Come Over to the Dark Side

It's tearing the universe apart and causing scientists to tear out their hair. Isn't it time you got to know dark energy?

By RICHARD MONASTERSKY

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A mysterious force is ripping through physics and astronomy departments these days. It has splintered long-cherished theories and has caused otherwise rational scientists to latch onto a weird and ugly notion.

Such is the might of dark energy, a hypothesis so powerful it dwarfs even the Star Wars merchandising campaign.

In the space of seven years, the dark-energy revolution has rewritten textbook entries on how the universe operates and what will ultimately happen to the cosmos. Yet dark energy is a nebulous concept, one that has thus far flummoxed some of the smartest researchers on the planet. "The fundamental physics of dark energy is a complete mystery to us right now," says Sean Carroll, an assistant professor of physics at the University of Chicago.

With a pull even stronger than that of gravity, the concept has captured the attention of scientists across the globe, who are brainstorming new theories and aiming their telescopes at the mystery to search for some chink in its opacity. When the National Research Council listed the top new developments in astronomy this year, dark energy took first place. "It is now clear that dark energy exists, and that it governs the destiny of the universe, even if its nature remains completely unknown," the council reported.

To help shed some light on dark energy, we've compiled a list of fundamental questions that sentient residents of the universe should know the answers to.

What is dark energy?

The bad news is: Nobody knows. The good news is that grant-hungry physicists can instantly boost their chances of success by inserting "dark energy" into the title of a proposal.

The term "dark energy" is one that cosmologists dreamed up to explain a strange, repulsive type of gravity that is blowing up the universe, causing it to grow faster as it ages. Since the 1930s, astronomers have known that the universe is expanding, but they believed that gravity was slowing down that growth because galaxies tug on each other.

In 1998, however, studies of distant stellar explosions, called supernovae, indicated that they were farther away than they should be. Something, it seemed, was causing the cosmic expansion to speed up.

"It's as if you threw a ball up in the air and, just when you would expect to see it turn and fall back, it starts moving upward further and faster," says Licia Verde, an assistant professor of physics at the University of Pennsylvania, who is trying to measure aspects of dark energy.

Confronted by such a strange discovery, physicists suggested that a repulsive force, which they called dark energy, was speeding up the cosmic growth. And while they don't know what dark energy is, they know there is a lot of it. Judging from various pieces of astronomical evidence, researchers estimate that ordinary matter makes up only 5 percent of the universe. Another 25 percent is so-called dark matter, which emits no light and is hard to pin down but otherwise behaves like matter. "We don't have a clue what is the remaining 70 percent," says Ms. Verde.

Chalk that remainder up to dark energy. It's a different beast from everything we know because it has no mass and it doesn't clump up in regions, like matter or dark matter. Most intriguingly, it doesn't thin out when the universe expands.

Why were scientists so quick to jump on the bandwagon?

Normally scientists are a conservative lot. They sign onto new paradigms only after their old ones grow so rickety that they topple under the weight of countervailing facts and withering debates. But the dark-energy revolution was a bloodless coup, one that took only a few years to win over most scientists.

It helped that the idea wasn't new. Einstein cooked up a somewhat related concept in 1917 because he thought that the universe was static, neither growing nor shrinking. So he introduced a constant to an equation to balance against the tug of gravity. When the astronomer Edwin P. Hubble discovered that the universe was actually expanding, Einstein threw out his "cosmological constant" and later called it his biggest blunder. Over the decades since, various researchers have reintroduced the cosmological constant or similar terms, but they always seemed outlandish.

In the 1990s, though, some physicists tried to revive the idea. Three years before the supernovae data confirmed that the universe's growth was speeding up, Lawrence M. Krauss, of Case Western Reserve University, and Michael S. Turner, of the University of Chicago, wrote a paper titled "The Cosmological Constant Is Back." They argued that various knotty issues, such as the age of the universe and the amount of matter it holds, argue for the existence of some additional type of energy with characteristics like Einstein's cosmological constant. Other researchers also suggested reviving the constant.

When the two groups studying supernovae independently found evidence of the accelerating universe, researchers quickly latched onto the cosmological constant as a potential answer. "It is truly amazing how quickly the switch turned," says Mr. Krauss, a professor of physics and astronomy.

What is the source of dark energy?

Physicists now entertain two possibilities: Dark energy is a constant, as Einstein suggested, or it is a slowly changing type of energy, dubbed "quintessence."

The cosmological constant would be an energy that comes from empty space. Although hard to fathom, the concept holds that even the vacuum of the universe is quivering with energy. According to the rules of quantum mechanics, particles and their antiparticle siblings -- like electrons and positrons, respectively -- should be popping into existence and then annihilating each other all throughout space. These shadowy objects don't survive long, the rules say, but they do create a buzzing energy in empty space that could fuel the cosmic acceleration.

Quintessence would work differently. It's more like an intrinsic heat stored in the universe, says Mr. Krauss. Although the universe would cool over time, the process has not happened quickly enough to drain all of the energy from the empty regions of the universe.

At present, physicists can't pick between the cosmological constant and quintessence -- but some find the idea of a varying dark energy easier to stomach.

Why don't physicists like the idea of a vacuum filled with energy?

The cosmological constant gives researchers headaches. Although quantum theory says empty space should have energy, the relevant equations predict that the energy should be 10120 times more powerful than the tiny value consistent with the rate of acceleration seen today. At the theoretically predicted value, the universe would have flown apart so rapidly that no atoms would ever have formed. No molecules, no galaxies, no Starbucks. So theorists had been in the habit of tweaking their equations to make the vacuum energy zero, which allows them to justify the fact of their own existence.

