Supernovae remained attractive as potential standard candles for a couple of reasons. They're bright enough to be visible from the farthest recesses of space, meaning that astronomers can use them to probe deep into the history of the universe. And they operate within human time frames, their luminosity rising and falling over the course of weeks, meaning that, unlike most astronomical phenomena (such as the formation of a solar system or the coalescing of galaxies into a cluster), supernovae offer a soap opera that astronomers can actually watch.
But supernovae were also problematic for at least three reasons. As the LBL group put it, "they are rare, they are rapid, and they are random." In our own Milky Way galaxy, supernovae pop off at an average rate of maybe once a century. So astronomers pursuing supernovae have to devise a way to look at a great number of galaxies, whether individually in quick succession or in great gulps of the sky all at once—or, ideally, both: great gulps in quick succession. Supernovae also require a fast response. Once astronomers identify a supernova, they have to move quickly to do the necessary follow-up studies—not always possible when time on telescopes is assigned months in advance. And they're random. You never know where or when one's going to go off, so even if you could reserve follow-up time on telescopes months in advance, you wouldn't know whether you would have a supernova worth studying on that date.
Perlmutter and Pennypacker were already testing new subtraction software on the 60-inch telescope at Mount Palomar when they first heard that they weren't alone. The idea of chasing supernovae for clues about cosmology was half a century old, and now the widgets were out there to make that chase a reality, so they weren't entirely surprised that another group had been using distant supernovae to try to determine how much the expansion of the universe was changing over time.
Trying, and failing. From 1986 to 1988, three Danish astronomers, with the help of two British astronomers, took turns making a monthly trek to a 1.5-meter telescope at the European Southern Observatory in La Silla, Chile. They calculated that if they looked not at one galaxy at a time but at clusters of galaxies, they could beat the once-a-century-per-galaxy odds of finding the right kind of supernova. They selected clusters with well-established distances. And they timed their searches carefully, choosing the nights just before and after a new moon so that they were able not only to capitalize on dark skies but to compare images about twenty days apart, a period that, through happy coincidence, corresponds to the natural life (or, more aptly, death) cycle of the kind of supernova they wanted.
They found one. Possibly a second, on what would be their final night of observing, though they didn't bother to follow up that detection. They were ahead of their time, and, after two years, their time was up. Their telescope turned out to be too small for their purposes, their rate of discovery too low. Unless they could garner access to a larger telescope and a more powerful detector, they would need dozens of years to collect a suitable sample. Even at the rate of one good supernova a year, they would still need ten years, minimum, to complete their program.
For the members of the Berkeley supernova team, the news of this failure raised a potentially fatal question: How could they reassure the review committees at the Center for Particle Astrophysics that their group could succeed where the Danish collaboration had failed?
First, the team stressed that the Danes had succeeded in finding a distant supernova—so distant that it broke the redshift record for a supernova, 0.31, meaning that it exploded about a quarter of the way back to the beginning of the universe (or 3.5 billion years ago). Second, the Berkeley team would be using better instrumentation. The Anglo-Australian Telescope in Siding Spring, Australia, had a 3.9-meter mirror, more than double the diameter—or four times the aperture, the light-collecting area—of the one the Danish team had used in Chile. And LBL was commissioning a much larger camera.
For insurance, Pennypacker invited Gerson Goldhaber onto the project. In 1933, when Goldhaber was nine, his family fled Germany. He lived in Cairo and then Jerusalem before emigrating to the United States to attend graduate school at the University of Washington. Goldhaber had worked at LBL since 1953, playing key roles on the Bevatron and then, for the past twenty years, collaborating with the Stanford Linear Accelerator Center. He had made important discoveries. He had guided teams that won the Nobel in Physics. As Pennypacker reasoned, "They would never shut down anything he did."
Problems began even before they could start observing. The contractor constructing the camera delivered a mirror that didn't fall within "tolerances," as opticians call the allowable imperfections. The second cut was spoiled when cleaning fluid spilled on the mirror. Finally, the third cut of the mirror worked.
Pennypacker, however, had ordered a camera without a filter, figuring that the more light he got, the better—and not understanding that if you want to compare the brightness of an object on different days, you need to observe in different filters in order to "equalize" the light level. After the crack technical crew at LBL designed an after-the-fact filter wheel, Pennypacker handed it to a graduate student, sent her to Australia, and provided her with the number to give to customs. When she got to customs, she went up to the two clerks and said, "I have this number." They looked at each other, then back at her. "Number for what?" one of them said. (A few days later the director of the observatory in Australia managed to convince the authorities to unconfiscate the wheel.)
