The State of Cosmic Ray Transport Theory
Cosmic rays are an energetically important component of the interstellar medium of galaxies and likely of the circumgalactic medium and intracluster medium. The correct treatment of cosmic ray dynamics in these environments is one of the biggest theoretical uncertainties in galaxy formation. Observations show that cosmic rays are confined to the Milky Way for a lifetime much longer than the light-crossing time across the galaxy, implying efficient scattering by small scale fluctuations in the magnetic field. The origin of this scattering is a long-standing theoretical puzzle. In this talk I will describe theoretical progress on understanding the origin of the scattering that regulates cosmic ray transport.