Dynamical Black Holes: Lessons from the Fluid-Gravity Correspondence

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For stationary black holes it is universally agreed that entropy is proportional to horizon area. It is not so clear what the relationship is for dynamical black holes. In such spacetimes the event horizon is teleologically defined while the apparent horizon is non-unique. Thus even if one believes that entropy continues to be well-defined and proportional to horizon area, there are many possible areas to choose from. In this work I will review some recent work that I have done with M. Heller, G. Plewa and M.Spalinski that examines this issue from the perspective of the fluid-gravity correspondence. In this quasi-equilibrium regime the slowly evolving black brane horizons on the gravity side are dual to a perturbed fluid flow. The fluid manifestations of the various horizon definitions can be compared and the blackbrane mechanics is dual to hydro-thermodynamics. In particular, the uncertainty as to which is the "correct" horizon can be interpreted as dual to the inherent freedom in defining an entropy current.