Nothing on the tree-less plains of western Argentina seems to expend much energy. Cattle stand nearly motionless as they graze on the thin grass, which grows slowly in the dry heat and high altitude. A cylindrical water tank with a small solar panel and a skyward-facing antenna sits unobtrusively in the nearly motionless landscape. But hidden within this scene is plenty of drama. At any given moment, millions of projectiles from deep space are raining down, penetrating every object in their path. Each particle then vanishes without a trace—unless it happens to pass through the water tank, where it causes a minute spark visible to scientists thousands of miles away.
The tank is one of 1,600 spaced out at one-mile intervals over 1,100 square miles of land, an area bigger than Rhode Island. Collectively they make up the Pierre Auger Cosmic Ray Observatory, a $50 million physics experiment to study bits of atomic shrapnel that blast out from some of the most violent places in the universe. These energetic particles, called (somewhat misleadingly) cosmic rays, tell revealing tales about the exploding stars and black holes that have shaped galaxies and seeded the cosmos with the essential elements of life.
Traditional telescopes are blind to many of these cataclysms. Some 600 miles to the north, atop Chile’s high mountains, some of the world’s greatest observatories are surveying the distant universe in breathtaking detail, and yet they have little new to say about the inner core of a quasar, the edge of a stellar shock wave, or clumps of dark matter. Visible light and radio waves do not or cannot escape from such regions. Cosmic rays, which fly straight from the site of the conflagration, can.
To attain a new perspective on the cosmos, astronomers are teaming up with particle physicists to develop clever ways of detecting these wayward particles. Pierre Auger’s water tanks represent one way to do it; experiments borne by balloon, like the Advanced Thin Ionization Calorimeter, are another. “To think of these devices as ‘telescopes’ is revolutionary,” says Dan Hooper, a high-energy astrophysicist at Fermi National Accelerator Laboratory (Fermilab), outside Chicago. “Telescopes are not just something you look through and point at something. You have to be pretty creative.”
The first thing to know about observing the universe with cosmic-ray eyes is that Earth’s atmosphere destroys these particles before they reach the ground. A cosmic ray—usually a proton, but sometimes other particles—will slam into air molecules 50 miles or so above the surface, rapidly shedding energy and giving rise to a shower of billions of electrons, positrons, and muons that rain down onto the terrain below.
That is where Pierre Auger’s network of water tanks comes in. The scientists chose them as detectors precisely because light moves more slowly in water than it does in air. A particle will come screaming through the atmosphere at close to light speed; as soon as it passes into the water, it finds itself in violation of nature’s speed limit. Whenever electrically charged particles go faster through an insulating material (like water) than the speed of light would allow, they disrupt nearby electrons, causing a flash of light (known as Cherenkov radiation).
Scientists know that a particle shower has occurred when multiple tanks detect flashes at the same time. By combining data on the precise timing of the flashes from all the tanks, Auger physicists can reconstruct the collision that took place high in the atmosphere and determine the energy and direction of the original cosmic ray. That’s why they call it a telescope.
In the six years that the Pierre Auger Observatory has been in operation in Argentina, it has detected 1.6 million particle showers. Recently scientists traced the origin of a few such showers to violent supernovas in the galaxy M82, located 12 million light-years away in the constellation Ursa Major. These star explosions are among the most powerful events ever observed—each one emits so much light that it can outshine an entire normal galaxy. And yet the cosmic rays that supernovas emit are ho-hum—just medium power by astrophysical standards. Nearly all the showers that scientists measure are like this: interesting, but unspectacular.
What makes physicists sit up in their chairs are those rare particles that are really, really energetic—100 million times more potent than the ones from M82. Such extreme particles are among the rarest things known to physics; they come down over any given square mile only two or three times per century. That is why the Auger network must cover so much ground. So far, says Paul Mantsch, a physicist and project manager at Auger, the observatory has seen about 50 of these ultrahigh-energy cosmic rays. (For comparison, the Hubble Space Telescope picks up many thousands of photons, or particles of light, from even the dimmest galaxies it observes.)
Scientists were startled when they started seeing these extreme particles in 1991 at the Fly’s Eye Observatory in Utah. “There is no known mechanism—not in nature and certainly not from man—to speed up particles that much,” Mantsch says. It is also not clear how those particles ever made it to Earth. Cosmic rays with this much energy should interact with microwave radiation in interstellar space and grow weaker. Yet the Utah particles and the “Auger 50,” which traveled pretty close to the speed of light, seemed to arrive intact. So for Mantsch and his colleagues, the question is acute: Where did these impossible particles come from?
