"Breakthrough of the Year"
NERSC and the Fate of the Universe:
Science Magazine Names Supernova Cosmology Project "Breakthrough of the Year"
By Jon Bashor, jbashor@lbl.gov
January 4, 1999
When the National Energy Research Scientific Computing Center (NERSC)
moved to Berkeley Lab in 1996, a computational science program was created to encourage
collaborations between physical and computer scientists. The Supernova Cosmology
Project's
work was one of the first projects funded; it demonstrates how high-performance
computing
can accelerate scientific discovery.
With the recognition by Science magazine of the Supernova Cosmology
Projects scientific breakthrough, along with other collaborations, Berkeley Lab
establishes itself as home to one of the leading computational science centers in the
country.
"This summer we burned lots of time on the T3E," says team member Greg
Aldering of NERSCs contributions. "They gave us help developing our
algorithms,
and they gave us confidence in our methods."
The Cray T3E was particularly important, Aldering says, "because we spent a lot
of
time doing fits." To analyze their data from 40 supernovae for errors or biases,
the
team used the 512-processor Cray T3E-900 supercomputer to simulate 10,000 exploding
supernovae at varying distances, given universes based on different assumptions about
cosmological values; these were then plotted and compared with real data to detect any
biases affecting observation or interpretation.
"One thing we needed to establish about our model -- and did establish -- is
that
the mass of the universe couldnt go negative," says Aldering.
A completely separate line of inquiry, but one essential to the Supernova Cosmology
Projects search method, was to study the characteristics of type Ia supernovae. To
make meaningful comparisons of nearby and distant type Ias -- in other words, to affirm
their usefulness as standard candles -- the light measurements from the more distant
supernovae, with larger redshifts, were compared with the redshifts of closer ones.
These
measurements were then altered slightly to examine the effects of dust along the
line-of-sight, and to test slightly different explosion scenarios. These simulations
were
compared with the team's observations to make sure these matched their theoretical
calculations.
Because the real measurements involved readings taken many times over a 60-day period
from 40 supernovae, making the comparisons "is a task you only want to send to a
supercomputer," says Berkeley Lab postdoctoral fellow Peter Nugent.
Nugent, who ran all of the simulations and analyses on the T3E for the project, said
the Cray supercomputer was also used to make sure that the error bars presented in the
research were reasonable. In addition to chi-square fitting, researchers also employed
bootstrap resampling of the data. Here they plotted the mass density of the universe and
the vacuum energy density based on data from 40 supernovae. Then they began resampling
the
data, taking random sets of any of the 40 supernovae and finding and plotting the
minimum
value for each parameter. The resampling procedure was repeated tens of thousands of
times
as an independent check on the assigned error bars.
"Currently this work takes about an hour using 128 processors on the T3E,"
Nugent says. "It's wonderful to be able to run six or seven of these in just one
day
and then compare the results."
Those results include the designation by Science of research revolutionary
in
its field. In addition, Supernova Cosmology Project team leader Saul Perlmutter was also
honored with an invitation to address the recent supercomputing conference SC98,
sponsored
by the Institute of Electrical and Electronic Engineers, where he discussed the melding
of
cosmology and computational science at Berkeley Lab.
Science magazine names
Supernova Cosmology Project "Breakthrough of the Year"
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