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It has been known that a non-perfect fluid that accounts for dissipativeviscous effects can evade a highly anisotropic chaotic mixmaster approach to asingularity. Viscosity is often simply parameterised in this context, so itremains unclear whether isotropisation can really occur in physically motivatedcontexts. We present a few examples of microphysical manifestations ofviscosity in fluids that interact either gravitationally or, for a scalar fieldfor instance, through a self-coupling term in the potential. In each case, wederive the viscosity coefficient and comment on the applicability of theapproximations involved when dealing with dissipative non-perfect fluids. Uponembedding the fluids in a cosmological context, we then show the extent towhich these models allow for isotropisation of the universe in the approach toa singularity. We first do this in the context of expansion anisotropy only,i.e., in the case of a Bianchi type-I universe. We then include anisotropic3-curvature modelled by the Bianchi type-IX metric. It is found that aself-interacting scalar field at finite temperature allows for efficientisotropisation, whether in a Bianchi type-I or type-IX spacetime, although themodel is not tractable all the way to a singularity. Mixmaster chaoticbehaviour, which is well known to arise in anisotropic models includinganisotropic 3-curvature, is found to be suppressed in the latter case as well.We find that the only model permitting an isotropic singularity is that of adense gas of black holes.

Microphysical manifestations of viscosity and consequences for anisotropies in the very early universe