Embracing the dark side: looking back on a decade of cosmic acceleration.

On Jan. 12, 1998, just before leaving for his honeymoon, astronomer Adam Riess e-mailed his colleagues that the universe appeared to be completely dark and utterly repulsive. Fortunately, he was talking about a matter of gravity.

Riess was part of a team of astronomers viewing distant supernovas to study the expansion of the universe. Researchers have known since the 1920s that the universe is expanding, with distant galaxies fleeing from each other at a rate proportional to their distance. That expansion, driven by the energy released during the Big Bang, ought to have been decelerating ever since, braked by the mutual gravity of all the matter in the cosmos.

But that's not what Riess, along with astronomers from a rival team, had found. Instead of slowing, cosmic expansion was speeding up. Gravity had somehow transformed from an attractor to a repeller, forcing matter to fly apart at an ever-faster rate.

"I still recall feeling very excited--excited that it was true and also very anxious, because most things you discover in science are wrong," says Riess of the Space Telescope Science Institute in Baltimore.

But with another team, led by Saul Perlmutter of the Lawrence Berkeley (Calif.) National Laboratory, coming to the same conclusion, astronomers had to accept--and even embrace--the notion that gravity has a flip side.

Some kind of invisible, mysterious substance--which University of Chicago cosmologist Michael Turner dubbed dark energy--fills the universe, turning gravity's pull into a comic push. This mystery material, thought to pervade all of space, comprises 74 percent of the universe's mass and energy.

Understanding dark energy "is the most profound problem in all of science," says Turner. Solving it even might unite quantum theory--the subatomic realm--with gravity, which operates over the largest distances imaginable.

But 10 years after dark energy's discovery, scientists still have "no killer theory" to explain its existence, says theorist David Weinberg of Ohio State University in Columbus. Astronomers are beginning to embark on a host of new observations that might help solve the puzzle. In the meantime, theorists have no dearth of ideas.

Cosmic acceleration has been variously proposed as originating from the quantum version of empty space, posited as a leftover from the brief epoch of rapid expansion at the birth of the universe, or attributed to gravity leaking away into extra, hidden dimensions.

Then there are the weird explanations.

"This is the time to get all the ideas," says Turner, "Because some are just crazy, but one of them might be right."

INTO THE DARK Dark energy wasn't called dark energy before 1998, but a similar idea had come in and out of vogue among some cosmologists for years. Then, in the late 1990s, cosmologists faced a crisis. On the one hand were findings from studies of the cosmic microwave background, the radiation left over from the Big Bang, indicating that the universe was fiat: Parallel lines would never meet. That meant the total density of cosmic energy and matter had to equal a critical value. On the other hand, measuring the amount of mass in the universe by observing galaxy clusters told a different story: There wasn't nearly enough matter to make the universe fiat.

That's one reason why many scientists readily embraced cosmic acceleration, says Turner. Dark energy would provide the missing stuff--something other than matter--that would keep the cosmos fiat. "Everyone was excited in 1998 because this seemed to be the missing piece of the puzzle. It made all of cosmology work," says Turner.

Evidence since then has strengthened the case for cosmic acceleration. By examining the brightness of Type la supernovas both nearby and far back in cosmic time, Riess, Perlmutter, and colleagues have reconstructed the history of the universe's expansion.

Dark energy, or repulsive gravity, was always present, but initially unimportant. The youthful universe, though expanding, was relatively compact and dense. The high-mass density enabled gravity's tug to reign supreme. But the continuing expansion of the universe diluted the density of matter. Eventually, about 5 billion years ago, the cosmic push of dark energy won the tug-of-war against gravity's pull, and cosmic expansion began to accelerate.

A CONSTANT MYSTERY Studies so far hint that dark energy might have a constant density, spread evenly throughout space. That would resemble the cosmological constant, a feature that Albert Einstein inserted into his theory of gravitation in 1917. After the discovery that the universe wasn't static, Einstein disowned the term. But maybe he was right after all.

Einstein's cosmological constant would be a property of empty space. And that, in turn, could tie dark energy to the tiniest realms of space. According to quantum mechanics, the laws that govern the behavior of subatomic particles, empty space isn't really empty. It seethes with pairs of particles and antiparticles that constantly pop in and out of existence. That activity imbues the nothingness of space with an energy. Moreover, that energy could be just the type to flip the switch on gravity.

