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Shadow Universe

Travis Rathbone

Dark matter makes up much of the cosmos, yet no one knows exactly what it is. Soon, physicists may finally solve one of science’s biggest mysteries.Dark matter makes up much of the cosmos, yet no one knows exactly what it is. Soon, physicists may finally solve one of science’s biggest mysteries.

“Turn here. Take the dirt road on the right. You’ve got to see this.” I park my rental car, and Rick Gaitskell directs me to a makeshift wooden observation deck overlooking the Trojan mine in Lead, South Dakota, just a mile down the road from his home. In the thickening twilight, we watch a phalanx of Caterpillar earthmovers scooping up and carting away chunks of a mountain, creating a large terraced pit. Nearby, a flat-crested ridge rises where the trucks have recently piled up rock from an earlier dig. Their piercing headlights mirror the glow of Venus, hovering just above the horizon.

“It’s incredible,” Gaitskell says. “There’s no stopping it. They are literally moving mountains in search of gold.” I try to read his expression in the dim light. At first, I assume he is expressing camaraderie with the diggers at the Trojan site. Technically speaking, he is a physics professor at Brown University, but it isn’t much of a stretch to say that he is also a fellow prospector.

Gaitskell leads a team that has just switched on the Large Underground Xenon (LUX) experiment, a hulking particle detector located almost a mile deep in the nearby Homestake Gold Mine. In effect, he is panning for dark matter, the invisible—and, for now, hypothetical—stuff that makes up five sixths of the mass of the cosmos. If he finds it, the Nobel committee will very likely come calling. Discovering just one dozen dark particles would be enough to throw all of modern physics for a loop. Considering the LUX experiment cost about $10 million to build, that puts the effective price of dark matter at, oh, about one million trillion trillion dollars per ounce. This is off-the-charts precious material.

“I’ve been looking for dark matter for 23, no, 24 years now,” he says. And he is not alone; the search for dark matter has grown into a small industry, albeit one that does not yet have a product to sell. “Every experiment has reported essentially negative results. No one even knows for sure if the damn stuff really exists. Those fellows,” Gaitskell says, nodding to the pit, “know exactly where the gold is.” I realize now he is not feeling empathy for the miners. He is feeling envy.

Then his expression brightens: “This time, the outcome could be different. After about two weeks of operation, we expect LUX to surpass the sensitivity of the current world-leading experiment. After that, it should be sensitive to dark-matter particles in a way that no previous direct detection has been. Meanwhile, other experiments are closing in on dark matter on multiple fronts.”

All around us, a second reality binds the universe and gives it order.Below where we’re standing, the trucks continue their restless maneuvers. Above, the stars of the Big Dipper shimmer into view. All around us, a second reality apparently binds the universe together and gives it order. No one in all of history has seen that invisible fabric. But in the next two years, scientists like Gaitskell may bring it into view.

Touching Dark Matter!

If you want to put your hands on something as subtle as dark matter, the first thing you need to do is get away from everything that may be blocking your path. That’s why my trip to the LUX experiment begins with an ear-popping elevator ride down the old Homestake Mine’s Yates Shaft. The surface world is awash with high-speed atomic fragments emitted by the sun, by supernovas exploding in deep space, even by distant black holes. With each second of my descent, that chaos fades. After a 10-minute drop, I reach 4,850 feet and walk out into a brightly lit maze of whitewashed tunnels. Until 2002, Homestake was still an active gold mine. Now, it has been repurposed as the Sanford Underground Research Facility. Only here is the surface world remote enough for LUX to do its job.

The history of dark-matter research has followed a similar trajectory, as scientists have stripped away the visible aspects of the universe to determine what else is out there. It began in the 1930s, when Swiss astrophysicist Fritz Zwicky measured the motions of galaxies and realized that even after he accounted for all the stars and gas, something seemed to be left over: massive clumps of unseen material yanking galaxies around at high speeds. He called it dunkle Materie.

Large Underground Xenon (LUX) detector

Photomultiplier tubes inside the Large Underground Xenon (LUX) detector in South Dakota can detect photons emitted when dark-matter particles collide with the nuclei of xenon atoms.

