The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality - Richard Panek (2011)
Part IV. Less Than Meets the Eye
Chapter 10. The Curse of the Bambino
"I'M JUST GOING to watch this for a little while."
"Because of the funny noise?"
"Because it stepped backward."
"It stepped backward?"
"It stepped backward."
"It stepped backward." A pause. "That's impossible."
The two graduate students were staring at a pinkie-sized shaft—a device that was turning gears that were turning gears inside a copper cylinder that extended some thirteen feet underground. The shaft was rotating, or "stepping," clockwise in tiny tick, tick, ticks. A counterclockwise step might have been impossible, but the first student had seen it with his own eyes. Now he needed to see it again.
He jammed his hands in his pockets. Then he took his hands out of his pockets and crossed his arms. Next he leaned one hand against a concrete pole. Then he grabbed a swivel chair and rested a knee on the cushion. He didn't take his eyes off the shaft. Another intern wandered past, asked what they were doing, and joined the staring contest.
The shaft was the first to blink. After ten minutes it stepped backward again.
The three students marched over to the indoor shack where the other members of the team were huddling in the air conditioning. Their presence pushed the shack to its capacity: eight. The first graduate student announced his finding to Les Rosenberg, one of the leaders of the project. Rosenberg, bushy-bearded and balding, smiled, but not really.
"That's impossible," he said.
"Oh, it's just the software," said yet another member of the team, not even glancing up from a desktop computer.
Still, Rosenberg had to see for himself. Soon four physicists, hands in pockets, were staring at the shaft. Tick. Tick. Tick. Tick. Tick. Tick. Tick.
And so went the search for dark matter one summer afternoon in 2007 in a tin-roofed hangar in the California desert forty miles east of the Bay Area—officially, Building 436 of Lawrence Livermore National Laboratory, but more commonly, "the shed." The experiment was state-of-the-art, though at the moment it was more state-of-the-workbench. The interns were working from blueprints they'd spread on the concrete floor, and they were variously wielding wire cutters and wrenches, drill bits and hammers and a hacksaw. Drips, dents, flakes, scrapes, and spills decorated the tables and metal shelves. The "To Do" list on the whiteboard hanging next to the shed entrance was numbered 1 to 8, though the 8 was on its side: infinity. After lunch, the software guy fixed the software glitch: infinity minus one. The experiment was nearly twenty years in the making—this incarnation of the instrument would be the second—and it had another decade or so to go. But in the end, after the experiment had run its course, the world would know whether one of the two leading candidates for dark matter actually existed.
Even as Vera Rubin and her galaxy-motion-measuring colleagues in the 1970s were converging on the evidence for "missing mass" and prompting cosmologists to ask the inevitable question What is it?, parallel developments in particle physics were coincidentally coming up with a possible answer: Not the stuff of us. Not the stuff of atoms—the protons and neutrons, collectively called baryons, that have been forming and re-forming familiar matter from the first instant of the universe. Other stuff instead, also left over from the first instants of the universe, but not forming and re-forming—not interacting with itself or any other matter. Stuff that was weighing down the universe just by being there in abundance, but not doing much else. In the 1970s theorists were coming up with these hypothetical particles by the bushel in an effort to solve some problems with the standard model of particle physics. But when they looked at the properties such particles would have, they noticed that two in particular would exist in just the right proportion to make up the amount of matter in the universe that was "missing."
One was the axion, the particle that the physicists in "the shed" were hoping to detect. If it existed, then it did so by the trillions per cubic centimeter, and several hundred trillion would be threading their way through your body right now. Physicists are used to the idea of particles passing through seemingly solid objects; a neutrino could pass through a light-year of lead without coming into contact with another particle. But as with the search for the other leading dark-matter candidate, called the neutralino, the trick with the axion was to catch it.
Karl van Bibber started chasing the axion in 1989, when he was still on the prodigy side of forty. Three years later he recruited Rosenberg, a former student of his at Stanford whom he considered "an absolute genius" and "a world-class experimentalist," to join him in the Axion Dark Matter Experiment (ADMX). Van Bibber grew up in Connecticut, a fan of the Boston Red Sox, the team that infamously hadn't won a World Series since 1918. He spent his childhood hearing about how the Red Sox sale of Babe Ruth to the Yankees in the 1919–20 off-season had cursed the ball club. When the Red Sox went to the World Series in 2004, van Bibber's enthusiasm proved contagious; Rosenberg, who already felt some fondness for the team from his years on the faculty at MIT, joined his colleague in rooting for the Red Sox. Van Bibber's screen saver on his desktop computer at Livermore was a floating compendium of newspaper headlines from the Red Sox World Series victory in 2004: "Ghost Busters!" "BELIEVE IT!" "SEE YOU IN 2090!" He and Rosenberg long ago agreed that being a Red Sox fan was good training for being an axion hunter.
