The source of the highest energy cosmic rays, with energies above 10^19 eV, is an enigma. It is hard to see how conventional acceleration mechanisms (such as active galactic nuclei) can reach these energies. Furthermore, protons of such high energies are little affected by magnetic fields, and thus their arrival directions point back to their sources, but no generally agreed-upon sources are seen in those directions. This puzzle motivates the study of nontraditional sources. Topological defects, such as domain walls, strings, and monopoles, may have been produced by phase transitions in the early universe. Cosmic strings may also arise from superstring theory. Because such objects naturally have a very high energy scale, it is possible for them to release very massive particles which decay to produce the observed cosmic rays. It is also possible for some topological defects to move very rapidly and to produce very strong magnetic fields, and thus to act as acceleration sources. This project will calculate, by analysis and computer simulation, the expected cosmic ray fluxes from the various topological defect sources, for comparison with existing observational data and the results of new experiments, in particular the Auger observatory, now in operation in Argentina. The most exciting possible outcome of this work would be an explanation of observed events in terms of a specific source model. But even a negative result would lead to important bounds on topological defect models. In either case, we would learn important information about early-universe cosmology and fundamental physics at the highest energy scales
