The research team's simulations started with a cube of WDM particles initially distributed almost uniformly across a theoretical portion of the universe. For the Linux cluster simulations, this cube consisted of 17 million particles in a grid with 512 cells on each side. The grid area was a randomly picked volume of space, 20 megaparsecs per side. An average galaxy in this grid would be made from a volume of three megaparsecs and would occupy about 10 kiloparsecs. The researchers use Tree Particle Mesh (TPM) code to calculate the gravitational force that the particles have on each other over a series of time steps. Over time, particles are attracted to each other and fall toward each other to form dark matters halos. Galaxies form within these dark halos, explains Bode.

Closeup of the dark matter halo surrounding a typical galaxy. In the CDM model (left), the large galaxy has many smaller satellites surrounding it. In the WDM model (right), there are fewer of these satellites. So far, this research suggests that the WDM model fits better with how matter is actually dispersed in the universe. The density of matter increases closer to the center of the halo, from slightly denser than average (blue) to more than a million times denser (white). Click each image to enlarge it.

The 17 million-particle run took eight days using 128 processors full time. Early in 2002 the team began another set of simulations using 134 million particles in a billion-cell grid. That run is expected to take two months using 128 processors continuously. The TPM algorithm gives Bode and Ostriker a mathematical shortcut for looking at the gravitational force the WDM particles exert on each other. In a world of unlimited computing power, the most straightforward way to compute this force would be to calculate Newton's law of gravitation on each pair of particles. However, since the current computations involve roughly 1016 pairs of particles—that's 10 quadrillion pairs—such calculations are impractical if not impossible. TPM breaks this massive problem down into many smaller ones. It uses a tree code to calculate short-range gravitational forces (forces within the many halos produced by gravity). Then it employs a fixed particle mesh to calculate long-range gravitational forces (forces exerted from outside the halos, including forces in the voids between halos).

"Preliminary indications are that the warm dark matter model could provide a better description of the universe as we've actually observed it compared to the cold dark matter model, in which case it is a clue to what dark matter is," says Bode. "What exactly it is, we still don't know."

The dark matter mystery is still unsolved, but, thanks to more detailed simulations, the clues are coming quickly. With a little more detective work, some answers should be revealed.

This research is supported by the National Science Foundation and the National Computational Science Alliance.