We consider the advantages of and the problems associated with hypotheses to explain the origin of ultrahigh energy cosmic rays (UHECR: E > 10 EeV) and the " trans-GZK " cosmic rays (TGZK: E > 100 EeV) both through " old physics " (acceleration in cosmic sources) and " new physics " (new particles, topological defects, fat neutrino cross sections, Lorentz invariance violation).

This is an introductory review about the on-going search for a signal of Lorentz Invariance Violation (LIV) in cosmic rays. We first summarise basic aspects of cosmic rays, focusing on rays of ultra high energy (UHECRs). We discuss the Greisen-Zatsepin-Kuz’min (GZK) energy cutoff for cosmic protons, which is predicted due to photopion production in the Cosmic Microwave Background (CMB). This is...

We propose that local Lorentz invariance is spontaneously violated at high energies, due to a nonvanishing vacuum expectation value of a vector field φμ, as a possible explanation of the observation of ultra-high energy cosmic rays with an energy above the GZK cutoff. Certain consequences of spontaneous breaking of Lorentz invariance in cosmology are discussed.

We analyze the existing evidence that BL Lacertae objects (BL Lacs) are sources of the highest-energy cosmic rays. We argue that three independent signatures observed in the real data — (1) improvement of correlations with corrections of trajectories for the Galactic magnetic field; (2) connection between γ-ray and UHECR emissions; (3) non-uniform distribution of correlating rays over the sky —...

The maximum distance bound for ultrahigh energy cosmic rays (UHECR) with energies above the Greisen-Zatsepin-Kuzmich cutoff ∼ 10 eV relaxes significantly if there is some mechanism that forces UHECR to propagate in discrete intervals, rather than continuously, through intergalactic space. In particular, intervals as small as a femtometer relax the bound for protons by an order of magnitude and ...

Abstract. In this paper we discuss the anti-GZK effect that arises in the framework of the diffusive propagation of Ultra High Energy (UHE) protons. This effect consists in a jump-like increase of the maximum distance from which UHE protons can reach the observer. The position of the jump is independent of the Intergalactic Magnetic Field (IMF) strength and depends only on the energy losses of ...

Explanations of the origin of ultra-high energy cosmic rays are severely constrained by the Greisen-Zatsepin-Kuz’min effect, which limits their propagation over cosmological distances. We argue that possible departures from strict Lorentz invariance, too small to have been detected otherwise, can affect elementary-particle kinematics so as to suppress or forbid inelastic collisions of cosmic-ra...

Quantum-gravity effects may introduce relevant consequences for the propagation and interaction of high energy cosmic rays particles. Assuming the space-time foamy structure results in an intrinsic uncertainty of energy and momentum of particles, we show how low energy (under GZK) observations can provide strong constraints on the role of the fluctuating space-time structure.