Viscous modes, isocurvature perturbations and CMB initial conditions
  When the predecoupling plasma is thermodynamically reversible its
fluctuations are classified in terms of the adiabatic and entropic modes. A
different category of physical solutions, so far unexplored, arises when the
inhomogeneities of the viscosity coefficients induce computable curvature
perturbations. The viscous modes are explicitly illustrated and compared with
the conventional isocurvature solutions.
