Cosmological perturbations of a perfect fluid and noncommutative
  variables
  We describe the linear cosmological perturbations of a perfect fluid at the
level of an action, providing thus an alternative to the standard approach
based only on the equations of motion. This action is suited not only to
perfect fluids with a barotropic equation of state, but also to those for which
the pressure depends on two thermodynamical variables. By quantizing the system
we find that (1) some perturbation fields exhibit a noncommutativity quite
analogous to the one observed for a charged particle moving in a strong
magnetic field, (2) local curvature and pressure perturbations cannot be
measured simultaneously, (3) ghosts appear if the null energy condition is
violated.
