During the time that the matter in the universe was fully ionized before about 300,000 years after the Big Bang, whenever matter tried to gravitationally collapse into clumps, the matter was so tightly 'coupled' to the cosmic background radiation via electromagnetic forces, that the clumps would dissipate. The radiation field produced so much friction against which the matter had to work, that clumps smaller than about 10^15 times the mass of the Sun could never survive.
Bulk viscosity is a term used to describe the electromagnetic 'friction' which the ionized matter experienced through its interaction with the cosmic fireball radiation field. All electromagnetic interactions produce their own photons as the charged particles are accelerated or decelerated by the various plasma electromagnetic fields, so in principle, bulk viscosity produces its own 'background radiation field' which adds to the cosmic background radiation field.
The observations made by the NASA/COBE satellite have shown, however, that the cosmic background radiation field is such a perfect 'planck-type spectrum' that there are now very strong limits to just how much dissipation there could have been by bulk viscosity. This process would have affected the shape of the cosmic background radiation by making its spectral shape look slightly different than a black body with a temperature of 2.76 degrees K. The so- called 're-Comptonization parameter' measures this deviation, and the limits show that there could have been no additional sources of photons generated in the universe after about the first year of cosmic expansion, other that the cosmic background radiation itself.
The bottom line is that bulk viscosity generates photons, but the measurements show no traces of these in the cosmic background. Therefore, this mechanism was simply not that important in the dynamics of matter during the early history of the universe.