Understanding dark energy（暗力量） is the number one issue in explaining the universe, according to Salman Habib, of the Laboratory's Nuclear and Particle Physics, Astrophysics and Cosmology group. "Because the universe is expanding and at the same time accelerating, either there is a huge gap in our understanding of physics, or there is a strange new form of matter that dominates the universe – 'dark energy' – making up about 70 percent of it," said Habib. "In addition, there is five times more of an unknown 'dark matter' than there is ordinary matter in the universe, and we know it's there from many different observations, most spectacularly（令人吃惊地，引人注目地）, we've seen it bend light in pictures from the Hubble Space Telescope, but its origin is also not understood."

Even though it's looking at only a small segment of the "accessible" universe, Habib's "Roadrunner Universe" model requires a petascale（千万亿次） computer because, like the universe, it's mind-bendingly large. The model's basic unit is a particle with a mass of approximately one billion suns (in order to sample galaxies with masses of about a trillion suns), and it includes 64 billion and more of those particles.

The model is one of the largest simulations of the distribution of matter in the universe, and aims to look at galaxy-scale mass concentrations above and beyond quantities seen in state-of-the-art sky surveys.

"We are trying to really understand how to more completely and more accurately describe the observable universe（可观测宇宙）, so we can help in the design of future experiments and interpret observations from ongoing observations like the Sloan Digital Sky Survey-III. We are particularly interested in the Large Synoptic Survey Telescope (LSST) in Chile, in which LANL is an institutional member, and DOE and NASA's Joint Dark Energy Mission (JDEM)," said Habib. "To do the science in any sort of reasonable amount of time requires a petascale machine at the least."

The Roadrunner Universe model relies on a hierarchical（分层的，体系的） grid/particle algorithm（算法） that best matches the physical aspects of the simulation to the hybrid（混合的，杂种的） architecture of Roadrunner. Habib and his team wrote an entirely new computer code that aggressively exploits Roadrunner's hybrid architecture and makes full use of the PowerXCell 8i computational accelerators. They also created a dedicated analysis and visualization software framework to handle the huge simulation database.

"Our effort is aimed at pushing the current state of the art by three orders of magnitude（数量级） in terms of computational and scientific throughput（吞吐量）," said Habib. I'm confident the final database created by Roadrunner will be an essential component of dark universe science for years to come."