On the hunt for a new theory of cosmic ray transport

Detailed energy spectra of cosmic rays (CRs) measured close to Earth provide powerful constraints on the physics of their transport in the Milky Way. These spectra can be modeled reasonably well assuming diffusive propagation in the Galaxy with a prescribed power-law energy dependence. However, the microphysical origin of such transport remains unclear. In this presentation, I will argue that popular existing models of CR transport in magneto-hydrodynamic turbulence, which assume pitch-angle scattering by volume-filling small-amplitude magnetic-field fluctuations, are theoretically uncertain and not in good agreement with observations. This suggests that the microphysical theory of CR propagation needs to be revisited. I will discuss possible new models of CR transport that may bridge the gap between theory and observations. In particular, I will argue that CR propagation may depend on the geometry of small-scale magnetic field reversals and scattering in intermittent regions of large field-line curvature.