Item

Abstract

It has been known that a non-perfect fluid that accounts for dissipative
viscous effects can evade a highly anisotropic chaotic mixmaster approach to a
singularity. Viscosity is often simply parameterised in this context, so it
remains unclear whether isotropisation can really occur in physically motivated
contexts. We present a few examples of microphysical manifestations of
viscosity in fluids that interact either gravitationally or, for a scalar field
for instance, through a self-coupling term in the potential. In each case, we
derive the viscosity coefficient and comment on the applicability of the
approximations involved when dealing with dissipative non-perfect fluids. Upon
embedding the fluids in a cosmological context, we then show the extent to
which these models allow for isotropisation of the universe in the approach to
a 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 anisotropic
3-curvature modelled by the Bianchi type-IX metric. It is found that a
self-interacting scalar field at finite temperature allows for efficient
isotropisation, whether in a Bianchi type-I or type-IX spacetime, although the
model is not tractable all the way to a singularity. Mixmaster chaotic
behaviour, which is well known to arise in anisotropic models including
anisotropic 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 a
dense gas of black holes.