But there's a huge catch. The quantum energy from empty space, physicists calculate, is way too big--120 orders of magnitude too large--when compared with the amount astronomers are measuring. "It's not too strong to say [this mismatch] has been the bone in our throat for a long time," says Nobel laureate Steven Weinberg of the University of Texas at Austin. "The problem is not why there is dark energy; the problem for physicists is why it is so incredibly small."

Then there's what's called the cosmic coincidence problem. No one can explain why the energy density associated with the cosmological constant should have a magnitude comparable to the density of matter. "That's why I think a cosmological constant is pretty wacky," says Rocky Kolb of the University of Chicago. "I refer to it as the cosmo-illogical constant."

Some theorists, including Steven Weinberg, are hoping string theory may help. String theory posits that every subatomic particle is represented by a vibrating "string." The theory allows for a landscape of different, parallel universes, disconnected from each other. Each universe possesses a different value of the cosmological constant. According to that theory, humans happen to live in a universe where the cosmological constant is small, but not zero. Were it much bigger, cosmic acceleration would have begun so early that galaxies, stars, and planets wouldn't have had time to coalesce. In other words, in a universe that had a different value for the cosmo-logical constant, nobody would be alive to observe it.

This "anthropic" explanation for the cosmological constant may be something that cosmologists will have to accept, says Weinberg. But for others, it's anathema. "To me, this is like pulling up the white flag and saying you give up," says Kolb. "And I am not ready to do that yet."

VARIATION ON A THEM E Other researchers have conjectured that dark energy changes with time. Whether or not it does so could drastically alter the fate of the universe.

In one time-varying version, the density of dark energy will continue to grow, and the universe will end in a Big Rip. Not only will groups of galaxies continue to flee from each other at an accelerated rate, but every individual galaxy, star, and planet will be ripped apart in some 50 billion years, says Robert Caldwell of Dartmouth College in Hanover, N.H.

A model developed by Paul Steinhardt of Princeton University and his collaborators not only seeks to understand why dark energy might vary but also ponders a "radical departure from the way we understand the universe," he says. Developed in collaboration with Neff Turok of the University of Cambridge in England and other colleagues, the model posits that the cosmos has no beginning and no end. In this universe, the world as we know it is confined to a membrane. Nearby lies a partner universe, confined to another membrane and separated from us by a gap tinier than the diameter of an atomic nucleus. Interactions between these two parallel universes create fresh generations of dark energy.

The two membranes move and are attracted to each other, eventually colliding. On the rebound, as they pull apart, energy is added to the system, akin to the gravitational energy stored in a ball that's been pushed uphill. That stored energy is the dark energy.

TINKERING WITH GRAVITY Explaining cosmic acceleration in terms of dark energy--a property of empty space that creates repulsive gravity--is only one possibility. Theorists are also pondering the possibility that Einstein's theory of gravity isn't quite right: His theory may break down when applied to the largest distance scales.

In one modification of Einstein's theory, proposed by Gia Dvali of New York University and his colleagues, gravity would grow weaker over large distances because it leaks out into other, unseen dimensions (SN: 5/22/04, p. 330). These researchers model the observable universe as confined to a three-dimensional membrane. All the stuff in the cosmos resides within that membrane, as do all the forces of nature--with the exception of gravity. Gravitons, the subatomic particles that transmit gravity, could escape the membrane, traveling a small distance into another dimension. With gravitons exiting, gravity would weaken at great distances. The smaller gravitational grip mimics dark energy's ability to rev up cosmic expansion.

Astronomers could put that theory to the test by observations in our own solar system, Dvali says. Leaky gravity would cause the moon to tilt, or precess, slightly as it orbits Earth. New experiments to accurately gauge the Earth-moon distance, bouncing laser light off reflectors left on the moon by Apollo astronauts, could discern the predicted precession.

Dvali's team has now extended the theory to more than a single extra dimension. Previous attempts to do so had failed, he notes, because such theories resulted in unwanted "ghost" particles that have physically unrealistic properties, such as negative energy. Dvali and his colleagues recently calculated that if the number of dimensions into which gravity leaks varies with distance, the theory isn't haunted by ghosts.

In a very different theory, put forward by Jae-Weon Lee of the Korea Institute for Advanced Study in Seoul and his collaborators, the universe is, in effect, a giant black hole.

