Seeing the invisible

In astrophysics, dark matter is akin to the idiomatic 800-pound gorilla – a dominating influence that throws its weight around, dictating how stars and galaxies move. But unlike gorillas, dark matter is invisible and its nature is elusive. Physicists can only surmise its existence from its effects on visible matter.

“If you didn’t have dark matter the stars would not be able to move at the velocity they’re moving at. They would fly apart,” says Michael Kuhlen, a postdoctoral researcher at the University of California, Berkeley. Physicists theorize that “halos” of invisible matter host galaxies, providing the gravity that holds them together.

Kuhlen is part of a research team using Jaguar, Oak Ridge National Laboratory’s world-leading Cray XT computer, to help understand dark matter distribution. Their Via Lactea II model first gained attention when it elucidated the “lumpy” nature of the dark matter halo enfolding a galaxy like the Milky Way.

Via Lactea II was made possible with a grant of 1.5 million processor hours on Jaguar, provided through INCITE, the Innovative and Novel Computational Impact on Theory and Experiment program, supported by the Department of Energy’s Office of Science. The researchers, led by Piero Madau of the University of California, Santa Cruz (UCSC), now have a 5 million processor-hour INCITE grant to run their next, more detailed dark matter simulation.

There’s a lot to detail. As much as 83 percent of the matter comprising the universe is dark matter, physicists say. Researchers have offered several theories describing it, but the one gaining the most acceptance casts the mysterious material as a low-temperature fundamental particle that interacts only weakly with ordinary matter – except through gravity.

Via Lactea II is based on this cold dark matter representation. Although its name is Latin for Milky Way, it’s not designed to precisely simulate evolution of our galaxy but of one similar to it, Kuhlen says. “By similar, we mean it has the right mass, the right rotation curve and roughly the correct accretion history” – the process that formed the galaxy.

The latest INCITE grant will enable more precise simulations. Instead of resolving structures on a scale of hundreds of parsecs (a parsec is equal to about 3.26 light-years, or about 19 trillion miles), the group’s model could have a resolution in the tens of parsecs. It’s like switching from a microscope capable of enlarging objects by 10 times to one capable of enlarging objects by 100 times.

“We could actually resolve scales in these dark matter halos that also are accessible to observational astronomy. We can get a correlation between what observers see and what we can predict,” potentially providing further data on dark matter’s nature, Kuhlen says.