Auger results on clustering of > ∼ 60 EeV ultra-high energy cosmic ray (UHECR) ions and the interpretation of the γ-ray spectra of TeV blazars are connected by effects from the extragalactic background light (EBL). The EBL acts as an obscuring medium for γ rays and a reprocessing medium for UHECR ions and protons, causing the GZK cutoff. The study of the physics underlying the coincidence betwe...

The High Resolution Fly’s Eye (HiRes) experiment has observed the GZK cutoff. HiRes observes two features in the ultra-high energy cosmic ray (UHECR) flux spectrum: the Ankle at an energy of 4 × 10 eV and a high energy suppression at 6×10 eV. The later feature is at exactly the right energy for the GZK cutoff according to the E1/2 criterion. HiRes cannot claim to observe a third feature at lowe...

In this general introduction to Ultra High Energy Cosmic Rays (UHECRs) we discuss the propagation of UHE protons and the GZK feature that is expected approaching 10 eV for homogeneously distributed sources. We also briefly present the effects of the propagation on other particles that can play the role of UHECRs. With the help of numerical simulations for the propgation of UHECRs, we show that ...

Assuming that there is no GZK (Greisen-Zatsepin-Kuzmin) cut-off and that super-GZK cosmic rays correlate with AGN (Active Galactic Nuclei) at cosmological distances, it is speculated that a relic superheavy particle (X) has its lifetime enhanced by sequestration in an extra dimension. This sequestration is assumed to be partially liberated by proximity of merging supermassive black holes in an ...

The High Resolution Fly’s Eye (HiRes) experiment has observed the GZK cutoff. HiRes’ measurement of the flux of cosmic rays shows a sharp suppression at an energy of 6×10 eV, exactly the expected cutoff energy. We observe the “ankle” of the cosmic ray spectrum as well, at an energy of 4× 10 eV. However we cannot claim to observe the third spectral feature of the ultrahigh energy cosmic ray regi...

The apparent lack of suitable astrophysical sources for the observed highest energy cosmic rays within approximately 20 Mpc is the "Greisen-Zatsepin-Kuzmin (GZK) paradox." We constrain representative models of the extragalactic magnetic field structure by Faraday rotation measurements; limits are at the microG level rather than the nG level usually assumed. In such fields, even the highest ener...

The conjecture that the GZK puzzle might be related with nontrivial structure of the inelastic defect of total cross sections in scattering from nuclei has been suggested.

We point out that the presence of energetic cosmic rays above the GZK cutoff may be explained by fundamental non-linearities in quantum mechanics at the Planck level.

We propose a new probe of cosmic relic neutrinos (C$\nu$B) using their resonant scattering against cosmogenic neutrinos. Depending on the lightest neutrino mass and energy spectrum flux, Standard Model vector meson (such as hadronic $\rho$) resonance can be produced via $\nu\bar{\nu}$ annihilation. This leads to distinct absorption feature in flux at an solely determined by mass, apart from red...