Courtesy Lux Dark Matter

These days, the evidence for dark matter is everywhere. An invisible factor makes galaxies rotate faster than expected. It makes clusters of galaxies bend and distort passing starlight more than they should. It even seems to explain how those galaxies formed in the first place. Supercomputer simulations show that diffuse clouds of ordinary matter in the early universe did not have enough gravity to pull together into the orderly galaxies and galaxy clusters seen today. Run the same simulations with a dark component stirred in and everything comes together just right.

What dark matter is, nobody knows. But physicists can tell you exactly what it is not: ordinary atoms of the variety that make up you, me, and everything else in the visible world. Some of the most persuasive proof comes from measurements of the cosmic microwave background, the afterglow of the big bang. Right after that moment of birth, the whole universe was ringing like a bell, and just as a bell’s tone reflects its size and shape, so the pattern of cosmic ringing reveals exactly what material was present in the early universe. The humbling answer: 15 percent of the matter was and still is visible, 85 percent dark. (Even more bizarre, dark and visible matter together account for only one third of the total mass; the rest seems to be an unknown form of energy embedded in space itself.)

“When I was in college and heard that 85 percent of the universe was missing, I knew that was what I wanted to study,” says Nicole Larsen, a Yale graduate student who works at the Sanford facility. Larsen and I are standing on a metal grate, eyeing the top nine feet of the two-story LUX detector. All the cool-looking stuff—the plumbing that keeps equipment clean and chilled, the electronics that collect and process data—is up here on level two.

We walk downstairs, and I take in the slight anticlimax of the detector itself. It looks a lot like a giant water tank. It is, in fact, a giant water tank. It holds 70,000 ultrapure gallons that block natural radioactivity emitted by the surrounding rock. Suspended inside the tank, out of sight, is a 70-inch-tall, two-ton titanium freezer containing 800 pounds of liquid xenon cooled to –170°F.

Considering the complexity of the underlying science, the concept behind LUX is strangely simple. “Whatever dark matter is, it certainly is in particle form,” Gaitskell says. According to the leading physics theory, dark matter is a weakly interacting massive particle, or WIMP. Sooner or later, a passing WIMP should randomly smack into an atom of ordinary matter, sending the atom flying. It would be like the invisible man going out for a jog and revealing himself by accidentally running into another jogger. When that happens to a xenon atom inside LUX, it emits a flash of light and gives off a slight electric charge. Detectors inside the tank look for that telltale pair of signals, while software weeds out the noise of everything else.

Of the 10 other competing experiments, all rely on the same basic collision principle: Spot a signal, find a WIMP, identify dark matter, bag a Nobel. Will LUX be the one to win the prize?

Gaitskell groans. “The reality is that you’re mostly trying to identify mundane signals that only look like dark matter. You’re out there on the bleeding edge of technology, so often you have to learn how your detector operates as you go,” he says. That is a recipe for errors and controversial claims, of which there have been plenty over the past two decades. Many other dark-matter experiments have reported intriguing but vague sightings. One, called DAMA, based in Italy, claims to have 10 years’ worth of observations tracking dark-matter particles blowing past Earth. Competing teams have not found a source of error, but neither have they been able to confirm the result.

“Everyone is after them, trying to drive a stake through the heart of DAMA,” says Juan Collar of the University of Chicago, who leads another dark-matter detector called COUPP, now firing up in the Vale Creighton Mine near Sudbury, Ontario.

Gaitskell is eager for more cut-and-dried answers. “It doesn’t make any sense to me to build an experiment that isn’t going to be better than everything that’s come previously,” he says, “so we planned a detector that was substantially bigger and more sensitive.” The search will begin with a 60-day shakedown test this year, followed by a 300-day run. By then, LUX will be deep into unexplored territory, surpassing the sensitivity of previous searches by about a factor of 10.

Creating the Dark Matter

Experiments designed to detect dark matter directly, such as LUX, are appealing because they are so intuitive: Either something goes bump inside the detector or it does not. But their simplicity comes with some serious limitations. If the dark particles are significantly lighter than expected, they may not show up in the detector. Even if they do, the detectors can tell you only a little about their properties.

If you really want to understand the physics of dark matter, you need to create it in the lab so you can study it and figure out what makes it tick. And if you want to start making an exotic new particle that no one has ever seen, you need to book a flight to Geneva, head down into another tunnel, and get to work at the Large Hadron Collider (LHC).

