Reconciling cosmic-ray transport theory with phenomenological models
  motivated by Milky-Way data
  Phenomenological models of cosmic-ray (CR) transport in the Milky Way (MW)
can reproduce a wide range of observations assuming that CRs scatter off of
magnetic-field fluctuations with spectrum $\propto k^{-\delta}$ and $\delta
\sim [1.4,1.67]$. We study the extent to which such models can be reconciled
with current microphysical theories of CR transport, specifically
self-confinement due to the streaming instability and/or extrinsic turbulence
due to a cascade of MHD fast modes. We first review why it is that on their own
neither theory is compatible with observations. We then highlight that CR
transport is a strong function of local plasma conditions in the multi-phase
interstellar medium (ISM), and may be diffusive due to turbulence in some
regions and streaming due to self-confinement in others. A multi-phase
combination of scattering mechanisms can in principle reproduce the main trends
in the proton spectrum and the boron-to-carbon ratio (B/C). However, models
with a combination of scattering by self-excited waves and fast-mode turbulence
require significant fine-tuning due to fast-mode damping, unlike
phenomenological models that assume undamped Kolmogorov turbulence. The
assumption that fast modes follow a weak cascade is also not well justified
theoretically, as the weak cascade is suppressed by wave steepening and
weak-shock dissipation even in subsonic turbulence. These issues suggest that
there may be a significant theoretical gap in our understanding of MHD
turbulence. We discuss a few topics at the frontier of MHD turbulence theory
that bear on this (possible) gap and that may be relevant for CR scattering.
