The bulk speed of relativistic cosmic rays is limited by frequent scattering off hydromagnetic waves. In models of galaxy evolution, it is frequently assumed, then, that the cosmic ray population is strongly coupled to magnetic perturbations and volumetrically influences the gas through its pressure gradient; however, zooming in on the multiphase ISM, we find an "obstacle course" of density inhomogeneities that induce cosmic ray collisions, ionization-dependent transport, and macroscopic decoupling from waves. So how do cosmic rays navigate and influence such a medium, and can we constrain their transport with observations?
In this talk, I'll present two sets of idealized, high-resolution MHD simulations that probe cosmic ray interactions with multiphase gas. First, I'll show how cosmic ray fronts sample and exchange energy and momentum with a clumpy ISM, illuminating the key roles of magnetic field topology and cloud interfaces. Second, I'll briefly discuss cosmic ray interplay with compressive turbulence and the efficiency of turbulent reacceleration in ISM, CGM, and ICM environments. Of note, we find that turbulent reacceleration in the ISM is negligible for cosmic rays with energies up to ~200 GeV, which alleviates tension between observations and cosmic ray propagation models.