The 4-Percent Universe

by Richard Panek

  But what about over time? A new syllogism presented itself: One, the universe is expanding; two, the universe is full of matter attracting other matter through gravity; therefore, the expansion must be slowing down. The lingering challenge to aspiring cosmologists was no longer Why wasn't the universe collapsing? It was Will it collapse?

  Ever since Hubble's discovery of evidence for an expanding universe, astronomers had known how to measure how much the expansion was slowing down, at least in principle. Hubble had used Henrietta Swan Leavitt's period-luminosity relation for Cepheid variable stars to determine distances to nearby galaxies. And he had used the redshifts for those galaxies as equivalent to their velocities as they moved away from us. When he graphed those distances against those velocities, he concluded they were directly proportional to each other: the greater the distance, the greater the velocity. The farther the galaxy, the faster it was receding. This one-to-one relationship showed itself as a straight line on a 45-degree angle. If the universe were expanding at a constant rate, that straight line would continue as far as the telescope could see.

  But the universe is full of matter, and matter is tugging gravitationally on other matter, so the expansion can't be uniform. At some far reach of the Hubble relation, the galaxies would have to deviate from the straight line. The graph of their points would begin to gently curve toward brighter luminosity. How much they deviated from the straight line would tell you how much brighter they were at that particular redshift than they would be if the universe were expanding at a constant rate. And how much brighter they were—how much nearer they were—would tell you how much the expansion was slowing.

  To make that distinction, astronomers would need to continue to graph distance against velocity. For the velocity axis, they could still use redshift. For distance, however, they had a problem. Cepheid variables are visible only in relatively nearby galaxies. For distant observations astronomers would need another source of light that had a standard luminosity, celestial objects they could plug into Newton's inverse-square law as if they were so many candles at greater and greater distances.

  And ever since Hubble's discovery of evidence for an expanding universe, astronomers had also known about the standard-candle candidate that the Berkeley team would decide to pursue. In 1932, the British physicist James Chadwick discovered the neutron, an uncharged particle that complements the positively charged proton and the negatively charged electron. In 1934, the same Caltech astrophysicist who had recently suggested that galaxy clusters might be full of dark matter, Fritz Zwicky, collaborated with the Mount Wilson astronomer Walter Baade on a calculation showing that, under certain conditions, the core of a star could undergo a chain of nuclear reactions and collapse. The implosion would race inward at 40,000 miles per second, creating an enormous shock wave and blowing off the outer layers of the star. Baade and Zwicky found that the surviving ultracompact core of the star would consist of Chadwick's neutrons, weighing 6 million tons to the cubic inch and measuring no more than 60 miles in diameter.

  Astronomers had already identified a class of stars that suddenly flared brighter, then dimmed, and they had named this phenomenon "nova," for "new star" (because its sudden brightening might suggest it was "new" to us). Baade and Zwicky decided that their exploding star deserved a classification all its own: "super-nova."

  Almost at once, Zwicky initiated a search for supernovae. He helped design an 18-inch telescope that became the first astronomical instrument in use on Mount Palomar, and soon newspapers and magazines across the country were keeping a running tab of how many "star suicides" his survey had discovered. Baade, meanwhile, suggested that because supernovae seemed to arise from the same class of objects, they might serve as standard candles, though he cautioned in a 1938 paper that "probably a number of years must elapse before better data will be at our disposal."

  The wait turned out to be half a century. In 1988, the National Science Foundation awarded the University of California, Berkeley, six million dollars over five years to establish the Center for Particle Astrophysics. The center would take multiple approaches to the mystery of dark matter. One was to try to detect particles of dark matter in the laboratory. Another looked for signs of dark matter in the cosmic microwave background. A third approach explored dark matter through theory. And another group would try to determine how much matter was out there, dark or otherwise, by using supernovae as standard candles.

  The skepticism was sure to be high: physicists doing astronomy? Pennypacker and Perlmutter knew they would eventually have to convince the astronomy community, as insular and guarded as any in science, that physicists working at a particle physics institution could be capable in their line of work. But first they were going to have to convince themselves.

