Grand unified theories, topological defects, and ultrahigh-energy cosmic rays

The ultrahigh-energy (UHE) proton and neutrino spectra resulting from collapse or annihilation of topological defects surviving from the grand unification (GUT) era are calculated. Irrespective of the specific process under consideration, the UHE proton spectrum ``recovers'' at {~}1.8×{}10¹¹ GeV after a partial Greisen-Zatsepin-Kuz'min ``cutoff'' at {~}5×{}10¹⁰ GeV and continues to a GUT-scale energy with a universal shape determined by the physics of hadronic jet fragmentation. The shape of the UHE neutrino spectrum is, however, sensitive also to the cosmological evolution of the defects involved.