The discovery is Nobel-worthy—the universe is expanding faster and faster, driven by a mysterious force called dark energy. But who deserves the credit?
“Your job as a scientist is to figure out how you’re fooling yourself,” Saul Perlmutter declares. The famed astrophysicist is sitting in the cafeteria at Lawrence Berkeley National Laboratory, eating a falafel. Normally he talks at a machine-gun pace, but his speech, between bites, is measured. He glances out a big picture window toward the Berkeley hills and the fog-veiled universe beyond. “Our brains are … so good at seeing patterns that we sometimes see patterns that aren’t there.”
Perlmutter and his colleagues have spent two decades looking for patterns in the night sky—specifically, patterns in the spatial distribution of distant, dying stars that suddenly brighten, and then fade. They hope to resolve an ancient puzzle: How will the universe end? Eleven years ago, in the autumn of 1997, they uncovered a big piece of the puzzle. But their discovery was so unexpected that they worried the patterns were illusory. They checked and rechecked their data, searching for some subtle error that might have misled them. A mistake would make them look like fools. But if they waited too long to report their results, rival teams might beat them to announcing the discovery and perhaps to winning a Nobel Prize.
Their shocking discovery was “dark energy,” a mysterious repulsive force that apparently makes the universe expand faster and faster over time. Dark energy now threatens to undermine fundamental beliefs about physics, cosmology, perhaps even the nature of scientific discovery.
Ironically, Albert Einstein, who foresaw so much of modern physics and cosmology, indirectly anticipated the discovery by Perlmutter and his team. In the 1910s, Einstein proposed his general theory of relativity, which attributed the gravitational tug of masses like Earth and the Sun to “warps” or curvature in what he called space-time. But he was annoyed to discover that the theory contained a hidden implication—the universe, which appeared at the time to consist of a single galaxy (the Milky Way), is either contracting or expanding. Nonsense! Anyone could look at the night sky and see that the cosmos was neither expanding nor contracting. So Einstein eliminated the implication by adding to his equations what he called “a slight modification,” the infamous “cosmological constant.” It was a kind of anti-gravitational force that, he thought, would counteract any tendency of the universe to expand or contract.
But by 1929, using evidence from a powerful new telescope, Pasadena astronomer Edwin Hubble discovered that the universe is in fact expanding. Other galaxies are retreating from the Milky Way. Red-faced, Einstein realized that if he had trusted his original equations, he could have predicted the discovery of the expanding universe. He abandoned the cosmological constant, denouncing it as his “greatest blunder.”
Based on his discovery, Hubble proposed an exciting project—an effort to forecast the long-term fate of the universe. Would it continue expanding forever? Hubble suggested that astronomers answer the question by, in effect, weighing the universe. He wanted scientists to determine the overall mass and gravity of the cosmos by using telescopes to map the distribution of galaxies (mass) across the universe. He assumed three possible fates for the universe, depending on how much mass it contained. In the first scenario, the universe is a heavyweight—massive and jam-packed with galaxies. These galaxies exert so much gravitational tug on each other that the expansion rate is quickly slowing and will eventually reverse and the cosmos will collapse like a botched soufflé. The second possibility is that the universe is a middleweight. In that case, expansion will eventually slow to a crawl, but not contract. In the final scenario, the universe is a lightweight with relatively few galaxies. Gravity isn’t nearly strong enough to overcome the explosive energy of the Big Bang, so expansion will continue forever.
Hubble’s goal proved much harder to achieve than expected. It was difficult to map the distances and distribution of galaxies with any reliability. Well into the 1990s, scientists were reporting widely divergent values for the present rate of cosmic expansion, the so-called Hubble constant, which is determined by analyzing how light from retreating galaxies shifts toward the red end of the spectrum. (The “redder” the shift, the faster the rate of retreat.) Still, astronomers remained convinced that if and when they found the solution, it would fit into one of Hubble’s three scenarios.
To find convincing evidence for any of the expected options would have been exciting and philosophically fascinating, Perlmutter says. “What I didn’t expect was that the answer would be None of the above.” Thanks to him, and other pioneering scientists, astronomers opted for a fourth option: The cosmic expansion rate is accelerating. Contrary to what almost everyone assumed in the 20th century, the fate of the universe isn’t decided solely by mass and gravity; they play second fiddle to dark energy, whatever the heck that is. Although it resembles Einstein’s cosmological constant, instead of holding the universe together, the mystery force is blowing it apart.
