This article provides a short description of the Pierre Auger Observatory adapted from the original Pierre Auger Observatory website, including links to further resources.

On the vast plain known as the Pampa Amarilla (yellow prairie) in western Argentina, a new window on the universe is taking shape. There, the Pierre Auger Cosmic Ray Observatory is studying the universe's highest energy particles, which shower down on Earth in the form of cosmic rays. While cosmic rays with low to moderate energies are well understood, those with extremely high energies remain mysterious. By detecting and studying these rare particles, the Auger Observatory is tackling the enigmas of their origin and existence.

Cosmic rays are charged particles (usually a proton or a heavy nucleus) that constantly rain down on us from space. When a cosmic ray particle reaches Earth, it collides with a nucleus high in the atmosphere, producing many secondary particles, which share the original primary particle's energy. The secondary particles subsequently collide with other nuclei in the atmosphere, creating a new generation of energetic particles that continue the process, multiplying the total number of particles. The resulting particle cascade, called "an extensive air shower," arrives at ground level with billions of energetic particles extending over an area as large as 10 square miles.

The acceleration of most low energy cosmic rays is related to various types of magnetic fields in space. These magnetic fields are known to exist on the sun, in the solar wind, and in the remnants of supernova explosions in our Milky Way Galaxy. Interactions of charged particles with these fields can account for cosmic rays with energies ranging from 1 billion (109) electron volts to 10 thousand trillion (1016) electron volts. Occasionally, however, a cosmic ray with an energy above 10 million trillion (1019) electron volts is detected. It would take 10 million Tevatrons, the world's largest particle accelerator, to achieve energies as high as these remarkable cosmic rays! There is no scientific consensus on how or where cosmic rays with these ultra-high energies originate. With unprecedented collecting power and experimental controls, the Auger Observatory has been gathering the data needed to solve those puzzles.

Auger scientists face a challenge, however, because the highest energy cosmic rays are extremely rare. Cosmic rays with energies above 1019 eV arrive on Earth at a rate of only 1 particle per square kilometer per year. The especially interesting cosmic rays, which have energies of over 1020 electron volts (equivalent to the kinetic energy of a tennis ball traveling at 53 miles per hour, but packed into a single proton!), have an estimated arrival rate of just 1 per square kilometer per century! In order to record a large number of these remarkable events, the Auger Observatory has created a detection area in western Argentina's Mendoza Province that is the size of the state of Rhode Island (USA), or a bit larger than the country of Luxembourg.

The Auger Observatory is a "hybrid detector," employing two independent methods to detect and study high-energy cosmic rays. One technique detects high energy particles through their interaction with water placed in surface detector tanks. The other technique tracks the development of air showers by observing ultraviolet light emitted high in the Earth's atmosphere.

The first detection method uses the Observatory's main visible feature - the 1,600 water tanks that cover an enormous section of the Pampa and serve as particle detectors. Each 3,000-gallon (12,000 liter) tank, separated from each of its neighbors by 1.5 kilometers, is completely dark inside - except when particles from a cosmic ray air shower pass through it. These energetic particles are traveling faster than the speed of light in water when they reach the detectors; therefore, their electromagnetic shock waves produce Cherenkov light that can be measured by photomultiplier tubes mounted on the tanks. Extensive air showers contain billions of secondary particles and can cause nearly simultaneous bursts of light in more than five tanks. Scientists can determine the energy of the primary cosmic ray particle based on the amount of light they detect from a sample of secondary particles. Slight differences in the detection times at different tank positions help scientists determine the trajectory of the incoming cosmic ray.

The charged particles in an air shower also interact with atmospheric nitrogen, causing it to emit ultraviolet light via a process called fluorescence, which is invisible to the human eye - but not to the Auger Observatory's optical detectors. The observatory's second detection method uses these detectors to observe the trail of nitrogen fluorescence and track the development of air showers by measuring the brightness of the emitted light. To the fluorescence detectors, a cosmic ray looks like a UV light bulb rocketing through the atmosphere at the speed of light, with an ever-increasing brightness that can reach up to four watts as the cascade grows to its maximum size. Using a grid of focusing mirrors to collect the light, cameras can view the air shower up to 15 kilometers away. The Auger Observatory's fluorescence detectors are much more sensitive than the human eye and can "see" distant air showers develop. Occasionally, a cascade will occur in a place where two fluorescence detectors can record it, which allows for very precise measurements of the direction the cosmic ray came from.

Employing these two complementary observation methods provides the Auger Observatory with high quality information about the types of particles in the primary cosmic rays. Comparing results from the different types of detectors also helps scientists reconcile the two sets of data and produce the most accurate results about the energy of primary cosmic rays. The fluorescence detectors are able to detect the total energy of an air shower, which is approximately equal to the energy of the primary cosmic ray. Total cosmic ray energy is more difficult to determine with the surface detectors, which sample a small fraction of the energy of an air shower. Comparing data from the two methods is similar to comparing the results of a political poll and the results of an actual election, allowing scientists to better understand data from both detection methods and work on increasing the accuracy of both techniques. While the fluorescence detectors only work on clear, moonless nights, the surface detectors are always operating regardless of atmospheric conditions.

The Auger Observatory is in the final stages of construction and has been collecting data since early 2004 near Malargüe, Argentina, a town in Mendoza Province that lies just east of the Andes Mountains. A matching site will also be built in southeastern Colorado, USA, providing nearly uniform coverage of the skies in the northern and southern hemispheres. If cosmic rays are found to arrive from specific directions, the Auger Observatories will be able to identify and study possible cosmic ray sources all over the sky with equal sensitivity. If discrete sources are not found, the full-sky coverage provided by the two sites will be essential for determining whether cosmic ray arrival directions are characterized by subtle large-scale patterns in the universe, or whether they are completely arbitrary.

The Auger Project was first proposed in 1992 by Jim Cronin and Alan Watson. Today, more than 280 physicists from more than 70 institutions around the world are collaborating to build the southern site. The 17 participating countries are sharing the $50 million construction budget, each providing a minor part of the total cost. Alan Watson and Giorgio Matthiae are the current project spokespersons, and Jim Cronin is the spokesman emeritus.