Some Current Theoretical Issues around Ultra-High Energy Cosmic Rays
  We address some current theoretical issues around ultra-high energy cosmic rays. We recall that scenarios producing more gamma-rays than cosmic rays up to high redshift can in general only provide a sub-dominant contribution to the ultra-high energy cosmic ray flux. This includes extra-galactic top-down and the Z-burst scenarios. Finally we discuss the influence of large scale cosmic magnetic fields on ultra-high energy cosmic ray propagation which is currently hard to quantify. The views presented here represent the authors perspective. 
Guenter Sigl