To have any hope of catching an axion they had to build a radio receiver that could track a signal with a "strength" in the vicinity of a trillionth of a trillionth—or 1/1,000,000,000,000,000,000,000, 000th—of a watt. That's three orders of magnitude fainter than the final transmission from the Pioneer 10 spacecraft in 2002, when it was seven billion miles from Earth and well on its way out of the solar system. But with Pioneer 10, scientists at least knew the signal's frequency; they knew where to turn to on the radio dial.
Karl van Bibber didn't have that luxury. But he did have an advantage over other dark-matter hunters: He'd know his prey if he saw it.
How do you see something that is dark, if by "dark" you mean, as astronomers beginning in the 1970s and 1980s did, "impossible to see"? How do you do something that is, by your own definition, impossible to do?
You don't. You rethink the question.
For thousands of years, astronomers had tried to apprehend the workings of the universe by looking at the lights in the sky. Then, starting with Galileo, they learned to look for more lights in the sky, those that they couldn't see with their eyes alone but that they could see through a telescope. By the middle of the twentieth century, they were expanding their understanding of "light," looking through telescopes that saw beyond the optical parts of the electromagnetic spectrum—radio waves, infrared radiation, x-rays, and so on. After the acceptance of evidence for dark matter, astronomers realized they would need to expand their understanding of "look." Now, if they wanted to apprehend the workings of the universe, they would have to learn to look in a broader sense of the word: to seek, somehow. To come into some manner of contact with. Otherwise, they could do only what ancient astronomers had been compelled to do, in the absence of instruments that extended one of the five senses: Save the appearances. Think. Theorize.
And theoretical was all that dark matter was. From the start, the evidence for it was indirect. We "knew" it was there because of how it affected stuff we could see. The obvious answer to what it was, was more of the same—more of the stuff we would be able to see, if only it weren't so distant, or so inherently dim, that it foiled our usual means of observation. Ockham's razor argued for a universe that consists of matter we already know—matter made from baryons—not matter we don't. Maybe, as Vera Rubin liked to joke, dark matter was "cold planets, dead stars, bricks, or baseball bats."
In 1986, Prince ton's Bohdan Paczynski suggested that if these massive objects we couldn't see did exist in the halo of our own galaxy—where astronomers thought most of the Milky Way's dark matter resided—we could recognize their presence through a technique called gravitational lensing. In 1936, Einstein had suggested that a foreground star could serve as a lens of sorts on a background star. The gravitational mass of the foreground star would bend space, and with it the trajectory of the light from the background star, so that even though the background star was "behind" the foreground star from our line of sight, we would still be able to see it. "Of course," Einstein wrote in an article, "there is no hope of observing this phenomenon directly." To the editor of the journal he privately confided, regarding his paper, "It is of little value."
Einstein, however, was thinking small. He was still stuck in the universe in which he'd come of age. But the universe was no longer swimming only in the stars in our galaxy; it was swimming in galaxies. A few months after Einstein published his brief paper on the subject, Fritz Zwicky pointed out that rather than a foreground star, a foreground galaxy could serve as a gravitational lens. And because a galaxy had the mass of billions of stars, "the probability that nebulae which act as gravitational lenses will be found becomes practically a certainty."
In 1979, that prediction came true when astronomers found two images of the same quasar thanks to the gravitational intervention of a galaxy. The advent of CCD technology and supercomputers, Paczynski realized, might allow astronomers to make gravitational-lens detections on the small scale that Einstein had described, then dismissed. Paczynski reasoned that if, from our line of sight, a dark object in the halo of our galaxy—a Massive Compact Halo Object, or MACHO—passed in front of a star in a neighboring galaxy, the gravitational effect of the dark foreground object would cause the light from the background object to appear to brighten. In 1993, two teams reported that after monitoring the brightness of millions of stars in the Large Magellanic Cloud, they had likely observed three such events—an impressive exercise in astronomy, but not a rate of discovery that suggested a Milky Way halo teeming with dark and massive objects made of baryons.
