Astronomy Prof. Gus Evrard likes to think big. He regularly wraps his head around mind-boggling numbers and impossible distances, mentally and virtually moving around places so quickly that not even light could keep up with him — all in a day’s work.

His mission: to figure out how the universe came to look as it does today. In order to do so, he must inhabit a world very different from the one we’re familiar with. His tool: computing power and lots of it. In his own words, Evrard says he “makes virtual universes in the computer.”

In last month’s Nature, the Virgo consortium, an international group that Evrard is affiliated with, unveiled the results of the most detailed simulation of the universe ever performed, from its early history to the present day. Dubbed the Millennium Run, the group linked 512 IBM processors in parallel and ran them at full tilt for 28 days in Germany. In the process the computer cranked out 20 terabytes of data, or enough to fill around one thousand iPods.

And only then did the post-processing of all that information begin. On other workstations, scientists combed the data for patterns of large-scale structures and other interesting formations like galaxies, clusters and local groups.

The Millennium Run followed the evolution of the virtual universe from a few hundred thousand years after the Big Bang, when it was still a nearly-uniform soup of hot matter, up to the present day, billions of years later. This fast-forwarding of the universe was achieved using “the N-body simulation technique,” Evrard explained.

That the early universe was only nearly uniform is of principal interest. Scientists believe that it was the few tiny imperfections, perhaps from the quirks of quantum physics, way back then that served as the seeds from which all later structure arose. Otherwise, our universe would still be a completely uniform one where matter couldn’t accrete — a boring cosmic soup.

Fortunately, this pattern of slight imperfections is still imprinted in the sky and can be detected in the microwave range of the electromagnetic spectrum. Other projects such as the Wilkinson Microwave Anisotropy Probe have peered into the deep past, and the Virgo consortium has put it to good use, including that pattern of perturbations at the start of the simulation.

Among its findings, the simulation revealed that large-scale structures arose much earlier than was previously expected. The strategy now is to corroborate its findings with actual surveys of the sky, such as the Sloan Digital Sky Survey, painstakingly conducted by taking measurements of the real sky.

The simulated universe was a cube, two billion light-years on each side, and filled with over 10 billion individual particles.

“The twist here is this simulation only models dark matter,” Evrard said. Each of the particles represented a uniform blob of dark-matter, that mysterious stuff that scientists believe outnumbers regular matter — all the stuff we can see on Earth — by more than five to one.

The astrophysicists chose dark matter to simplify things, as it only interacts gravitationally, whereas visible matter can lead to nasty complications like supernovas, X-ray jets and humans.

“It’s ironic that we can simulate better the part that we can’t see. That’s tragic as well, of course,” Evrard said.

The bottleneck was, is and will be raw computing speed. The physics behind it is sound. Since the 1970s, cosmological simulations have more or less paralleled the rise of computing power and Moore’s law, doubling every 16 months. The Millennium Run actually outperforms this trend of improvement by a notable margin.

A baby Millennium Run is in the works though.

“We’re rerunning a simulation with visible matter built into it, but it’s not going to be as high-resolution a run,” said Evrard. And if the rate of increase of computing power keeps up, scientists hope to run a similar simulation with detail down to the level of individual stars.

Evrard is as comfortable with the extremely large as with the extremely small.

“That’s what’s cool in this business, you’re working on the very largest scales, but the seeds of them were once tiny. You and I and everyone in Ann Arbor was once contained within a region smaller than the atomic nucleus.”

The Millennium Run website, http://www.mpa-garching.mpg.de/galform/millennium/, features movies, a fly-through of the simulation showing an otherwise-impossible trip of billions of light years in a few seconds and a breathtaking zoom-in from a godlike expanse down to the level of individual galaxies.

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