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

Science News, Feb 2, 2008 by Ron Cowen

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.