Scientists love a mystery, because solving a mystery in nature means the opportunity to learn something new about the universe. High-energy cosmic rays are just such a mystery. Something out there - no one knows what - is hurling incredibly energetic particles around the universe. Do these particles come from some unknown superpowerful cosmic explosion? From a huge black hole sucking stars to their violent deaths? From colliding galaxies?
We don't yet know the answers, but we do know that solving this mystery will take scientists another step forward in understanding the universe.
Cosmic rays are fast-moving particles from space that constantly bombard the earth from all directions. Most of the particles are either the nuclei of atoms or electrons. Of the nuclei, most are single protons - the nuclei of hydrogen atoms - but a few are much heavier, ranging up to the nuclei of lead atoms. Cosmic ray particles travel at nearly the speed of light, which means they have very high energy. Some of them, in fact, are the most energetic of any particles ever observed in nature. The highest-energy cosmic rays have a hundred million times more energy than the particles produced in the world's most powerful particle accelerator.
Lower-energy cosmic ray particles that strike the earth come from within our own Milky Way Galaxy. They may originate, directly or indirectly, from the supernova explosions that mark the deaths of many stars. These explosions throw out fast-moving magnetic fields which reflect charged particles. Cosmic ray nuclei gain energy when they collide with such a moving reflector. At a magnetic shock, where the magnetic field slows abruptly, particles can become trapped between two reflectors. Like a ping-pong ball caught between two converging paddles, the nuclei make many reflections, and the energy gained in each reflection grows as their energy increases. This "magnetic shock acceleration" model was first proposed by the great physicist Enrico Fermi as an explanation for the acceleration of most cosmic rays. The process has been observed in magnetic shocks in the solar wind that flows out from our sun, producing cosmic rays of modest energy. The stronger moving magnetic fields produced in supernova explosions could provide the energy for most other cosmic rays.
Even these shocks are not strong enough, however, to accelerate the highest-energy cosmic rays. While no one knows their source, there are compelling reasons to believe that they must originate outside our Milky Way galaxy - but where?
Wherever they come from, the highest-energy particles hold secrets to the origin of their enormous energies, many millions of times greater than any earthbound particle accelerator can create. Fermi's acceleration mechanism provides an explanation for cosmic ray energies perhaps as high as 1015 eV. Acceleration mechanisms for cosmic rays of higher energies are not understood.
Observational evidence supports the view that cosmic rays with energies up to about 3 x 1018 eV originate within our galaxy. Above this energy, most cosmic rays may be coming from outside the Milky Way. The highest-energy cosmic rays are not deflected much by the weak magnetic fields in our Galaxy, yet they do not arrive preferentially from the disk of the Milky Way or the side of the sky toward the center of the Galaxy. This strongly suggests an extragalactic origin. Although we have not confirmed any source in the cosmos that can produce such energies, several hypotheses have been proposed. These include radio galaxy hot spots and active galactic nuclei (AGN) jets.
Because the highest-energy cosmic rays are deflected very little by the magnetic fields in our galaxy - and even less by the much weaker fields in intergalactic space - we ought to be able to look back in the direction of the cosmic rays to find their origin. So far, however, none of the cosmic ray events with energies above 1020 eV point back to a possible source in the cosmos! Where have they come from? The mystery deepens when we realize that, unless the source is fairly close to our Milky Way Galaxy (within 100 million light years or so), collisions with the low-energy microwaves that pervade the universe would reduce cosmic ray energies to levels below 1020 eV before they ever reached Earth. The sources must be relatively nearby, but the arrival directions do not point to any known astrophysical powerhouses.
Cosmologists, who study the structure and dynamics of the universe, offer another possible explanation for the mysterious source of the highest-energy cosmic rays. Cosmologists postulate a universe filled with relics left over from the Big Bang - hypothetical objects, called topological defects, with names like "cosmic strings," "domain walls," and "monopoles." Although these strange objects figure prominently in theories of the evolution of the universe, we have no experimental evidence to show that they really exist. However, if they did exist, and if they sometimes collapsed, their collapse could produce enough energy to create very high-energy cosmic rays. If we could make the connection between high-energy cosmic rays and the collapse of topological defects, it would provide experimental evidence for these topological defects and a great step forward in understanding the early universe.
To learn about the nature of high-energy cosmic rays, scientists measure their energy and their direction as they arrive from space. Cosmic rays of modest energy are measured directly by sending detectors to heights above most of the earth's atmosphere, using high-flying balloons and satellites. For high-energy cosmic rays, however, it is more efficient to exploit the atmosphere, measuring each cosmic ray indirectly by observing the shower of particles it produces in the air.
An air shower occurs when a fast-moving cosmic ray particle strikes an air molecule high in the atmosphere, creating a violent collision. Fragments fly out from this collision and collide with more air molecules, in a cascade that continues until the energy of the original particle is spread among millions of particles raining down upon the earth. By studying the air showers, scientists can measure the properties of the original cosmic ray particles.