Ultra high energy cosmic rays

The origin of Ultra High Energy Cosmic Rays (UHECR) is reviewed. Protons and nuclei with energy higher than (3-10) ‐ 10 19 eV are not confined by galactic magnetic fields and are expected to have large disc anisotropy in contrast to observations. Extragalactic protons and nuclei from the remote sources must have a cutoff in their energy spectra due to interaction with microwave photons. The absence of such a cutoff in the observed spectrum constitutes the UHECR problem. Astrophysical and elementary particle solutions of the UHECR problem are reviewed. The uniform distribution of the sources, GRBs and the sources with local overdensity are shown to be in contradiction with observational data. The viable astrophysical model with AGN as the sources explains very well the observed spectra up to E ∼8 ‐ 10 19 eV. The calculated spectrum has the features of interaction with microwave radiation in the form of the dip, due to electron-positron pair production, and beginning of steepening due to pion production. These features are seen in the observed spectrum. However at energy E >1 ‐ 10 20 eV there is the excess of the observed events. Elementary particle solutions to the UHECR problem include superheavy relic particle production at the inflationary stage, production of UHE particles by topological defects (TD), models of Z-bursts and others. These models can have spectra without cutoff.