It is hard to imagine a scientific revolutionary who looks and acts less like a rabble-rouser than Saul Perlmutter. Now nearing 50, he’s an amiable, polite, bespectacled man with thinning hair. Author-astrophysicist Donald Goldsmith described him as “Woody Allen with a Ph.D.” Perlmutter was born in 1959 in Champaign-Urbana, Illinois, and grew up in Philadelphia. His father was a professor of chemical engineering at the University of Pennyslvania, and his mother was a professor of social work at Temple University. Their household was filled with talk of politics and arts. Perlmutter learned to play the violin and loved to sing. After entering Harvard as an undergraduate, he considered jointly majoring in physics and philosophy, but realized “there’d be no time for anything else.”
Still, when Perlmutter arrived at Berkeley as a graduate student in physics in the early 1980s, he hoped to do research “that would address a deep philosophical question.” His doctoral adviser was physicist Richard A. Muller, who was planning to use robotic telescopes to look for supernovae and a hypothetical star called Nemesis, which Muller suspected triggered mass extinctions on Earth by steering comets toward the inner solar system every 26 million years. Perlmutter joined that project, where physicist Carl Pennypacker was developing a robotic telescopic search at Berkeley’s Leuschner Observatory in Lafayette. Over the next few years, their hard-working robot observer detected 20 “nearby” supernovae. Although the mystery star was never found, the supernova investigations opened the long, winding road to a historic discovery.
Perlmutter and others began trying to map the distances to supernovae of a type known as “Ia,” which occur when a white dwarf star draws too much mass from its orbiting companion and explodes from the pressure. Type Ias are among the brightest supernovae, so they’re visible from many billions of light years away. Unlike other supernovae, nearly all type Ias generate roughly the same amount of light. In theory, that makes them ideal for measuring distances to faraway places in the cosmos, because the apparent brightness of an object declines over distance by a predictable amount and can therefore be calculated with a simple equation. As a result, astronomers consider type Ias a “standard candle” for mapping the distribution of galaxies across the cosmos.
There were problems, though, as Perlmutter and his associates quickly learned. Some questioned whether type Ias really behave consistently. They pointed out that an astronomer who gazes upon the most distant type Ias is, in effect, gazing through a time machine—seeing the universe as it was billions of years ago. But how can we know whether the universe behaved back then as it does today? If primordial type Ia supernovae acted differently, perhaps they are not such trustworthy markers after all.
To answer the skeptics, Perlmutter set about finding and carefully comparing as many type Ias as possible. But they occur very rarely. None had been seen in our own galaxy for several centuries, and, in even other galaxies, the short-lived supernovae are hard to spot. Undaunted, Perlmutter and Pennypacker established a collaboration with British and Austrailian astronomers at the Isaac Newton Telescope in the Canary Islands. By accessing the telescope’s imagery via the Internet, Perlmutter was able to study new type Ia supernovae from his office in Berkeley. In 1992 they discovered a type Ia at redshift 0.46, from a time nearly halfway back to the Big Bang—a new record. Meanwhile, new scientists gravitated to his team, by then known as the Supernova Cosmology Project (SCP).
One was Gerson Goldhaber. Then in his late 60s, Goldhaber was already one of the grand old men of Berkeley and of world physics, the offspring of a distinguished family of three generations of researchers. In the 1950s, ’60s, and ’70s he had participated in discoveries of new subatomic particles. By adding Goldhaber to the team, Pennypacker says, they acquired a noted physicist who was “extremely well respected by everybody and extremely reliable.” With funding precarious, Pennypacker reasoned that “it would be much harder for the Lab to shut us down because Gerson was involved.”
Competitors also emerged. The most important was the High-Z Supernova Search Team (z is the astronomical term for galactic redshift), founded in 1994 at Harvard. High-Z member Adam Riess later moved west to the Berkeley astronomy department, where he joined ranks with supernova expert Alex Filippenko, formerly of the SCP team. Independently of Perlmutter’s group, Riess and Filippenko began their own search for type Ia exploding stars.