  Lawrence Berkeley National Laboratory had invented accelerator particle physics as the world had come to know it. In the late 1920s the physicist who would become the lab's namesake, Ernest Lawrence, conceived of an accelerator that shot particles not in straight lines, as linear accelerators did, but in circles. Strategically placed magnets would deflect the particles just enough to prod them to follow the closed curve, around and around, faster and faster, to higher and higher levels of energy. Lawrence's first "proton merry-go-round"—or cyclotron—was five inches in diameter, small enough to fit inside any room bigger than a broom closet in the physics building on campus. In 1931 he'd moved his operations into an abandoned building, the former Civil Engineering Testing Laboratory—the first official site of the Berkeley Lab "Radiation Laboratory." By 1940, a version of the cyclotron had reached a diameter of 184 inches, and the experiment had outgrown the Rad Lab. Lawrence secured from the university a promontory above the campus. But his legacy wasn't just the complex of buildings that over the decades would come to line Cyclotron Road. It was all the particle accelerators in various places around the world that circle underground, miles-long snakes devouring their tails.

  What fun was that? You could be a cog in the biggest wheel in the history of the planet, you could even be the most important cog, but you'd still be a cog—assuming you lived long enough for the next generation of accelerators to come online. Luis Alvarez had been a key contributor to the construction of the Bevatron, a proton accelerator with a 400-foot circumference that opened at LBL in 1956. But his was not the kind of mind that aspired to cogdom a couple of decades hence. On his own he used principles of physics in a frame-by-frame analysis of the Zapruder film of the assassination of President Kennedy to see whether the "one bullet, one gunman" hypothesis was tenable. (It was.) He probed an Egyptian pyramid with cosmic rays to learn whether it held secret passages. (It didn't.) He wanted to mount a "High Altitude Particle Physics Experiment," but the LBL director at the time refused to give him access to lab funds. Not because of what the experiment would be doing—particle physics was what LBL did. Rather, the problem was how HAPPE would be doing it: aboard a balloon. "We are an accelerator lab," the lab director Edwin McMillan, a Nobel laureate in Physics, told Alvarez. "If we stop doing accelerator physics, our funding will disappear."

  For Alvarez, turning your back on a particle physics experiment because it went up in the air instead of around and around betrayed a lack of imagination. Alvarez quit the leadership of his own LBL physics group in protest and got funding elsewhere, including from NASA, to see if HAPPE could fly. (It crashed.)

  By the mid-1970s, Alvarez had his own Nobel Prize in Physics, and the lab had a new director. One day Alvarez was reading a magazine article about an experiment being done by an acquaintance of his. Stirling Colgate, toothpaste scion by birth and thermonuclear physicist by choice, had planted a 30-lnch telescope on a surplus Nike missile turret in the New Mexico desert. The plan was to program it to automatically look at a different galaxy every three to ten seconds. The telescope would then transmit the information through a microwave link to the memory of an IBM computer on the campus of the New Mexico Institute of Mining and Technology, eighteen miles away. There, software w
ould search the images for supernovae.

  Automated astronomy wasn't entirely new. The era of the lone observer standing on a mountaintop, eye to eyepiece, staring into the abyss, was coming to an end. Since the invention of the telescope in the first decade of the seventeenth century, astronomers and telescope operators had guided their instruments by hand. Now astronomers were writing computer programs that manipulated the motions of the telescope—and did so with far greater sensitivity than the human hand, and eventually without the need for a human presence on site at all. The University of Wisconsin had an automated telescope. So did Michigan State University, and MIT too.

  Supernova searches weren't new, either. They had been a staple of astronomy since Zwicky's initial observing program in the 1930s. The 18-inch telescope on Mount Palomar that he helped design was still in use for that purpose, as was a nearby 48-inch, along with telescopes in Italy and Hungary.