Then again, maybe the problem wasn't some unobservable matter but the observable effect—gravity. In 1981, Mordehei Milgrom, of the Weizmann Institute in Rehovot, Israel, arrived at Modified Newtonian Gravity, or MOND—a mathematical formula that he claimed described the light curves for galaxies just as well as, and probably better than, the presence of some sort of mystery matter. It did not, however, describe galaxy clusters very well.
But even if it had, physicists had already recognized a seemingly less obvious yet, somewhat paradoxically, more persuasive solution to the dark-matter problem than either the stuff we know or modified gravity: stuff we don't know.
As part of his inner space/outer space research, David Schramm as well as his students had discovered that deuterium (an isotope of hydrogen that has one neutron in the nucleus instead of none) could only be destroyed in stars rather than created (as other elements could be). Therefore, all the deuterium in the universe today must have been present in the earliest universe, and you could conclude that the present amount was at least the primordial amount. Through further calculations you could figure out how dense with baryons the early universe must have been in order for that minimum amount of deuterium to have survived that primordial period. The denser the baryonic matter, the steeper the drop in the deuterium survival rate. In order for at least this much deuterium to have existed in the early universe, the density of baryonic matter must have been at most a certain amount. This analysis therefore revealed an upper limit on the density of baryonic matter. (Schramm and Turner came to call deuterium a "baryometer.")
By similar reasoning and calculations, you could arrive at a lower limit for baryonic matter. Helium-3 (two protons plus a neutron) could only be created in stars rather than destroyed, so you could conclude that the present amount was at most the primordial amount. Then you could calculate how dense with baryons the early universe must have been in order for that maximum amount of helium-3 to have survived, and from that amount you could arrive at a lower limit on the density of baryonic matter.
By using particle physics to set upper and lower limits on the density of baryonic matter in the universe, Schramm and others converged on an omega for baryonic matter of about 0.1.
That amount, however, said nothing about non-baryonic matter.* Soon observations "weighing" the universe on different scales began converging on a number of their own—an omega in the 0.2 range, and perhaps higher. That disparity alone—0.1 baryonic matter versus 0.2 total matter—provided evidence for the existence of more than black holes and baseball bats in the halos of galaxies or suffusing galaxy clusters. The universe needed non-baryonic matter. And in a Big Bang model, such matter could come from only one place—the same place as the protons and neutrons and photons and everything else in the universe: the primordial plasma.
Even if particle physicists didn't know what these particles were, they knew that, like all the other particles that have been streaming through the universe since the first second of the universe, they had to be either fast or slow. Particles that were very light and moved at velocities approaching the speed of light—relativistic velocities—were called hot dark matter. Particles that were heavier and therefore more sluggish, attaching themselves to galaxies and moving at the same pace as the stars and gas, were called cold dark matter. And those two interpretations came with a crucial test.
In the early 1980s, astronomers hadn't yet detected the primordial ripples in the background radiation that would have corresponded to the so-called seeds of creation—the gravitational gathering grounds that would become the structures we see in the current universe. Even so, theorists knew that if those ripples did exist, then the two models of dark matter—hot and cold—would have affected them in different ways, leading to two opposite evolutionary scenarios for the universe.
Hot dark matter—particles moving at relativistic velocities—would have smeared the primordial ripples to large volumes, like a downpour on sidewalk chalk. In a universe full of matter gathering around those vast swaths, larger structures would have formed first. These vast gobs of matter would then have had to break up over time into the specks we see today—galaxies. The universe would have had a top-down, complex-to-simple history.
Cold dark matter—particles moving at a small fraction of the speed of light—would have sprinkled the primordial ripples much more subtly and affected the evolution much more slowly. Structure in that universe would have started as specks, or galaxies, and worked its way up to larger and larger structures. The universe would have had a bottom-up, simple-to-complex history. The observations in the early 1980s indicating that the Milky Way is part of a Local Supercluster, or that superclusters are separated by great voids, provided enough support for the cold-dark-matter model that most theorists abandoned the hot-dark-matter model by the middle of the decade. Then astronomers began using redshift surveys to map the universe in three dimensions, beginning in the late 1980s with the dramatic Harvard-Smithsonian Center for Astrophysics sighting of a "Great Wall" of galaxies. From 1997 to 2002, the Two-degree-Field Galaxy Redshift Survey, using the 3.9-meter Anglo-Australian Telescope, mapped 221,000 galaxies; beginning in 2000, the Sloan Digital Sky Survey, operating on the 2.5-meter telescope at the Apache Point Observatory in New Mexico, mapped 900,000 galaxies.
