The 4-Percent Universe

by Richard Panek

  Despite its name, the Hubble constant wasn't a constant—a value unchanging over time. It told you only how fast the universe was expanding now—its current rate of expansion—and for this reason astronomers sometimes referred to it as the Hubble parameter. It told you nothing, however, about how much the expansion rate was changing over time. That value—Sandage's second number—astronomers called the deceleration parameter because it would tell you to what extent the universe was slowing down. From the Hubble parameter you could extrapolate backward into the past and, depending on the amount of matter in the universe, derive the universe's age. From the deceleration parameter you could extrapolate forward into the future and, depending on the amount of matter in the universe, derive the universe's fate. In that sense, there were only two numbers to measure in cosmology: the alpha and the omega of the universe.

  Both measurements would require a standard candle, and at the time that Suntzeff received his Carnegie Fellowship in 1982, Sandage (along with Gustav Tammann) had settled on supernovae. Sometimes Suntzeff and Sandage would be in Chile at the same time, at the Carnegie's Las Campanas facility, Sandage working on one telescope, Suntzeff another, and Sandage would ask Suntzeff to check whether a speck on a photographic plate was really a supernova. A dozen times Suntzeff swung the telescope to perform follow-up observations, and eleven times he had to break the news to Sandage: no supernova. In the end, Sandage figured out that he literally lacked the "proper equipment." The photographic plates were flawed. When he couldn't get Kodak to meet his exacting specifications for supernova searches, he abandoned the project.

  But by then Suntzeff himself had become intrigued by supernovae. On cloudy nights at the observatory he would retire to the library and catch up on the supernova literature, or seek advice from Uncle Allan, as everybody called Sandage. The time was coming for Sandage to pass down to the next generation the program that Hubble had passed down to him. He had lost some sight in his right eye, the one he had pressed to an eyepiece for four decades, and his sense of balance was off, a hazard on an observing platform dozens of feet off a concrete floor. Soon he would have to pack up his eyepiece, pocket his napkin ring, and come down from the mountain.

  Besides, Sandage could see that his way of doing astronomy was coming to an end. For the first two centuries after the invention of the telescope, astronomers had to rely on nothing other than the light that hit their eyes at any one moment, and then that light was gone. Astronomers could draw what they had seen. They could capture it in words. They could record measurements to designate the location of an object or describe its motions. But what they saw—the light itself, the visual representation of the object in a moment in time—was gone.

  The invention of photography in the mid-1800s radically changed that relationship between observers and their observations. For astronomy, photographs had an obvious advantage over the eye. A photograph preserved what an astronomer saw. It preserved the light itself, and therefore the image of the object at one particular moment. Astronomers could refer back not only to what they had drawn or captured in words or recorded as math, but to what they had actually seen. And then so could any other astronomer, now or in the future.

  But photography didn't just allow astronomers to collect light. It allowed them to collect light over time. Light didn't just land on the photographic plate; it landed and stayed there, and then more light landed and stayed there, and then more light. The sources of light were so faint your eyes couldn't see them, even with the help of a telescope, but the photographic plate could, because it was acting not like a moment-to-moment sensor but like a sponge. It could soak up light all night long. The longer the exposure, the greater the amount of light on the plate; the greater the amount of light, the deeper the view.

  But now the charge-coupled device, or CCD, promised to do for the photographic plate what the photographic plate had done for the eyeball. A CCD consists of a small wafer of silicon that collects light digitally; one photon creates one electrical charge. A photographic plate is sensitive to 1 or 2 percent of the available photons; a CCD can approach 100 percent. For any aspect of astronomy, the advantages were obvious. Digital technology meant that you could process the images with computers, and more light meant that you could see farther and collect data faster. But for supernova searches, as Sandage explained to Suntzeff, the CCD came with a bonus.

