Abstract
A broad class of generalized gravity theories can be cast into Einstein gravity with a minimally coupled scalar field using a suitable conformal rescaling of the metric. Using this conformal equivalence between the theories, we derived the equations for the background and the perturbations, and the general asymptotic solutions for the perturbations in the generalized gravity from the simple results known in the minimally coupled scalar field. Results for the scalar and tensor perturbations can be presented in unified forms. The large-scale evolutions for both perturbations are characterized by corresponding conserved quantities. The simple result for the scalar perturbation is possible mainly due to our proper choice of a gauge-invariant combination which corresponds to the perturbed scalar field in the uniform-curvature gauge.
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