In those surveys and others, astronomers found that the farther across the universe they looked—and therefore the farther back in time—the less complexity they saw. Which is another way of saying that the closer they got to the present, the more complexity they saw. Galaxies formed first, at redshifts of 2 to 4—or roughly nine to twelve billion years ago. Then those galaxies gathered into clusters, at redshifts of less than 1—or less than roughly six billion years ago. And now, today (in a cosmic sense), those clusters are gathering into superclusters. Matter clumped first in small structures, and those small structures continued to gather together. The universe has apparently had a bottom-up, simple-to-complex history, consistent with theoretical cold-dark-matter models.
Still, what those surveys mapped were sources of light. They showed where the galaxies were, leaving scientists to infer where the dark matter was. In 2006, the Cosmic Evolution Survey, or COSMOS, released a map of the dark matter itself. The survey studied 575 Hubble Space Telescope images of instances in which two galaxies or clusters of galaxies lined up one behind the other. Like the microlensing technique that the MACHO surveys had used, weak gravitational lensing relied on a foreground concentration of mass to distort the light from a more distant source. Unlike microlensing, however, weak gravitational lensing recorded not individual events, as objects passed in front of other objects, but ongoing relationships between objects that were, for all practical purposes, stationary relative to each other—galaxies or clusters of galaxies. The light from a foreground object told astronomers how much mass appeared to be there. The gravitational-lensing effect on the background object told them how much foreground mass was there. The difference between the two amounts was the dark matter.
The COSMOS map not only covered an area of the sky nine times the diameter of the full moon, but was three-dimensional; it showed depth. It was like the difference between a map that shows only roads and a map that also shows the hills and valleys that the roads traverse. And because looking deeper into space means looking back in time, the COSMOS map showed how those hills and valleys got there—how the dark matter evolved. According to this "cosmopaleontology," as the team called this approach, the dark matter collapsed upon itself first, and then those centers of collapse grew into galaxies and clusters of galaxies—again, an image consistent with the bottom-up, cold-dark-matter formulation.
Perhaps the most dramatic, and certainly the most famous, indirect evidence for the existence of dark matter was a 2006 photograph of a collision of two galaxy clusters, collectively known as the Bullet Cluster. By observing the collision in x-rays and through gravitational lensing, Douglas Clowe, then at the University of Arizona, separated visible gas from invisible mass. The visible (in x-ray) gas from both clusters pooled in the center of the collision, where the atoms had behaved the way atoms behave—attracting one another and gathering gravitationally. Meanwhile, the invisible mass (detectable through gravitational lensing) appeared to be emerging on either side of the collision. It was as if dark-matter boxcars from both clusters had raced, ghostlike, right through the cosmic train wreck.
The photograph appeared around the world, and the Bullet Cluster became synonymous with dark matter. The false color helped: NASA assigned the visible gas pinkish red and the invisible mass blue. The headline on the press release also helped: "NASA Finds Direct Proof of Dark Matter."
But that wasn't quite true. Even leaving aside the dubious use of the word "proof," the "direct" was subject to debate—and had been closely parsed during the writing of the press release. The problem was that astronomers had been saying for a generation that dark matter dominated baryonic matter in the universe. Now they were saying that dark matter dominated baryonic matter in the universe. "It's not 'direct,'" Clowe conceded. "A true dark-matter direct detection would be catching a particle."
So how could you catch one? How could you capture the evidence that, as Mike Turner liked to say, "you could put in a bottle and bring to the aunt from Missouri who's saying, 'Show me'"? First, you would have to know what to look for—or "look" for.
By the late 1970s, theorists had finished fashioning the standard model of particle physics, an explanation of the relationships among three of the four fundamental forces in the universe—electromagnetism, weak interaction (or weak nuclear force), strong interaction (or strong nuclear force). The particles themselves came in two types, bosons and fermions—those that, respectively, can and cannot occupy the same quantum space. Some theorists proposed a "supersymmetry" between bosons and fermions; each boson would have a fermion partner, and vice versa. The photon, for instance, got a photino superpartner, the guage boson a guagino, the gluon a gluino. And the neutrino got a neutralino.
