NUMBER 8
High-Energy Cosmic Rays

Earth's atmosphere is regularly bombarded by very small numbers of incredibly energetic cosmic ray particles, a type discovered less than a decade ago. No one can explain where they come from or why.

Less energetic cosmic ray particles have long been known to bathe our planet. Hundreds slam into every square yard (or square meter) of Earth's atmosphere every second. Many are thought to originate within our galaxy, spawned by large exploding stars called supernovae.

Earth's atmosphere destroys the particles, protecting us. (Astronauts on space walks, on the other hand, are exposed to the potentially deadly particles and must limit time spend outside their craft.)

A decade ago, a new type of cosmic ray particle was discovered, one far more energetic (and much more rare) than the more common type previously known.

"How these microscopic particles achieve macroscopic energies is one of the most pressing questions of high-energy astrophysics," says Niel Brandt, assistant professor of astronomy and astrophysics at Penn State. "These particles cannot travel far through the universe because interactions with the cosmic microwave background quickly sap their energy, so they must be created relatively nearby."

But no one can find the source.

Brandt and others speculate that the particles might come from some unknown super powerful cosmic explosion, from a huge black hole, or could possibly represent exotic remnants of the early universe.

Return to the Main Weird Page

More About High-Energy Cosmic Rays

When an aged star explodes (in a supernova) protons and neutrons are sent streaming out into space. Some of these particles are accelerated to nearly the speed of light. These particles, coming from relatively nearby supernovae, are thought to gain speed as they pass through magnetic fields in our galaxy, or when the shock wave of the supernova source gives them a boost.

But higher-energy cosmic rays, if they came from outside our galaxy, should be decelerated by a relatively low-energy background of microwaves that permeate the Milky Way, researchers say. This implies that the particles are generated within the Milky Way, or nearby.

The discovery of these high-energy cosmic rays stunned researchers in 1991. They were dumbfounded when a collection device above the Utah desert measured particles coming from space that were six times more energetic than any theory could account for.

Cosmologists have suggested that the particles might be accelerated by mysterious leftovers from the formation of the universe, things with names like "cosmic strings" and "domain walls." These objects, only theorized to exist, might collapse and create enough energy to produce the particles, researchers say.

If scientists can figure out where high-energy cosmic rays come from, they would gain clues to the formation of the universe.

Brandt said the enigmatic sources should be revealed within a decade. "Even after we understand these sources, however, the highest energy cosmic rays will remain a remarkable creation of nature," he said.

But studying the particles is a challenge. As few as one per week falls on any given square kilometer (0.6 miles) of the atmosphere, so a large detection system is needed.

Answers may come with the Pierre Auger Project in Argentina, an array of 1,600 particle-detector stations that will cover an area about 10 times the size of Paris. Project collaborators plan to begin cosmic-ray observations in 2001, but full operation won't get underway until 2003. Because cosmic rays don't normally reach Earth, scientists will measure the energy released by their collision with the atmosphere.

Return to the Main Weird Page . Next Weird Thing