  We describe the cosmological dynamics of perfect fluids within the framework
of effective field theories. The effective action is a derivative expansion
whose terms are selected by the symmetry requirements on the relevant
long-distance degrees of freedom, which are identified with comoving
coordinates. The perfect fluid is defined by requiring invariance of the action
under internal volume-preserving diffeomorphisms and general covariance. At
lowest order in derivatives, the dynamics is encoded in a single function of
the entropy density that characterizes the properties of the fluid, such as the
equation of state and the speed of sound. This framework allows a neat
simultaneous description of fluid and metric perturbations. Longitudinal fluid
perturbations are closely related to the adiabatic modes, while the transverse
modes mix with vector metric perturbations as a consequence of vorticity
conservation. This formalism features a large flexibility which can be of
practical use for higher order perturbation theory and cosmological parameter
estimation.
