The Pierre Auger Observatory has a detection area of 3,000 km², so large that it is best seen by airplane. A space-based sucessor with a detection area hundreds of times greater is already being planned: the JEM-EUSO will be attached to the International Space Station in 2013. It will use large volumes of the earth’s atmosphere to detect and observe particles colliding with planet’s magnetic field. All images courtesy Pierre Auger Observatory

Some people enjoy living life at the edge, such as participants in extreme sports. At the other extreme are those who relish watching rare events.

Among the latter are astronomers at the Pierre Auger Observatory, a multi-national collaboration to detect the 'light-signature' given off as these cosmic rays hit particles in our atmosphere.

Based in Argentina, the observatory monitors ultra-high energy cosmic rays —  spectacular examples of some of nature’s most powerful forces. These rays consist of protons or atomic nuclei travelling near the speed of light from 300 light-years away (over one thousand trillion miles or 1 with 15 zeroes after it). In comparison, our closest star, Alpha Centauri, is only 4.37 light years away.

Mystery

A phenomenon predicted in 1963, called the Greisen-Zatsepin-Kuzmin limit, or GZK said that if cosmic particles travel from far enough away at the right energy, they will impact photons in the Cosmic Microwave Background, causing them to lose their energy and momentum. According to the theory, particles above a certain energy should not be visible from Earth. However, some ultra-high energy cosmic rays appear to break this limit; no one has a satisfactory explanation why.

Some of these particles have energies so high that they cannot even be contained within our entire galaxy's magnetic field and must be extra-galactic in origin. Suspected source of these particles include gamma-ray bursts from supernovae or energetic remnants from the universe’s creation. Other candidates are galaxies with a higher-than-average luminosity at their cores. This increased activity within a galaxy's nucleus is probably due to a super-massive back hole: as the black holes absorb all the surrounding matter, accretion disks form, and within these tempestuous regions  huge amounts of radiation and particles are ejected.

Researchers at the observatory have confirmed that the radiation of these particles is not uniform in all directions, which means that scientists should more easily trace the origin of these particles back to their original source, as the particles have effectively moved in a straight line through deep space.

Simulated ultra-high energy cosmic ray airshower of a proton with an energy of 1019 electron volts. The colors represent different kinds of particles such as photons, electrons and muons.

From the Fly's Eye

But there are still more questions.

Pierre Auger Observatory scientist Lukas Nellen said: "Work is in progress on determining the nature of the ultra-high energy cosmic rays. Are they protons or iron or something in-between?"

He and his colleagues use devices callled photomultipliers to track the light flashes produced when cosmic rays strike a detector. They also use grid computing to simulate the ‘air showers’ produced when these collisions cause a cascade of secondary particles to be produced.

In 1991, a particle was observed by the Fly’s Eye Observatory in Utah with an energy of 51 joules. Called the ‘Oh-My-God particle,’ it flabbergasted astronomers. It had the equivalent kinetic energy of a tennis ball travelling at 96 kilometers or 60 miles per hour, condensed into a point billions of times smaller than the width of a human hair.

Even the most powerful particle accelerator in the world, the LHC, can only accelerate protons to a maximum energy of 14 TeV (teraelectronvolts). These cosmic rays have 20 million times more energy. Nellen said “As human beings, we are all interested in the ‘most extreme;’ e.g.  sports, engineering, the animal kingdom. In our case, it's physics.”

Probing these cosmic-rays is the extreme sports of physics and like Formula One racing, things get exciting when there is a crash, or when an ultra-high energy particle collides into Earth's atmosphere.

—Adrian Giordani, iSGTW