cosmological origins the result should be a relative absence of particles above $\sim$ 100 EeV. If the source spectrum is sufficiently flat above this “GZK” feature, one might also expect a flux pile up around 100 EeV as well. The experimental situation is unclear at the moment, since groups using complementary techniques for analyzing extensive air showers have obtained conflicting spectral properties above 100 EeV. The HiRes experiment, using atmospheric fluorescence measurements seems to see the GZK feature (Abu-Zayyad, 2002). The AGASA muon detector reports a spectrum that actually hardens towards the highest energies (Takeda, 2002). I will consider this an open question in the following discussions. The Auger Observatory, which will apply both techniques simultaneously and will have a substantially enhanced effective target area, should resolve this question in the next several years, in any case.

A related propagation issue is whether the UHECR events point back close to their sources; that is, whether intergalactic magnetic fields have any appreciable influence on UHECR trajectories. This obviously determines the degree to which the observed distribution mimics the source distribution. Magnetic fields in extragalactic space are not well known. In some rich clusters there is evidence for fields of at least 