The theories can explain a giant vacuum energy or none at all. But nobody knows of a way to get a tiny cosmological constant like the one suggested by the supernovae data. "A nonzero cosmological constant is ugly beyond all belief," says Mr. Krauss, "because it's small. And it's too hard."

What if dark energy doesn't exist?

"We might be completely fooled by the data," says Gia Dvali, a professor of physics at New York University. He is one of several researchers who suggest that something even more radical might explain the cosmic acceleration.

Mr. Dvali thinks that it might be time to revise Einstein's general-relativity theory of gravity. "I really think there is a modification of gravity," he argues. "Whatever is happening at those distances, there is something strange happening with gravity, which is not related to dark energy."

It gets even weirder when Mr. Dvali goes into detail. Gravity, he suggests, leaks into unseen dimensions when it traverses huge distances. That leakage weakens gravity in our three-dimensional universe, so the cosmos accelerates faster than expected.

This hypothesis is an outgrowth of string theory, which holds that every fundamental particle is made of tiny vibrating bits of energy called strings. As a consequence of the theory, physicists propose that the universe holds seven or eight additional dimensions beyond the three that we inhabit. All forces and particles aside from gravity are trapped in our three-dimensional space, which physicists call a membrane, or "brane" for short. Gravity is free, however, to slip off our brane into extra dimensions, says Mr. Dvali.

How can we tell if gravity is leaking from our universe?

Just look at the moon, says Mr. Dvali. If the extra dimensions exist, they affect gravity in a couple of ways. At cosmic distances, gravity could leak from our brane. But at shorter distances, gravity is trapped here, and the presence of extra dimensions slightly boosts gravity's power. Theoretically, that should have an effect on the moon, nudging its position by about a millimeter from where it would otherwise be.

Scientists are working on ways to measure the moon's position that accurately. The Apollo astronauts left mirrors on the moon, and researchers can bounce laser beams off those mirrors to gauge the distance. This method is accurate to within a centimeter, says Mr. Dvali, but in the near future, scientists might be able to push their techniques to the point at which it could detect subtler alterations in gravity's strength.

Is it possible that the dark energy concept is just a big mistake?

There is a chance that the exploding stars have fooled everybody, says Alain Blanchard, a professor of astrophysics at the Paul Sabatier University-Toulouse III, in France. To date, he says, only one piece of direct evidence points to the presence of dark energy: the distant supernovae, which appear farther away than they should be.

"That's a reasonable assumption," says Mr. Blanchard, "but I find it's not enough to conclude that 70 percent of the universe is composed of strange stuff."

To try to trace the universe's evolution, he has taken a different tack, studying giant clusters of galaxies that emit X-rays. Those studies suggest that the universe holds far more matter than most cosmologists have assumed. In fact, with all that matter in the universe, there is no need to invoke dark energy, he says: "When you introduce something really different, you have to really investigate all the possibilities. For the cosmological constant, it's my impression that people have adopted it too fast."

Mr. Blanchard is not winning over other physicists, however. "It's my belief that he's just plain wrong," says Chicago's Mr. Carroll. "The very widespread consensus is not only that the universe is expanding, but we have evidence of the acceleration from multiple, completely independent lines of evidence."

How will we eventually find answers?

Researchers are going to opposite extremes -- from the biggest objects in the universe to the smallest -- in search of clues to dark energy.

On the large scale, astrophysicists are measuring giant clusters of galaxies, which were formed out of dense regions in the infant universe. By surveying the number of clusters back in time, they hope to make a growth chart of the universe. Another type of expansion data will come from measuring the brightness of many more supernovae, going back further and further in time. If researchers can accurately gauge the growth of the universe, they can tell what kind of dark energy is driving the acceleration -- either the vacuum energy of the cosmological constant or a changing force like quintessence.

"If you would find a deviation from the vacuum energy, that would be very significant because that would be something new, something we have had no knowledge of up until now," says Martin Kunz, an assistant professor of theoretical physics at the University of Geneva, in Switzerland, who is using many types of data to try to distinguish among different models of dark energy.

Another source of answers could come from the world's largest particle accelerator, under construction by the European Organization for Nuclear Research, or CERN. When that facility goes on line, in 2007, physicists will smash together protons at energies beyond the intensity of any previous experiments. They will be looking for new kinds of matter, called supersymmetric particles, which are thought to be the unseen partners of fundamental particles like electrons and quarks.

Physicists have theorized that supersymmetric particles could be part of the missing dark matter in the universe. At the same time, those particles might also point the way toward understanding what dark energy is, says Mr. Kunz. "If they do find signs of supersymmetry, that would be very important for research into dark energy," he says.

Mr. Carroll is betting that such particle experiments will yield the next big development in physics. But he doesn't want to wager much on that forecast. "My track record and everybody else's track record at predicting these things is abysmally bad," he says.

For now, all the uncertainty has given a jolt of energy to physicists, even as they worry about shrinking budgets. "Occasionally a science reaches a precipice -- a junction where all paths seem to lead to confusion," concluded a panel of the National Academy of Sciences in a report on the future of physics. "These crises are often a precursor to major conceptual breakthroughs. By any measure, cosmologists and physicists now find themselves in such a (wonderful) quandary."