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.