Large Hadron Collider (LHC)

Particles crashing together in the Large Hadron Collider (LHC) could produce dark matter. The particles would likely escape undetected, so physicists will look for signs of energy they carried away.

Courtesy CERN

That’s what physicist Joe Lykken of Fermilab, a U.S. national laboratory for particle physics, has been doing for the past six years. It’s what thousands of his colleagues have been doing too. Despite all the breathless headlines about the Higgs boson, finding it was something of a secondary achievement for the LHC. Peter Higgs predicted the existence of that particle nearly half a century ago to fill in the gaps in the overarching framework of particle physics known as the Standard Model. Most researchers in the field considered the reality of the Higgs boson a foregone conclusion. (One MIT physicist privately confessed he was “depressed a little” that the Higgs fit the model so well.)

The real goal of the LHC is to grapple with some of the big questions that the Standard Model does not address. Atop that list: Why is gravity so weak compared with the other forces? Why is matter arbitrarily divided into two classes of particles—exemplified by photons and electrons—that behave according to different rules? And, yes, what is dark matter?

It turns out all these questions may be related through a theory called supersymmetry. “We’ve all agreed for the last 30 years that supersymmetry was the most obvious thing for nature to do,” Lykken says, because it restores balance to particle physics and points the way toward a long-sought “theory of everything.” Supersymmetry predicts that there is an as-yet undetected third family of particles that link the two we know. Conveniently, that family includes particles that fit the description of dark matter: massive, stable, and invisible. The process of proving that supersymmetry is correct should therefore have the happy byproduct of creating dark matter and nailing down its exact properties. That is where the LHC comes in.

At the LHC, physicists race beams of protons through a 17-mile-long underground ring, accelerate them to 99.9999991 percent the speed of light, and crash them together. At those speeds, the protons contain a staggering amount of energy: The beam contains the equivalent energy of a Toyota Corolla driving at nearly the speed of sound. After the collision, that energy has to go somewhere. What happens is that it spontaneously turns into matter, creating a spray of particles. (The equivalence of matter and energy—the soul of e=mc2—is everyday reality in the subatomic world.) Any kind of particle that can be created by that much energy could be present in the spray.

The great hope of researchers like Lykken is that dark-matter particles are in the mix. Finding them is exceedingly difficult, because the particles themselves probably fly through the LHC’s instruments unseen. “Instead, you look for what we call ‘missing energy signatures,’ ” Lykken says. “That tells you there is one or more particles that we didn’t detect directly.” It is yet another form of chasing shadows.

So far, those experiments, which have been taking place ever since the LHC began smashing protons together in 2010, have turned up nothing. “I think it’s fair to say that people were a tad surprised that an instrument of the scale and audacity of the LHC didn’t see evidence of supersymmetry,” Gaitskell says. Some physicists started grumbling about abandoning the theory, but Lykken is not terribly concerned. Due to a number of technical mishaps—most notably a spill of more than six tons of liquid helium in 2008—the LHC has been operating at about half-power. Last February, engineers shut down the machine for a major upgrade.

Particle physicists have simulated what a dark-matter event at the Large Hadron Collider might look like. Proton beams colliding within the circled region produce jets of ordinary particles. They may also create dark matter, which eludes direct detection but carries away energy (represented by the red arrow) that can be deduced later.

Courtesy Matevz Tadel/University of California at San Diego

In 2015, the LHC will restart at full energy. In technical terms, it will go from 8 trillion electron volts to 14 trillion electron volts, but conceptually, it’s fair to say that physicists plan to turn up the volume to 11. “The LHC we just ran for two years was the backup plan after we had the accident,” Lykken says. “So it’s not fair to be disappointed that we didn’t find supersymmetry at the LHC, because the real LHC—the one we always advertised—hasn’t actually happened yet.”

Dark-matter searches at the reborn LHC will take many forms. Pauline Gagnon, an Indiana University research scientist who works at CERN, the European physics consortium that built the collider, is exploring speculative “hidden valley” models, wherein an entire parallel world of dark particles would phase in and out of view in the LHC. Another place to look for dark matter, she notes, is in the particles created when a Higgs boson decays. That notion shows just how quickly the discovery process in particle physics is advancing. Last year, the Higgs was the cause célèbre, inspiring hyperbolic news stories and geek revelry. By 2015, the Higgs will be a familiar part of the landscape. It will be the dirt and sand that needs to be washed away by those panning for dark-matter gold.

