Researchers explore mysteries of dark matter, energy

Abram Katz , <a href=www.zwire.com>Register Science Editor 06/08/2003 Photo courtesy of NASA Earth was less than a speck in the old universe of planets, stars, galaxies, black holes, and quasars. Now it turns out that our whole universe of visible matter, and gravity is but a mote in the cosmos. Much of 20th century physics may have to be modified to accommodate two enormous oddities — dark matter and dark energy. The latest research suggests that this "dark," undetectable stuff constitutes 99 percent of the mass of the universe. Dark matter is mysterious, but it follows the standard laws of gravity. Dark energy, on the other hand, is an enigmatic force that seems to behave like gravity in reverse. While gravity should be slowing the expansion of the universe, dark energy is accelerating the spread. "We have no clue what’s going on. That’s exciting," said Charles Baltay, Higgins professor of physics and professor of astronomy at Yale University. Baltay and colleagues at Yale, Indiana University, and at a Venezuelan university and an observatory hope to shed some light on dark matter and energy. Baltay will use the world’s largest telescopes and other instruments to observe quasars on the edge of visible space. These incredibly bright beacons will help astronomers and physicists chart the geometry of the universe, estimate the mass of dark matter, and probe dark energy. Baltay said the universal domination of dark matter and energy makes human science seem like a puny enterprise. "The stuff we’ve been studying is a lousy 1 per cent of what’s out there," he said. Scientists calculate that dark matter comprises about 30 percent of the universe and dark energy around 70 percent, meaning visible matter is indeed trivial. Dark matter at least behaves like normal everyday matter — it just happens to be almost impossible to detect. German astronomer Fritz Zwicky posited the existence of dark matter in the 1930s. Zwicky saw stars exceeding the expected speed limit around galaxies. That could only happen if the galaxy had 10 times more mass, which could not be accounted for by stars, dust or gas. At roughly the same time, Edwin Hubble realized that stars and galaxies were all speeding away from each other. Albert Einstein and others supposed the universe was static, but here it was expanding. Einstein had added a "cosmological constant" to his equations to save the universe from gravitational collapse. When it became clear that the universe was expanding, Einstein removed the cosmological constant, calling it his biggest blunder. Meanwhile, expansion raised the question of the universe’s mass. A certain mass would produce sufficient gravity to pull the spreading galaxies back together in a big crunch. If the universe were too light, matter would keep expanding. And should there be exactly the right amount, expansion would slow but never quite stop. This is the "critical mass." Depending on the mass, the universe could be "closed," "open" or "flat." Closed meant collapse and curved space. Open meant eternal expansion and negative curvature, and flat implied perpetual slowing and uncurved flat space. The amount and mass of dark matter became an important element in cosmology. This whole model was thrown into disarray in the late 1990s. Researchers at Princeton, the University of California at Berkeley and Harvard measured how fast a certain type of supernova was receding. The astronomers found the universe is not merely expanding. It’s accelerating. "Your first reaction is ‘That’s ridiculous,’" Baltay said. A force like repulsive gravity must be pushing matter apart. Einstein’s disavowed cosmological constant appeared real, after all. The weird reverse gravity became known as dark energy. Since energy and mass are equivalent, the observations suggest that dark energy accounts for about 70 percent of the mass of the universe. "The universe is more complicated. When we finally understand it, we’ll have a nice simple picture again," Baltay said. Baltay and colleagues hope to clarify what’s out there, using a technique that Einstein would appreciate. Massive things like galaxies can bend light enough to act like optical lenses. A quasar provides the light. If the quasar is behind a galaxy, or a clump of dark matter, observers on Earth will see multiple images of the quasar. This is called gravitational lensing. Lensing without a visible galaxy suggests the presence of dark matter. "We’ll look at a lot of lenses to see if there is dark matter, if the lens is invisible," Baltay said. Knowing the distance to the quasars and the mass of the lenses, researchers can measure angles like a prospector and calculate the degree to which space is curved, Baltay said. "By drawing big triangles in space, we can see the geometry of space," he said, including dark matter and energy. First, the researchers will use a large array of charge-coupled devices attached to a Schmidt telescope in Venezuela to survey the sky for quasars and lensing. The 200-inch telescope at Mount Palomar and the world’s largest telescope, the 10-meter Keck telescope in Hawaii, will be used to observe and confirm the quasars. As part of the project, Baltay and researchers also will make their own measurements of supernovas. Because only about one in 1,000 quasars are "lensed" the experiment will require viewing hundreds of thousands of quasars.

Abram Katz can be reached at akatz@nhregister.com or 789-5719.