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.

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 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 Lab.

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.