UHECR propagation and interpretation of UHECR data
Charged particles of extraterrestial origin with energies in excess of 1018 eV (known as ultra-high-energy cosmic rays, UHECR) were first observed in the 1960s, and current-generation UHECR detectors have collected over 105 such events. Nevertheless, the interpretation of these data is not straightforward, and the sources of such particles are still unknown (but widely believed to be extragalactic). Complications include interactions of these particles with cosmic background photons, deflections of their trajectiories by magnetic fields, and the large systematic uncertainties and model-dependence of current determinations of their mass composition. In my talk, I summarize the state of the art in UHECR research, the main obstacles to answering currently open questions, and the outlook for the near future.