The 4 Percent Universe

by Richard Panek

  Creature comforts presuppose the existence of creatures, and you don't put creatures into an environment that supports literally no other creatures unless you've got a really good reason. The National Science Foundation thought it did: science you couldn't do anywhere else on Earth. Less than half a mile from the South Pole Telescope, construction on the IceCube Neutrino Detector was redefining "telescope" by pointing its detectors not up at the sky but down through the earth. It was going to cover a square kilometer and consist of a series of eighty or so cables garlanded with sixty sensors each and descending (with the help of massive hot-water drills to clear the passage) about a mile below the snow. Those sensors should be able to observe the kinds of particles from space that can rip through the Earth's atmosphere, zip through the surface on the other side of the planet, and just keep going through crust, mantle, and core without interacting with anything else—unless, in a few cases, they smacked into an atom in the pure ice below the polar surface. (It wasn't a dark-matter experiment, but some of those particles might even be evidence of two dark-matter particles annihilating each other.)

  In 1991, the NSF began a collaboration with the University of Chicago on the Center for Astrophysical Research in Antarctica. The purpose of CARA was to establish an observatory at the South Pole that would serve as a permanent base for millimeter and submillimeter astronomy—the Dark Sector, a tight cluster of telescopes, less than a mile from the station, where light and other sources of electromagnetic radiation would be kept to a minimum. (Not far away were the Quiet Sector, for seismology research, and the Clean Air Sector, for climate projects.)

  Holzapfel had been part of CARA almost from the start. He arrived at the University of Chicago as a postdoc in 1996, and although he returned to Berkeley as a faculty member in 1998, he continued to collaborate with CARA, and in particular with John Carlstrom, the director of CARA and the dean of Antarctic astronomy. Growing up in Pittsburgh, the son of an accountant and a teacher, Holzapfel developed an affinity for science for reasons he never understood. It was often science "as antisocial as science can be"—lots of high-voltage explosives and electrocution accidents. But it was also quiet, private science, like building a crystal radio and having a hard time believing that he was listening to chatter from the other side of the world.

  And then, as part of CARA, he got to listen for whispers from the other side of the universe. "Very excitable," restless in the extreme, the size-16 sneaker at the end of a crossed leg always vibrating as if agitating for more, Holzapfel helped conceive the experiments, design and run the instruments, and interpret the data from a series of CARA telescopes. Starting in the late 1990s, those experiments refined COBE's measurements even further season after season, right through the culminating project for CARA, the Degree Angular Scale Interferometer (DASI).

  At first, DASI was no different (broadly speaking) from the others. It looked for patterns in the CMB—the temperature, the fluctuations—and found them. In April 2001, Carlstrom announced that DASI had indeed detected the telltale pattern of acoustic waves predicted by inflation; just as a musical note has overtones, the fetal cry of the universe should have three peaks.

  The following year, however, DASI looked at the polarization—the direction of the photons as they decoupled from matter. The temperature and fluctuations told you where the matter was when the universe was 400,000 years old; the polarization told you how it was moving. Once again, the new cosmology faced a test. As the DASI team said in their PowerPoint presentations:

  if it's not there at the predicted level, we're

  back to the drawing board

  It was there at the predicted level. No surprise, but a relief nonetheless.

  And then came the Wilkinson Microwave Anisotropy Probe. In 2003 WMAP released its first set of data: another baby picture of the universe, a gentle riot of hot reds and cool blues representing the temperature variations that are the matter-and-energy equivalent of the universe's DNA. The match between simulations and data? Exact, only more so (if that were possible).

  The South Pole Telescope was looking at the background radiation, too. But its mission wasn't just to do more of the same. It wasn't just documenting the radiation from the Big Bang in greater and greater detail, setting tighter and tighter margins of error for the next generation of CMB detector to beat. It wasn't using the CMB as an end in itself—a passive map, flattened on a celestial tabletop.

  Instead, the SPT astronomers were using the CMB as a means—an active tool, one that would probe the evolution of the universe.

