Developments in quasihydrodynamics

At its core, hydrodynamics is a many-body low-energy effective theory for the long-wavelength, long-timescale dynamics of conserved charges in systems close to thermodynamic equilibrium. It has a wide range of applications spanning from nuclear physics, astrophysics, cosmology, and more recently strongly-interacting electronic phases of matter. In solid-state systems, however, symmetries are often only approximate, and softly broken by the presence of the lattice, impurities, and defects, or because the symmetry is accidental. Therefore, the hydrodynamic regime must be expanded to include weak non-conservation effects, which lead to a theory known as quasihydrodynamics. In this thesis we make progress in understanding the theory of (quasi) hydrodynamics, with a specific focus on applications to condensed matter systems and their holographic description.