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Mysteries that still need to be solved
What Are Cosmic Rays?
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.
Where Do Cosmic Rays Come From?
An active galactic nucleus (AGN) containing a black hole, a possible source of high-energy cosmic rays.
No one knows the source of the highest-energy cosmic ray particles. Most lower-energy cosmic ray particles that strike the earth come from within our own Milky Way Galaxy. Many probably come from the exploding stars we call supernovae. Over time, some cosmic ray particles pick up energy from moving magnetic fields they encounter as they wander around the galaxy. The great physicist Enrico Fermi first provided an explanation for the acceleration of most cosmic rays. In Fermi's cosmic ray accelerator the protons "bounce" off moving magnetic clouds in space. Despite the random directions of both the cosmic rays and the clouds, over time the cosmic rays gain energy. This process is well understood for low-energy cosmic rays accelerated by magnetic fields produced by the sun. In our own galaxy, scientists believe that the strong moving magnetic fields produced in supernova explosions provide the energy for acceleration. Scientists believe that the very highest-energy cosmic ray particles come from sources beyond the Milky Way -- but where?
Where Do They Get Their Energy?
Colliding galaxies, another possible source of high-energy cosmic rays.
Wherever they come from, the highest-energy particles hold secrets to the beginning of the universe, because of the mystery of their enormous energies, so many millions of times greater than any earthbound particle accelerator can create. We know of no source in the cosmos that could produce such energies, not even the power released by the most violent exploding stars.
Fermi's acceleration mechanism provides an explanation for cosmic ray energies perhaps as high as 1015 eV. However, the size of the galaxy and the magnetic field strengths available within it are not great enough to accelerate particles to energies higher than that. More powerful natural accelerators, therefore, must be responsible for the cosmic rays observed at still higher energies; and these powerful accelerators must lie outside our galaxy. In fact, cosmic rays of energies beyond 1020 eV cannot be easily contained by the weak magnetic fields within the galaxy, a fact that also suggests their extragalactic origin.
Cosmological Questions
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.
Cosmologists -- scientists 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.
How Do We Learn About Cosmic Rays?
To discover the source of cosmic rays, scientists measure their energy and their direction as they arrive from space, and learn something about their nature. To measure cosmic ray particles directly requires sending detectors to heights above most of the earth's atmosphere, using high-flying balloons and satellites. However, we can also detect cosmic rays indirectly on the surface of the earth by observing the showers of particles they produce 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.
The Pierre Auger Project will construct two 3,000 square kilometer grids of detectors spaced at 1.5 kilometer intervals. One array in the Northern Hemisphere and a second in the Southern Hemisphere will measure air showers for the whole sky, in the attempt to track high-energy cosmic rays to their unknown sources. For almost three quarters of a century, cosmic ray research has solved important scientific problems -- and uncovered new ones. In that tradition, the Pierre Auger Project will try to answer the questions of the highest-energy cosmic rays, one of the great scientific mysteries of our time.
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