The 4 Percent Universe

by Richard Panek

  Suntzeff, they decided, would be in charge of the observing. He would find the supernova candidates and do the follow-up measurements. Schmidt would be in charge of the analysis. He would take some existing software and create a new code that would clean the images, do the subtraction, and isolate the supernovae.

  Suntzeff turned to Schmidt. "How long will it take you to write the new code?"

  Schmidt could carry himself as a cocky young astronomer, but his wiseacre's side-of-the-mouth way of talking suggested not so much arrogance as irony. Suntzeff preferred to think of him as constitutionally optimistic. Yet even Schmidt had to hesitate. Then he reminded himself: Saul's doing it.

  "Two months," he answered.

  On his return to Harvard, Schmidt disappeared into his office for hours at a time, day after day, week after week, writing the code. But he also circulated through the halls, stopping colleagues and dropping into offices, letting a select group know that he and Nick Suntzeff were putting together a team to catch Saul. In each case he got the same response, expressed with the same level of eagerness: Can I be part of that?

  Robert Kirshner wouldn't even have to ask. He had been a student of supernovae since 1970, longer than some of his students had been alive. At forty-four, he was now an elder statesman in astronomy, the chairman of the Astronomy Department at Harvard. He had long experience getting money out of the National Science Foundation, reserving time on the world's best telescopes, and helping to set policy for the Space Telescope Science Institute, the science and operations center for the Hubble Space Telescope. He was one of the world's foremost supernova experts, as well as the mentor to several generations of supernova experts—the graduate students he had recruited and the postdocs he had hired for his private duchy within the Harvard-Smithsonian Center for Astrophysics, half a mile up Garden Street from Harvard Square. When Nature received the Danish group's preliminary results in 1988, it was Kirshner whom the journal asked to privately review the paper and then publicly write an accompanying news analysis. When Berkeley's Center for Particle Astrophysics convened an External Advisory Board and needed a supernova guru, it was Kirshner who got the call. When Perlmutter et al. submitted a paper to the Astrophysical Journal Letters analyzing their 1992 supernova, it was Kirshner whom the editors asked to serve as referee.

  To all his peer evaluations Kirshner brought a deep skepticism, born of his own decades of experience, about the ability of anyone to perform the near-surgical task of supernova analysis. Kirshner could be amusing; in casual conversation he often made exaggerated facial expressions, adopted accents, whinnied at his own jokes. His talks at conferences were reliably witty and well-attended. But when it came to supernovae, and to what you needed to know to do supernova astronomy, Kirshner could be exacting, even bruising.

  But he had a point—several points, actually. If you wanted to do supernovae, you had to know spectroscopy—the analysis of an astronomical object's spectrum of light that identified its chemical composition as well as its motion toward or away from you. You had to know photometry—the tedious, difficult determination of an object's brightness. You had to account for dust, either within the supernova's host galaxy or somewhere along the line of sight between the supernova and the observer. Sometimes dust was there, sometimes not. If it was there, it would dim or redden the light from the supernova. And if you didn't know the extent to which dust was polluting the light, you wouldn't know how much to trust your data.

  For the Berkeley supernova group, however, Kirshner reserved a special level of skepticism. As far as he was concerned, they were doing poor work that was giving his area of expertise a bad name.

  From the start, Kirshner had his doubts about a bunch of particle physicists trying to do astronomy, adopting it as if it were a hobby rather than a science you needed to perfect over a lifetime. So far he'd seen nothing to ease those concerns. In the 1980s, Richard Muller had diverted time from the supernova survey at the Leuschner Observatory to pursue his Nemesis project. The discovery of a companion star to the Sun, if he made it, would be momentous, but it was so unlikely that the effort seemed almost a capricious use of precious telescope time. In 1989, Muller, Pennypacker, and Perlmutter got the attention of astronomers around the world by concluding that the famous supernova 1987 A—the first naked-eye supernova in four hundred years—had left behind a pulsar, a neutron star spinning hundreds of times per second. The "evidence" turned out to be an instrument error. And then came the embarrassment that Kirshner got to witness for himself, as a member of the External Advisory Board: a three-year attempt to find distant supernovae at the Anglo-Australian Telescope that had come up empty.

