The 4-Percent Universe

by Richard Panek

  Saul was going to be using their data to beat them at their own game.

  Extending the Type Ia search to higher redshifts, upon the completion of the nearby survey, had always been a possibility. Now it had become urgent. But they would have to move fast if they were going to collect enough distant supernovae of their own.

  Brian Schmidt was the beneficiary of an oil-money education: a high school in Alaska that hired PhDs to teach teenagers. Schmidt hadn't been the best physics student in his class; he reckoned that two other students were superior. But he was the one who wound up putting physics to use, and he attributed the difference to "passion." If he said he could write the code for a high-redshift supernova search in two months, then he would write the code in two months. And he did, sort of. He wrote some new code and patched in some existing code from Phillips and Suntzeffs CCD observations of 1986G, and on the basis of the resulting test "data," he and his team received three runs of two nights each on the 4-meter telescope at Cerro Tololo early the following year, in February and March of 1995. By that time, Schmidt was in the process of moving to Australia with his wife and their three-month-old—he would be living in Canberra and working at the Mount Stromlo and Siding Spring Observatories—and he didn't have travel money. But he figured he could manage the observations with the help of on-site collaborators and the Internet.

  That's when he had his first epiphany: Test data ain't the real thing.

  In February 1995 the team began taking the reference images—the initial images that they would subtract from later images. Sometimes the software wouldn't run. When it did run, Schmidt found himself trying to debug software and download images on a 100-byte-per-second link. Often he would have to guess what the problem was and do minuscule modifications of code, which his collaborators in Chile would implement, and then inevitably they'd all be talking on the phone, Cambridge and La Serena and Canberra, and Schmidt would wind up staying up all night while his wife, caring for the baby, hovered nearby. Finally Schmidt instructed his collaborators in Chile to send a set of data to him on tape so that he could examine it himself.

  It never arrived. Somewhere between Santiago and Siding Spring, it vanished.

  That's when he had his second epiphany: From now on, I will always go to Chile.

  "From now on," though, presupposed that they would find a distant Type Ia. The first night of follow-up observations, February 24/25, was clear, but the "seeing"—the term astronomers use for atmospheric conditions—was poor. The second night, March 6/7, had excellent seeing and produced six candidates, but on closer inspection none turned out to be a supernova. The third night, March 24/25, was, except for one brief period, overcast.

  In the Observing Proposal that the team had submitted the previous September, Schmidt and Suntzeff had written, "Based on the statistics of discovery from the Calán/Tololo SN survey, we can expect to find about 3 SNe Ia per month." Surely, they told themselves now, they should find at least one. Yet they were heading into their final night, and they'd gotten nothing. Were they doing something wrong? Did the software harbor a glitch? Or had they just been unlucky?

  On the final night, March 29/30, a 16-pixel-by-16-pixel smudge of light squeezed through the Internet pipeline between Chile and Australia. Schmidt squinted at it, then squinted at it some more, but he couldn't be sure. He picked up the phone and called Phillips and Suntzeff and told them to take a look and tell him what they thought. They took a look and agreed that it sort of seemed like a supernova, maybe. But they couldn't know for sure until they got the results of the crucial follow-up spectroscopy, from the 3.6-meter New Technology Telescope in La Silla, Chile.

  Bruno Leibundgut—whose talk in 1989 had inspired the earlier survey—was in charge on Sunday, April 2. Late that night he called Mark Phillips with a bad-news/good-news report.

  First the bad news. "It's very faint," he said.

  But the good news was that at least the smudge was still there, indicating that it was indeed a supernova.

  So: They'd been merely unlucky.

  The team continued to sort through the data from the March runs, and by Wednesday of that week they had found that the candidate supernova from the final night had also appeared on the last field from the third night of observing, just before the clouds closed them down. "This is very encouraging," Phillips e-mailed the team.*

  Thursday morning, Chile time, Schmidt sent out a progress report to all the members of the collaboration. He suggested they start thinking about submitting the observation to the International Astronomical Union circular, a standard procedure. He also suggested that they start thinking of what to call the collaboration—"a catchy name (or at least an accurate one if we cannot be catchy)." They were moving forward as a team, though he still wished, he added, that they had a redshift for the galaxy.