  The usefulness of a supernova for cosmology depends in large part on its light curve, a graph that shows the rise and fall of the luminosity of a supernova over time. Every supernova light curve rises abruptly over a matter of days as the supernova climbs toward maximum luminosity, then falls gradually as the supernova fades. But because each type of supernova releases its own distinctive cocktail of elements (hydrogen or no hydrogen, for example) and emerges out of a specific process (explosion or implosion), its light rises and falls in a signature pattern. To trace that pattern, you want to know when the curve peaks—when the brightness reaches maximum—so you need to be fortunate enough to discover the supernova on its way up. To chart the curve, you then need to make multiple follow-up observations—the more observations, the more data points you can plot on the graph; the more points, the more reliable the curve. But those observations are reliable only insofar as you can be sure how bright the light from the supernova is, and the accuracy of that measurement depends on how well you can distinguish the supernova light from the light of the host galaxy. A technology that allows you to make more observations and then quantifies those observations pixel by pixel can go a long way toward reducing the margin of error. The speed and precision of CCD technology, Sandage said, were going to make a light curve into a graceful, unambiguous arc—to the eye of a photometrist like Suntzeff, a work of art.

  Suntzeff was already familiar with CCD technology. When he completed his Carnegie Fellowship in 1986, he became a staff astronomer at the Cerro Tololo Inter-American Observatory (a division of the U.S. National Optical Astronomy Observatory) in Chile; he was recruited by Mark Phillips, a good friend from graduate school—they'd both been at the University of California, Santa Cruz, back in the 1970s. Suntzeffs first mission was to install a CCD on a telescope, and he teamed up with Phillips to test the equipment on supernova 1986G. Suntzeff would do the observing and photometry, and Phillips would do the comparisons with light curves from other supernovae.

  Suntzeff expected the result to be historic. As far as he and Phillips knew, theirs was the first "modern" light curve, meaning the first one obtained with a CCD. Historic though it was, the result was disappointing. The light curve for 1986G seemed to be significantly different from other Type Ia light curves. The supernova appeared to be fainter than it should be at its redshift, and the light curve looked as if it rose and fell more steeply than other Type Ia curves.

  Part of the problem with being a scientific pioneer is that you have a compromised historical sample. The only light-curve comparisons that Phillips and Suntzeff could make had to come from photographic plates. They didn't know whether their odd CCD light curve said more about Type Ia supernovae or about CCD technology. Still, the two astronomers were confident enough in their corrections to the data that they concluded, in a paper they published the following year, that Type Ia supernovae probably varied too much in luminosity to serve as standard candles.

  But as frustrating as the result was, Phillips and Suntzeff also sensed an opportunity. Their job would be to convince the community that Type Ia supernovae weren't standard candles—or to convince themselves that they had been wrong, and that Type Ia were standard candles after all. Either way, the two astronomers were in the supernova game now.

  Their timing couldn't have been better. On February 23 of the following year, 1987, a supernova went off right overhead. SN 1987 A appeared in the Large Magellanic Cloud, one of the few galaxies visible to the unaided eye—and only from the Southern Hemisphere. It was the first unaided-eye supernova since 1604, and among astronomers it prompted a worldwide viewing party. It wasn't a
Type Ia, the explosive kind of supernova that Phillips and Suntzeff had studied. It was a Type II, the implosive kind. Still, on the basis of their access to a CCD in the Southern Hemisphere and their co-authorship on the SN 1986G paper, they found themselves assuming the role of what they facetiously called "the local supernova experts."

  In July 1989, they attended a two-week supernova workshop at their alma mater, UC Santa Cruz. The topic for the first week was 1987 A, but since the workshop was sure to attract just about every supernova expert in the world—all fifty of them—the organizers added a second week on supernova topics other than 1987 A. By this point, virtually everyone at the conference had been working on 1987 A nonstop for more than two years. They had results: observations, interpretations, theories. But they also had core-collapse-supernova fatigue. What about explosive supernovae? What about Type Ia? The first week of the meeting would be for work; the second week, fun.

  Sometimes at conferences the most productive work happens in the hallway between sessions, or over a beer in the evening. For Suntzeff, it happened during a conversation with an old friend. "There are only two numbers to measure in cosmology!" Uncle Allan boomed at him, and while Suntzeff didn't think much of the comment at the time, he recalled it later, back home in La Serena, when a junior staff member at the observatory mentioned an idea for a project.

