If you're trying to determine whether the universe will collapse or continue expanding -- and you plan on announcing your findings to the world -- it's a good idea to crosscheck your work. That's what the Supernova Cosmology Team has done. To analyze their data from 40 supernovae for errors or biases, and to simulate 10,000 exploding supernovae, they used the Cray T3E supercomputer at NERSC.
Their conclusion?
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 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 Lab.
Using several ground-based telescopes, the Hubble Space Telescope, and the NERSC T3E, 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 user "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.
Berkeley Lab astrophysicist Saul Perlmutter, with supernova 1987a in
the background. (Supernovae image: © Anglo-Australian Observatory, David Malin,
photographer)
To double-check their work, the supernova team had to compare the light from nearby supernovae with that of the distant ones. 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 as well as slightly different explosion scenarios. The measurements were then compared to make sure the team's observations matched their theoretical calculations. Because the measurements involved readings from 40 supernovae taken many times over a 60-day period, making the comparisons "is a task you only want to send to a supercomputer," said 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.
"This work takes about an hour using 128 processors on the T3E," Nugent said. "It's
wonderful to be able to run six or seven of these in just one day and then compare the
results."
The first two images, from an Earth-based telescope, show a small region
before and just after the appearance of a Type Ia supernova that exploded when the
universe was about half its present age. The third image shows the same supernova as
observed by the Hubble Space Telescope. Because their intrinsic brightness is predictable,
such supernovae help to determine the deceleration, and so the eventual fate, of the
universe. (Perlmutter et al., The Supernova Cosmology Project)
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 much difference between earlier and more recent events, Nugent said. The conclusion is that these supernovae are good standard candles for acomparative measurements. See http://panisse.lbl.gov/public/ for more information.