  That evening, they did. Mario Hamuy had examined the spectra that Leibundgut had obtained four nights earlier and reported the result to Phillips, who relayed it to the rest of the collaboration: The host galaxy, and with it the supernova itself, showed a redshift of 0.48, placing it at a distance of 4.9 billion light-years—and setting a new supernova record.

  They couldn't yet tell if it was a Type Ia, meaning that they didn't yet know if it would be useful in determining the rate of deceleration. Leibundgut would have to keep pounding at the data before they could say anything with confidence.

  But as Leibundgut wrote to Schmidt that day, "We are rolling again. What a change a supernova can make."

  They had beaten Saul at beating them at their own game.

  6. The Game

  POP! SN 1994F went off.

  Pop! SN 1994G went off.

  Pop! SN 1994H went off.

  The Berkeley team had hung numbered tags around the necks of the champagne bottles, "1" through "6," one for each supernova they had discovered during their most recent run at the Isaac Newton Telescope, December 1993 through February 1994, plus a "0" for their 1992 supernova. The members of the Supernova Cosmology Project—as they were now calling themselves—had gathered at Gerson Goldhaber's house in the Berkeley hills. They laughed at themselves for being "lightweights"; they probably couldn't finish even two bottles. But the champagne inside the bottles wasn't the point, of course; the number of bottles was. It had taken the team four years to get their first supernova. Now, it had taken them three months to get their next six.

  But they weren't celebrating only the supernovae. They were toasting their survival. Carl Pennypacker was no longer part of the team. Pushed? Jumped? Who knew? At least they still had a team. In the fall of 1994 the Center for Particle Astrophysics and the LBL Physics Division had convened a Project Review Committee to determine whether the supernova search should continue. Even when the committee decided favorably, Bernard Sadoulet cut the CfPA's contribution to the supernova budget by half and gave it to his own project; Robert Cahn, the head of the LBL Physics Division, then informed Sadoulet that he was cutting LBL's contribution to Sadoulet's dark-matter experiment by half and giving it to the Supernova Cosmology Project. Not only was the SCP solvent for a change, but they had a guardian angel at LBL, a division director who understood why a physics lab might want to do a distant supernova search. And now they'd gone and gotten six more supernovae. They weren't just in the distant supernova game, they were the game.

  Pop! SN 1994al. Pop! SN 1994am. Pop! SN 1994an.

  The party didn't last long.

  For years Bob Kirshner, as a member of the supernova project's External Advisory Board, had been saying that the LBL collaboration didn't know what it was doing—that team members weren't taking dust into account or paying sufficient attention to photometry or concerning themselves with whether Type Ia supernovae were standard candles. He didn't seem to understand that for LBL such considerations were beside the point—or weren't yet the point. The team was just hoping to prove it could do what it was trying to do: detect supernovae distant enough to be useful for doing cosmology. Their early efforts were what team me
mbers called, on various occasions, "demonstration runs" that were part of a "pilot search" in an "exploratory program." Then when they did find the 1992 supernova, Kirshner's objections as referee on that paper held up publication until 1995, when a more sympathetic referee, Allan Sandage, approved it. Breezing into Berkeley from Harvard, Kirshner seemed oblivious to the growing consternation, frustration, and anger not only at his objections but at him. A colleague of his on the External Advisory Board, speaking at a cosmology conference, characterized Kirshner's contribution to the discussion of the LBL approach: "No! This could not work! It couldn't possibly discover these high-redshift supernovae!"

  And now here Kirshner was, saying, Well, maybe.

