  It is difficult for conventional sources to accelerate cosmic ray particles
to the highest energies that have been observed. Topological defects such as
monopoles and strings overcome this difficulty, because their natural energy
scale is at or above the observed energies. Monopoles connected by strings are
a particularly attractive source, because they would cluster in the galactic
halo and thus explain the absence of the GZK cutoff. Heavy monopoles connected
by light strings could last for the age of the universe as required. Further
observations might support this model by detection of the anisotropy due to the
halo, or might refute such models if strong clustering of arrival directions or
correlations with known astrophysical objects are confirmed. All top-down
models must contend with recent claims that the percentage of photons among the
cosmic rays is smaller than such models predict.
