2 - The Michigan Daily - Tuesday, July 5, 2005
pp
Computing power helps remake universe
By Kingson Man
Daily Staff Reporter
Astronomy Prof. Gus Evrard likes to
think big. He regularly wraps his head
around mind-boggling numbers and
impossible distances, mentally and vir-
tually 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 uni-
verse 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 computercranked out 20 tera-
bytes 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-forward-
ing 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 imperfec-
tions, 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 - aboring cosmic soup.
Fortunately, this pattern of slight
imperfections isstill 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, includ-
ing 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 expect-
ed. 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 mea-
surements 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 regu-
lar matter - all the stuff we can see on
Earth - by more than five to one.
The astrophysicists chose dark mat-
ter to simplify things, as it only interacts
gravitationally, whereas visible matter can
lead to nasty complications like superno-
vas, 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 sim-
ulations have more or less paralleled the
rise of computing power and Moore's law,
doubling every 16 months. The Millen-
nium Run actually outperforms this trenc
of improvement by a notable margin.
A baby Millennium Run is in the works
though.
"We'rererunningasimulation withvis-
ible matterbuilt into it, but it's not goingto
be as high-resolution a run," said Evrard.
And if the rate of increase of computin
power keeps up, scientists hope to run -
similar simulation with detail down to the
level of individual stars.
Evrard is as comfortable with the
extremely large as with the extreme-
ly 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 oth-
erwise-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.
Rich clusters of galaxies and light distribution can be seen in this
large-scale simulation of the universe.
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