Some mysterious source from beyond the galaxy periodically
zaps Earth with high-energy cosmic rays from all directions.
So far, the two leading experiments devoted to the
detection and understanding of these rays have yielded
contradictory results.

“Right now theorists are having a lot of fun because we
have two possible outcomes, and maybe a third one, that
both experiments are incorrect and there’s a third
explanation,” said Angela Olinto, Associate Professor
in Astronomy & Astrophysics at the University of Chicago.

A new experiment will likely pierce the secrets of these
high-energy cosmic rays and resolve the data conflict
within a few years, said Olinto, who will discuss the
mystery in a session on cosmic-ray astrophysics on
Sunday, Feb. 16, at the American Association for the
Advancement of Science annual meeting in Denver.

High-energy cosmic rays are subatomic scraps of matter
that fly through the universe at nearly the speed of
light. These particles hit Earth’s atmosphere with the
energy of a tennis ball traveling at 167 miles an hour,
Olinto said.

The Auger experiment is operated in Argentina by a
collaboration of more than 250 scientists in 16 nations.
The project was initiated by James Cronin, a Nobel
laureate and University Professor Emeritus in Physics
at the University of Chicago, and Alan Watson of the
University of Leeds. Cronin and Watson named the project
after Pierre Auger, the scientist who discovered cosmic
rays in 1938. Auger conducted research at Chicago in
1942, launching hot-air balloon experiments from the
University’s Stagg Field to study cosmic rays.

High-energy cosmic rays are relatively rare. They occur
only once a century over a given square kilometer patch
of land — thus the large size of high-energy cosmic ray
experiments. When complete, the Auger experiment will
consist of a grid of electronic instruments that covers
3,000 square kilometers, an area more than half the size
of the state of Delaware.

When a cosmic ray strikes Earth, it reacts with atoms in
the atmosphere to create a cascade of a billion particles
that shower the ground. Utah’s existing High Resolution
Fly’s Eye detects cosmic rays by observing the fluorescent
light they cause when they strike the atmosphere. Japan’s
current Akeno Giant Air Shower Array detects the cascade
of secondary particles when they strike the ground. Auger
will use both techniques on a larger scale and should
resolve the data discrepancy between HiRes and AGASA.

For years it seemed that cosmic rays emanated from all
over the sky, but that is no indication of where they
originally came from. That is because the rays are
electrically charged and magnetic fields deflect their
paths as they travel through the universe. But at the
very highest energies, the rays will travel a direct
line from their source to Earth, enabling scientists
to pinpoint their origin.

AGASA has begun to see a hint of clusters of particles
coming from the same region. AGASA also sees cosmic
rays at unexpectedly high energies.

“That is really exciting, because if AGASA is really
finding particles above the energy where we thought
they wouldn’t be, then there’s a whole new class of
cosmic particle accelerators that nobody has predicted,”
Olinto said.

HiRes, meanwhile, has detected neither clustering nor
cosmic rays at unexpectedly high energies. But Auger
should be able to settle the discrepancy as it grows
from its current 30 detectors to 1,600 detectors in the
coming years. “I believe AGASA has a better case for
being correct,” Olinto said. “I also hope AGASA is
correct — it will be a lot more fun.”

Some scientists have proposed that the high-energy
cosmic rays are produced by jets of matter emitted by
supermassive black holes or by gamma-ray bursts, which
are the most powerful explosions in the universe.
Another possibility is topological defects, stresses
and strains comparable to faults and folds in the
Earth’s crust that periodically release tremendous
energies generated early in the history of the universe.
Spinning neutron stars within the Milky Way galaxy are
yet another explanation, according to a proposal put
forward by Olinto, Pasquale Blasi of the Astrophysical
Observatory in Arcetri, Italy, and Richard Epstein of
Los Alamos National Laboratory.

“A very young neutron star could be spinning so fast,
3,000 times a second, that its strong magnetic fields
could hit these particles, almost like a baseball bat,
to incredible energies,” Olinto said.

Only time will tell which theory, or another one entirely,
proves correct.

“Right now we have a lot of fun things to debate,” she said.