We consider cosmological scenarios originating from a single imperfect fluid
with bulk viscosity and apply Eckart's and both the full and the truncated
M\"uller-Israel-Stewart's theories as descriptions of the non-equilibrium
processes. Our principal objective is to investigate if the dynamical
properties of Dark Matter and Dark Energy can be described by a single viscous
fluid and how such description changes when a causal theory
(M\"uller-Israel-Stewart's, both in its full and truncated forms) is taken into
account instead of Eckart's non-causal theory. To this purpose, we find
numerical solutions for the gravitational potential and compare its behaviour
with the corresponding LambdaCDM case. Eckart's and the full causal theory seem
to be disfavoured, whereas the truncated theory leads to results similar to
those of the LambdaCDM model for a bulk viscous speed in the interval 10^{-11}
<< c_b^2 < 10^{-8}. Tentatively relating such value to a square propagation
velocity of the order of T/m of perturbations in a non-relativistic gas of
particles with mass m at the epoch of matter-radiation equality, this may be
compatible with a mass range 0.1 GeV < m << 100 GeV.Comment: 23 pages, 7 figure