Even when the team did perform supernova searches, the subsequent logistics were daunting. The on-site computers didn't have sufficient bandwidth for the Berkeley project's purposes, so team members had to take the computer data to the Sydney airport, fill out volumes of paperwork, and put the cargo on the next plane to San Francisco. There, someone else had to fill out more paperwork before claiming the package and driving it across the bay to Berkeley. Total travel time: forty-eight hours. Then the physicists at Berkeley needed another two days to search the images for supernova candidates, and then another day to study finding charts—maps that show all the known objects in a section of the sky—to see if they really were supernovae. Five days is a long time to wait if you want to schedule follow-up observations of an object that is fast disappearing. Before long, however, they managed to figure out a way to get their data to Berkeley without air travel: A team member at LBL would call up NASA, specify when the supernova search would need to be transmitting data from Australia, and ask if someone at NASA would please turn on the Internet then.
Not that the quality of the data or a delay in transmission mattered. In the course of two and a half years, Pennypacker and Perlmutter secured a dozen nights on the Anglo-Australian Telescope; of those, nine and a half were cloudy or had poor atmospheric conditions, or were needed for testing. And although they did manage to identify six candidates, the final count of actual supernovae was worse than the Danes': zero.
The Berkeley team had come up with an ambitious three-year plan, and they'd executed it. They had stretched the existing technology as far as they could under the circumstances, tweaking each widget until it had nothing left to give, and their efforts simply weren't enough.
Every few months the supernova search had to justify its existence as part of the Center for Particle Astrophysics to an internal Program Advisory Committee. Every few months it also had to justify its existence to an External Advisory Board. That justification now took the form of the team's ability to secure time on the 2.5-meter Isaac Newton Telescope, in the Canary Islands, off the northwest coast of Africa. The telescope was slightly smaller than the one in Siding Spring, but the camera would be bigger, and the weather would be better. Muller himself made the pitch to Bernard Sadoulet, the director of the center: "Look, two or three years from now, we will be delivering supernovas. We will be making real measurements. We will have results. I guarantee that. By the time the initial funding for the Center for Particle Astrophysics runs out, we will have something real to show. And you must understand that. You must know that."
The supernova s
earch got its reprieve. If you were a veteran of the project, you could consider yourself on probation yet one more time. Newcomers, however, wondered what they'd gotten themselves into. Group meetings were held in an office where there weren't enough chairs. You might find yourself sitting at a computer, quietly typing away, when a higher-up would tell you that the project had exceeded its computer allotment and to either shut down the computer now or someone would be along shortly to pull the plug. One graduate student read the recruitment brochure, liked the idea of "weighing the universe," and committed to the project—only to learn that the search had yet to produce one supernova. A postdoc arrived for his first day on the job to find a note on his desk from Perlmutter, saying that he'd gone to the Canary Islands and asking the postdoc to use a finding chart to choose fields for Perlmutter to target. The postdoc stared at the note. He had trained as a particle physicist; he didn't know what a finding chart was.
And then Pennypacker—his words—"blew up the budget." He had developed a habit of spending money he hoped would materialize in the next round of budgets. This time, however, he misread a ledger, spent money he didn't have, and then spent it again.
By his own admission, Pennypacker wasn't leadership material, at least not of the kind required to run an unorthodox and high-risk project. People loved collaborating with him; he was enthusiastic and affable and smart and visionary. The ability to translate those virtues into the words that a review committee or an administrator wanted to hear, however, eluded him. So did a fundamental understanding of administrative details. If the supernova project were to continue, he was made to understand, it would have to do so under different leadership.
Robert Cahn, the new director of the Physics Division at LBL, first approached the senior researcher on the project, Gerson Goldhaber. But for Goldhaber the chance to work on supernovae had represented a freedom from the kind of responsibilities he'd held for four decades at behemoth particle accelerators. Muller had moved on. The next choice was Saul Perlmutter. Cahn consulted with Muller: Was the kid ready? Muller thought maybe so. Twice, Muller said, he'd had the experience of going to Perlmutter with what he thought was a conceptual breakthrough, and Saul had said, "Ah! That's a very interesting idea," then pulled out a notebook and flipped to the page where he'd already seen the idea through and found that it didn't work.
In March 1992, Perlmutter went to the Isaac Newton Telescope to take the first set of images. In late April and early May, Pennypacker went to the INT to make follow-up observations of the same fields while Perlmutter stayed in Berkeley and waited for data to arrive via the BITNET. As the sun was setting over the Atlantic—midafternoon in Berkeley—Perlmutter and a couple of team members would settle into the seats before the high-quality image display in the deliberately overcooled computer center in the basement at LBL, bundled in sweaters and jackets, and start to sort through the software. By ten or eleven in the evening, Perlmutter was alone and the images would begin to emerge on his screen. Each image held hundreds of galaxies; by the end of the night he would collect dozens of images. He printed out each one, just in case. Sometimes the computer told him that a blip of light had appeared that hadn't been there the previous month, and he would bend close to the screen and try to figure out what was wrong. The view of the wide-field camera distorted the geometry, so he didn't trust blips near the edge of the frame. Sometimes a blip would be too near the center of a galaxy, meaning that its light would be impossible to distinguish from the background light during follow-up observations. Sometimes the blip turned out to be an asteroid. One night he found a blip that he couldn't discount, and he had to wonder what obvious error he was overlooking, but he couldn't think of one, so he asked himself what subtle error he was overlooking, but he couldn't think of one, and then he wondered what he was doing wrong, when suddenly he realized, "Wait—this is what we're supposed to be looking for": the potential supernova you can't throw away.