Hunting The Dark Matter!

While most dark-matter sleuths hunker underground, Samuel Ting focuses his research 200 miles above the planet, at the International Space Station. Ting has no interest in waiting for dark particles to knock into an atom or to shoot out of a detector here on Earth; he wants to track them down in space, on their own turf, by picking up the visible trail they may leave behind.

At first blush, that may sound like a contradiction. If something is dark, how can it be visible? But just as other particles may be able to create dark matter, dark matter may sometimes give rise to other particles. In particular, current theory suggests that if two WIMPs collide, they destroy each other, producing a burp of gamma rays and detectable particles in the process.

Those particles would have some unusual characteristics. For one thing, they would consist equally of matter and antimatter, most likely electrons and their inverted twins, positrons. For another, those particles could carry any amount of energy up to a certain point but never more, a limit set by the amount of energy contained within the original dark-matter particle. Since mass and energy are equivalent, that maximum energy could reveal the dark particle’s mass. So the visible signal, if you use the term “visible” loosely, looks like this: an unexpected flux of positrons that obey a very strict energy limit. “You will know it has a dark-matter nature, because that distribution can come only from particle physics,” Ting says.

On Earth, positrons are destroyed the moment they touch ordinary matter, so the only way to pick up the dark-matter signal, Ting says, is to search for it in the vacuum of space. Not surprisingly, the idea of launching a giant particle detector above the atmosphere generated a lot of skepticism at first. “Nobody thought this could be done in space,” he says. Ting fought for 17 years, through a space shuttle catastrophe, numerous funding challenges, and several daunting technical setbacks, to make it happen. Finally, in 2011, astronauts installed Ting’s 18,500-pound, $2-billion Alpha Magnetic Spectrometer (AMS) on the main truss of the International Space Station.

This past spring, Ting released data from the first 25 billion particle detections. He strikes a tone of stoic optimism about the ambiguous results. The AMS does not see a telltale energy cutoff—what Ting calls “the cliff”—although there is maybe a little hint of the flattening before the cliff. Also encouraging: “Our data are coming from all directions,” Ting says, which is consistent with diffuse dark matter but not with a nearby astronomical object, like a collapsed star, that happens to be spitting out positrons. And he notes that he has only 8 percent of the data that he plans to collect between now and 2028, which will enable him to map cosmic matter and antimatter at energies similar to the collisions in the LHC. “Nobody has ever been there before,” Ting says.

ISS

Dark-matter particles could destroy one another when they collide. The Alpha Magnetic Spectrometer, mounted on the truss of the ISS, looks for antimatter produced by those impacts.

Courtesy NASA

For particle searches, the AMS is by far the best game going—and probably will be for another couple of decades. But other researchers are following the dark-matter trail through space, seeking out the gamma rays that should also appear when two dark-matter particles collide. The approach requires less heroic measures and a lot less patience: NASA already has a space-based telescope, the Fermi observatory, capable of detecting such high-energy bursts of light.

In fact, the scientific literature is glutted with provocative radiation-signature claims. Over the past few years, several groups have declared that Fermi is picking up at least four different kinds of gamma-ray signals that don’t match any known object or process. That seems like tantalizing evidence of dark matter, but the reports disagree on many of the details. And many of the purported sightings are so faint that they are difficult to distinguish from instrumental effects or random cosmic noise. Even stranger, some of them should not be detectable at all according to conventional theories of dark matter.

Douglas Finkbeiner of Harvard University, who has spent much of the past year trying to make sense of one of these possible dark-matter signals, doesn’t hide his frustration. “It’s a hard game,” he says. “I would summarize by saying that things are confusing at the moment.” Yet in that confusion, he sees progress toward a deeper truth: The fact that the various experiments do not quite match up may indicate that there is more than one answer to the dark-matter puzzle.

“I don’t see how dark matter can just be one little particle living off on its own, having nothing to do with anything else,” Finkbeiner says. “I think it’s going to open the floodgates to a whole new field of physics.”

Testing the Shadow Universe!

