Physics & Math / Cosmology

Photography by  Kaz Arahama

Corey S. Powell has been an editor at Discover since 1997. This article is adapted from his just-published book, God in the Equation. Copyright © 2002 by Corey S. Powell. Reprinted by permission of The Free Press, a division of Simon & Schuster, New York.

Saul Perlmutter darts around his modest office at Lawrence Berkeley National Laboratory, a cluster of drab buildings nestled in the hills above the University of California campus. With his edgy movements, shaggy hair, and Woody Allen-ish gestures, he could be mistaken for a computer programmer. But it's soon clear that these institutional-lab white walls and gray steel bookshelves— even the rolling landscape outside— are only a minuscule part of who he is. Riffling through a stack of journal reprints and computer printouts, Perlmutter fishes out an article titled "Measurements of Omega and Lambda from 42 High-Redshift Supernovae." During the past 10 years, working in step with a rival group of scientists centered at Harvard University, Perlmutter and his collaborators have peered to the far edge of what astronomer Edwin Hubble called "the dim boundary— the utmost limits of our telescopes." The results, summarized in this innocuous-sounding document, have rewritten the saga of the Big Bang. They offer both a new chronicle of how the universe has evolved and an unnerving prophecy of how it may end.

When he set out on his cosmic quest, Perlmutter was still in his twenties, full of improbable ambition. “It goes back to childhood,” he says. “I’ve always been interested in the most fundamental questions.” He began by studying subatomic particles, but by 1983 he was fed up with complicated physics experiments that took years to execute. He sought a different path to universal truth and found it in astrophysics.

German-born astronomer Walter Baade suggested the idea in 1938 as he worked at the Mount Wilson Observatory in California. Then as now, astronomers estimated the distances to galaxies by studying Cepheid variables, an unusual class of stars whose brightness rises and falls predictably: The longer the period of variation, the more luminous the star. But even the most powerful telescopes of the time could detect Cepheids only in a handful of nearby galaxies. Supernovas, in contrast, are so brilliant that they can be seen across the entire universe. Formed when a star self-destructs, supernovas exist for only a few weeks before fading away; but for those few weeks, they shine more brightly than a billion suns. If all supernovas are essentially the same, Baade reasoned, their light can be used as “standard candles” to reckon cosmic distances.

But the supernovas were not as standard as Baade hoped. He soon learned that some are much more luminous than others. If observers did not understand the nature of those variations, their distance measurements could be off by more than a factor of two. By the time Perlmutter began his quest, a number of researchers—among them supernova guru Robert Kirshner of Harvard—had identified that a class of exploding stars could light a path through such difficulties. Dubbed Type Ia, these supernovas form when middleweight stars like the sun grow old and burn out, leaving behind a white dwarf star. Normally, a white dwarf is stable. But if it has a companion star, it can grab material from its partner and keep growing more massive. Eventually, it hits a point at which gravity can no longer support its bulk. The star implodes, setting off a titanic thermonuclear blast.

There are so many galaxies—about 100 billion—that today’s

telescopes could detect a supernova every few seconds

Type Ia explosions have a distinctive light pattern, or spectrum, that makes them easy to identify. As luck would have it, they are also the most luminous supernovas. Perlmutter and his Berkeley Lab colleague Carl Pennypacker decided to see if these stars could, at last, provide the kinds of cosmological revelations that Cepheid variables could not. The two researchers persuaded a few graduate students and colleagues to help and in 1988 began the Supernova Cosmology Project. But they weren’t the only ones drawn to supernovas. Soon they would find themselves in a heated competition.

Supernovas are among the rarest of celestial events. The last one seen in our galaxy was recorded by Johannes Kepler in 1604, five years before Galileo turned his first telescope skyward. In any one galaxy, a Type Ia explosion lights up just once every 300 years or so. But on a cosmic scale, the numbers pile up quickly. There are so many galaxies in the universe—about 100 billion—that today’s largest telescopes could in principle detect supernovas every few seconds. The problem is where to look.

The detectors, known as charge-coupled devices, or CCDs, record every iota of light they receive as digital fields of ones and zeros. Perlmutter decided that if images were converted to digital data, they could be searched to find a single supernova in a field full of galaxies. He would begin by recording the light from a patch of sky. Then, a few weeks later, he would record it again and subtract the binary numbers in the first image from those in the second. If everything stayed the same, nothing but background noise would remain. But if something new appeared—if a star exploded and brightened—it would pop out immediately. That was the idea, anyway. In practice, nobody could make it work. Perlmutter spent long hours writing software to combine, clean up, and analyze the images. “A lot of time you think, ‘Boy, you’re spending your whole life on this stupid computer,’” he says, laughing.