Black holes have an event horizon, a one-way membrane where a ray of light that gets too close will fall into the gravitational monster and never return. Space-time in the vicinity of a black hole is so warped that it can create particle-antiparticle pairs out of the vacuum. Occasionally, one member of the pair will fall back into the hole while its counterpart will escape into space. To a distant observer, it appears that the black hole is radiating.

Lee and his colleagues suggest that as the universe expands, it creates a cosmic version of a black hole--event horizon, a region of space from which distant observers will never see a light signal. If a particle-antiparticle pair is created at this horizon, one particle may fall toward it while the other heads toward the distant observer. In effect, the cosmic-event horizon radiates, and Lee's team says the radiation could be just the right amount to drive the accelerated expansion.

Caldwell disagrees: "There is an energy associated with the cosmic horizon," he notes, but the amount is negligible and has the wrong form to be dark energy.

Kolb has an entirely different idea. He and his colleagues propose that the lumpy structure of the universe gives rise to cosmic acceleration.

Astronomers have known for years that the universe has a weblike structure, consisting of vast voids surrounded by filaments where galaxies congregate. There's still an average density to the universe, but "we model the evolution of this inhomogeneous universe as if we live in a homogeneous place," Kolb says, and "technically, that's not correct." His team is investigating whether the lumps exert what he calls a "back reaction" on the cosmos, mimicking the antigravity effect of dark energy.

"Right now, we are not able to make a prediction, so no one will take us seriously," Kolb admits. "Inhomogeneity is sort of a wacky idea. It would mean that the way we've done cosmology since 1922 has been slightly off."

OBSERVERS TO THE RESCUE? A suite of new experiments may determine whether dark energy is real, or if general relativity itself must be modified.

Last September, the National Research Council recommended that a dark-energy probe be the first spacecraft that NASA will launch in its Beyond Einstein series of missions.

Jointly sponsored by NASA and the Department of Energy, the project has attracted three proposals, which will now duke it out. The $1 billion winner would be launched around 2015.

The Supernova/Acceleration Probe would study the expansion history of the universe by recording some 2,000 Type 1a supernovas a year, using a mirror about the same size as that of the Hubble Space Telescope and the biggest camera ever launched into space. The probe would employ a second method to hunt for dark energy. As seen from Earth, the gravity of any massive object bends the path of a light ray emitted by a body, such as a galaxy, that lies directly behind it. The shape of the background-object galaxy is distorted as the light passes through this gravitational lens. The greater the rate of cosmic expansion, the larger the volume that exists between distant galaxies and Earth. Cosmic acceleration therefore ups the chances that light from a distant galaxy will encounter a distorting lens en route to Earth.

A second mission, known as ADEPT, would use the echoes of a primordial cosmic symphony to examine cosmic expansion. The interaction between gravity, matter, and radiation in the early universe set up acoustic oscillations, cosmic sound waves that left their imprints on the distribution of galaxies across the sky. By recording the positions of 100 million distant galaxies, astronomers hope to discern the length of the sound waves and use them as a cosmic ruler for measuring the rate of expansion of the universe when it was less than half its current age.

A third mission, the Dark Energy Space Telescope, would use a near-infrared telescope to detect more than 3,000 Type la supernovas over a 2-year period. It would then survey a large chunk of the sky to determine how the distribution of galaxies has evolved since the Big Bang.

It is critical to measure cosmic acceleration in as many ways as possible, notes Turner. If any two methods come up with different answers, it could indicate that Einstein's conception of gravity needs modification.

Turner says he's optimistic that within 15 years, a combination of space- and ground-based telescopes devoted to studying dark energy will at least partially crack the mystery.

"This puzzle seems to be [related] to a number of other puzzles, it's the nexus," says Turner. "We can't understand the universe until we discover what dark energy is."

Back in 1998, veteran astronomer Nick Suntzeff, now at Texas A&M University in College Station, had a similar sentiment when he sent an e-mail reply to his younger colleague, Riess: "I really encourage you to work your butt off on this.... You probably never will have another scientific result that is more exciting come your way in your lifetime."

[ILLUSTRATION OMITTED]

TUG OF WAR--Observations of distant Type la supernovas, which act as cosmic mile markers, reveal that the cosmic push of dark energy was always present in the universe but didn't begin overwhelming the pull of dark matter until about 5 billion years ago. That's when the cosmos began revving up its rate of expansion,

Composition of the Universe

Atoms          4%
Dark Matter   22%
Dark Energy   74%

Note: Table made from pie chart.