And Now, the Biggest Question in the Universe
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'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."