The neutralino—even before the axion or MACHO—turned out to be an attractive candidate for dark matter. Theorists' calculations predicted how many of these neutralinos would have survived to the present universe, and they predicted the mass of the neutralino, and when they added up those two numbers, the answer was nearly identical to the best estimates of the amount of dark matter. Aesthetically, physicists liked that the neutralino wasn't ad hoc; nobody invented it to solve the problem of dark matter. The neutralino would just be there, and its connection to dark matter was a bonus.
The trouble with the neutralino, from an observer's perspective, was that it interacted only through the weak force. Hence the name that Mike Turner bestowed on this class of dark-matter candidates: Weakly Interactive Massive Particle, or WIMP.* A WIMP wouldn't interact through electromagnetism, meaning that we couldn't see it in any wavelength. It also wouldn't interact through the strong nuclear force, meaning that it would rarely interact with atomic nuclei. The key word, though, is "rarely."
The very occasional exception was the opening that dark-matter detectives needed. It allowed them to take evidence that would be inaccessible to our senses and transform it into evidence that would be accessible. They still wouldn't be able to see the WIMPs themselves, but they would theoretically be able to see two aftereffects of a WIMP-nucleus interaction. One would be a minuscule amount of heat from the agitated nucleus. The other would be an electric charge from loosened electrons. Neither of those aftereffects in itself would be enough to identify a neutralino. But the combination of the two in a single event would be a signature unique to the particle.
To "look" for these effects, however, scientists would have to adopt another kind of "telescope," one that was new to astronomy: the laboratory.
One of the start-up programs at the Center for Particle Astrophysics in the late 1980s (along with the experiment that would become the Supernova Cosmology Project) was an effort at this kind of detection, the Cryogenic Dark Matter Search, or CDMS. In order to stabilize the target atoms—germanium, in this case—the detector had to maintain a temperature of .07 of a degree Fahrenheit above absolute zero. And in order to block out cosmic rays and other offending ordinary particles, the detector had to be shielded.
Under the leadership of the Center for Particle Astrophysics director Bernard Sadoulet, the CDMS project began life in a shallow site on the Stanford campus, seventy feet below ground, or roughly the equivalent of several hard turns in a subterranean parking garage. The problem wasn't getting a ping—a reading that showed an interaction with the nucleus of the germanium atom. Pings it got. The depth was sufficient to block out cosmic rays but not muons, which are like a heavy version of the electron. Muons penetrated the seventy feet of rock, hit the detector, and made neutrons, which leave a signal similar to the neutralino's but aren't, alas, neutralinos. The problem was getting the right kind of ping.
There was nowhere to go but down. In 2003, the successor detector, CDMS II, began operating under half a mile of rock in a former iron mine in northern Minnesota. By then CDMS had inspired a generation of similar detectors, though the high cost, large scale, and long data gestation for CDMS prompted researchers to consider cheaper, faster approaches. Many of the second-generation detectors relied on the noble gases argon, neon, and xenon, which don't need to be cooled to anywhere near absolute zero to turn into a usable liquid form, and which are far less expensive. In 2007 the XENON10 experiment, a 15-kilogram tank of liquid xenon operating in the underground laboratory at Gran Sasso, Italy, established itself as a viable rival with the release of results at a far more sensitive level than CDMS II had yet been able to reach.
Back in 1992, Sadoulet had told a journalist, "I may be bragging, but I think we're close." Sixteen years, numerous rotations of graduate students and postdocs, and two generations of detector later, a group of twelve CDMS team members gathered at his home to await a "blind" analysis of their data—a test of whether the latest research they had done would coalesce into a quantifiable result. According to their calculations, over the preceding year the CDMS II particle detector should have registered no more than one or two "hits" from stray subatomic particles of ordinary matter. The fewer hits they saw, the more confidently they could eliminate a segment of WIMP phase space—the graph that showed all reasonable combinations of size and mass. Like the settlement of a frontier by pioneers, the elimination of each swath of the graph left a narrower region to explore. At precisely midnight, they gathered around a computer in Sadoulet's living room, "unlocked" the data, and waited for the answer to bloom into view.
A cheer went up—not unlike the spontaneous applause that greeted a team member later that month at a UCLA symposium on dark matter when he stood before a hundred or so colleagues from around the world and re-created, via PowerPoint, the revelation of non-detection. CDMS II had leapfrogged back into the lead, leaving a XENON10 team member to interrupt his own PowerPoint presentation later that morning to sigh, "I guess this graph is about forty-five minutes out of date."
