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In recent years there has been a lot of interest in discussing framedependences/independences of the cosmological perturbations under the conformaltransformations. This problem has previously been investigated in terms of thecovariant approach for a single component universe, and it was found that thecovariant approach is very powerful to pick out the perturbative variableswhich are both gauge and conformal invariant. In this work, we extend thecovariant approach to a universe with multicomponent fluids. We find thatsimilar results can be derived, as expected. In addition, some otherinteresting perturbations are also identified to be conformal invariant, suchas entropy perturbation between two different components.

Conformal invariant cosmological perturbations via the covariant approach: multicomponent universe