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By June 1994, the groundwork had been laid for a truly massive computation, one that would model the way dark matter and baryonic gas interact to generate large scale structure on the order of clusters and superclusters. The model incorporated not only dark matter but also baryonic gas, at last enabling the cosmology group at NCSA to run a simulation capable of making theoretical predictions that could be tested against hard observational data.
The simulation, when visualized, portray the universe as it would be seen in X-rays. The resulting patterns in the data can then be compared with actual X-ray observations made of distant galaxies by ROSAT, the orbiting NASA-European Space Agency satellite, as well as NASA's Advanced X-Ray Astronomy Facility, scheduled for launching in 1997.
Jeremiah Ostriker, Princeton University,
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The simulation computes the heating of cosmic gas (primordial hydrogen and helium) as it is pulled into the gravitational wells caused by dark matter. Gas falling into the wells collides with matter, causing shock waves that superheat the intergalactic gases in which galaxies are thought to "swim." The superheated gas emits X-rays detectable by space-born observatories.
It used 60% cold dark matter, 30% hot dark matter and 10% baryonic gas in its recipe, and, using KRONOS, computed the behavior of gas as it interacts gravitationally with 50 million particles of mixed dark matter in a 512^3 grid. The simulation required all 16 gigabytes of the Connection Machine 5's memory.
Each of the CM-5's 512 processors was assigned a block of 64^3 computational cells and 100,000 dark matter particles. Each cell in this grid spanned 1 million light years. The CM-5, running in dedicated mode for 40 hours, churned out numerical solutions f or gas velocity, temperature and density in 134 million cells.
As such, this gas plus dark matter simulation ranks among the largest of its kind ever undertaken. Not surprisingly, the cosmologists had to use advanced visualization techniques to make sense of the data. They used the Cosmic Explorer an advanced suite of visualization tools for ex ploring cosmological datasets to create a series of pioneering movies at NCSA.
The predicted pattern of X-ray emission corresponds quite closely with those observed by ROSAT, although the model under-predicts the amount of gas relative to dark matter in the clusters by a factor of two or more. And the match between observed and predicted patterns becomes much less certain beyond distances correspo nding to a red shift of 0.5 where data remains very sparse.
Efforts are now underway to improve by a factor of 10 the resolution of the simulated, X-ray emitting gas clusters. Aside from tapping the massively parallel architecture of the CM-5, a variety of other types of supercomputers will be tested for their ability to sustain computations of this scale.
By the late 90s, when NASA plans to launch its Advanced X-ray Astronomical Facility, there'll be a wealth of simulated data to compare against the X-ray universe.
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