Think of today's astrophysicists as cosmic detectives. As better instruments make it possible to look at clusters of stars and galaxies billions of light years from earth, astrophysicists are able to see back in time to when the universe was relatively youthful. With this vision, they can compare their observations with theoretical simulations that attempt to explain cosmic events in the early universe. These glimpses of the past, as well as the computer simulations, are clues that will hopefully lead to answers to some of the most fundamental questions of science: Why is cosmic material dispersed in clumps rather than distributed evenly across the universe? What is the nature of that mysterious, unseen matter that makes up most of the universe's mass, commonly referred to as dark matter?

Paul Bode, a research scientist at Princeton University, and Jeremiah Ostriker, a Princeton astronomy professor and a member of the Alliance Cosmology team who also holds the Plumian Professorship of Astronomy and Experimental Philosophy at the University of Cambridge, concern themselves with the latter two questions. They have so far logged 25,000 hours on the Alliance's Platinum Linux cluster at NCSA in an effort to describe the properties of dark matter in ways that fit logically with what other astronomers have observed in the universe.

Scientists have postulated the existence of dark matter for more than 25 years, even though they have never been able to see it. In the observed universe, galaxies and clusters of galaxies spin fast—so fast that they must contain more matter than what can be seen. The outer portions of spiral galaxies rotate around their galaxy centers with such speed that they would fly apart if only their visible matter were holding them together. Likewise, galaxies inside galaxy clusters move in relation to each other at speeds that are faster than what would be induced by the gravity of the galaxies' visible matter. There simply must be more matter, undetectable to human optics, holding together these spinning disks, scientists theorize.

They coined the term dark matter to describe this invisible mass. Dark matter is believed to surround galaxies in invisible halos. The exact nature of dark matter, including its exact mass, how it interacts with other particles, how fast it moves, and how it obeys the general laws of physics, are questions that still need to be answered. Bode and Ostriker believe that the key is to figuring out the characteristics dark matter would need to create a universe that looks like the one we see.

"We may think of these very large cosmological simulations as a means of test driving new fundamental theories of physics," explains Ostriker. "We put into the computer some new, specific but speculative theory, start with initial conditions given by microwave observations of the deep past, and then calculate forward. The test is to see if the final computed universe looks like the real one."

"One of the questions we have is: How dense and how energetic would dark matter need to be in order for the universe to be the way it is?" adds Bode. "The density of the universe is not uniform, and we believe dark matter provides the extra gravitational pull to attract more matter together into the dense clumps of galaxies that we observe."

Access Online | Posted 4-9-2002