  The construction of the South Pole Telescope required shipping 260 tons of material, first to Christchurch, New Zealand (the treaty-agreed port of entry to Antarctica), then to McMurdo Station, on the perimeter of the continent, and then, at the rate of 10,000 pounds per LC-130, on twenty-five flights to the Pole itself. And because much of the technology at the Pole is singular, there were no economies of scale. If you were the graduate student who had to tighten a bolt during the construction phase, you couldn't just grab a precalibrated industry tool. You had to take off your gloves and use your hands, learning what one-sixteenth of a turn felt like.

  Astronomers like to say that for more pristine observing conditions they would have to go into outer space—and Holzapfel thought of the people who wintered there as astronauts of a sort. Each year two grad students or postdocs pulled Pole duty on the SPT. Twice a day, six days a week, from February to November, the two "winter-overs" layered themselves with thermal underwear and outerwear, with fleece, flannel, double gloves, triple-thick socks, padded overalls, and puffy red parkas, mummifying themselves until they looked like twin Michelin Men. Then they trudged in darkness across the same plateau of snow and ice as the summer crew to look for the silhouette of the South Pole Telescope's 10-meter dish, except instead of trying to spy it through a whiteout, they identified it by how it blocked out a backdrop of more stars than any hands-in-pocket backyard observer has ever seen. The telescope gathered data and sent it to the desktops of distant researchers; the two winter-overs spent their days working on the data, too, analyzing it as if they were back home. But when the telescope hit a glitch and an alarm on their laptops sounded, they had to figure out what the problem was—fast.

  They had to know what to do if—as happened once during the dark months—the instrument started making noises like a sledgehammer on steel: go outside, climb into the dish, and relubricate one of the bearings. Or a fan might break because the atmosphere was so arid that all the lubrication evaporated, and then the computer would overheat and turn itself off, and suddenly the system would be down, and nobody would have any idea why, and the telescope would be losing observing time at the rate of thousands of dollars an hour. And if the winter-overs couldn't fix whatever was broken, it would stay broken; planes don't fly to the Pole from February to October (the engine oil would gelatinize).

  The job of summer crew members like Holzapfel was to prepare the instrument so that the "astronauts"—the winter-overs—didn't encounter any surprises during their six-month "spacewalk." Once a year the summer team would bring the SPT's detector in for a checkup—"in" being the control room below the telescope.

  You could think of the SPT as a matryoshka doll. The outermost doll was a shield surrounding the antenna, to block as much light from the ground as possible. Next was the antenna, a 10-meter parabolic dish. Hovering above the dish, attached to a boom, was a "retractable boot," a long, rectangular metal container, which contained the receiver cabin, which held the receiver, which received the CMB photons through a window that opened on a secondary mirror, which bounced the photons toward six wedge-shaped wafers in a circle, like pizza slices, each wafer containing 160 bolometers, each bolometer containing a detector: the gold-plated spiderweb to catch CMB photons and, at the web's center, a superconducting film 30 micrometers in diameter, or about half as thick as a human hair.

  The grad-student-to-be put down her knitting and tapped her keyboard. To get at the innermost
dolls, the team had to first turn the antenna to position the retractable boot over the roof of the laboratory building, then lower the boot until the receiver box nestled snugly over a matching panel in the roof of the control room. Inside the control room, the summer crew opened the ceiling and, using chains and brute force, extracted the receiver and gently guided it to the control room floor. After waiting thirty hours or so for the cryostat to warm up to room temperature, they got out their tools. At that point, one Pole veteran turned to another and said, "Do you know how many screws there are?"

  "No. Hundreds. But the sad thing is, I've put each of them in three times."

