Abraham Loeb can check the expansion of the universe from this domed telescope at Harvard's Center for Astrophysics. He calculates that distant galaxies will move away from us so fast they will gradually fade from sight. Staff photo by Kris Snibbe

The end
Frozen future forecast for the universe

By William J. Cromie
Gazette Staff

The good news is that the universe will last forever. The bad news is that we will be seeing less and less of it as galaxies fade and become frozen in time.

Until four years ago, the best minds on Earth could not figure out if the universe will keep expanding the way it has been for about 14 billion years, or if gravity will pull all the stars and galaxies together into a life-ending crunch.

Today, there is more or less of a consensus that universal expansion will go on forever, with galaxies and clusters of galaxies moving away from each other so fast that gravity cannot ever pull them together. Since 1998, astronomers have found good evidence that this expansion is accelerating, that galaxies are separating at ever-increasing speed.

One morning in his bathroom, Abraham Loeb, a professor of astronomy at Harvard, began thinking about the consequences of such a runaway separation. "What will people, looking at the sky billions of years from now, see," he wondered. "I started doing some calculations - they took a day or two - and I got some interesting results."

Eventually, the rate of expansion will approach the speed of light, 186,321 miles a second. As far as scientists know, nothing can move faster than that in a local area. But over the gigantic scale of the universe, galaxies moving at close to the speed of light can separate at a relative speed that exceeds the universal limit. At this point, light coming from distant galaxies never catches up to telescopes on Earth because the planet is moving away too fast.

As that happens, images of faraway galaxies become frozen in time.

"As a consequence of the general theory of relativity, galaxies won't disappear; they won't fall over the horizon," Loeb notes. Their images just sit on the horizon like the still frame of a motion picture showing a cowboy riding off into the sunset. But because Earth keeps moving away, the galaxies eventually appear fainter and fainter.

"Fifty to 100 billion years from now, the universe will be a dark and lonely place," Loeb concludes.

ET can't call home

When galaxies reach the horizon of the universe, astronomers will no longer be able to obtain information about them. "The more distant a galaxy is, the earlier it will exit our universe," Loeb points out. "Once it does, we can no longer see how it ages."

As the expansion accelerates, less and less information will be available about the late history of distant galaxies. Some of them have already moved beyond our horizon. The light we still receive from them left billions of years ago, and the light they emit now will never reach us.

The farthest known light source, a highly energetic quasar some 13 billion years old, appears to us as it looked 1 or 2 billion years after the beginning of the universe, because it has taken all that time for its light to reach us. "We'll see it for another 4 billion years or so, until it's 5 billion years old," Loeb estimates. "Then it will become a frozen, fading image."

The fact that the universe will look dark and lonely in the year 1000000002002 may be a moot point, since our own star, the sun, will burn out long before then. It's about 5 billion years old now, and most astronomers don't think it has enough fuel to burn more than 5 billion more years.

But a fact like that doesn't faze a man with Loeb's imagination. "By that time," he muses, "newer stars will dominate our galaxy. We could move to the neighbor of one of these stars, or produce our own heat and light with nuclear energy."

If humanity survives, about 50 billion years in the future, Earthlings will only see galaxies that are now our closest neighbors. That's because the gravitational attraction between the Milky Way and its neighbors is stronger than the mysterious force that is pulling the universe apart.

In fact, Loeb points out, the nearest galaxy to us, the Andromeda Nebula, is getting closer. It is now just visible without the aid of a telescope, but the Milky Way and Andromeda are expected to collide before the year 50000002002. The collision won't generate a tremendous explosion. Apart from crashing pockets of gas creating huge shock waves, both galaxies are "empty" enough for their stars to slide into the unoccupied spaces. Instead of looking like the flat spiral formed by today's Milky Way, the new super galaxy - call it Milkyomeda - will be shaped like an elliptical globe

A matter of missing matter

"The accounting of material in the universe is quite embarrassing," Loeb admits. At present, astrophysicists believe that all the galaxies, hot gases, stars, and planets we can see make up only about 5 percent of matter in the universe. Another 25 percent or so is dark, cold, invisible material. We know it's there from the gravitational pull it exerts on the motion of galaxies, but no one knows just what "dark matter" is. A full 70 percent consists of so-called dark energy present in the vacuum of space - in the in-between spaces once thought to be totally empty.

The great challenges in cosmology today, says Loeb, are to identify the nature of dark matter, find out where dark energy comes from, and explain the origin of the speedup in expansion.

The latter is so small it took centuries to detect. Will it always be that small? Could it speed up or slow down? If the dark vacuum is unstable and its density becomes negative, gravitational attraction would take over, ending the universe in a Big Crunch.

Loeb and post-doctoral fellow Jeremy Heyl studied the possibility of such a catastrophe. They conclude that we don't have to worry about it, at least not for a while.

Loeb's calculations and speculations will be published in the Feb. 15 issue of the physics journal Physical Review D. His conclusions are already available on the Internet, however, and they are drawing comments from other scientists.

Freeman Dyson, a futurist at the Institute for Advanced Study in Princeton, N.J., suggests that people on Earth should start thinking about how to bring advanced civilizations together before they are permanently moved away from each other. He wants to get everyone together by launching rockets to places not participating in the expansion, such as galaxies in the gravitational neighborhood of the Milky Way. Alternately, it might be possible to bring planets together in such a way that their gravitational mass exceeds the inexorable acceleration.

Loeb smiles at that solution. "Dyson's underlying assumption is that it's desirable to bring different civilizations together," he comments. "But human history teaches us that the potential for peace is associated with giving each culture as much space as possible. An expanding universe provides an infinite amount of space."

Copyright 2002 by the President and Fellows of Harvard College