  Since the discovery of the large angular scale anisotropies in the microwave
background radiation, the behaviour of cosmological perturbations (especially,
density perturbations and gravitational waves) has been of great interest. In
this study, after a detailed and rigorous treatment of the behaviour of
gravitational waves in viscous cosmic media, we conclude that the damping of
cosmological gravitational waves of long wavelengths is negligible for most
cases of physical interest. A preliminary analysis suggests that similar
results hold for density perturbations in the long wavelength limit. Therefore,
long wavelength cosmological perturbations have not been practically affected
by viscous processes,and are good probes of the very early Universe.
