  Simple linear second-order differential equations have been written down for
FRW cosmologies with barotropic fluids by Faraoni. His results have been
extended by Rosu, who employed techniques belonging to nonrelativistic
supersymmetry to obtain time-dependent adiabatic indices. Further extensions
are presented here using the known connection between the linear second-order
differential equations and Dirac-like equations in the same supersymmetric
context. These extensions are equivalent to adding an imaginary part to the
adiabatic index which is proportional to the mass parameter of the Dirac
spinor. The natural physical interpretation of the imaginary part is related to
the particular dissipation and instabilities of the barotropic FRW
hydrodynamics that are introduced by means of this supersymmetric scheme