  Mario Hamuy had arrived at Cerro Tololo as a research assistant on February 27, 1987—three days after 1987 A blossomed in the Large Magellanic Cloud. The original, pre-1987 A plan for Hamuy was that, as the new hire, he would go to the mountain and spend a few days acclimating himself to the instruments. Instead, the director of the observatory sent him to the mountain to observe 1987A and only 1987A. By the time he returned to La Serena a month later, Hamuy was, if not yet a bona fide supernova expert, at least a supernova enthusiast above and beyond his characteristic enthusiasm.

  Now he explained to Suntzeff and Phillips that he had attended a talk at Santa Cruz by Bruno Leibundgut, a Swiss astronomer with a fresh PhD. Suntzeff and Phillips knew Leibundgut from observing runs in Chile in the early and mid-1980s, when he was a graduate student working with Gustav Tammann. They had attended his talk, too, and Leibundgut told them afterward that he'd been a late addition to the schedule. Bob Kirshner had recently hired him as a postdoc at Harvard, starting that fall, and at some point during the meeting Kirshner had turned in his seat and casually asked Leibundgut when he was giving a talk. Leibundgut answered that he wasn't; Kirshner told him that he was, now: He could have Kirshner's slot. And so Leibundgut wound up telling the world's supernova experts about his doctoral thesis: a template for Type Ia supernovae suggesting that they might be standard candles after all.

  For Phillips and Suntzeff, the talk was part of a long-term, ongoing conversation in the supernova community. They had made their own contribution through their work on 1986G. For Hamuy, however, the talk provided a vision for the future. Listening to Leibundgut, he recalled that his graduate school advisor at the University of Chile, José Maza, had coordinated a supernova survey in the late 1970s and early 1980s. Maybe the time had come to revive the idea of a supernova survey from the Southern Hemisphere, this time using the superior CCD technology. On his return from Santa Cruz, he had approached Maza with the idea, and Maza agreed to help. Now Hamuy wanted to know what Phillips and Suntzeff thought.

  Phillips told him he thought it might be a good idea, but, he cautioned, a supernova survey from the Southern Hemisphere had to be something more than a supernova survey from the Southern Hemisphere. At which point Suntzeff thought, "There are only two numbers to measure in cosmology."

  Maybe they were wrong in thinking that Type Ia supernovae were not standard candles. Maybe Leibundgut, who after all had been studying other Type Ia while they were busy with the Type II 1987 A, was right. And if he was right, then maybe they could use nearby Type Ia supernovae to measure the Hubble parameter—the current rate of the universe's expansion. And if that program worked, they could go to farther supernovae to measure the deceleration parameter—the rate at which the expansion was slowing down.

  Hamuy devised the logistics. The survey would be a collaboration between two observatories, Cerro Calán, the university's observatory, in Santiago, in the middle of the city, and Cerro Tololo, his current employer—hence the Calán/Tololo survey. Ideally, a supernova search would combine the widest-field camera with the latest CCD technology, but that option wasn't available to the collaboration. Instead they had to choose between a telescope that couldn't accommodate a CCD camera but had a wide-field view and a telescope that could accommodate a CCD but had a narrow-field view. They chose the wide-field, no-CCD view, the 24-inch Curtis Schmidt Telescope on Cerro Tololo. When hunting prey as rare and elusive as supernovae, the more galaxies you can grab at a time, the better your chances of finding even one, and in identifying supernova candidates, quantity still trumped quality. The photographic plates were big—eight inches by eight inches—and covered a patch of sky equal to one hundred full moons. For the follow-up observations, they would use the narrow-field CCD, a telescope with only a one-moon view, but that window was wide enough for performing photometry and spectroscopy on a supernova for which you already knew the specific coordinates.