For corroboration he had to wait until his collaborators showed up in the morning, and even then it wasn't a moment for celebration, partly because they couldn't be absolutely sure, and partly because the work had just begun. The data was worthless without follow-up observations that would tell them whether it was indeed a supernova, and if so, at what redshift.
Some of those observations they could do on their own, in the days that followed, while they still had time on the INT. Others required them to find out which astronomers were observing on the big telescopes around the world, figure out whether anyone in the LBL operation might be friends with them, then phone them in the middle of an observing run they had been planning for six months or a year, to plead with them to drop everything and point their telescope somewhere else. In this regard, Perlmutter was singularly talented. Nobody worked the middle-of-the-night phone calls to astronomers on other continents like he did. He was persistent, and he was persuasive, and he was impervious to rejection or insult. He literally wouldn't take no for an answer. Sometimes the plea elicited a laugh, sometimes an outburst of anger. But sometimes the plea elicited data—just enough to tell them that the blip was still there, and fading. They had a supernova.
Still no cause for celebration. Again, the data was meaningless for cosmology unless they knew how distant the supernova was—its redshift. For that, they would need a spectroscopic analysis. Twelve times, at four observatories around the world, astronomers agreed to make the follow-up observations. Eleven times the weather didn't cooperate. The twelfth, the instrument malfunctioned.
As spring stretched into summer, Pennypacker began to think of his team as characters in The Treasure of the Sierra Madre: fortune hunters wandering the desert in search of gold. And they find it—the prospectors discover their vein; the astronomers detect their supernova. And then the gold dust slips through their fingers and blows away in the wind. Walter Huston or Humphrey Bogart or Tim Holt says Thanks anyway to a pal in a faraway observatory, then slowly hangs up the phone.
One night in late August, Pennypacker and Perlmutter called Richard Ellis, a friend to the team as well as a British veteran of the Danish observations in the late 1980s. Ellis snapped at them. Didn't they know that observing conditions in the Canary Islands had been bad lately, and that he and the other observers were already inundated with requests for make-up observations from astronomers who had actually had time on the telescope—unlike the Berkeley team?
Then he went and made the observation. On August 29, 1992, Ellis took out his finding chart and, working on the 4.2-meter William Herschel Telescope, he and a postdoc took two half-hour spectra. When they were done, Ellis got Pennypacker on the phone in Berkeley.
The old record redshift, the one set by the Danish team, had been 0.31, corresponding to roughly 3.5 billion years ago. The new record redshift was 0.458, or 4.7 billion years ago.
Pennypacker let out a whoop. Six years after he and Perlmutter had discussed collaborating on a search for cosmologically significant supernovae, they hadn't found the weight or the shape or the fate of the universe.
But they were in the game.
5. Staying in the Game
IN EARLY 1994, a couple of astronomers got to talking. Brian Schmidt had just completed a doctoral thesis on supernovae at Harvard's Center for Astrophysics, and he was thinking about ideas for his next project as a postdoc. Nicholas Suntzeff had been an astronomer at the Cerro Tololo Inter-American Observatory in Chile since 1986, and he had been working on a supernova survey since 1989. As supernova specialists they had both been following the efforts of Berkeley's supernova project. Now, as they sat in the air-conditioned computer room at the observatory headquarters in the Chilean coastal town of La Serena, Schmidt mentioned that he'd been thinking about putting together a team to go up against LBL's.
Suntzeff didn't hesitate: "Can I be part of that?"
Now that, Schmidt thought, is the mark of a good problem in science. It's not when people say, "Oh, that's interesting." It's when they say, " Ooo, can I be part of that?"
Schmidt had to give Saul Perlmutter and the Berkeley team credit. They had seen that, thanks to advances in technology, supernovae might finally be used to do cosmology, and they were succeeding against enormous odds. They had been in the right place at the right time. But were they the right team?
Like many other astronomers, Schmidt had been skeptical that physicists—even physicists-turned-astrophysicists—would be able to consistently find distant supernovae. But even after the LBL team had found its first supernova, Schmidt and other astronomers remained skeptical that physicists-turned-astrophysicists—no matter how brilliant—could perform the kinds of follow-up observations and analyses that routinely strained even their own hard-won expertise. Seemingly everybody in the supernova game had been on the receiving end of a middle-of-the-night phone call from Saul, asking them to drop everything and perform a follow-up observation of a supernova candidate. Perlmutter had gotten a reputation in the community for being preternaturally persistent. But in Suntzeff's experience, every time he slewed his telescope to Perlmutter's target, the field was empty. "Must be too faint," Suntzeff would say diplomatically.
Schmidt and Suntzeff grabbed the nearest blue-and-gray sheet of IBM computer printout, flipped it over, and began scribbling. They continued the conversation in Suntzeff's office later that day and the next day as well, laying out their plan of attack.
The 4-Percent Universe Page 9