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Mercury, January/February 2003 Table of Contents

Type Ia Supernova 1998bu. Courtesy of Nicholas Suntzeff (NOAO), et al.

by Jennifer Birriel

Astronomers have pinned their conclusion of an accelerating universe on observations of Type Ia supernovae. But truth be told, they don't even know for sure what causes these explosions.

In 1929, Edwin Hubble stunned the world with the announcement that galaxies are rushing away from one another, which implies that our universe is expanding. This news came much to the chagrin of Albert Einstein, whose general relativity equations predicted cosmic expansion. Einstein had firmly believed that the universe was static and had even added a fudge factor, dubbed the cosmological constant, to make his equations yield a static universe. After Hubble's announcement, Einstein publicly declared the cosmological constant to be the "greatest blunder" of his scientific career.

The cosmological constant fell into and out of favor over the next several decades, revived on occasion to solve some cosmological conundrum, only to fall into disfavor again. Ultimately, astronomers expected that gravity would slow the expansion of the universe.

But in 1998, the cosmological constant gained a new, almost celebrity status. Two groups of astronomers -- the High-Z Supernova Search Team led by Brian Schmidt and the Supernova Cosmology Project led by Saul Perlmutter -- presented strong evidence that our universe is expanding at an ever-increasing rate. The two teams based their accelerating universe conclusion on observations of distant Type Ia supernovae. These recent results represent the first convincing evidence for the existence of the infamous cosmological constant -- a cosmic "anti-gravity" that will ultimately drive apart all the galaxies in the universe to infinite distances apart at speeds greater than the speed of light. So far, cosmic acceleration has withstood the critical scrutiny of the astrophysical community.