Dark matter perturbations and viscosity: a causal approach
  The inclusion of dissipative effects in cosmic fluids modifies their
clustering properties and could have observable effects on the formation of
large scale structures. We analyse the evolution of density perturbations of
cold dark matter endowed with causal bulk viscosity. The perturbative analysis
is carried out in the Newtonian approximation and the bulk viscosity is
described by the causal Israel-Stewart (IS) theory. In contrast to the
non-causal Eckart theory, we obtain a third order evolution equation for the
density contrast that depends on three free parameters. For certain parameter
values, the density contrast and growth factor in IS mimic their behaviour in
$\Lambda$CDM when $z \geq 1$. Interestingly, and contrary to intuition, certain
sets of parameters lead to an increase of the clustering.
