Cosmological perturbations in generalised gravity theories: formulation

The author presents a simple way of deriving cosmological perturbation equations in generalised gravity theories which accounts for metric perturbations in a gauge-invariant way. The author uses an imperfect fluid formulation of the perturbation equations developed in Einstein gravity and absorb all new contributions as effective fluid quantities. The author applies this approach to the f( phi , R)- omega ( phi ) phi ,c phi c Lagrangian which includes most of the gravity theories employing a scalar field and scalar curvature. The relation between the proposed method and the conformal transformation method is discussed. Background and perturbation equations are displayed for specific gravity theories which can be recovered as special cases from the above general Lagrangian.