Abstract

Supernovae ultimately deposit _{10% of their total energy in a population of relativistic cosmic rays. Because these particles lose energy to radiation only slowly compared to the} 90% of the supernova energy that is deposited in the interstellar medium as heat, they are a potentially important feedback mechanism in galaxies despite their comparatively small energy budget. Their effectiveness, however, depends crucially on the poorly-understood plasma processes that couple them to the background gas, which spans a huge range of properties from cool and mostly-neutral atomic and molecular gas to hot ionised medium, with most of the mass over most of cosmic time residing in the former phase. We lack a complete theory for cosmic ray-gas coupling in part due to the paucity of observational constraints on cosmic ray transport, which are limited beyond the Solar neighbourhood, and even scarcer for extragalactic systems. In this talk I discuss recent progress towards connecting microphysical models for cosmic ray transport to observable predictions for CR-driven non-thermal emission, and the implications of these results for theories of cosmic ray transport. I conclude by exploring some of the new frontiers in this area that will be opened by the advent of the CTA and, later in the decade, the SKA .