It was some indication of just how difficult the WIMP problem was that even a null result was cause for celebration. Later that day, one of the team leaders graciously accepted congratulations on his team's work as he boarded an elevator. "Of course," he added, softly, as the doors closed, "a detection would have been better."
Nineteen months later, he got his wish. The next "unblinding party" for CDMS II was also its last. In the interim, that incarnation of the experiment—five towers of six detectors each—had been decommissioned to make way for an upgrade: SuperCDMS. When the team "opened the box" on that last round of data, they expected the result to be more of the same: plenty of nothing. Instead, they got two "somethings": one from August 5, 2007, the other from October 27, 2007.
A null result would have made a definitive statement, excluding one more phase space for future experiments to investigate. Two detections, however, occupied a particle physics purgatory. Statistically, that number wasn't enough even to claim "evidence for," let alone the "discovery" that five events would have justified. If both events were due to background noise such as cosmic rays or radiation from within the mine, then you were unlucky. If both events were indeed the "edge of the signal," and a competing collaboration such as XENON100 (the successor to XENON10, already up and running when CDMS II opened the box) wound up seeing a statistically satisfying number of events and got to claim the discovery ... then you were still unlucky. As one graduate student said, expressing his disappointment at not getting a null result, "We would have totally dominated!"
"We're actually in the game to see something," Jodi Cooley had to remind him. The coordinator of data analysis for the experiment, she had joined the collaboration as a postdoc at Stanford five and a half years earlier, and she had secured her first faculty position, as an assistant professor at Southern Methodist University, two months earlier. By the standards of a Bernard Sadoulet, she was a newcomer to the dark-matter game. But she was also enough of a veteran that she had tired of celebrating the sighting of nothing.
Still, she knew what that grad student meant. In a way, Cooley told herself (though not the grad student), a total of two detections was "the worst-case scenario."
The collaboration spent the next few weeks running the results through data quality checks. Were the detections well inside the detector, where stray radiation was less likely to reach? Yes. Did a detection come during a time when the instruments had been behaving smoothly? Yes. Did a detection come at the same time as another detection—a double-WIMP detection that would have defied belief? No. Did the two detections occur on the same detector? No. In the end, the team subjected the results to more than fifty checks, and both detections passed every test.
The quality of the results was strong. It was the quantity that was the problem. The collaboration just didn't have enough events to let them know what they'd seen.
Still, they'd seen something. That fact alone made the result more worth reporting to the community than a null result would have been. The collaboration would have published a paper on the results no matter what the outcome, but this something—these two somethings—merited a more direct interaction with the community. The collaboration scheduled simultaneous presentations at Fermilab and Stanford the following month, as well as smaller educational sessions at other institutions in the collaboration. The subject of dark matter was tantalizing enough that the talks were sure to attract some attention.
They had no idea.
Within days, rumors about the result were dominating the particle physics slice of the blogosphere. "Dark matter discovered?" "Has Dark Matter Finally Been Detected on Earth?" "Rumor has it that the first dark matter particle has been found!" "¿Se ha descubierto la materia oscura en el CDMS?" "Pátrání po supersymetrické skryté hmotě." " dark matter ."
The team realized that by scheduling all the sessions for one day they had inadvertently given the impression that there was about to be a before-and-after moment in science. There wasn't. At best there would be a sort-of-before-and-sort-of-after-but-we-won't-really-know-until-some-other-experiment-reinforces-our-results-and-even-then-today's-announcement-would-be-seen-in-retrospect-asat-best-a-hint-of-detections-to-come moment. They "pre-poned" the Fermilab and Stanford announcements, moving them up a day, separating them from the more casual sessions, hoping to lower expectations.
Too late. If they did announce a null result, wrote one blogger, "the Thursday speakers will be torn to pieces by an angry mob, and their bones will be thrown to undergrads." To which "Anonymous" added in the Comments section on the same website, "Independent of the rumors,* I have it from a very well-known physicist that CDMS will in fact announce that they have discovered dark matter tomorrow." Discover magazine live-blogged Cooley's standing-room-only presentation, prefacing the tick-tock with: "Personally, I have heard rumors that they have either 0, 1, 3, or 4 signal events."
Well, no, no, no, and no. "THE NUMBER IS TWO!!!!"
