Electron hydrodynamics beyond momentum and viscosity

The key feature of recent experimental realizations of hydrodynamic transport in metals has been the presence of a velocity field in the long wavelength description of the system. This field arises due to a long-lived momentum density and a signature consequence are non-Ohmic viscous effects in transport. In this talk I will discuss various scenarios in which additional collective modes, beyond momentum, can influence the long-wavelength description of the material. These modes can arise due to (in general fluctuating) order parameters such as superfluidity, as well as additional conserved quantities such as spin, transverse momentum or imbalance modes, and lead to a different classes of unconventional, hydrodynamic transport. I will give an overview of experimental signatures of these regimes.