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

Abstract

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. Although there is now little doubt that a suppression of the spectrum exists near the energy predicted, it is by no means certain that this is a manifestation of the GZK-effect as it might be that this energy is also close to the maximum to which sources can accelerate particles, with the highest-energy beam containing a large fraction of nuclei heavier than protons. The way forward is briefly mentioned.

40 pages with 21 figures Accepted for publication in Reports on Progress in Physics September 2013

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• Detection of ultra-high energy cosmic ray showers with a single-pixel fluorescence telescope
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• Measurement of the cosmic ray spectrum above $4\times 10^{18}$ eV using inclined events detected with the Pierre Auger Observatory
• Can transition radiation explain the ANITA event 3985267?
• Transition radiation at radio frequencies from ultra-high energy neutrino-induced showers
• The flux suppression at the highest energies
• Determination of cosmic-ray primary mass on an event-by-event basis using radio detection
• Ultra high energy cosmic rays and possible signature of black strings
• Inverse problem for extragalactic transport of ultra-high energy cosmic rays
• Effects of large-scale magnetic fields on the observed composition of ultra high-energy cosmic rays
• All-sky neutrino point-source search with IceCube combined track and cascade data
• Luminosity of ultrahigh energy cosmic rays and bounds on magnetic luminosity of radio-loud active galactic nuclei
• Gamma Ray Bursters and Black Holes in the Gravity's Rainbow
• Ultra high-energy cosmic rays from beyond the Greisen-Zatsepin-Kuz'min horizon
• On the inverse problem for extragalactic cosmic ray nuclei with energies $10^{18}$ to $10^{20}$ eV
• Ultra-high energy cosmic rays and the extragalactic gamma ray flux