  By now, the telescope had been taking data for two seasons; shortly before heading south, Holzapfel had signed off on a paper reporting the serendipitous discovery of three galaxy clusters using the SZ method. In that method, the CMB provided a backlight of sorts on the foreground evolution of the universe. How the photons in the CMB had changed over the course of their journey through the universe would tell researchers how the universe itself had changed. The clusters that Holzapfel and his colleagues discovered by identifying this change—they were aiming for a thousand—would then undergo further scrutiny from other telescopes to determine their redshifts. When astronomers pieced together the abundances (determined from the SZ effect) and the distances (from redshift) of those clusters, they hoped to see the influence of dark energy on the growth of large-scale structure throughout the history of the universe—the same tug of war between dark energy and gravity that the other methods of defining dark energy were trying to detect. And that past was prologue. How the galaxy clusters had grown over the history of the universe would help astronomers predict which side would win that tug of war in the future.

  Galaxy clusters were the largest gravitationally bound structures in the universe. Since gravity gathered smaller structures into larger ones, and gravity was now losing the tug of war with dark energy, it was reasonable to assume that galaxy clusters would also be the latest-forming gravitationally bound structures in the universe. And as dark energy took a greater and greater toll, they would also be the last-forming such structures.

  Holzapfel thought of these clusters as the proverbial canaries in a coal mine. If the density of dark matter or the properties of dark energy were to change, the abundance of clusters would be the first thing to reflect that change. The South Pole Telescope should be able to track that change over time. At so many billion years ago, how many clusters were there? How many are there now? And then compare them to your predictions—your computer simulations—until they matched.

  Holzapfel already had a hunch what that match would be. It was where all the methods of defining dark energy—the supernovae, the BAO, the weak lensing—were converging: the cosmological constant. He would have to abide by whatever the data said, but he didn't have to like it. He would prefer the other ending, the one where the universe collapses and then bounces back—the ending that speaks of rebirth and reminds us of seasons. Instead, the story of the universe appeared to be heading toward the conclusion you could see, metaphorically, everywhere you turned at the South Pole: cold and empty and eternal: the clusters receding until we won't see them anymore. A hundred billion years from now we'll be left with just one cluster, our Local Group, and no clue that anything else is out there.

  Like many astronomers, Holzapfel found that outcome "depressing." Not for some woe-is-me reason; he already considered himself "existentially challenged." He was perfectly content to liken life to a Russian novel, in which a depressing future can be as exciting as happily-ever-after. His concern was more professional. He didn't like a story of the universe that ended with his profession—cosmology—dying out.

  But then, nobody ever said the universe had to be benign.

  12. Must Come Down

  THEY NEEDED SOMETHING to write on—fast. The discussion had progressed to the point where words wouldn't do. They needed numbers, signs, the propulsive force of mathematical symbols flying across a surface. The table of theorists got up and joined the several other clutches of theorists at work on the only blackboard in the room. Still, there was plenty of space for all. The blackboard was "full wall," as they liked to say at the Perimeter Institute for Theoretical Physics. Blackboards in offices were full wall. Blackboards in the hallways, blackboards in nooks off the hallways, blackboards in outdoor courtyards—all full wall. The blackboard in the café reached floor to ceiling, and stretched the length of the room. The theorists had all turned their backs on the café tables, on the windows, on the view of the sunset over the treeline of a city park. Here, there, along the wall, they hunched forward, peering at the hieroglyphs appearing on the board, gesturing their concerns, voicing their corrections. The new group, however, had no chalk. No matter. They simply bent close to the blackboard and waved their hands, their fingers describing arcs in the air. They didn't need chalk. For them, the equations were there.

  From across the café, Brian Schmidt watched. "They're really going at it," he said to nobody in particular. Then he produced a cell phone and took a picture.

