A covariant approach to parameterised cosmological perturbations
  We present a covariant formulation for constructing general quadratic actions
for cosmological perturbations, invariant under a given set of gauge symmetries
for a given field content. This approach allows us to analyse scalar, vector
and tensor perturbations at the same time in a straightforward manner. We apply
the procedure to diffeomorphism invariant single-tensor, scalar-tensor and
vector-tensor theories and show explicitly the full covariant form of the
quadratic actions in such cases, in addition to the actions determining the
evolution of vector and tensor perturbations. We also discuss the role of the
symmetry of the background in identifying the set of cosmologically relevant
free parameters describing these classes of theories, including calculating the
relevant free parameters for an axisymmetric Bianchi-I vacuum universe.
