Ultra high energy cosmic rays: the theoretical challenge

The origin of the highest-energy cosmic rays remains a mystery. The lack of a high-energy cutoff in the cosmic-ray spectrum together with an apparentl y isotropic distribution of arrival directions have strongly constrained mo st models proposed for the generation of these particles. An overview of th e present state of theoretical proposals is presented. Astrophysical accele rators as well as top-down scenarios are reviewed along with their most gen eral signatures. The origin and nature of these ultra high energy particles will be tested by future observations and may indicate as well as constrai n physics beyond the standard model of particle physics. (C) 2000 Elsevier Science B.V. All rights reserved.