Or as Cooley carefully explained, "The results of this experiment cannot be interpreted as significant evidence for WIMP interactions, but we cannot reject either event as a signal." Theirs wasn't a detection. It wasn't a null result. It was a neither-nor conclusion that taught the portion of the world that cared about such things a lesson that Jodi Cooley, Bernard Sadoulet, and a certain graduate student had already learned the hard way:
If you let it, dark matter will break your heart.
"I'm in love with the axion!"
Les Rosenberg didn't care who knew it. When the mood struck, he wasn't afraid to declare his affection to the world. Karl van Bibber—fit; never took the elevator when stairs were an option; looked like Leonard Nimoy, in a good way—was a bit more discreet; the word he repeatedly used about his relationship to the axion was "smitten." Like the Red Sox—and unlike the WIMP—the axion seemed to inspire a certain kind of blind devotion and underdog identification.
The natural mathematical match between the neutralino and dark matter—how many neutralinos would have survived the primordial conditions, multiplied by the predicted mass of a neutralino, equaling the best estimates of the current density of dark matter—had always made it the favorite candidate among physicists. The longer it remained undetected, however, the more the community was willing to consider alternatives. The axion might not have been as obvious a match, but it was a match nonetheless.
Like WIMPs, the axion was a hypothetical particle that fell out of an adjustment to the standard model. In 1964 physicists discovered the violation of a certain kind of symmetry in nature—in part, that the laws of physics wouldn't hold if a particle and its antiparticle traded places. In 1975 the physicists Frank Wilczek and Steven Weinberg independently realized that a particle with certain properties could solve the problem. "I called this particle the axion, after the laundry detergent," Wilczek once explained, "because that was a nice catchy name that sounded like a particle and because this particular particle solved a problem involving axial currents."*
Unlike WIMPs, however, the axion was not a massive particle. The neutralino would be fifty to five hundred times the mass of a proton; the axion would be one-trillionth the mass of an electron, which itself was 1/1,836th the mass of a proton. If axions existed, they would be a trillion times lighter than an electron, making the chance of their interacting—or coupling—with baryonic matter, as van Bibber said, "vanishingly small." But in 1983 the physicist Pierre Sikivie realized that while the axion, unlike the neutralino, couldn't couple with matter, it could interact with magnetism. Under the influence of a strong enough magnetic field, an axion could disintegrate into a photon—and that's what a detector could detect.
In 1989 van Bibber attended a meeting at Brookhaven National Laboratory, on Long Island, where Adrian Melissinos, of the University of Rochester, asked a couple of dozen physicists whether they wanted to participate in the construction of such a detector. He went around the table: "Are you in or out?" Van Bibber was in. When Melissinos had finished surveying the scientists, van Bibber pointed out that Melissinos hadn't polled himself. Was he in or out?
"Oh, this is too much like hard work," Melissinos said. "This is for you young guys."
And so van Bibber found himself leading an experiment that might outlive his professional life. ADMX was a highly magnetized resonant cavity. If axions were entering it, then they would interact with the magnetism and disintegrate into photons. If they disintegrated into photons, then they wouldn't be able to pass back through the casing of the cavity. Instead, they would remain inside, bouncing off the walls, emitting a faint microwave signal. That signal was what ADMX should be able to detect. In other words, ADMX was a radio receiver.
By 1997 he and Rosenberg had a prototype up and running. The following year they published a paper that put the community on notice: You could actually do Sikivie's strong-magnet, resonant-cavity axion experiment. Their impression was that the success of the prototype in 1998 surprised the community; it stunned them, anyway. That instrument—a waist-high copper cylinder—still sat in a corner of the shed. Once, the public relations department at Livermore contacted van Bibber about displaying it in the visitors' center, and they asked him to send a photo. He did, and he never heard back.
But it remained a thing of beauty to him. As a kid, van Bibber wanted to be a cartoonist, just like his father, Max, who drew the Winnie Winkle comic strip. Karl's drawings, however, were disturbing, and his parents thought he might be emotionally troubled, until they realized he was colorblind. His father did nonetheless influence his choice of profession. One day he brought home from Manhattan a science textbook, and Karl, then in his early teens, performed experiment after experiment until he'd exhausted the book. He was, well, "smitten."
For van Bibber, ADMX was a low-energy physics version of one of those experiments. It was usually a ten- to fifteen-person collaboration among friends, including the half-dozen kids who were doing the heavy lifting, literally. In collider physics, thousands of students could spend their entire educational careers writing the software for an experiment they might never be able to touch. "Cannon fodder," van Bibber called them. But here in the shed, the kids could spend their summers in T-shirts and shorts, griping about the heat. (Once, just for fun, the group moved a thermometer up a ladder one step at a time, and at each rung the temperature rose one degree, until it peaked at 118°F.) And as they worked, they could find out whether they, like van Bibber, nonetheless got a kick out of building a detector that could find a signal equivalent to the cosmic microwave background ... plus one photon.