  What distinguished Colgate's project was the combination of the two ideas—automated telescope, supernova search. An astronomer looking for a supernova has to compare images of the same galaxy several weeks apart to see whether a dot of light has emerged. Traditional supernova searches required developing photographic plates by hand, then comparing them to previous exposures by eye, both time-consuming processes. In Colgate's system, a television-type sensor would replace photographic plates, and a computer would compare images almost instantly. In the end, his experiment didn't work. His hardware was fine; he had been able to make a telescope that could point where and when he wanted it to point. The problem was the software. The computer code required the equivalent of a hundred thousand FORTRAN statements, and Colgate was working more or less alone. Still, Alvarez saw, the potential was there. No: The inevitability was there—the era of the remote supernova survey.

  Alvarez looked up from the magazine. "Stirling has been working on this project," he said, handing the article to a postdoc and former graduate advisee of his, Richard Muller. "And I think he's abandoning it. Talk to him. See what's going on."

  By the time Perlmutter arrived at LBL in 1981 as a twenty-one-year-old graduate student, the stories of Muller's battles with the bureaucracy were legion. Year after year, LBL leadership would tell Muller that this was the last time he'd be getting funding, because if the Department of Energy was going to cut anything from the LBL budget, it was going to be the speculative astrophysics project, and then that money would be gone from the LBL coffers for good. But Muller had read Jim Peebles's book Physical Cosmology, and he understood that the astronomy of the very big and the physics of the very small were becoming closer, even indistinguishable. It was the era of the cosmic microwave background. Of quasars, those mysterious sources of extremely high energy from the depths of the distant universe. Of pulsars, which provided evidence not only that Zwicky and Baade had been right all those decades earlier about the existence of neutron stars, but that these stars were spinning at a rate of hundreds of times a second. You couldn't study any of these phenomena without thinking about high-energy physics. Astronomy might not be the kind of high-energy physics that the lab had ever pursued, but it was quickly becoming the kind that the lab had always done. From the point of view of a Luis Alvarez or a Richard Muller, they weren't drifting toward a new discipline. The discipline was drifting toward them.

  If anything, Perlmutter thought of himself as a born physicist—someone who wanted to know how the world worked on the most fundamental level, to discover the laws that united all of nature. Science courses had always been the easy part of his education; he almost hadn't needed to think about them. He studied science, enjoyed it ... and then had plenty of time to indulge other interests. And what he thought about, when he wasn't thinking about the laws that united nature, were the "languages" that united humanity—literature, math, music. So maybe he was a born philosopher, too. His parents were both professors—his father taught chemical engineering, his mother social work—who had elected to raise their family in an ethnically mixed neighborhood in Philadelphia, and he grew up listening to his parents and their friends talking about social issues and the latest books and films. He began practicing violin in the second grade. He joined the chorus. In high school he challenged himself to learn how to think like a writer—to learn the nature of narrative. And when he arrived at Harvard College in 1977, he assumed he would double-major in physics and philosophy.

  The "physics stuff," he soon realized, was starting to get hard. College physics bore little relation to high school physics. He concluded that if he pursued both physics and philosophy, he would have no time for other courses, let alone a social life. He would have to make a choice.

  If he chose philosophy, he couldn't do physics. But if he chose physics, he would still be doing philosophy, because in order to do physics you had to ask the big questions. Before science was science (the study of nature through close observation), it was philosophy (the study of nature through deep thought). Even as science had accumulated all manner of empirical scaffolding over the past few centuries, the guiding impulse of the scientist had remained constant: What is our relationship to the natural world? When Perlmutter joined Richard Muller's physics group at LBL in 1982 and had to choose among the eclectic programs Muller was directing that were now acceptable to the institution—using planes to sample carbon in the atmosphere, measuring the gravitational deflection of starlight by Jupiter—he selected the nascent supernova survey because it seemed the kind of project that might lead to the biggest questions of all. Just as you were automatically doing philosophy if you were doing physics, you were automatically doing physics if you were doing astronomy. Instead of the nature of narrative, Perlmutter would be exploring the narrative of nature.

