Cosmological perfect fluids in higher-order gravity
Abstract
We prove that in Robertson-Walker space-times (and in generalized Robertson-Walker spacetimes of dimension greater than 3 with divergence-free Weyl tensor) all higher-order gravitational corrections of the Hilbert-Einstein Lagrangian density F(R, R, ... , k R) have the form of perfect fluids in the field equations. This statement definitively allows to deal with dark energy fluids as curvature effects.
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