Lonely universe: runaway expansion will lead to dark times ahead.

Early in 1998, our perception of the universe got a jolt. Two independent teams of researchers studying distant supernovas found evidence that the cosmos is doing more than merely expanding. The teams discovered that it is expanding at an ever-faster rate (SN: 4/7/01, p. 218). Opposing the tug of gravity, there seems to be some mysterious force, dubbed dark energy, that stretches space-time and pushes galaxies apart.

Although the evidence is still under debate, scientists are beginning to explore the full consequences of living in a revved-up universe. And as several cosmologists have come to realize, antigravity has a downside.

Because gravitational attraction was dominant when the universe was younger and objects were closer together, runaway expansion didn't begin until about 5 billion years ago. But to those who are superlatively farsighted, this relatively new trait of the universe couldn't have more profound consequences.

That's the thought that came to Harvard cosmologist Avi Loeb one morning last year, after being kept awake most of the night by his own personal reminder of the future, his 1-month-old daughter. Loeb realized that in a revved-up universe, galaxies eventually would recede from each other at faster than the speed of light. Although the laws of physics hold that nothing can move faster than light in any local region of the universe, two widely separated galaxies, pushed apart by the expansion of space-time between them, can have a relative velocity that exceeds the local limit.

When that happens, all possibility of communication between those galaxies, or even visual contact between them, dies. At that point, light emitted by one of the galaxies will never ever catch up to the other.

Rather than seeing the opposite galaxy vanish from sight immediately, an observer in one galaxy will see the light from the other gradually grow dimmer like a dying ember (see box page 140). Ultimately, the remote galaxy will become so dim that not even the most sensitive telescope will be able to detect it. In the end, evidence of all but the nearest galaxies will become forever inaccessible.

If would-be stargazers in the Milky Way were to be observing the universe billions of years from now, they would see a dark and desolate canvas, says Loeb. Our galaxy's closest neighbors, those whose gravitational attraction had managed to resist the repulsive force of dark energy, already would have merged with our own into a new supergalaxy. As for the other 50 billion galaxies in the universe, none would be visible. The supergalaxy would appear as an island unto itself, note Loeb and his Harvard colleague Kentaro Nagamine in an article they recent posted on the Internet (http://xxx.lanl.gov/abs/astro-ph/0204249).

The most remote galaxies now known are the ones whose light now reaching us was emitted when they were in their infancy, some 13 billion years ago. But as astronomer Edwin Hubble discovered in the 1920s, the most distant galaxies are also those that speed away the fastest. As a consequence, these bodies would be the first to disappear for Milky Way observers in a revved-up universe. That should happen in about 50 billion years.

"The radiation emitted by the galaxies once they reach the age of 4 to 6 billion years will never reach us due to the acceleration of cosmic expansion, and so we will never know what these sources look like as they age," says Loeb, who reported his calculations in the Feb. 15 Physical Review D.

Thinking about the long-term effects of antigravity may seem little more than an academic exercise, admits cosmologist Gus Evrard of the University of Michigan in Ann Arbor. On the other hand, with so many astronomers peering at distant galaxies and trying to figure out what the universe looked like long ago, "it's refreshing instead to look at the future and figure out what the universe will look like billions of years from now" he says.

For their part, Evrard and his Michigan colleagues, including Michael Busha, are beginning to analyze what dark energy will do to the overall architecture of the universe.

Galaxies today are distributed in a three-dimensional cosmic web, bunching along huge filaments that are separated by giant voids. Over time, notes Evrard, as more and more galaxies have clustered together, the web has become ever more intricate.

But in the far future, he says, as galaxies recede from one another faster and faster, those that already haven't bunched together will no longer have the opportunity to do so. The web itself will begin to unravel.

Ultimately, Evrard adds, the web will lose all its structure and become a cosmic vapor.

However, there could be an upside to all this gloom. By the time the Milky Way loses sight of all its neighbors, more than a hundred billion years in the future, any sentient beings left in the galaxy may finally have figured just what dark energy is and how it has been pushing galaxies apart.

RELATED ARTICLE: Freeze frames: final images in a rewed-up universe will be indelible.

Imagine two galaxies fleeing from each at a faster and faster rate, finally reaching a relative speed that outpaces light. Why doesn't each galaxy disappear suddenly from the other's view? The answer lies in the some of the peculiarities of Einstein's general theory of relativity, says Harvard University cosmologist Avi Loeb.

As the relative speed between two galaxies approaches the speed of light, an observer in one galaxy would see a clock in the other ticking ever more slowly, eventually stopping altogether, he notes. At that point, the galaxy doesn't vanish from sight. General relativity predicts instead that a final image will persist indefinitely, as the last frame of a film might remain flickering forever on a movie screen. As time passes, that last frame dims. The increasingly distant galaxy's image gradually shifts to longer and longer wavelengths until no telescope could detect it.

Exactly the same phenomenon seems to happen when an object falls into a black hole, says Loeb. From the point of view of an outside observer, a clock on the doomed object would tick more and more slowly, so it would take forever for the body to actually reach the point of no return--the so-called event horizon. Observers will never see the object actually disappear into the black hole. Instead, they'll see the object approach the event horizon at an increasingly slow rate until the object appears to be at a final standstill. --R.C.