  Hundreds of thousands of dollars: money enough to fund dozens of more modest and more practical astronomical projects: empty.

  It wouldn't be entirely fair to say that particle physics operates according to the principle Get funding first, ask questions later. But it wouldn't quite be inaccurate, either. Projects in particle physics routinely involve dozens, hundreds, even thousands of participants, and require machines that manufacture ultra-energetic pyrotechnics that the universe hasn't seen since its megacompact first fraction of a second of existence. The Berkeley supernova search wasn't operating on that scale, but other projects at LBL were, and the lab itself had long been the world's foremost proponent of that work ethic. Particle physicists can somewhat afford to bulldoze ahead, confident that between their billion-pound hardware and their collective brainpower they'll find the answer to any question they might ask. And the first question that the LBL team had asked was: Can we find distant supernovae?

  It was, Kirshner thought, the wrong question to ask first. The right one was whether distant supernovae were worth finding. Could they really serve as standard candles?

  The recent history of astronomy held a couple of cautionary tales for the standard-candle-bearer. Having discovered evidence for the expansion of the universe, Edwin Hubble spent much of the last twenty years of his life working under the assumption that galaxies might be standard candles, even though they weren't entirely uniform. Maybe they were similar enough that he could use them to discern the universe's shape and fate. Walter Baade, one of his Mount Wilson colleagues as well as Fritz Zwicky's collaborator on the 1934 "super-nova" paper, argued that Hubble had it backward: "You must understand the galaxies before you can get the geometry right." Allan Sandage, Hubble's protégé and, upon his death in 1953, his successor at the Mount Wilson and Palomar Observatories, would later write, "Hubble clearly understood this, but rather than be stopped because this part of his subject was 30 years before its time, he pushed ahead with an abandonment known to pioneers in any milieu who try to reach Everest without proper equipment."

  Then it was Sandage's turn. For a quarter of a century, he and the Swiss astronomer Gustav A. Tammann pursued an alternate candidate for standard candles. If galaxies themselves weren't uniform enough, then maybe clusters of galaxies were—or, more precisely, the brightest galaxy within each cluster. But this proposition, too, suffered from an insufficient understanding of galaxy mechanics. Some galaxies would grow dimmer with age, as their stars died out, while other galaxies would grow brighter with age, as they merged with smaller galaxies. Unable to reliably tell the difference, and wary of other factors they couldn't begin to guess, Sandage and Tammann turned back from the summit. "Essentially," Sandage announced to his colleagues in cosmology in 1984, at a conference on the expansion rate of the universe, "we have failed."

  Kirshner never passed up an opportunity to point out that the same fundamental lack of understanding of underlying processes could easily sabotage the usefulness of supernovae for cosmic measurements. Already astronomers had determined that supernovae belong to two classes—and possibly more.

  One class was the kind that Zwicky and Baade had prophesied—one that results in the birth of a neutron star. It was the kind Zwicky assumed he was finding in his 1930s survey of "star suicides." In 1940, however, Rudolph Minkowski at Mount Wilson too
k a spectroscope of a supernova that was different from the spectroscopic analyses of Zwicky's supernovae. Minkowski's supernova showed the presence of hydrogen. Zwicky's supernovae did not. They were clearly different types of supernovae.

  Since then astronomers had come to think that one type of supernova—the type that Zwicky and Baade had predicted in 1934, that Zwicky thought he was observing in 1936 and 1937, and that Minkowski did observe in 1940—was the result of a chain of nuclear processes in a star several times the mass of the Sun, leading to a 40,000-miles-per-second implosion.

  The other type—the type that Zwicky observed—begins life as a hydrogen-rich star like our own Sun. As it ages, the Sun will shed its outer hydrogen layer while its core contracts under gravitational pressure. In the end, only the core will remain—a shrunken skull called a white dwarf, with the mass of the Sun packed into the volume of Earth. If a white dwarf had a companion star (and most stars in our galaxy do), then at this point it might start to siphon gas off the other star. In the 1930s, the Indian mathematician Subrahmanyan Chandrasekhar calculated that when a star of this kind reaches a certain size—1.4 times the mass of the Sun, or the Chandrasekhar limit—it will begin to collapse of its own weight. The gravitational pressure will destabilize its chemical composition, leading to a thermonuclear explosion.

