Scientists hope to glean secrets behind cosmic rays

By Ethan Frenchman

High-energy cosmic rays, single subatomic particles that occasionally enter the atmosphere, have 100 million times the energy produced in Earth’s most powerful particle accelerator. The Pierre Auger Observatory, a high-energy cosmic ray detector in Argentina managed by the U of C’s Fermilab, was inaugurated earlier this month to study such rays.

The brainchild of James Cronin, a professor emeritus of astrophysics at the University and a Nobel laureate in physics, the observatory is expected to “shed light on one of the lasting mysteries of the universe,” said University professor of astrophysics Rocky Kolb, who is unaffiliated with the project.

The goal of the observatory, which is approximately half the size of Rhode Island, is to “make use of nature’s accelerator” to potentially discover the source of the rays and even “some new physics that is at an energy scale beyond the reach of terrestrial accelerators,” Kolb said.

Cosmic rays are widely believed to be “messengers of the most extreme part of the universe,” said Angela Olinto, a University professor of astronomy and astrophysics and Auger researcher.

“They could be coming from the largest mass black holes in the centers of galaxies, but they could also be some relic from the early universe—for example, dark matter, or even some effect coming from extra dimensions. Really understanding the origins of these particles should open up a lot of questions,” Olinto said.

According to James Pilcher, chair of the Fermi Institute, Cronin began stressing the need for the observation of high-energy cosmic rays 13 years ago. Since then, Cronin has put together an international team of 250 scientists from 16 countries to work at the $50 million observatory.

High-energy cosmic rays are extremely rare, entering a given square mile only once per century. Furthermore, each ray’s impact is small, perceptible most often within a 10-kilometer radius. Researchers constructed the largest observatory possible to maximize their number of observations.

Located in the town of Malargüe in northwestern Argentina, Auger is a hybrid observatory able to record two different sets of data. Sixteen hundred 3,000-gallon plastic tanks of purified water are dispersed evenly across the array’s 1,600 square miles.

When a high-energy cosmic ray enters a tank, it creates a flash of light called the Cherenkov effect which is caused by the particle traveling at different speeds in water and in air.

Observation of the Cherenkov effect is combined with a specially designed fluorescent camera that, said Olinto, must essentially capture “a light bulb moving at the speed of light.” The camera will take pictures of the single particle entering the atmosphere, interacting with other atoms, and causing a cascade of secondary particles that resembles a fireworks display.

Complementary data collected from the fluorescent cameras as the particle moves through the atmosphere, creating secondary particles, and from the water tanks as the particles hit the ground are expected to give researchers a comprehensive observation of high-energy cosmic rays, according to Pilcher.

Plans are currently in the works to create a second observatory in Colorado to collect observations from the Northern sky. Because of the array’s low cost and expandable design, Olinto hopes that the Colorado facility will be significantly larger than its counterpart in Argentina, at approximately 22,000 square kilometers, roughly the size of New Jersey.

Participating scientists are also taking an active interest in the local community of Malargüe, which “is delighted to have this observatory there,” Pilcher said. “[The scientists] have had science fairs and hosted local high schools. There is even a school being named after James Cronin.”