And Now, the Biggest Question in the Universe

Physicists have put their hopes on the Large Hadron Collider, an $8 billion, 17-mile-long racetrack beneath the border between Switzerland and France designed to slam protons into one another at nearly the speed of light. More than 7,000 scientists from 80 countries will work at the complex, which began ramping up to full power this month. Physicists hope it will solve fundamental questions: What did the universe look like as it was being born? What is the interplay between the four fundamental forces -- electromagnetism, the strong and weak nuclear forces, and gravity? Is there an undiscovered particle that gives everything mass?

If they can answer these questions, they could crack the puzzle of dark matter. But dark energy is still terra incognita.

Dark Energy

The astronomer Edwin Hubble discovered decades ago that other galaxies seemed to be moving away from us in the Milky Way. The farther away they were, the faster they were going. But astrophysicists figured that gravity would cause the universe's expansion to gradually slow down.

Then in 1998, two teams upended that theory by looking at Type Ia supernovae, rare stellar explosions that briefly release tremendous, consistent amounts of light. By carefully measuring how the light from these events shifted toward the red end of the visible light spectrum -- similar to the Doppler effect that causes the sound of a train whistle traveling away from you to drop in pitch -- they found that the expansion had accelerated in the last few billion years. Empty space itself seemed to be acting as a force driving things apart, and to be growing stronger as the universe got bigger.

Albert Einstein hinted at the existence of a mysterious force like this when he proposed the concept of a cosmological constant, a fudge factor representing an unknown that he thought would keep the universe in a steady state. After Hubble made his observations, Einstein repudiated the constant as the greatest blunder of his career. Now it looks as if he spoke too soon.

But cosmologists are the first to admit they have no clue what dark energy really is. It could be Einstein's cosmological constant. Others theorize that it's quintessence, a form of anti-gravity that changes over time. Some think it could be evidence for string theory, the idea that the universe is composed of tiny strings and dimensions we can't see. Or it could be a sign that our model of physics has flaws that aren't apparent until one looks at things on a cosmic scale.

Finding the answer has become one of NASA's top priorities. The Joint Dark Energy Mission, an effort involving NASA and the Energy Department, seeks to send a new telescope into space tailored to find out more about dark energy. So far, three candidates have emerged: the Dark Energy Space Telescope, or Destiny; the Advanced Dark Energy Physics Telescope, known as ADEPT; and the SuperNova Acceleration Probe, or SNAP.

All would detect thousands of Type Ia supernovae, allowing cosmologists to be more precise in their understanding of what dark energy is and how it may have evolved. The proposals would also use other methods. ADEPT would measure ancient sound waves that rumbled through the early universe, leaving in their wake tiny variations in how galaxies are distributed across huge volumes of space. SNAP and Destiny would use gravitational lensing to measure how the distribution of matter across the universe could be affected by dark energy.

Charles L. Bennett, a professor of physics and astronomy at Johns Hopkins University who is leading the ADEPT team, said the mission would probably be selected by the middle of next year, and the telescope would be in space a decade from now.

"We should have a whole lot more data 10 years from now," Bennett said. "I can't help but feel we'll know a lot more about it, and possibly the definitive answers by then. We've got a full frontal assault on it going on in the community."

For Saul Perlmutter, a University of California at Berkeley cosmologist who led one of the teams that discovered dark energy and is now leading the SNAP effort, a final answer is tantalizingly near, but it was finding the question that was the real joy.

"We've gotten amazingly close to what we feel is a full picture," he said. Dark energy, he said, was "the little crack in the blind that gives us some insight into what we've never thought before. For a physicist, for a scientist, that is one of the ultimate pleasures, to see the beginnings of a mystery that you can explore."