The incredible energy of the cosmic rays could be an important clue, Mantsch says. In order to arrive at such a speed, their source must be near-at-hand, in cosmological terms—within 250 million light-years. In 2007 astronomers working at Auger traced some of the ultrahigh-energy cosmic rays to nearby active galactic nuclei, the turbulent centers of violent galaxies. The source of all that commotion is thought to be a giant black hole, billions of times more massive than our sun. As these monsters feast on clouds of gas, their intense magnetic fields could thrust jets of high-energy particles out into space at virtually the speed of light. The scientists, though, are not yet entirely convinced; recent data have been inconclusive. “The active galactic nuclei seemed to make sense, but now it’s a lot more up in the air,” says New York University physicist Glennys Farrar, who works with researchers at the Auger Observatory.
Some scientists think the sources of these impossible particles may be more exotic. UCLA astrophysicist Rene Ong has done sky surveys of possible cosmic-ray emitters using the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona. He finds that most cosmic rays come from well-known objects that produce other forms of radiation, too—black holes emit X-rays, for instance, and supernovas glow with visible light. But Ong has found several regions that send out cosmic rays but apparently little else, like a lightning storm in a cloudless sky. Physicists have dubbed these mystery sources, which now number in the dozens, dark accelerators. “Are they something new? We don’t know,” Ong says. “We’ve been introducing as many mysteries as we’ve solved.” Astronomers will now be training their best instruments on these parts of the sky.
While Ong hunts for the source of the most powerful cosmic rays, other physicists are using the particles to pursue another cosmic mystery: dark matter, an invisible form of mass that rarely interacts with normal particles. We know of its existence only by its gravitational effect, even though it may outweigh ordinary matter five to one. “Whoever finds it gets a trip to Stockholm,” says Joel Primack, a dark-matter theorist at the University of California at Santa Cruz.
How do you find something you cannot see? “You need a different type of telescope,” Fermilab’s Hooper says. A cosmic-ray detector may be just what is needed. Hooper wants to search for evidence of dark-matter particles colliding with one another. They would not emit visible light, but they might release energetic particles that scientists can detect.
In fact, physicists may have come across this signal already. Over the past few years, balloon and satellite cosmic-ray experiments have found high-energy electrons and their positively charged counterparts, positrons, in concentrations much higher than they would expect to see from the sun and other known sources of cosmic rays within our galaxy. Some theorists have attributed this strange excess to nearby pulsars—fast-spinning stellar remnants—but Hooper suspects that it comes from interactions between dark particles as they whip around the Milky Way.
To obtain more definitive answers, scientists need a complete census of the kinds of cosmic rays traveling through space, how fast they are moving, and the directions from which they are coming. Such precision calls for a detector that can take in cosmic rays directly. This July the Alpha Magnetic Spectrometer experiment (AMS) will start doing just that. AMS will dock with the International Space Station, where it will be able to intercept particles before they strike Earth’s atmosphere. If the source of the excess particles is a single object like a pulsar, the experiment should reveal a subtle bump in the number of cosmic rays coming from one direction. A uniform distribution of particles, in contrast, would suggest that they originate in dark matter spread throughout the Milky Way. Such a finding would turn today’s ideas about particle physics upside down. “If dark matter is producing these cosmic rays,” Hooper says, “then you have to rule out the vast majority of the models we have now.”
Scientists always crave better equipment, but the study of cosmic rays truly pushes the limits. The Auger Observatory is already the biggest and most sensitive cosmic-ray detector ever made, yet if scientists want to unravel the mystery of ultrahigh-energy cosmic rays, they will need a sample much larger than 50. They will also need much better readings on where in the sky the particles come from. Fortunately, help is on the way. In 2013 Japan plans to send the Extreme Universe Space Observatory (pdf) to the International Space Station, where it will look down at Earth for the flashes of ultraviolet light that occur when cosmic rays strike the atmosphere. On the ground, Auger scientists hope to build another observatory in Colorado—dubbed Auger North—that would cover seven times the area of the Argentine array. Between them, the two Auger projects would be able to scan the entire sky for cosmic rays.
Because these projects are expensive, scientists are also pursuing smaller, outside-the-box approaches. Looking out the window of her New York University office recently, Glennys Farrar saw something familiar to any New Yorker: wooden tanks of water on the roofs of nearby buildings. Farrar hopes to take advantage of the roughly 14,000 tanks that dot the city like alien spaceships. If she has her way, in a few years many of them will be retrofitted with about $5,000 worth of light sensors and electronics designed to detect minute flashes of light, like the Auger tanks. Cosmic-ray hunters will have made it to Broadway.