Goldhaber says he noticed something odd in 1997 while analyzing the team’s 20 distant type Ia supernovae. The objects tended to be dimmer than they should have been if cosmic theories were correct. “This worried me at the time, but not sufficiently, I’m afraid,” he says. Everyone had assumed that the large-scale distribution of type Ias would show signs of a cosmic deceleration as the galaxies gravitationally tugged on each other. Incredibly, his charts revealed the deceleration was negative. Eleven years later, Goldhaber recalls the discovery with enthusiasm: “Negative deceleration means acceleration!” The retreat of the galaxies is speeding up. Some mysterious force, a kind of anti-gravity, is counteracting the gravitational pull of cosmic matter.
After “many checks and rechecks” of his data, Goldhaber announced his analysis of 38 type Ia supernovae at the SCP team’s weekly meeting on September 24, 1997. He advised his colleagues to double-check his results: “I’ve been known to make mistakes,” he acknowledged—a comment that was italicized in the meeting notes. By November, he says, “everyone became convinced that this astonishing result was indeed correct.” In December, Perlmutter presented the team’s preliminary findings in speeches at Berkeley and Santa Cruz. The following year, in 1998, Science magazine named the discovery of the expanding universe “Breakthrough of the Year.”
But does it make sense to say that Goldhaber—or, for that matter, any single person “discovered” dark energy at a specific time? Members of the various teams disagree on who discovered what, and when. This is typical in science, as the often-numerous participants in a historic discovery review their actions and debate whose contributions were decisive. There’s plenty at stake—awards, for example. So far the many awards given to dark energy researchers have usually been distributed among figures from the High-Z and SCP teams. There is no doubt that Perlmutter’s team collected much of the data on what later turned out to be dark energy, but some members of other teams feel they also deserve credit for independently analyzing the data and because there was some collaboration among the groups. Even within Perlmutter’s group, there is disagreement over Goldhaber’s claim that he presented the first clear evidence for a cosmological constant, a.k.a. dark energy, in September 1997. To date, the controversy has been muted, in part because many of these scientists still work together.
The fact is, scientific breakthroughs often owe to the toil of dozens, hundreds, even thousands of workers, from Ph.D.s to lab techs, sometimes over many decades. Astronomy has long been seen as a “small science,” but that’s fast changing: As a discipline, it is increasingly dependent on big, expensive research projects bankrolled by the federal government. This autumn, for example, in a joint collaboration, NASA and the Department of Energy are expected to formally request proposals for a future satellite that will explore dark energy. Perlmutter and his colleagues propose building a satellite called SNAP, the Supernova Acceleration Probe. The project will be “Big Science” incarnate, costing hundreds of millions of dollars.
With colleague Michael Levi, Perlmutter took me to see a full-scale wooden model of the probe, inside a large machine shop on a hillside at LBNL. We donned construction helmets to enter the building. The model is a precise replica of SNAP, right down to its complex astronomical mirror and its half-billion-pixel electronic imager. Patting one of its struts, Perlmutter acknowledged that a few skeptics have cautioned that dark energy might be so subtle an effect that all efforts to explain it will fail. Despite its cost, there is no guarantee the project will reveal the underlying nature of dark energy.
There is no known physical explanation for dark energy—indeed, no one has a clue what dark energy is; it defies all existing paradigms. But Perlmutter defends SNAP and other projects that seek to understand it. “Until you look, you don’t know.” Still, he says skepticism is vital to scientific progress: “You spend 95 percent of your time looking for every possible way that you could be wrong…. That’s why you get the ‘Mr. Spock’ characterization of the unenthusiastic, wet-blanket scientist, because there is the absolute need to be your own worst skeptic.”
Because scientific progress now requires multiple and varied contributors, Perlmutter believes the question of who discovered dark energy and when is ultimately an empty one. Science is a group enterprise; and because groups change their collective minds more slowly than individuals, historic discoveries usually unfold over months or years, not moments. “There’s no ‘aha!’ moment,” he says. “It’s more like ahhhhhhhhhh … haaaaaaaaaa!”
