2002: Features (print version)
Inside the Box
the universe's 13 billion years sheds light into the heart of its darkness.
Sharla A. Stewart
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.
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.)
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.
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."
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.
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.
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."
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
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.
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.
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.
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.
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
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."
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?
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."
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.