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More on Topological Hydrodynamic Modes

Based on previous work that topologically nontrivial gapless modes in relativistic hydrodynamics could be found by weakly breaking the energy momentum conservation, in this paper, we study the holographic system which produces the same hydrodynamic modes. In the hydrodynamic system, one possibility to obtain the energy momentum non-conservation is to couple the system to external gravitational fields, i.e. to observe the system in a special non-inertial frame. Similar to what happens in the hydrodynamic system, a non-inertial frame version of holography indeed produces the same topologically nontrivial gapless hydrodynamic modes. We also generalize the study of topological modes in relativistic hydrodynamics to the case with one extra U(1) current and find that more complicated topological phase diagrams could exist when we consider more possibilities of the mass terms. We also discuss the possible underlying mechanism for this topological change in the spectrum when being observed in a non-inertial reference frame.