Abstract
Possible analogies between vacuum state and quantum fluid provide a model to study vacuum energy density induced by thermal corrections, spacetime curvature, boundary conditions and quantum back-reaction. We find that vacuum energy density in this quantum fluid model is not naturally of the order of the matter energy density. We show how higher-order corrections in quantum back-reaction can also contribute to vacuum energy density, and how the cosmological expansion is a manifestation of a universe out of mechanical equilibrium. This last fact implies that simple thermodynamic arguments are not enough to explain the cosmological constant problem because the calculation of the associated vacuum energy density requires first the knowledge of the underlying microscopic physics of vacuum.
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