  Through a telescope on Earth, the two types would look the same, even though one is an implosion and the other is an explosion. But a spectroscope would show the difference—hydrogen or no hydrogen, Type II or Type I. For astronomers, the uniformity of Type I supernovae offered the possibility that this type might be a standard candle. Since these supernovae all began as a single kind of star, a white dwarf, that had reached a uniform mass, the Chandrasekhar limit, maybe their explosions had the same luminosity.

  In the 1980s, however, the clear distinction between Type I and Type II began to blur. Spectroscopic analysis of three supernovae—one each in 1983, 1984, and 1985—showed that they consisted of huge amounts of calcium and oxygen, consistent with the interiors of massive stars that end their lives as Type II supernovae, but no hydrogen, consistent with white dwarfs that end their lives as Type I supernovae. Some astronomers, including Kirshner, suggested that they were seeing a third type of supernova, essentially a hybrid of the other two. It was the product of a core collapse that had already lost its outer shell: a hydrogen-free implosion.

  They added this specimen to the Type I column, calling it Type Ib. The old Type I, a thermonuclear explosion with no hydrogen, was now Type Ia.

  In 1991, even that classification—Type Ia—began to blur. On April 13, five amateur observers in four locations around the world discovered a supernova designated 1991T.* On December 9, an amateur astronomer in Japan discovered a supernova designated 1991bg. Follow-up spectroscopic observations by professional astronomers—including Kirshner, on April 16, for 1991T—showed that they were both Type Ia supernovae. But their luminosities differed widely. Supernova 1991T was much brighter than the usual Type Ia at its particular distance, and 1991bg was much dimmer than the usual Type Ia at its particular distance. Astronomers could rule out the possibility that they were simply miscalculating distances: The dimmer supernova was ten times dimmer than a supernova observed in 1957 in the same galaxy.

  Astronomers began to suspect that while each supernova in the universe might be a Type Ia, Type Ib, or Type II, the types themselves might be more like families. The supernovae within a family share traits, but they're not identical; they're more like siblings than clones. For astronomers hoping to adopt Type Ia supernovae as standard candles, Kirshner wrote, the problem "was serious and real." You couldn't ignore it.

  And the Berkeley group didn't ignore it. In her 1992 doctoral thesis a team member summarized the collaboration's general attitude toward the problem: "There is still some contention" about "whether individual SNe Ia do not fit the model," but, she added, echoing the chorus that Kirshner had heard from the LBL group again and again, "it is clear that the overwhelming majority of SNe Ia are strikingly similar."

  Clear? Not to Kirshner, and he was the expert—a "realist," as he liked to call himself, not a wishful thinker.

  In his role as a member of the External Advisory Board of the Center for Particle Astrophysics since the late 1980s, Kirshner emphasized that the Berkeley search team hadn't yet found a supernova, needed to be careful about photometry, couldn't account for dust—and didn't know whether Type Ia supernovae were standard candles.

  Then in 1992 the LBL group found their first supernova. In his referee's report for Astrophysical Journal Letters, Kirshner complained that they still needed to be careful about photometry, still couldn't account for dust—and still didn't know whether Type Ia supernovae were standard candles. All they had shown, he thought, was that one could find supernovae distant enough that one could, in principle, do cosmology with them. But the Danes had done that, too, and they'd done it four years earlier. What the LBL team hadn't shown, in Kirshner's reading of the paper, was that one could find supernovae distant enough that one could in fact do cosmology with them.

  He sent the paper back for a simple reason: "They hadn't yet learned anything about cosmology"—basically, that you couldn't assume exploding white dwarfs were perfect standard candles. They weren't perfect standard candles. The best you could hope was that somebody, someday, would figure out whether Type Ia supernovae, however imperfect, might be just good enough.

  In high school in Marin County in the late 1960s, Boris Nicholaevich Suntzeff Evdokimoff played on the same varsity soccer team as his good friend Robin Williams. At Stanford in the 1970s, he regularly competed on the tennis court with—and lost to—Sally Ride. What was really cool, though, was that as a Carnegie Fellow in the early 1980s he got to talk astronomy with Allan Sandage.

