If you're trying to determine whether the universe will either
collapse or continue expanding - and you plan on announcing your
findings to the world - it's a good idea to cross-check your work.
That's what the Supernova Cosmology Team has done. And to analyze
their data from 40 supernovae for errors or biases, they used the
Cray T3E supercomputer at NERSC. Along the way, they used the machine
to simulate 10,000 exploding supernovae.
Our universe, which began with the Big Bang, will never come to
a standstill or collapse in a Big Crunch, but will expand forever,
according to findings announced earlier in January 1998 by Saul
Perlmutter, leader of the international Supernova Cosmology Project
and a member of the Center for Particle Astrophysics based at Berkeley
Using several ground-based telescopes plus, more recently, the
Hubble Space Telescope and a NERSC computer, the Supernova Cosmology
Project has determined that the universe was expanding faster seven
billion years ago (roughly half the time since the Big Bang) than
it is today. Although expansion has slowed, the deceleration is
not enough to suggest that gravity can bring outwardly rushing galaxies
and other celestial matter to a halt.
"On the basis of both the ground-based data and the new Hubble
data, we find evidence for a universe which may ultimately expand
indefinitely," Perlmutter said.
The evidence comes from observing Type Ia supernovae in very distant
galaxies. To look at a distant object in space is to look into the
distant past. To measure that distance, astronomers use "standard
candles,"objects whose intrinsic brightness is the same wherever
they are found. Type Ia supernovae at their maximum brightness can
be brighter than entire galaxies, bright enough for their light
to have traveled billions of light-years and still be visible.
While the evidence for the findings came from astronomical observations,
Perlmutter's team also used one of NERSC's T3E supercomputers to
double-check their work.
For example, the team must compare the light from nearby supernovae
with that of the distant ones. To make meaningful comparisons, the
light measurements from the more distant supernovae (which have
been shifted to the red part of the spectrum due to the expansion
of the universe) and the closer ones (which are in the blue) were
altered slightly to examine the effects of dust along the line-of-sight
to the supernovae and slightly different explosion scenarios. Then
they were compared to make sure the team's observations matched
their theoretical calculations. Because the 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 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, 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."
The group also used the T3E to simulate the explosions of 10,000
supernovae at varying distances, given a universe with a particular
cosmology, in an effort to study their observation techniques. The
cosmological values from the fits to the simulations were then plotted
and compared with their known input to determine any biases which
could have influenced the interpretation of the original data.
Finally, Nugent is tapping NERSC for help in preparing a paper
in which he and researchers from the University of Oklahoma compare
spectra from nearby and distant supernovae. They are studying whether
or not the environments in which the supernovae occur influence
how they explode. One theory holds that supernovae which exploded
several billion years ago, in metal-poor environments, may look
quite different from those which are used as calibrators, which
occur relatively nearby in more metal-rich environments.
So far, the results show not a lot of difference between earlier
and more recent events, Nugent said. The conclusion is that these
supernovae are good standardized candles for comparative measurements.