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Abstract:
Cosmic ray particles are observed to $3 10^{20}$ eV, with no
sign yet of any cutoff, as would be expected from interaction
with the microwave background - of these particles were protons.
The various proposals to accont for the partciel energies,
their nature, their origin, their spectrum, and the transition
from Galactic cosmic rays will be discussed. The most
cautious proposal is still that powerful radio galaxies with
their hot spots account for all these events. However,
the lack of an observed cutoff suggests that at least at
the highest energies there may be a transition to a new
source population, either through supersymmetric partner
particles, or through Big Bang relic decay, or through
energetic neutrinos. The transition region in energy
to Galactic cosmic ray particles is also under dispute,
and there, at about $3 10^{18}$ eV, we may just see heavy
nuclei (say, C to Fe) all the way down to the knee, at
$3 10^{15}$ eV.
From $10^{14}$ eV to $3 10^{20}$ eV there is no general agreement
as to the origin of cosmic rays, but there are proposals that
may account for all theior properties quite successfully.
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