  At least the LBL team had a six-year head start—surely that counted for something. Besides, their faith in Type Ia as standard candles had been rewarded. First Mark Phillips had demonstrated that the light curve for an inherently dimmer supernova falls off sooner—its descent is steeper—than the curve for an inherently brighter one. Then the Berkeley team had arrived at their own variation on his technique. They made Type Ia light curves uniform by treating them like images in a funhouse mirror—stretching them "fatter" or compressing them "thinner" until they fit an idealized Type Ia template. (The team often took advantage of an LBL photocopier that could distort images in precisely that manner.) If Type Ia supernovae were less a type than a family, then each member of that family was less a standard candle than a calibrated candle.

  And now, three years after proving to themselves that they could find a distant supernova, they had gone ahead and figured out how to find supernovae on a regular basis. After discovering the three in early 1994 on the Isaac Newton Telescope, they found three more with the Kitt Peak 4-meter telescope, in the mountains southwest of Tucson, Arizona. By June 1995 they had accumulated eleven distant Type Ia in total, and they were ready to make their first major statement to the community in the form of four papers at the NATO Advanced Study Institute Thermonuclear Supernovae Conference in Aiguablava, on the Mediterranean coast of Spain: They had figured out a way to discover Type Ia supernovae whenever they wanted.

  They called it the "batch" method. Just after a new moon they would make as many as a hundred observations, each image containing hundreds of galaxies as well as, if possible, clusters of galaxies. In one several-day run they could gather tens of thousands of galaxies. Two and a half to three weeks later, just before the next new moon, they would return to those same tens of thousands of galaxies. In an update of the old galaxy-by-galaxy blinking technique, computer software would subtract the earlier reference image from each new image, searching the hundreds of galaxies for the new dot of light that might signal the emergence of a supernova. Then, again by using new software, the astronomers could determine that same night whether that dot actually was a supernova. They could then relay those coordinates to team members waiting at other telescopes, who would perform the necessary spectroscopy and photometry using telescopes on which they'd reserved time months earlier. (Having a successful track record worked wonders with time allocation committees.) You might not know in advance exactly where a supernova was going to go off, but you knew that one or more would. In effect, they had figured out how to "schedule supernova explosions"*—just like that.

  Pop, pop, pop.

  So Berkeley had a six-year head start. So what? Schmidt and Suntzeffs team had astronomers—professionals who didn't need to learn how to do photometry and spectroscopy, who needed only to do them well and then to make improvements where necessary.

  A lot of Schmidt and Suntzeffs team had been at the NATO meeting, too. (The conference was organized by a former postdoc of Kirshner's.) The Harvard and Chile guys regarded the Berkeley team with some incredulity. "'I just heard about it, and I just thought about it, so—this is my subject!'" is how Kirshner characterized the SCP's attitude toward supernovae. SCP team members were talking about timing their observations to the new moon—as if astronomers hadn't been doing just that for thousands of years. The "batch" method? Radio astronomers were taking that approach in the 1960s. Supernovae on demand? José Maza was delivering that in the 1970s.

  Naturally, with all these scientists pursuing the same goal, meeting in the same place, there had been talk of a collaboration. But some of the members of Schmidt and Suntzeffs team left Spain with the impression that, as Kirshner said, "working together meant working for them." Why would one of the world's most knowledgeable supernova specialists want to be subordinate to Saul Perlmutter, Type Ia neophyte and ten years his junior? For that matter, why would any of these purebred astronomers put themselves in a position where they might be reporting to purebred physicists? Perlmutter was talking about how "rare," "rapid," and "random" supernovae were. And they were! But Schmidt and Suntzeffs team preferred to put the emphasis on "dimness," "distance," and "dust"—how to tell whether a supernova is intrinsically dim, dim because it's distant, or dim because of dust. While the physicists were worrying about how to find distant supernovae, the astronomers were worrying about what to do with the distant supernovae once they found them.

  Which they had (or, at least, a distant supernova). But they still couldn't be sure what type it was. The problem had been there from the start. In the same April 6 e-mail that Leibundgut sent to Schmidt celebrating the difference that one supernova can make, he also mentioned, almost as an aside, that "the 'supernova' spectrum still has a lot of galaxy in it"—that the light from the apparent supernova was difficult to separate from the light of the host galaxy. The spectrum could tell you the redshift of the galaxy, and therefore the redshift of the supernova residing in it. But in order to see the spectrum of the supernova itself, you were going to have to isolate its light.

