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 $$F(R,\square R, \ldots , \square ^k R)$$ 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.

中文翻译：

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高阶引力中的宇宙学完美流体

我们证明，在罗伯逊-沃克时空（以及维数大于 3 且具有无散度外尔张量的广义罗伯逊-沃克时空）中，希尔伯特-爱因斯坦拉格朗日密度 $$F(R,\ square R, \ldots , \square ^k R)$$ F ( R , □ R , … , □ k R ) 在场方程中具有完全流体的形式。该陈述明确允许将暗能量流体作为曲率效应处理。