Conformal invariant cosmological perturbations via the covariant
  approach
  It is known that some cosmological perturbations are conformal invariant.
This facilitates the studies of perturbations within some gravitational
theories alternative to general relativity, for example the scalar-tensor
theory, because it is possible to do equivalent analysis in a certain frame in
which the perturbation equations are simpler. In this paper we revisit the
problem of conformal invariances of cosmological perturbations in terms of the
covariant approach in which the perturbation variables have clear geometric and
physical meanings. We show that with this approach the conformal invariant
perturbations are easily identified.
