The Extragalactic Sources and Ultra High Energy Cosmic Rays

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Cosmic Rays with energies above 1018 eV are denoted as Ultra High Energy Cosmic Rays (UHECRs). Their Larmor radius is so large that their arrival direction could correlate with the position of the source, if their origin is Galactic. The non-observation of anisotropies from the Galactic plane, even at the highest energies, and the evidence for a large scale anisotropy above 8 × 1018 eV pointing away from the Galactic plane, strengthens the conjecture that UHECRs are likely produced in extragalactic objects. The method used to detect UHECRs is based on the use of the Earth’s atmosphere as a target. Secondary charged particles are measured by arrays of particle detectors at ground level and the fluorescence light produced by the longitudinal development of the showers by mean of particular optical telescopes. The current generation of UHECR detectors, the Pierre Auger Observatory in Argentina and the Telescope Array experiment in Utah, are hybrids, with both surface detector arrays and fluorescence detectors observing at the same site. The goal of these very large experiments is to measure accurately the flux of UHECRs, to understand their nature and to provide evidence of correlations of arrival directions at the highest energies with the large-scale distribution of matter in the near-by Universe. After an introduction on the large-scale structure of the Universe, all these aspects are covered in this chapter.