Quasars or gut: probing theories with cosmic rays

Abstract During the last few decades half a dozen detectors of various sizes and techniques observed a few tens of cosmic rays with energies exceeding 10 J (∼ 5 × 1019eV). At the same time, but with an increasing rate, innumerable articles were steadily produced on the subject by theorists and phenomenologists, and almost as many and often redundant papers have been reviewing them. There is no doubt that the end of the cosmic-ray energy spectrum triggers an uncommon excitation in several communities: astrophysicists, cosmologists, and particle physicists, generating fruitful overlaps in their works. One may consider that it was not quite useful to add one more review to the existing list. However, it seems to us that the presently available (although scarce) experimental data were not fully used in the past for a serious evaluation of many proposed models. In parallel, new ideas are also being considered by the experimental groups preparing the next generation of experiments in the field, and some decisiv...