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How to build a virtual cosmos

This visualization of dark matter is one-thousandth of the gigantic Bolshoi cosmological simulation, zooming in on a region centered on the dark matter halo of a very large cluster of galaxies. (Credit: Chris Henze, NASA ARC)

If you're going to create a virtual universe, you're going to need a big computer — like the Pleiades supercomputer at NASA's Ames Research Center in California's Silicon Valley. Researchers have just made the most accurate computer simulation showing the evolution of large-scale structure in the universe, known as the Bolshoi simulation, available to astrophysicists around the world.

Bolshoi (which takes its name from the Russian word for "grand" or "big") took in data from ground-based and space-based instruments, including the best readings of the big bang's afterglow from NASA's Wilkinson Microwave Anisotropy Probe, or WMAP. Then it used 6 million CPU hours on Pleiades, ranked as the world's seventh-fastest supercomputer, to crunch all that data into a virtual representation of the universe evolving over time.

The time-lapse simulation occupies nearly 90 trillion bytes of memory, or the equivalent of nearly 10,000 typical movie DVDs.


The first two papers in a series describing the simulation have been accepted for publication in The Astrophysical Journal. "A lot more papers are on the way," one of the co-authors, physicist Joel Primack, said in a news release from the University of California at Santa Cruz.

So far, the simulation has been in close agreement with what astronomers are seeing in the actual universe.

"In one sense, you might think the initial results are a little boring, because they basically show that our standard cosmological model works," Primack said. "What's exciting is that we now have this highly accurate simulation that will provide the basis for lots of important new studies in the months and years to come."

The standard model suggests that only 4 percent of the universe's mass-energy content consists of ordinary matter — the kind that we can see. Another 22 percent is cold dark matter, which can be detected only by its gravitational influence. Physicists surmise that dark matter is made up of exotic particles that interact only weakly with ordinary matter, but they haven't yet identified any of those particles. It's the weightiness of dark matter that is thought to shape galaxy clusters into a "cosmic web," which you can easily see forming in the animation above. (Remember to go full-screen and HD for optimal effect, or check out this music-enhanced Vimeo version.)

The biggest constituent of the cosmos, at least based on current models, is dark energy: This mysterious energy, which is thought to account for around 74 percent of cosmic density, serves to counteract the force of gravity and cause the accelerating expansion of the universe. Its existence is required to reconcile cosmological theories with WMAP's observations as well as observations of distant supernovae — but no one has figured out what it is, which has led some astronomers to look for alternative theories.

Primack, who directs the University of California High-Performance Astrocomputing Center, said a close analysis of the Bolshoi simulation could help point the way to better explanations for the dark-energy effect.

"These huge cosmological simulations are essential for interpreting the results of ongoing astronomical observations and for planning the new large surveys of the universe that are expected to help determine the nature of the mysterious dark energy," he said.

The first paper based on Bolshoi analysis focuses on the role of dark-matter halos in the universe's development, while the second paper looks at Bolshoi's predictions for the abundance and properties of galaxies. The researchers have found that the simulation correctly predicts the number of galaxies as bright as our own Milky Way that have satellite galaxies as bright as the Milky Way's major satellite galaxies, the Large and Small Magellanic Clouds.

But this is just the tip of the iceberg: So far, less than 1 percent of the Bolshoi project's output has been released, Primack said. The Bolshoi simulation computes the evolution of a cubic volume measuring about a billion light-years on a side, following the interactions of 8.6 billion particles of dark matter. A variant of the simulation, called BigBolshoi or MultiDark, was run with the same number of particles in a volume 64 times larger. Another variant called MiniBolshoi is currently being run on Pleiades. It focuses on a smaller portion of the universe with higher resolution.

This all sounds pretty deep, but fortunately, the Bolshoi team has produced plenty of beautiful videos and illustrations that will delight even those who can't tell a baryon from a meson. For still more background about Bolshoi, check out the news releases from New Mexico State University, Ames Research Center and the High-Performance Astrocomputing Center.

Update for 5:50 p.m. Oct. 7: In a follow-up phone call, Primack told me that "the agreement between predictions that come from the simulations and the actual observations are really getting spectacular." The previous top-of-the-line virtual universe, known as the Millennium Simulation, showed galaxies as being "much more clustered than they actually are," he said, while the Bolshoi version is "bang-on." Primack said still more revelations are coming from the Bolshoi team. "It's like things are coming into sharp focus," he said.

More about cosmology:


Authors of "Halos and Galaxies in the Standard Cosmological Model: Results From the Bolshoi Simulation" include Anatoly Klypin, Sebastian Trujillo-Gomez and Joel Primack. Authors of "Galaxies in LCDM With Halo Abundance Matching: Luminosity-Velocity Relation, Baryonic Mass-Velocity Relation, Velocity Function and Clustering" include Trujillo-Gomez, Klypin, Primack and Aaron J. Romanowsky.

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