APRIL 2002: Features (print version)

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Thinking Inside the Box
>> Simulating the universe's 13 billion years sheds light into the heart of its darkness.

  Written by
  Sharla A. Stewart

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The movie doesn't take long -- 49 seconds, in fact. Yet in that short time all of existence unfolds on the computer screen: sky-blue pinpricks morph into brilliant blue filaments, glowing in a black expanse and contained, very neatly-and completely outside the realm of reality-within the three-dimensional parameters of a box.

The movie ends and then begins again, silently looping as Andrey Kravtsov, assistant professor of astronomy and astrophysics, explains what he's created: a high-resolution, fast-forward simulation of the entire 13 billion years of the universe-well, minus the first 100 million years, since the universe's earliest evolution is simple enough to be modeled on paper with linear equations, so there's no sense in devoting computational resources to simulating it. (The incandescent blue, on the other hand, seems to scoff at the observation reported this March that the universe is not even a pale turquoise but rather a go-with-everything beige.)

Like any good scholar, the 29-year-old Kravtsov begins by defining his terms. Pointing to a Z in the screen's upper left-hand corner, he explains with his Ukrainian accent, "Z is the redshift, a measure of time in astronomy." Because the speed of light is finite, the light now visible from galaxies was emitted long ago. And because the universe is expanding, this light is redshifted, meaning its wavelengths have shifted to longer, lower-energy, "redder" wavelengths. The greater the redshift, the further back in time the object we "see" existed. Kravtsov's simulation begins with 2 million particles at a redshift of about 30, or about when linear models begin breaking down, and ends at Z = 0. In lay terms, that's present day.

The catch is that Kravtsov's pinpricks and filaments can't, in fact, be "seen" at all. They represent nonbaryonic particles: dark matter. Which explains Kravtsov's color choice: the blue represents the "coldness" of dark matter. Unlike ordinary visible matter, dark matter is not made up of protons or neutrons and is thought to interact with ordinary matter only gravitationally. The absence of electromagnetic interaction between dark matter and baryons means that it does not glow the way that quasars, stars, supernovae-even people and trees, though in infrared-do. Astronomers now believe that nonbaryonic matter accounts for 85 percent of the matter in the universe. "So the evolution of the universe is actually dictated by dark matter," says Kravtsov. "We-the baryonic matter-just follow along."

His movie, one realizes, portrays the universe in the nude: what we'd see if we could don X-ray-er, enlightened-matter goggles-and look beyond the universe's flimsy baryonic clothes to watch the dark flexing of the powerful limbs and muscles beneath. And that's why Kravtsov's work is important: after he's written the equations and crunched the numbers, the simulated dark matter progresses along the redshift, with particles gravitating toward each other in ever increasing numbers, first creating galaxies and then clusters of galaxies and finally ending up in a configuration that remarkably resembles exactly where we-the baryonic matter-are today.

The scheme "works because it accounts for the structures we can see," says Bruce Winstein, the Samuel K. Allison distinguished service professor in physics, the Enrico Fermi Institute, and the College. "Dark matter, dark energy-we can't see these things in the lab or in the telescope, and yet they've become our two fundamental pursuits. So lacking the ability to observe them, we simulate them.

"This simulation," he continues, "wouldn't work without those two things in there. If we can figure out how the current structure came about, we can see what's moved it there. If the simulation had too much dark matter and dark energy, it wouldn't look the same."

Winstein, a particle-physicist-turned-astrophysicist, directs the seven-month-old Center for Cosmological Physics (CfCP), for which Kravtsov is among six new faculty recruits. (He comes from a Hubble fellowship at Ohio State University after earning a Ph.D. in astronomy and computer science at New Mexico State University in 1999.) Launched with a five-year, $15 million grant from the National Science Foundation, the CfCP is, according to its Web page, "devoted to exploiting the connections between physics at the smallest scale-interactions of the quarks and leptons-and at the largest scale-the constitution and birth of the cosmos itself."

Kravtsov's work marries the quests to understand the quark and the cosmos quite nicely. For the vast majority of the simulation, from Z = 30 to Z = 10, the matter seems to be distributed uniformly. But at the tiniest level there are irregularities, "primordial quantum perturbations" left over from the Big Bang that cause some particles to be ever so slightly closer to each other. These perturbations, says Kravtsov, are the "seeds of the structures" we see today.

"Gravity," he explains, "tends to make perturbations grow. As the density of matter in a perturbation increases, so does gravitational pull. Gravity thus leads to instability, and instability leads to growth of perturbations and collapse." That is how structures in the universe-filaments, clusters, galaxies, stars-form. And the collapse proceeds even as the universe expands. Moreover, it was recently discovered, the expansion is currently accelerating due to the presence of mysterious "dark energy," which dominates the energy density of the universe.

The trick to Kravtsov's simulation is that the box and the matter in it are "co-moving," or expanding with the universe so that the box appears to be the same size even as the universe expands. Kravtsov's simulation has one of the highest dynamic ranges among simulations of its kind-meaning that it's highly pixilated and therefore highly detailed, but only in the regions of interest: areas such as clusters and galaxies dense with matter.

"The question for us is always, What is the smallest scale you can follow compared to the size of the box?" he says. "In theory we would like to be able to push the algorithms to simulate the formation of stars." In reality, he's been able to "reach in" below a kiloparsec, or about 3,270 light years, roughly one-thirtieth the size of the Milky Way. The technique, called adaptive mesh refinement, is a trick engineers-specifically airplane designers-have used for 20 years, but it's relatively new to astrophysics.

After a year of creating the simulation's numerical code, Kravtsov spent a week running the code at the National Center for Supercomputer Applications in Urbana-Champaign, Illinois, churning the algorithms on 16 processors. The simulation's output was then compared against observed structures. His slimmed-down version is a 45-megabyte movie that runs on Windows Media Player.

In his office on Chicago's campus, Kravtsov double-clicks another movie file. The box rotates as the particles float toward one another, revealing from all angles the delicate beauty of the universe's filamentary structure. It looks exactly like the pulsating and branching blood vessels of the human body. "They actually have a lot in common," he agrees, "because they're both approximately fractal structures."

The structures freeze, and the question is itching to be asked: Well? What happens next? Will gravity keep pulling the Milky Way and Andromeda toward one another, as it has since Z = 3, until they collide-and if so, when will that happen? And when the box has gotten so big that nothing's close enough for gravity to pull it, what will happen then?

Kravtsov gives the pragmatic shrug of an experimental scientist: "Yeah, probably in 6 billion years they'll collapse. The simulation is designed for comparisons with actual observations, so to run it further would be a waste of computational resources."

He clearly doesn't see why you'd want to anyway-there's still too much to learn inside the box to think about straining its corners just yet.