NERSC Plays Key Role in Science Magazine's "Breakthrough of the Year"

The discovery that the universe is expanding at an accelerating rate, and thus is likely to go on expanding forever, was named Science magazine's "Breakthrough of the Year for 1998." The international Supernova Cosmology Project, headed by Saul Perlmutter of the Berkeley Lab Physics Division, calculated how fast the universe was expanding at different times in its history by comparing the distance of Type Ia supernovae with the redshifts of their home galaxies. To analyze the data from 40 supernovae for errors or biases, the cosmology team used the Cray T3E at NERSC.

Perlmutter's team used ground-based telescopes plus the Hubble Space Telescope and the T3E to determine the universe's rate of expansion. The discovery was confirmed by the High-z Supernova Search Team in Australia, who shared the magazine's honor. In addition to showing that there is not enough mass in the universe for gravitation ever to stop the expansion, the discovery implies that an unknown property of space, called the cosmological constant and first proposed by Albert Einstein, is acting to expand space itself.

To analyze the supernova data for errors or biases, the Supernova Cosmology team used the Cray T3E-900 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.

To make meaningful comparisons of nearby and distant Type Ia supernovae--in other words, to confirm their usefulness as "standard candles," objects whose intrinsic brightness is the same wherever they are found--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 the data matched their theoretical calculations.

The T3E was also used to make sure that the error bars presented in the research were reasonable. In addition to chi-square fitting, researchers 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.