Exact Friedmann Solutions in Higher-Order Gravity Theories
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We find the general behaviour of homogeneous and isotropic cosmological models in some fourth-order theories of gravity. Explicit, exact, general solutions are given for both empty universes and those filled with a perfect fluid. For the vacuum case, solutions are found with closed, open and flat geometries, whilst the perfect fluid solutions are all spatially flat. Both early and late-time limits are studied, and attractor behaviour towards simple power-law expansion is identified. Multiple solutions to the same theories, with the same matter content and topology are found. It is shown that these solutions exhibit great variety in their evolution.