  Alvarez had handed the idea of an automatic supernova survey to Muller on a whim, and now Muller handed that whim to his own postdoc, Carlton Pennypacker. By 1984, the Berkeley Automatic Supernova Search (BASS) team—Muller, Pennypacker, Perlmutter, and a few other graduate students—was operating on the 30-inch telescope at the university's Leuschner Observatory, in the hills a half-hour drive northeast of campus. Further technological advances that were particularly useful for supernova hunting were coming along all the time. When astronomers using photographic plates hunted for supernovae by eye, they used an optical device called a comparator. By rapidly switching back and forth between two images of a galaxy taken several weeks apart, the comparator would allow an astronomer to see whether any new pinpoint of light had appeared in the Interim. The comparator blinked the two images. New computer technology, however, allowed astronomers to take all the light from the earlier image and remove it from the later one. It subtracted the first image from the second. If the computer signaled that a telltale bit of light remained, then a real live human being analyzed the data. Sometimes the source of the bit of light was "local"—a fluctuation in the output, a cosmic ray from space striking the instrument, a subtraction error. Sometimes the source of the light was "astronomical"—asteroids, comets, variable stars. But once in a while the blip of light was a star erupting in a farewell explosion that stood out even against the background of all the tens or hundreds of billions of other stars in its host galaxy combined: that is, a supernova.

  Or a Nemesis. In 1980, Luis Alvarez, along with his son Walter Alvarez, had hypothesized that the mass extinction of the dinosaurs 65 million years ago, at the cusp of the Cretaceous and Tertiary periods, had been caused by a comet or asteroid impact that had disrupted the global ecosystem. Then, in 1983, a pair of paleontologists announced that they had discovered evidence of a cycle of mass species extinctions every 26 million years. The following year, Muller and some colleagues published a paper speculating on the existence of a companion star to the Sun—Nemesis. Every 26 million years, they wrote, the highly elliptical orbit of Nemesis would bring it relatively near the Sun, and its gravitational influence would draw comets from the farthest reaches of the solar system into the orbital paths of the planets nearest the Sun, including Earth.

>   The idea wasn't as fanciful as it might seem; studies of Sun-like stars had shown that about 84 percent were in binary systems, meaning that the Sun, if solo, would be an anomaly. Muller assigned Perlmutter the task of looking for Nemesis (or the Death Star, as the media called it), and in 1986 Perlmutter completed his thesis, "An Astrometric Search for a Stellar Companion to the Sun." But the two projects shared a telescope and some other hardware, some of which Perlmutter helped design, as well as the search software, much of which he wrote, and when he was invited to stay at LBL as a postdoc, Perlmutter looked to supernovae for inspiration. A few months earlier, on May 17, 1986, BASS had bagged its first supernova, and that was good enough for him.

  In 1981 the team had predicted a detection rate of one hundred supernovae per year, but scientific proposals were often overoptimistic. Besides, BASS supernovae were relatively nearby; they weren't going to be immediately useful for the big questions. Any changes in the universe's rate of expansion wouldn't be discernible unless astronomers could find standard candles in galaxies significantly farther than Hubble's sample, the deeper the better. How much those supernovae—and therefore their host galaxies—deviated from the straight-line Hubble diagram would tell astronomers the rate of deceleration. Thanks to BASS, Pennypacker and Perlmutter now knew they could do an automated supernova search; they had added two more supernovae in 1986 and another in 1987. But could they do an automated supernova search at cosmologically significant distances?

  Muller himself thought such a project might be premature. But he was also a scientist who for years had entertained Luis Alvarez's imaginative flights and was willing to risk his own scientific reputation on a search for a Death Star. He gave his consent, and Pennypacker applied for funding for a camera he wanted to mount on a telescope in Australia. Or, rather, Pennypacker commissioned the camera, then applied for the funding. But the heavens opened over Berkeley and the NSF showered millions of dollars on the Center for Particle Astrophysics, and even though the names on the supernova proposal were Richard A. Muller and Carlton Pennypacker, the search for the fate of the universe was, from the start, Pennypacker's and Perlmutter's.