Finkbeiner works at the Harvard-Smithsonian Center for Astrophysics, a bastion of scientists who study the universe through observation. Walk 10 minutes down Garden Street, make your way into the red-brick Jefferson Lab, knock on the door of Harvard physicist Lisa Randall, and you enter the world of theory. This is the place where fragments of the shadow world may begin to come together into a single, coherent picture.

“This is just a big new idea, so it was kind of fun to work on,” Randall says. She’s referring to a novel theory, unveiled this past summer, called “double-disk dark matter.” The name, which sounds like a failed Ben & Jerry’s flavor, doesn’t begin to do it justice. What Randall and her collaborators have done is rip away many of the assumptions that astronomers and physicists have made about dark matter, primarily out of a desire for simplicity.

Scientists have mostly assumed that dark matter is one particle—but, Randall asks, what if it is two or more mixed together? They have assumed that dark matter is largely inert, because it hardly interacts with visible matter—but what if dark matter can interact with itself in rich and complex ways? Randall describes the possibilities of the second kind of dark matter, and the hairs begin to stand up on the back of my neck. “There could be atoms, and there could be some sort of dark chemistry. There could be condensed objects, and then it’s possible they’d break into smaller ones,” she says. “It’s dark with respect to our light, but it might not be dark with respect to its own light.”

Randall has moved beyond metaphors. She is describing a literal shadow universe.

In this new view, the predominant dark component is still diffuse and largely formless, accounting for the observed motions of galaxies and all the other evidence that established the existence of dark matter in the first place. The second interactive component is very different. It collapses just like visible matter and so would form a dark disk embedded within the visible disk of the Milky Way—hence double-disk dark matter. That disk could be governed by its own interactions and its own forces. In principle, dark physics could lead to dark stars, dark planets, even dark life. “There are some crazier ideas, and we’re working on them,” Randall says. “It really is a whole new world.”

Randall and her collaborators began by considering whether some dark matter could be denser than expected, as the Fermi gamma-ray detections seem to indicate, while remaining unnoticed by the direct-detection experiments. Their theory would explain both parts: The dark disk would pull together into a concentrated, flattened form, but it would rotate in tandem with Earth and the rest of the galaxy, like neighboring horses on a merry-go-round. Particles in the dark disk would barely be moving relative to us and so would cause nary a peep in an instrument like LUX. In a yet-unpublished elaboration of the double-disk theory, Randall and physicist Matthew McCullough of MIT even explain why some underground detectors pick up dark matter while others do not.

Dark Map

The distribution of dark matter across the universe, measured by the Planck satellite, is projected onto an oval map of the full sky. Physicists identified dark-matter structures by the way their gravitational pull distorted the cosmic microwave background. Deep-blue areas are denser than light ones; gray areas indicate where light from our own galaxy was too bright for Planck to map more distant matter.

Courtesy ESA and the Planck Collaboration

To a resolutely data-oriented guy like Rick Gaitskell, all this theorizing sounds a bit wifty. “If I may paraphrase TV’s Dr. House, there are times when you have two theories if one doesn’t explain it, but it’s something you should resist,” he says. Juan Collar, Gaitskell’s rival at COUPP, is open to the philosophically opposite view: “If the universe that we can perceive is so rich, why wouldn’t this dark side of things be as rich or more?” To a particle scientist like Lykken, a universe full of diverse dark particles sounds not only possible but reasonable. “There is more dark matter than the matter we know about,” he says. “So why shouldn’t dark matter be at least as complicated?”

Fortunately, Randall’s ideas are also testable, and here too the answer could arrive within the next couple of years—possibly even before LUX, the LHC, and the AMS have a chance to weigh in. If there is a dark disk alongside the visible disk of our galaxy, it should have a measurable effect on the motion of surrounding stars. A new European space telescope called Gaia, slated at press time to launch in November 2013, is about to start making those measurements.

Just as Fritz Zwicky first glimpsed the dark universe while following the motions of galaxies 80 years ago, so Gaia may unveil a whole shadow world sitting right in front of us. My mind drifts back to the scene above the Trojan mine. Gaitskell and I were looking down, imagining the gold below. All along, the proof of the shadow universe might have been floating right above us, just waiting for someone to notice it amid the stars!

Source: PopSci