High-Energy Cosmic Rays and the Greisen-Zatsepin-Kuzmin Effect

Understand

Although cosmic rays were discovered over 100 years ago their origin remains uncertain.

• They have an energy spectrum that extends from 1 GeV to beyond 1020 eV, where the rate is less than 1 particle per km2 per century.
• Shortly after the discovery of the cosmic microwave background in 1965, it was pointed out that the spectrum of cosmic rays should steepen fairly abruptly above about 4 x 1019 eV, provided the sources are distributed uniformly throughout the Universe.
• This prediction, by Greisen and by Zatsepin and Kuzmin, has become known as the GZK-effect and in this article I discuss the current position with regard to experimental data on the energy spectrum of the highest cosmic-ray energies that have been accumulated in a search that has lasted nearly 50 years.