Dark goo: Bulk viscosity as an alternative to dark energy
  We present a simple (microscopic) model in which bulk viscosity plays a role
in explaining the present acceleration of the universe. The effect of bulk
viscosity on the Friedmann equations is to turn the pressure into an
"effective" pressure containing the bulk viscosity. For a sufficiently large
bulk viscosity, the effective pressure becomes negative and could mimic a dark
energy equation of state. Our microscopic model includes self-interacting
spin-zero particles (for which the bulk viscosity is known) that are added to
the usual energy content of the universe. We study both background equations
and linear perturbations in this model. We show that a dark energy behavior is
obtained for reasonable values of the two parameters of the model (i.e. the
mass and coupling of the spin-zero particles) and that linear perturbations are
well-behaved. There is no apparent fine tuning involved. We also discuss the
conditions under which hydrodynamics holds, in particular that the spin-zero
particles must be in local equilibrium today for viscous effects to be
important.
