Astronomy Colloquium

Cosmic ray propagation describes how cosmic rays travel through space, exchanging energy and momentum with the ambient medium as they go. Understanding how cosmic rays propagate is fundamental to using them for astrophysical probes, and for elucidating their role in star formation and AGN feedback, and in galaxy evolution. Advances in observation, computation, and theory are creating opportunities to develop and test our understanding of cosmic ray propagation theory as never before, and revealing that the success of existing theory is mixed. I will describe some recent work on resetting and testing cosmic ray propagation theory, show why it matters, and indicate where new development may be needed.