The Highest-Energy Cosmic Rays

The search for the origin of cosmic rays with energies above 10¹⁸ eV has made considerable progress during the past few years. With new data from the High Resolution Fly's Eye experiment in Utah and the Pierre Auger Observatory in Argentina, a suppression of the cosmic ray flux above 5 × 10¹⁹ eV consistent with the predicted suppression arising from interaction with the cosmic microwave background (the so-called GZK suppression) has been observed. Recent results from Auger seem to indicate that the arrival direction distribution of cosmic rays with energies above the GZK suppression is not isotropic. Progress toward a better understanding of the chemical composition of the cosmic rays is under way. A new generation of detectors is providing data of unprecedented quality and quantity. In this article, we review these recent developments and briefly outline how our view of the highest-energy cosmic rays has evolved over the past several years.