  Suntzeff loved historical connections in astronomy. A great-uncle of his had gone to school in Russia with Otto Struve, the descendant of a line of prominent astronomers. Struve fled Russia and the Bolsheviks at the time of the revolution and wound up in Turkey, impoverished, until a relative put him in touch with the director of the Yerkes Observatory in Wisconsin, who offered him a job as a spectroscopist. Struve later became director of the observatory, as well as McDonald Observatory in Texas and Leuschner Observatory in Berkeley. Suntzeffs family also fled Russia, though they headed in the other direction, to China and, eventually, San Francisco. There Suntzeffs grandmother reunited with Otto Struve. Small world.

  And now Nick Suntzeff would be doing his part to make astronomy a bit more intimate. He had applied for a Carnegie Fellowship for just that reason: to spend time with Sandage at the headquarters of the Carnegie Institution's Mount Wilson and Palomar Observatories. There, on an unassuming residential stretch of Santa Barbara Street in Pasadena, Edwin Hubble had figured out, in 1923, that the Milky Way was just one among a multitude of galaxies in the universe, and then, in 1929, that the universe was expanding. Allan Sandage arrived there in 1948, at the age of twenty-two, as a graduate student at Caltech. Over the next four years Sandage advanced from apprentice to assistant to Hubble's heir.

  "There are only two numbers to measure in cosmology!" Sandage often said to Suntzeff, evoking the title of an influential article he'd written for Physics Today in 1970, "Cosmology: The Search for Two Numbers." The first number was the Hubble constant. The 45-degree straight line that Hubble plotted for the distances of galaxies and their redshifts—the farther the galaxy, the greater its velocity receding from us—implied a relationship you could quantify. If you knew how distant a galaxy was, then you should be able to know how much faster it would appear to be receding, and vice versa.

  In the 1930s, Hubble himself estimated that galaxies were receding at a rate that was increasing 500 kilometers for every megaparsec (a unit of length in astronomy equal to 3.262 million light-years). That rate, unfortunately, corresponded to a universe that would be about two billion years old—which would make the universe younger than the three billion years that geologists
had pegged as the age of the Earth. This disparity did nothing to help cosmology's reputation as a nascent science. But Hubble himself regarded his observations only as a "preliminary reconnaissance"; to do cosmology properly, he would have to keep seeking nebulae as far as the 100-inch telescope on Mount Wilson would allow, and then, eventually, as far as the 200-inch telescope on Mount Palomar, outside San Diego, would allow.

  The 200-inch Hale Telescope saw first light in 1948, which happened to be the same year Sandage arrived at the Carnegie Observatories. But Sandage's timing was fortuitous in another way as well. The "monks and priests," as he called the first generation of Carnegie astronomers, were ready to retire. Up there, at the observatory on Mount Wilson, Sandage could dwell among his gods, astronomers who knew they'd "arrived," as Sandage would say, when they found their napkin not clipped to a clothespin but tucked inside a wooden ring inscribed with their name. And down here, on Santa Barbara Street, Sandage could inspect for himself "the plates of Moses"—the vast archives of photographic records, and an apt metaphor in more ways than one. Like Moses, Edwin Hubble had come down from the mountain bearing new laws of nature. But also like Moses, Hubble had to wander the desert for decades, only to die within sight of the Promised Land.

  Hubble suffered a major heart attack in the summer of 1949, at the age of sixty, just six months after making his initial observations with the 200-inch telescope. To his assistant fell the responsibility for executing one of the most ambitious scientific programs in history. Sandage found that the distances to the nearest galaxies were greater than Hubble had calculated, a correction that in turn affected Sandage's interpretation of more distant galaxies, which in turn affected his interpretation of even more distant galaxies. Distance dominoes fell as far as the 200-inch Palomar telescope could see. After Hubble's death in 1953, Sandage and his collaborators derived a Hubble constant of 180—a value he continued to revise downward over the decades, until, by the time Suntzeff arrived at the Carnegie Observatories in the early 1980s, he had satisfied himself that the Hubble constant was around 50 to 55.