Supernovae ultimately deposit ~10% of their total energy in a population of relativistic cosmic rays that subsequently interact with the interstellar medium (ISM) via magnetic forces. Because these particles lose energy to radiation only slowly compared to the ~90% of the supernova energy that is deposited in the ISM 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 bulk, neutral ISM. In this talk I introduce a new, physically-motivated model for the coupling between cosmic rays and the neutral, star-forming ISM, and show that it successfully predicts the gamma-ray spectra of resolved nearby galaxies, the galactic IR-gamma correlation, and the cosmological gamma-ray background. I conclude by exploring the implications of this model for the importance of cosmic ray feedback, demonstrating that this mechanism is likely unimportant for rapidly star-forming galaxies either today or in the early universe, but may be critical for local dwarfs and quiescent spirals.