  As the leader of the original High-z team, Schmidt was one of the dark-energy astronomers whose discovery nine years earlier had sent physicists down the byzantine path leading to this blackboard. Now he had entered the theorists' den. When the weeklong meeting on dark energy began, with a four-day conference at McMaster University, in nearby Hamilton, Ontario, several other astronomers had been in attendance. But today the setting had shifted seventy kilometers northwest to the Perimeter Institute, in Waterloo, Ontario, and the number of participants had thinned considerably. "I'm the last astronomer standing," Schmidt had said to the organizer of the Perimeter event, who answered, "No, you're not. What about Rocky?" Schmidt laughed. Rocky Kolb was as much an astronomer as Schmidt was a theorist, and Schmidt had made a point, in his lecture a day earlier, of identifying himself as a "dyed-in-the-wool astronomer." In the mid-1990s, when Schmidt had delegated the responsibilities for the suite of publications that would present the High-z results, a theorist had told him he would need to include a paper on something called the equation of state; Schmidt had shrugged, said "Okay," and invited his former Harvard officemate Sean Carroll to advise on the topic. Back then Schmidt hadn't even known what the term "equation of state" meant; now it seemed to be all anybody wanted to talk about, not just at the conference this week—in May 2007—but at every other dark-energy conference.

  Cosmology had a new number. Just as omega quantified the density of mass, the equation of state quantified the density of energy—specifically, the ratio of pressure to energy density. Cosmologists designated it as w. A cosmological constant would mean that w was exactly equal to -1; Einstein's lambda proposes that a given volume of space should have an inherent amount of energy per unit of volume, and that this energy suffuses the universe and remains constant over time. A w not -1 was quintessence. It would do ... something else.

  As the tenth anniversary of the discovery approached, the number of dark-energy meetings was only growing. As a resident of Australia, Schmidt had to travel halfway around the world just to get anywhere; at dinner a couple of nights earlier, he'd joked to his colleagues, "My average velocity for a year is 70 to 80 kilometers per hour." But the setting almost didn't matter. For the participants, the meetings and the message were becoming numbingly similar: the same chorus delivering variations on the same theme—a Mike Turner or a Saul Perlmutter here, a Rocky Kolb or an Adam Riess there, all of them looking for answers and coming up empty: a movable famine.

  Still, if you had to attend a conference, the Perimeter Institute at least had a marble-top bar where if you ordered a glass of wine, the bartender produced a wine list. Schmidt—who, in the thirteen years since he'd helped form the High-z collaboration in 1994, had graduated from scrappy postdoc to vineyard owner—approved. The Perimeter Institute began life in 2000 with a $100 million endowment from Mike Lazaridis, the founder of Research in Motion, which created the BlackBerry. The gu
iding principle was, as at many institutes, to give theorists a place to think free of distractions. The difference with Perimeter was that the freedom came with luxury. The interior design of the building alternated between full-wall windows and exposed concrete. A four-story atrium divided administrators from theorists. For lunch, the theorists could stop by the café, or they could stay in their office and order room service.

  The first part of the week, at McMaster, had consisted of the usual conference-style presentations: one talk after another in an auditorium seating a hundred or so participants. The Perimeter part of the week, however, would be a workshop: talks open to interruptions, catch-as-catch-can discussions in smaller groups, and the chance to keep on talking in the hallways and in the nooks and on the terrace and, always, over snacks and coffee and meals in the Black Hole Café. At dinner on the first evening at Perimeter, astronomer Schmidt and theorist Christof Wetterich, of Heidelberg University, fell into a discussion about a distinction Kolb had made earlier that afternoon, during the final lecture at McMaster.

  Kolb had begun with a meditation on how scientists think about cosmological models. To the astronomers of Copernicus's day (though not necessarily to Copernicus himself), a cosmological model was a representation of a world that made mathematical sense but might bear no relation to reality. Whether the Sun or the Earth was at the center of the cosmos didn't matter; what mattered was which object was more mathematically useful at the center of the cosmological model. Not so for the scientist of today. Over the past four centuries, scientists had learned that the accumulation of evidence could tell them which model was more correct—the one with the Earth at the center or the one with the Sun at the center. Today astronomers regarded the creation of a cosmological model as an attempt to capture "reality itself," Kolb said. "We really think that dark matter is a reality, and that dark energy is a reality." If they somehow turned out not to be, fine. But "that's really what we have to test."