The flux of ultrahigh-energy cosmic rays is very small, and it falls steeply with increasing energy. From 1012 to 1019 eV, the flux falls roughly like E −3, where E is the energy of the primary cosmic-ray particle hitting the top of the atmosphere. If the cosmic-ray spectrum continued indefinitely with an E −3 falloff, one would see only a few dozen cosmic rays per square kilometer per century with energies above 1019 eV. That's why observers studying ultra-high-energy cosmic rays want detection facilities with effective areas of thousands of square kilometers (see the article by Thomas O'Halloran, Pierre Sokolsky, and Shigeru Yoshida in Physics Today, January 1998, page 31).
By 1019 eV, the cosmic-ray flux is dominated by protons of extragalactic origin. In 1966, not long after the discovery of the cosmic microwave background, Kenneth Greisen at Cornell University pointed out that...
