By Nelson Hernandez
Washington Post Staff Writer
Thursday, September 25, 2008
Ten years ago, cosmologists discovered they were living a lie. The expansion of the universe was not slowing down as a result of gravity, as they had long believed: It was speeding up.
The working explanation, though by no means the definitive one, is that everything we know -- the whole cosmic fabric ranging from your coffee cup to the sun to entire clusters of galaxies -- is only about 1 percent of what's actually out there. Another 3 percent or so is hot interstellar gas we can see because it radiates X-rays and radio waves. Then things start getting weird. An invisible substance called dark matter, possibly phenomena such as giant black holes and unseen particles, is thought to compose 22 percent of the cosmos. Everything else, almost three-quarters of the total, is dark energy, a force that is apparently driving the universe apart.
The reaction among cosmologists was something like Keanu Reeves's in "The Matrix": Whoa!
"Your whole life as an astronomer, you learn that the universe is expanding, but it should be slowing down," said Tod R. Lauer, an associate astronomer at the National Optical Astronomy Observatory who is investigating dark energy. "But we find out it's speeding up. That's the most incredible shock we've had in cosmology in the last 40 years."
Gary Hinshaw, an astrophysicist at the NASA Goddard Space Flight Center, went even further. "It's probably the biggest surprise in cosmology in 100 years," he said.
Figuring out the nature of dark matter and dark energy is now the biggest question in the universe. Both could reveal the origin and fate of the cosmos, as well as the physical laws that govern it.
NASA and the Department of Energy, as well as research institutions in the United States and across the world, plan to spend billions of dollars to find the answers on Earth and in space. They'll use giant particle colliders and new telescopes that will look at exploding stars, ancient sound waves and gravity-induced distortions of light. Some scientists think finding an answer could lead to a revolution in human affairs no less significant than the discoveries of electricity and of the atom. But for most, it's simply the greatest challenge out there.
"I think it's one of the classic questions of human civilization: Where do we come from, and why are we here?" Hinshaw said. "It seems to me that if 96 percent of the stuff in the universe is foreign to us, it's pretty interesting for us to ask what that is."Dark Matter
Dark matter's existence was predicted in the 1930s, when the astronomer Fritz Zwicky focused on a clump of galaxies called the Coma cluster. Based on the amount of light coming from the cluster, which is one way of estimating how much mass and gravitational force it had, one would have expected the galaxies on the edge of the cluster to move more slowly than those on the inside. But the galaxies on the edge were moving much faster. Either our understanding of how gravity works was wrong or astronomers were somehow failing to account for a large chunk of mass.
Other astronomers later found the same phenomenon on a smaller scale. Stars on the edge of galaxies were orbiting much faster than would be expected based on the mass astronomers could see. More evidence came about a decade ago, when astrophysicists started looking at the effect of the missing mass on light itself by using a technique called gravitational lensing. If there is an object with large mass between us and a distant object we're looking at -- for instance, a clump of dark matter -- it can cause light from distant galaxies to seem to bend in a celestial fishbowl effect.
With dark matter's existence now generally accepted, scientists have turned to the problem of finding out what it is. Black holes and stars and large planets too dim to be seen could account for some of it. But physicists say the answer probably lies with a subatomic particle or with particles we haven't yet discovered.
"Dark matter is material that seems to interact by gravity but, as best as we can tell, does not emit any kind of light," said Richard Mushotzky, a senior scientist at NASA Goddard. "We can tell you where it is, but we cannot tell you what it is. This makes our physics friends very upset."
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."