GOVERNMENT

Increased Computational Power Leads To More Accurate Simulations: Cray today announced that researchers using the Cray XT3; supercomputer at Pittsburgh Supercomputing Center (PSC) have for the first time been able to integrate black holes into their large-scale cosmological simulations. Black holes and the high-energy quasars they spawn have been absent from the simulations until now because astrophysicists did not have the computational power to include them in the comprehensive models they use to study the birth of stars and formation of galaxies and other features of how the cosmos has evolved since the Big Bang. "The Cray XT3 supercomputer is ideal for these large-scale simulations because it has incredibly fast built-in communications," says Carnegie Mellon University astrophysicist Tiziana Di Matteo. "If we didn't have the bandwidth to communicate large chunks of data among the processors, it would have been really tough. I don't think we could run this simulation anywhere else right now." Previously considered rare, giant black holes with masses of a million to a billion times the mass of the sun have in recent years been observed at the heart of many galaxies. A black hole is formed when the matter in an exploding star collapses inward, creating such strong gravitational force that nothing, not even light, can escape. Massive black holes are thought to have formed in the early universe and have grown in mass by swallowing large amounts of interstellar matter. Simulations by Di Matteo and colleagues with PSC's Cray XT3 uncovered previously unknown relationships between the mass of black holes and the galaxies in which they reside and show that black holes have an important effect on the architecture and evolution of the cosmos. For the huge simulation on the Cray XT3 system, Di Matteo started with software called GADGET-2. To account for black holes, Di Matteo added code that "seeded" them at the centers of forming galaxies. She then added an equation to describe how the black holes accrete or absorb gas, which increases their mass and gravitation. Finally, she included calculations to account for the heating of gas surrounding the black hole. Di Matteo and her colleagues first applied this approach to two colliding galaxies with black holes at their centers, which revealed new behavior when the black holes were included. The success of the simulation led to a paper in the prestigious science journal Nature. The next step was to model a large portion of the universe at the same resolution and with the same spatial scale as the more limited, idealized computations that they had conducted previously. Such models can show astronomers where to point the Hubble Space Telescope and similar instruments so they can observe the actual formation of galaxies and associated black holes. One of the most exciting outcomes of the research is a "movie" that shows frame-by-frame how structures evolved in a large volume of the universe over 14 billion years. Black holes first appear when a universe is about 300 million years old, before galaxies have even formed. Matter in the universe then starts clumping together, and eventually black holes form in the center of most galaxies. PSC recently doubled the capacity of its Cray XT3 system to over 21 teraflops (trillion floating point operations per second). PSC's system was the first Cray XT3 supercomputer to be installed and became the leading performer among tightly coupled supercomputer architectures on the National Science Foundation's TeraGrid computing infrastructure.