Conformal invariant cosmological perturbations via the covariant
  approach: multicomponent universe
  In recent years there has been a lot of interest in discussing frame
dependences/independences of the cosmological perturbations under the conformal
transformations. This problem has previously been investigated in terms of the
covariant approach for a single component universe, and it was found that the
covariant approach is very powerful to pick out the perturbative variables
which are both gauge and conformal invariant. In this work, we extend the
covariant approach to a universe with multicomponent fluids. We find that
similar results can be derived, as expected. In addition, some other
interesting perturbations are also identified to be conformal invariant, such
as entropy perturbation between two different components.