  The workday for the Calán/Tololo collaboration would begin at sundown at the Curtis Schmidt Telescope, where the Cerro Tololo team would take the images and develop the photographic plates. At sunrise, they would put the plates on a truck, which would take the plates to a passenger bus, which would arrive, via the coastal highway, seven or eight hours later in Santiago. There the bus would be met by research assistants from Cerro Calán, who would bring the plates back to the observatory and then blink the previous night's images with reference images from a few weeks earlier. By the time the sun was setting and the dome was opening in Cerro Tololo, Hamuy, Phillips, and Suntzeff would have a list of supernova candidates they would be chasing that night with the CCD.

  The survey, however, wouldn't just discover supernovae. It would also improve the field's quality of observation and analysis by following up on the supernova discoveries of other astronomers, both professional and amateur. The team looked at the two odd 1991 supernovae—the surprisingly bright 1991T and the surprisingly dim 1991bg; Phillips became the lead author on an Astronomical Journal paper analyzing 1991T. Those two supernovae only reinforced his and Suntzeff's earlier suspicion that supernovae weren't standard candles. You could see the disparity at a glance—the light curves were just that different. The light curve belonging to the surprisingly bright 1991T rose and fell more gradually than the typical Type Ia light curve. The light curve belonging to the surprisingly dim 1991bg rose and fell more abruptly than the typical Type Ia light curve.

  The bright one declined more gradually. The dim one declined more abruptly.

  Bright ... gradually. Dim ... abruptly.

  The correlation jolted Phillips. Would it hold if he examined the light curves from a range of supernovae? If so, then maybe Type Ia didn't have to be identical in order to be useful for cosmology. Maybe how gradually or abruptly a light curve rose and fell could serve as a reliable indicator of its brightness relative to other Type Ia supernovae. And if you knew the relative luminosities among supernovae, then, through the inverse-square law, you would also be able to figure out relative distances. You would be able to use supernovae to do cosmology.

  Phillips recalled that while working on 1986G—the first CCD supernova light curve—he had consulted papers by Yuri Pskovskii from 1977 and 1984 positing a relation between the rise and fall of light curves and their absolute luminosities. But Phillips knew that the uneven quality of photographic plates made Pskovskii's hypothesis untrustworthy. Now, Phillips figured, he could use non-photographic studies to resolve the question.

  Throughout 1992 he collected light curves, including some of his own, that he felt satisfied the most stringent observational criteria, and then he subjected them to months-long analysis. One morning late
that year, he felt his preparations were over. The time had come to take the light curves, nine in all, and plot the data.

  One of the advantages of living in a relatively small city like La Serena, Chile, Phillips thought, was that you could walk home and have lunch with your wife. He usually didn't discuss his work with her. She wasn't particularly interested in astronomy, and he didn't particularly feel like talking about it at home. But that afternoon he made an exception.

  "I think," he told his wife, "I've discovered something important."

  One of the first astronomers to write Phillips with congratulations was none other than Bob Kirshner—a blessing from afar, a benediction from above. The Danish and Berkeley teams had both asked whether one could discover supernovae at distances sufficient to do cosmology. Their answers, in 1988 and 1992, were: Yes. Now the Calán/Tololo team had taken what Kirshner considered the scientifically responsible first step and answered the question of whether Type Ia were standard candles: No. But they might be the next best thing: candles you could standardize. You could correlate the decline rate of the light curve with the supernova's absolute magnitude.

  The next question, then, was: Could one detect distant Ia supernovae on a regular and reliable basis?

  In March 1994, out of nowhere, the Berkeley team answered that question decisively: Yes. In a stunning announcement, they said that between December 1993 and February 1994 they had discovered six distant supernovae in as many nights.

  When Schmidt sat down with Suntzeff in La Serena in late March and discussed the possibility of competing against Saul, the repercussions of the announcement were still reverberating in the supernova community, like one of the aftershocks perpetually roiling the Chilean countryside. Schmidt and Suntzeff themselves were reeling from a sickening realization. Since 1989, Calán/Tololo had collected fifty nearby supernovae, of which twenty-nine were Type Ia. The members of the collaboration would soon be publishing their value for the Hubble parameter, at which point their data would become public information, freely available. Eventually Berkeley was going to need nearby Type Ia to anchor the lower end of their Hubble diagram.