  First Mark Phillips tried. One week after notifying the team of Hamuy's calculation that the supernova was the most distant ever, he was ready to give up. "I've spent too much time the last few days looking at this," he wrote the team. "The conclusion I've reached is that the SN spectrum is of such low S/N"—signal-to-noise, useful supernova light versus the optical equivalent of static from the galaxy—"that it is impossible to tell what type it is."

  Leibundgut tried next. And tried. "And no to the spectrum," he wrote in an e-mail at the end of May. "I have tried several ways of extraction but without any improvements." When he got to the Aiguablava conference, he told his collaborators that he, too, was ready to give up. "I don't know what to do anymore," he said. "I'm not sure I can confirm it's Type Ia."

  "Crap!" Suntzeff said. "Saul's pulling in supernovae by the handful, and we only have one, and we can't even tell if it's a Ia!"

  At one point Leibundgut was discussing the problem with Phillips in the lobby of the hotel. The sea was outside. They were inside. Phillips turned to Leibundgut and said, "Why don't you subtract the galaxy?"

  "Subtracting the galaxy" is just about the first thing you do if you're trying to get the spectrum of a supernova. If you want to isolate the supernova light, you take a spectrum from the part of the galaxy containing the supernova, which is flooded with light from the galaxy, and then you take a spectrum from a different part of the galaxy, away from the supernova, and then you subtract the second reading from the first. Ideally, the spectrum of the supernova itself pops out.

  This supernova, however, had been so overwhelmed by galaxy light that Leibundgut hadn't tried the obvious. Nobody had. From Aiguablava he flew to Hawaii for another conference, and then home to Munich. He fiddled a little with the overall galaxy light, dividing its intensity by ten. Why ten? No reason. The spectrum from the galaxy would still be the same; he wasn't changing the quality of the data. He was just changing its intensity. He subtracted this spectrum from the supernova spectrum (which also contained the galaxy spectrum), and out popped a beautiful supernova spectrum.

  "The spectrum of 95K looks great!" Phillips wrote him on August 1. "I'm now very convinced that this was a genuine type Ia."

  They were back in the game. Now what they needed was to f
ormalize their existence as a team.

  From the start—during their first discussions in La Serena, in early 1994—Schmidt and Suntzeff knew what kind of team they would want. Theirs wouldn't resemble a particle physics collaboration. It wouldn't have the same rigid top-down hierarchy, the same plodding bureaucracy, the same assembly-line mentality. Instead, their collaboration would follow a traditional astronomy aesthetic. It would be as nimble, as independent, as Hubble on Mount Wilson or Sandage on Mount Palomar.

  Already that approach had paid off. Like the professional astronomers they were, they had asked what they considered the key question first: Are Type Ia supernovae really standard candles? Only when they knew that Type Ia could be calibrated did they actually go looking for a distant one. And they almost hadn't found it. But in the end they did find their high-redshift supernova, and it was indeed a Type Ia. They'd salvaged their collaboration, and maybe their credibility, by making the discovery "on the smell of an oily rag in a quasi-chaotic fashion," as Schmidt liked to say. The process hadn't been pretty—it was, Suntzeff thought, more like "anarchy"—but it was astronomy.

  And yet, astronomy itself was changing. The traditional go-it-alone aesthetic was disappearing. The diversity of the science and the complications of technology were forcing the field into greater and greater specialization. You couldn't just study the heavens anymore; you studied planets, or stars, or galaxies, or the Sun. But you didn't study just stars anymore, either; you studied only the stars that explode. And you didn't study just supernovae; you studied only one type. And you didn't study just Type Ia; you specialized in the mechanism leading to the thermonuclear explosion, or you specialized in what metals the explosion creates, or you specialized in how to use the light from the explosion to measure the deceleration of the expansion of the universe—how to perform the photometry or do the spectroscopy or write the code. A collaboration could easily become unwieldy.