ADMX was, in a real sense, a labor of love. Van Bibber loved that the axion was a high-risk career move. He loved that Rosenberg had the kind of "crazy streak" that allowed him to take that same risk. (For his part, Rosenberg called van Bibber the Mick Jagger of axions: the leader of the band, the salesman for the brand. And in 2006 the two of them even made the cover of Rolling Stone—or, at least, wrote a major article on ADMX for Physics Today.) Van Bibber loved that his collaboration was basically the only one in the world looking for the axion. He loved that the annual cost of the experiment was maybe 1 or 2 percent of the nearly one hundred million dollars spent on the two or three dozen WIMP experiments underway around the world at any one time.
But most of all, van Bibber loved that the axion signal would be so unfathomably faint. It meant that he was performing a seemingly paradoxical feat: "macroscopic quantum mechanics." He loved that if the axion was there, the instrument would detect it. You wouldn't know the frequency in advance, so the search of the microwave spectrum would have to be numbingly methodical. But when Phase II was over—Phase I ended in 2004—he would know: The axion exists, or the axion doesn't exist.
That certainty, van Bibber recognized, was something WIMP hunters could only envy—one of them a friend of his at the University of Chicago. Juan Collar was part of a generation that had joined the search for neutralinos in the 1990s. Since then he had abandoned the CDMS prototype. The acronym for his experiment, the Chicago Observatory for Underground Particle Physics (COUPP), which resided at a depth of a thousand feet in a tunnel at Fermilab, was significant: The p's were silent, as in "coup." Shaking his fist at an imaginary enemy, Collar would say, "It has the connotation of a terrible blow to the system"—the system being the whole cryogenic approach.
COUPP was less a technological advance than a throwback to an earlier era of physics: a bubble chamber. The chamber was filled with a superheated heavy liquid and outfitted with a camera; unlike other dark-matter experimenters, the COUPP team would have the thrill of seeing an actual visual result: a bubble. And bubbles they got: muons, again. Collar worried whether his generation—the particle physicists who had started out with such optimism in the 1990s, sure that they would be the ones to find the WIMP and win the race to discover dark matter—would stick around long enough to see the right kind of bubble, to hear the right kind of ping. He had his doubts. Sometimes at conferences Collar and his no-longer-on-the-prodigy-side-of-forty colleagues would convene at the hotel bar and "howl at the moon." And sometimes he would retreat to a downstairs lab at Chicago to play with a detector that had nothing to do with WIMPs, if only because when he put it next to a reactor, he would actually see a signal.
When Collar talked about his generation of researchers, he would say, "It gets kind of old, to look for a particle that might be there or not and always getting a negative result." A negative result from an experiment, after all, didn't mean that the neutralino didn't exist. It might mean only that theorists hadn't thought hard enough or that observers hadn't looked deep enough. Collar kept a graph taped to a wall in his office that showed the range where he and other researchers hoped the neutralino might reside, and sometimes he would find himself looking below the sheet of paper, at the blank wall. "If the neutralino is way down there," he would think, "we should retreat and worship Mother Nature. These particles maybe exist, but we will not see them, our sons will not see them, and their sons won't see them." And then he would think of his friend in the California desert. "Karl," Collar would tell himself, "knows he's going to get the job done, dammit."
But van Bibber, a generation older than Collar, had experienced a different kind of frustration: For years he and his fellow dark-matter hunters thought they owned the universe, if only they could find it. After 1998, they realized they owned maybe a quarter of the universe. Not bad, but van Bibber thought it was "sort of a rude demotion."
Still, he remained sanguine, as someone still in love, long into a marriage, does. When he reached his mid-fifties and thought about the possibility that ADMX would take another ten years, and that he might wind up with nothing, and that it might be the experiment that would close out his career—that he would have spent the latter half of his professional life in one way or another looking for the axion—he thought it still would have been a worthwhile pursuit. He hoped, of course, that his experiment would be the one that detected dark matter. But sometimes when he and his old friend and longtime colleague Les Rosenberg got to talking, they had to admit that after the Red Sox won the World Series, baseball was never the same.