UCSC Review Winter/Spring 1994
There's Something Dark Out There
Away from the haze of city lights, a truly dark night sky is both beautiful and mysterious. Thousands of stars shine sharply against the blackness, but the universe seems far richer and deeper than those brilliant flecks reveal. What does the black sky hide?
Some of the world's best astronomers are trying to answer that very question. Between the stars and galaxies, in the space that looks so empty to us, lurks the material astronomers seek: "dark matter." As its name implies, dark matter emits no light of its own. But of all the mass in the universe, at least 90 percent is dark, invisible to even the most powerful telescopes. The stars we wish upon are minor players in a universe dominated by things we cannot see.
Where is this dark matter? Some may be all around us, in the form of subatomic particles that scientists have not yet found. Black holes, both huge ones at the cores of galaxies and tiny ones throughout space, are another possibility. Planet-sized objects or dim stars may surround every galaxy in massive clouds.
Finding the dark matter is not just a casual pursuit, for astronomers think that it controlled how galaxies and clusters of galaxies formed and changed over time. Further, the exact amount of dark matter will determine the fate of the universe: to expand forever, or to recollapse in a "big crunch" many billions of years from now.
Those are big issues, and researchers at UCSC got big attention recently by making major advances in the quest to understand dark matter.
First, a study by astrophysicist Douglas Lin showed our Milky Way galaxy contains about ten times more mass than we can see in its stars alone. At a meeting in June, other researchers hailed Lin's work as convincing evidence that a shroud of dark matter does indeed surround the Milky Way, much as a ball of cotton envelops a tiny cottonseed.
Lin reached his conclusion by "weighing" the Milky Way in a clever way. The laws of gravity dictate exactly how a small object orbits around a large one; an important factor is the mass of the large object. To derive the Milky Way's mass, Lin measured the orbit of a nearby galaxy around the Milky Way with extraordinary precision.
That galaxy, the Large Magellanic Cloud (LMC), plods around the Milky Way at the leisurely pace of once every 2.5 billion years. Trying to detect that motion, says Lin, is like standing in San Francisco and trying to measure the speed at which a sapling grows in New York City. But thanks to advanced technology, that was fast enough for Lin and two other UCSC astronomers, Burton Jones and Arnold Klemola, to nail the LMC with a cosmic radar gun.
The team found detailed photographs of the LMC taken in 1974, then took pictures of the same part of the sky in 1989. Painstaking analysis of 250 stars in the LMC showed they had moved a minuscule amount with respect to galaxies in the background. Calculations gave the LMC's exact speed across the sky.
Other measurements allowed Lin to paint a complete picture of the LMC's egg-shaped orbit. The orbit requires that the Milky Way contain about 600 billion times the mass of our sun--five to ten times the mass of the galaxy's visible stars and gas.
"Obviously, there's more mass in the galaxy than meets the eye," Lin says. "The concept of dark matter used to be controversial, but we keep building evidence upon evidence."
Cosmologist Joel Primack explores dark matter not with telescopes, but with supercomputers. In October he and three colleagues, including UCSC alumnus Jon Holtzman of Lowell Observatory in Flagstaff, Arizona, published the best model of dark matter to date.
Primack's team tries to describe dark matter in a theoretical way. If tiny particles make up much of the dark matter, what precisely do they weigh? If there are clouds of larger objects, how evenly do they spread through the universe? Then, using those conditions, they create artificial "universes" of millions of particles in their computers and let the particles interact via the laws of physics. After billions of computations, the particles cluster together in clumps and filaments of "galaxies" separated by voids. The closer those results mimic the structures astronomers see in the real universe, the better the model.
Primack's model, called "cold plus hot dark matter," resembles the real universe quite well. "Cold" dark matter is anything that moved sluggishly soon after the Big Bang, while "hot" dark matter zipped along at close to the speed of light. Most previous models ignored hot dark matter entirely. Primack's recipe, however, calls for one part hot to two parts cold dark matter. A soupcon of other conditions makes the model a success--so far.
"We've stuck our necks out, and other people are shooting at our theory," Primack says. "However, the theory makes predictions about virtually every kind of object in the universe, so it is exquisitely vulnerable. We could easily be shot down by an observation tomorrow." The real value of such models, he adds, is to encourage astronomers and physicists to devise new ways of testing them.
It is a sobering thought that, after all of this century's scientific advances, the true face of most of the universe remains hidden from us. But more progress by researchers like Lin and Primack may lift that veil of darkness.
Karen Fox, a former intern in UCSC's Public Information Office, contributed to this article.