Constraints on the Intergalactic Transport of Cosmic Rays

Motivated by recent experimental proposals to search for extragalactic cosmicrays (including anti-matter from distant galaxies), we study particlepropagation through the intergalactic medium (IGM). We first use estimates ofthe magnetic field strength between galaxies to constrain the mean free pathfor diffusion of particles through the IGM. We then develop a simple analyticmodel to describe the diffusion of cosmic rays. Given the current age ofgalaxies, our results indicate that, in reasonable models, a completelynegligible number of particles can enter our Galaxy from distances greater than$\sim 100$ Mpc for relatively low energies ($E$ $< 10^6$ GeV/n). We also findthat particle destruction in galaxies along the diffusion path produces anexponential suppression of the possible flux of extragalactic cosmic rays.Finally, we use gamma ray constraints to argue that the distance to anyhypothetical domains of anti-matter must be roughly comparable to the horizonscale.Comment: 24 pages, AAS LaTex, 1 figure, accepted to Ap