Cosmic Rays

From Le Figaro, November 9, 2007
Marc Mennessier

THE MYSTERIOUS ORIGINS OF COSMIC RAYS

The Pierre-Auger Observatory in Argentina partly lifts the veil on the question of where the most energetic particles ever observed in the universe might come from.

What are they? Where do they come from and how do they manage to reach us? In spite of strenuous efforts to find answers since their discovery in 1938 by French physicist Pierre Auger, these three questions were without answer: ultra high-energy cosmic rays which provoke on contact with the atmosphere large bouquets of particles (electrons, photons...) had finally become one of the most fascinating enigmas of modern physics.

Without resolving everything, the study which is currently headlining the American publication Science, by the Pierre-Auger consortium of scientists of more than 300 researchers from 17 countries, is still a scientific landmark. 'We have come a long way towards resolving the mystery', indicated, in a release, physics Nobel Prize winner James Cronin from the university of Chicago who was, with his colleagues Alan Watson from Leeds (Great-Britain), and Murat Boratav, from the CNRS (national research) at the University Pierre et Marie Curie, one of the instigators of the Pierre-Auger Observatory. Thus from the greatest scientific instrument dedicated to the study of these strange cosmic phenomena, already operational, although the installation, well into the pampas, near the city of Mendoza (Argentina), has yet to be completed.

"We have managed to demonstrate that high-energy cosmic rays formed outside our galaxy, the Milky Way, that they probably emanate from galaxies with active centers relatively close to earth ( still at a distance of some hundreds of millions of light-years nonetheless...) and that they are mostly protons", summarizes one of the authors of the publication, Antoine Letessier-Selvon, research director for CNRS/IN2P3 ( the National Institute for nuclear physics and particulate physics) who was, as early as 1992, with Murat Boratav, one of the promoters of the Pierre-Auger Observatory in France.

To fully appreciate the importance of this discovery, one has to imagine minuscule specks of matter (protons or nuclei of heavier elements such as iron) racing through space at speeds close to that of light (about 300 000 kilometers per second). At unheard of energy levels. Of the thousands of cosmic rays detected since 2004 at the Pierre-Auger Observatory, abut 80 possessed an energy superior to 40 billion billion electron-volts, exaelectron-volts ( 4 followed by 19 zeros!). " That means that one of these particles possesses the energy sufficient to elevate by one degree Celsius the temperature of one gram of water which contains approximately ten thousand billion billion particles" explains Antoine Letessier-Selvon who proposes a second analogy. In its underground ring of 27kilometers, the future large accelerator being built for CERN (nuclear research), near Geneva, will be capable of producing proton streams with energy at 10^13 electron-volts (1 followed by 13 zeros): a level yet to be achieved on earth. "Consider that, to produce a cosmic ray of 57EeV, one would have to build an accelerator with a circumference of 270 million kilometers, that is to say 7 000 times the size of earth at the equator!"

GALAXIES WITH ACTIVE CENTERS

Where and how did the universe manage to accelerate particles to such energy levels, unattainable at scale on our own planet ? It becomes understandable why this question has troubled scientists for decades. And why 17 countries got together 54 million dollars to launch in 1999 the construction of the Pierre-Auger Observatory.

If cosmic rays of 'low' energy (less than 3 EeV still!) are spread-out through space
pretty much uniformly, the authors of the study have shown, by following their trajectory in earth's atmosphere, that this is not the case for the more energetic among them (more than 57Eev). The latter come from, in effect, relatively close regions of the sky, such as the super galactic incline, where the density of matter is higher and where one finds the most galaxies with active centers. Because of the presence of giant black holes which gobble-up matter in their centers, these galaxies could play the role of cosmic accelerators, but science is yet at a loss to describe the precise mechanisms by which this might be happening.

The weakness of deviations in trajectory (between 3 and 6 degrees) provoked by galactic and extra-galactic magnetic fields also suggests that comic rays are more likely composed of protons than nuclei of heavier elements. The latter, being more electrically charged, should deviate more. There again, it is impossible to conclude with certainty, because those magnetic fields which we know badly might be weaker than we assume and thus have a weaker effect on charged particles.

FOLLOW-UP

AN IMMENSE OBSERVATORY IN THE ARGENTINIAN PAMPAS

Situated near Mendoza, in a desert area, the Pierre-Auger Observatory follows in a non pre-defined manner, extra high-energy cosmic rays.