-acceleration.

Such a scenario leads to an even bleaker future than heat death, as of Dartmouth College in New Hampshire noted in 2002. He called dark energy of this type 鈥減hantom energy鈥� and showed that it would cause the already accelerating expansion of the universe to accelerate even more, resulting in a far more violent and rapid end.

In the most extreme case, Caldwell鈥檚 鈥渂ig rip鈥� could happen as soon as 22 billion years from now. Sixty million years before this ultimate end, the Milky Way would be pulled apart, at which point any Earthlings would see stars wink out. With three months left, the Earth would be torn from the sun. The planet itself would explode with 30 minutes to go, and atoms would disintegrate in the last 10^-19 seconds. Finally, space-time would give up the ghost too. 鈥淓verything will be completely destroyed,鈥� Wei says. 鈥淓ven space-time will be torn up.鈥�

Recently, others have explored alternatives to this dire fate (see guide). 鈥淎 bit of an industry has developed out of studying 鈥榬ips鈥�,鈥� Caldwell says.

One scenario is the 鈥渓ittle rip鈥�, in which dark energy grows more powerful fast enough that all bound structures 鈥� from galaxies to atoms 鈥� are torn apart, but slowly enough to avoid the ripping of space-time. With a similar end result, there鈥檚 also the 鈥減seudo rip鈥�, in which dark energy鈥檚 density increases for a time but then levels off to a constant. Still, both of these would leave behind a universe even colder and darker than in the heat-death scenario, because stars and galaxies would have been ripped apart.

鈥淚t just gets dimmer and darker and more depressing until everything burns out,鈥� says Scherrer, one of the authors of the little and pseudo rip scenarios.

Wei and colleagues wondered whether all rips were hopeless, or if there was another way dark energy could evolve that would let matter rise from the ruins. 鈥淭he fate of our universe is an unceasing topic of philosophy and religion,鈥� says Wei. 鈥淪o, as a cosmologist, considering this issue is very natural.鈥�

All the previous rip scenarios assume that dark-energy density can only increase. But the fact that it seems to be increasing right when human observers happen to be looking makes physicists suspicious. If the density changed once, why couldn鈥檛 it change again? So Wei and colleagues considered a case where the density went up for a time, and then came back down again.

The first stage of this scenario looks rather like the other rips, with most bound structures, from galaxies to atoms, eventually flying apart. But, before space-time rips, the density of dark energy starts to decrease again.

That would make the universe鈥檚 expansion accelerate to a lesser and lesser extent, so the cosmos should reach a point where the remaining fragments of matter find each other and clump together again due to gravity. The bigger the clumps, the stronger the gravity and the more likely other matter is to stick. Unlike all the other rip models, atoms and eventually galaxies have the chance to reform (). 鈥淭he gravitational force glues the ashes into new structures, and the universe can be rebuilt,鈥� says Wei.

鈥淭he gravitational force glues the ashes into new structures, and the universe can be rebuilt鈥�

Caldwell doubts that this would happen. 鈥淵ou鈥檇 find yourself in a cold and lonely universe. There would be lots of dark energy around, but the nearest atom would be far away. It鈥檚 going to take an awfully long time for particles to find each other.鈥�

Wei acknowledges that dark energy could still be accelerating the expansion of space-time, so it might take longer for matter to come back together than it did for it to fly apart. But if it does reform, the results could be very alien. While the existing universe, and its underlying space-time, was born in a big bang, the quasi rip does not destroy space-time. So the phoenix cosmos is essentially an old, recycled canvas with a new coat of paint. No one knows what that would look like. Wei reckons dark energy could even strengthen and weaken cyclically, ripping matter apart and letting it reform again and again.

Whether Wei鈥檚 quasi rip, or rips, are more likely than, say, Caldwell鈥檚 big rip can鈥檛 yet be determined, as all scenarios are consistent with the existing dark energy data. Observations of the cosmic microwave background 鈥� relic radiation from the big bang 鈥� that the team should release next year will help pin down the details.

There are also other fates that may await our universe, including a more sudden death due to the inherent instability of the vacuum or a cycle of births and deaths triggered by repeated collision with another universe that is part of a wider multiverse.

Caldwell advises against lying awake and worrying about which scenario will play out at this stage. 鈥淎 big rip is just one of many possible things that could, might, maybe happen,鈥� he says.