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
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.
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
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?
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 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
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
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 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.
The Pierre Auger Project will construct two grids of detectors. One array, in the Southern Hemisphere,
covers 3,000 square kilometers and has detectors spaced 1.5 km apart in a triangular pattern.
The second array, in the Northern Hemisphere, will cover an even larger area. Using both arrays,
we can measure air showers for the whole sky, with the goal of tracking high-energy cosmic rays
to their unknown sources. For nearly 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.