Expanding universe with positive bulk viscous pressures?

In a recent paper [M. Giovannini, Phys. Rev. D 59, 121301 (1999)] an approach was proposed according to which sufficiently large bulk viscous stresses which imply a violation of the dominant energy condition cause a modification of energy spectra of relic gravitons which in principle might be observable. We point out that this approach is unphysical since it is incompatible with the second law of thermodynamics at late cosmological times. © 2000 The American Physical Society.