I. Introduction
THE Pierre Auger Observatory [1] was designed to disentangle the puzzle of the origin and acceleration processes of the cosmic rays with energies above the GZK cutoff. Soon after the discovery of the cosmic microwave background (CMB) remnant of the Big-Bang, Greisen [2] and Zatsepin and Kuzmin [3] noticed that sufficiently energetic cosmic rays will loose energy scattering with its photons. The pion photoproduction has a characteristic attenuation length smaller than 50 Mpc for . Then, there should be a maximum observed energy in the cosmic ray spectrum, usually taken to be around . However, several events above have been observed. A straightforward interpretation is that the cosmic rays sources should be “local” , but there seems to be no unusual astrophysical sources such as active galactic nuclei, radiogalaxies or quasars within this distance in the direction of the observed events. This suggests that new physics or astrophysics is required to understand the nature, composition and propagation of the cosmic rays of highest energies.