1912

In a balloon at an altitude of 5000 meters, Victor Hess, the father of cosmic-ray research, discovered “penetrating radiation” coming from space. His was the first of many adventurous journeys made by physicists to study cosmic rays.

1927

Using a cloud chamber, Dimitry Skobelzyn photographed the first ghostly tracks left by cosmic rays.

1932

While watching the tracks of cosmic-ray particles passing through his cloud chamber, Carl Anderson discovered antimatter in the form of the antielectron, later called the positron. A positron is a particle exactly like an electron, but with an opposite, positive charge.

A debate raged over the nature of cosmic rays. According to a theory of Robert Millikan, they were gamma rays from space -- hence the name “cosmic rays.” But evidence was mounting that cosmic rays were, in fact, mostly energetic particles.

1937

Seth Neddermeyer and Carl Anderson discovered the elementary subatomic particle called the muon in cosmic rays. The positron and the muon were the first of a series of subatomic particles discovered using cosmic rays -- discoveries that gave birth to the science of elementary particle physics. Particle physicists used cosmic rays for their research until the advent of particle accelerators in the 1950s.

1938

Pierre Auger, who had positioned particle detectors high in the Alps, noticed that two detectors located many meters apart both signaled the arrival of particles at exactly the same time. Auger had discovered “extensive air showers,” showers of secondary subatomic particles caused by the collision of primary high-energy particles with air molecules. On the basis of his measurements, Auger concluded that he had observed showers with energies of 10¹⁵ eV -- ten million times higher than any known before.

1946

Groups led by Bruno Rossi in the USA and Georgi Zatsepin in Russia started experiments on the structure of Auger showers. These researchers constructed the first arrays of correlated detectors to detect air showers.

1949

Enrico Fermi put forth an explanation for the acceleration of cosmic rays. In Fermi’s cosmic-ray “shock” accelerator, protons speed up by bouncing off moving magnetic clouds in space. Exploding stars (supernovae) are believed to act as such cosmic accelerators, but they alone cannot account for the highest-energy cosmic rays.

1962

John Linsley and collaborators discovered the first cosmic ray with an energy of about 10²⁰ eV in the Volcano Ranch array in New Mexico, USA.

1966

In the early 1960s, Arno Penzias and Robert Wilson discovered that low-energy microwaves permeate the universe. Kenneth Greisen, Vadem Kuzmin and Georgi Zatsepin pointed out that high-energy cosmic rays would interact with the microwave background. The interaction would reduce their energy, so that particles traveling long intergalactic distances could not have energies greater than 5x10¹⁹ eV.

1967

An array of over 200 water-Cherenkov detectors covering 12 km² was operated for over 20 years starting in 1967 at Haverah Park in England. Although designed before the GZK prediction, the data have contributed in a major way to our understanding of cosmic rays at the highest energies.

1991

The Fly’s Eye cosmic-ray research group in the USA observed a cosmic-ray event with an energy of 3x10²⁰ eV. Events with energies of 10²⁰ eV had been reported in the previous 30 years, but this was clearly the most energetic.

1994

The AGASA Group in Japan and the Yakutsk group in Russia each reported an event with an energy of 2x10²⁰ eV. The Fly’s Eye event and these events are higher in energy than any seen before. Where did these three high-energy cosmic rays come from? None seem to point back to an astrophysical object that could impart such enormous energies.

1995

An international group of researchers began design studies for a new cosmic-ray observatory, the Pierre Auger Project, named in honor of the discoverer of air showers. The new observatory will use a giant array of detectors to detect and measure large numbers of air showers from the very highest-energy cosmic rays. Tracing high-energy cosmic rays to their unknown source will advance the understanding of the origin and evolution of the universe.

1999

The groundbreaking ceremony for the 19-nation Pierre Auger Project took place in Mendoza Province, Argentina. The Pierre Auger Observatory, designed to study the highest-energy cosmic rays with unprecedented precision, is located near the cities of Malargüe and San Rafael.

2005

The Pierre Auger Observatory celebrated its imminent completion and marked its scientific launch during the World Year of Physics.

2007

Scientists of the Pierre Auger Collaboration announced on 8 Nov. 2007 that active galactic nuclei are the most likely candidate for the source of the highest-energy cosmic rays that hit Earth. Using the Pierre Auger Observatory in Argentina, the largest cosmic-ray observatory in the world, a team of scientists from 17 countries found that the sources of the highest-energy particles are not distributed uniformly across the sky. Instead, the Auger results link the origins of these mysterious particles to the locations of nearby galaxies that have active nuclei in their centers. The results appear in the Nov. 9 issue of the journal Science.

2008

The Pierre Auger Observatory inaugurated the southern site of their observatory in Argentina, marking the end of the first phase of Observatory construction and the beginning of the second phase, which includes plans for a site in the northern hemisphere and enhancements to the southern-hemisphere site.

2010

The Pierre Auger Observatory inaugurates the High Elevation Auger Telescope (HEAT) which will observe medium-energy particle showers.

2011

The Auger Engineering Radio Array (AERA) is installed at the Pierre Auger Observatory.

Adapted with permission from the Pierre Auger Observatory's timeline of cosmic-ray history.