Cosmic rays affect galaxy evolution on all scales, from ionizing protoplanetary disks and molecular clouds to driving galactic outflows that alter the gas phase hundreds of kiloparsecs from the galactic disk. All models of cosmic-ray physics on “macro” scales (> pc) are sensitive to the assumed models of cosmic-ray scattering on “micro” scales (~ au), which are observationally and theoretically unconstrained. These effects are amplified in the circumgalactic medium, where models that fit existing data can vary by many orders of magnitude in their predictions for the cosmic-ray transport rate. In light of the many challenges facing existing models of cosmic-ray transport, I will present a novel category of “patchy” cosmic-ray scattering that is qualitatively different from the traditional “continuous” scattering models and discuss future observational constraints that could test these models.
