  The viscous inhomogeneities of a relativistic plasma determine a further
class of entropic modes whose amplitude must be sufficiently small since
curvature perturbations are observed to be predominantly adiabatic and Gaussian
over large scales. When the viscous coefficients only depend on the energy
density of the fluid the corresponding curvature fluctuations are shown to be
almost adiabatic. After addressing the problem in a gauge-invariant
perturbative expansion, the same analysis is repeated at a non-perturbative
level by investigating the nonlinear curvature inhomogeneities induced by the
spatial variation of the viscous coefficients. It is demonstrated that the
quasiadiabatic modes are suppressed in comparison with a bona fide adiabatic
solution. Because of its anomalously large tensor to scalar ratio the
quasiadiabatic mode cannot be a substitute for the conventional adiabatic
paradigm so that, ultimately, the present findings seems to exclude the
possibility of a successful accelerated dynamics solely based on relativistic
viscous fluids. If the dominant adiabatic mode is not affected by the viscosity
of the background a sufficiently small fraction of entropic fluctuations of
viscous origin cannot be a priori ruled out.
