  Collective behavior is one of the most intriguing aspects of the hydrodynamic
approach to electronic transport. Here we provide a consistent, unified
calculation of the dispersion relations of the hydrodynamic collective modes in
graphene. Taking into account viscous effects, we show that the hydrodynamic
sound mode in graphene becomes overdamped at sufficiently large momentum
scales. Extending the linearized theory beyond the hydrodynamic regime, we
connect the diffusive hydrodynamic charge density fluctuations with plasmons.
