Constraints on the Intergalactic Transport of Cosmic Rays

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Abstract
Motivated by recent experimental proposals to search for extragalactic cosmic rays (including antimatter from distant galaxies), we study particle propagation through the intergalactic medium (IGM). We first use estimates of the magnetic field strength between galaxies to constrain the mean free path for diffusion of particles through the IGM. We then develop a simple analytic model to describe the diffusion of cosmic rays. Given the current age of galaxies, our results indicate that, in reasonable models, a completely negligible number of particles can enter our Galaxy from distances greater than ~100 Mpc for relatively low energies (E < 106 GeV nucleon-1). We also find that particle destruction in galaxies along the diffusion path produces an exponential suppression of the possible flux of extragalactic cosmic rays. Finally, we use gamma-ray constraints to argue that the distance to any hypothetical domains of antimatter must be roughly comparable to the horizon scale.
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