Cosmological Perturbations and the Weinberg Theorem
  The celebrated Weinberg theorem in cosmological perturbation theory states
that there always exist two adiabatic scalar modes in which the comoving
curvature perturbation is conserved on super-horizon scales. In particular,
when the perturbations are generated from a single source, such as in single
field models of inflation, both of the two allowed independent solutions are
adiabatic and conserved on super-horizon scales. There are few known examples
in literature which violate this theorem. We revisit the theorem and specify
the loopholes in some technical assumptions which violate the theorem in models
of non-attractor inflation, fluid inflation, solid inflation and in the model
of pseudo conformal universe.
