Fredryk Von Caesar

Driving the known acceleration of the universe's expansion is a mysterious thing is called dark energy, thought of by scientists as anti-gravity working over large distances.

Conventional wisdom holds that the acceleration will proceed at a constant rate, akin to a car that moves 10 mph faster with each mile traveled. With nothing to cap the acceleration, all galaxies will eventually recede from one another at the speed of light, leaving each galaxy alone in a cold, dark universe within 100 billion years. We would not be able to see any galaxies outside our Milky Way, even with the most powerful telescopes.

That's the conventional view, remarkable as it sounds.

The Big Rip theory has dark energy's prowess increasing with time, until it's an out-of-control phantom energy. Think of our car accelerating an additional 10 mph every half mile, then every hundred yards, then every foot.

Before long, the bumpers are bound to fly off. Sooner or later, our hypothetical engine will come apart, regardless of how much we spend on motor oil.

Other theorists who have reviewed the Big Rip theory are not yet sold on the idea. Meanwhile, Caldwell's team has provided a precise countdown to total demise. The projected end is, reassuringly, 20 billion years away. If our species survives the next 19 billion years (and there are serious doubts about this, given our Sun's projected fate) here are some signs that scientists of the future will want to look for.

• A billion years before the end, all galaxies will have receded so far and so fast from our own as to be erased from the sky, as in no longer visible.
• When the Milky Way begins to fly apart, there are 60 million years left.
• Planets in our solar system will start to wing away from the Sun three months before the end of time.
• When Earth explodes, the end is momentarily near.

At this point, there is still a short interval before atoms and even their nuclei break apart. "There's about 30 minutes left," Caldwell said, "But it's not quality time."

And then what? Does the universe recycle itself? Is there something after nothing?

"We're not sure what happens after that," Caldwell says. "On the face of it, it would look like time ends."

Caldwell's study had humble beginnings. He and his colleagues, Marc Kamionkowski and Nevin Weinberg at Caltech, were considering how a sphere of matter collapses under its own weight to form a galaxy. In computer models, they tweaked with the dark energy factor and found that too much of it would actually prevent the sphere from collapsing. In extreme cases, the sphere exploded.

"That was our hint that there was something really unusual going on," Caldwell said.

It wasn't long ago, just before the accelerated expansion was discovered, that many cosmologists believed the universe might reverse course, that normal gravity would win, and that everything would fall back in a Big Crunch. More recently, solid observational data has all but assured the infinite-expansion model and the cold, dark, never-ending end.

The Caldwell group decided there might be a third possibility, leading to their new paper, which has been submitted to the Physical Review.

But there are many unknowns. It is not clear if the dark energy driving expansion is a force not currently described by physics, or if it is merely a different manifestation of gravity over huge distances. The repulsion could be a response to dark matter, unseen stuff that is known to comprise 23 percent of the universe, based on firm observations.

Dark matter has unknown properties, and it may be related to dark energy, Caldwell said. He notes that even Einstein considered that gravity might work repulsively, in a manner consistent with his theory of general relativity.

Dark energy, being quantified only recently, tends to be discussed as some strange new force, in addition to the four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces that govern atoms. But the repulsion is possibly just the way gravity behaves in the presence of dark energy, Caldwell said. In that sense, it is not a new force.

To turn dark energy into destructive phantom energy, Caldwell and his colleagues had to play around with a thing called the cosmological constant, a mathematical fix that Einstein applied to general relativity. Einstein later called it his greatest mistake, when Edwin Hubble found in the 1920s that the universe was expanding (seven decades later, that expansion would be seen accelerating).

The cosmological constant has been recently revived. Attempts to describe dark energy differ in how the density of dark energy varies with time. In some models, the density decreases slowly. For the cosmological constant, the density is a constant. For phantom energy, it must grow with time.

"We considered a more exotic form of dark energy which was more repulsive," as Caldwell explains is.

Abraham Loeb, a theoretician at the Harvard-Smithsonian Center for Astrophysics, has quantified the lonely effects of a forever-expanding universe. Loeb stands by that scenario, but he said Caldwell's idea is nonetheless interesting to explore.

"I think it's a logical possibility," Loeb told SPACE.com. But he cautioned that altering the cosmological constant goes against current consensus.

"If I had to place a bet, I would bet in favor of the standard cosmological constant," Loeb said.

If Caldwell's team is right, cosmology would undergo a revolution. Sci-fi ideas like wormholes and time travel might suddenly enter the realm of hard science. All of this could sort itself out pretty soon, Caldwell believes. Observations over the next few years may actually show whether his phantom energy is possible.

"Who knows if it is right or wrong," Caldwell said of his theory. "I think we'll find out pretty soon."

In fact, recent observations from NASA's WMAP space probe have pinned down the physics of the universe with surprising accuracy. A little wiggle room remains for the cosmological constant. Yet more WMAP data are expected over the next four years. Other missions, including one called the Supernova Acceleration Probe (SNAP), could provide answers, Caldwell said.

Even if the Big Rip is a big bust, there's no guarantee of a pleasant ending.

Paul Steinhardt, a Princeton University physicist, is, like Caldwell and Loeb, no stranger to strange ideas. Steinhardt advocates a cyclical universe, one that has no beginning or end but which instead is constantly starting over again.

Steinhardt theorizes within the generally accepted standards of the cosmological constant. He said the Big Rip is more exotic than most ideas but still conceivable, a projected possible result that is "straightforward and obvious for cosmologists."

Yet there is another entirely different possibility for the final moments of time as we know it.

In a theory put forth two years ago by Steinhardt and his colleagues, our universe is but a membrane, or brane, floating in a five-dimensional space. It is destined to collide dramatically with another brane. The idea, labeled the Ekpyrotic Universe, would replace portions of the Big Bang scenario while sticking to the presently accepted estimates of acceleration.

"Lest you get too optimistic, galaxies are destroyed in a far more violent way," Steinhardt said of the brane scenario. "They are vaporized at the next 'bang' -- the collision between branes so, you either rip them apart or you vaporize them."