Viscous dark matter growth in (neo-)Newtonian cosmology
  We assume cold dark matter to possess a small bulk-viscous pressure which
typically attenuates the growth of inhomogeneities. Explicit calculations,
based on Eckart's theory of dissipative processes, reveal that for viscous cold
dark matter the usual Newtonian approximation for perturbation scales smaller
than the Hubble scale is no longer valid. We advocate the use of a
neo-Newtonian approach which consistently incorporates pressure effects into
the fluid dynamics and correctly reproduces the general relativistic dynamics.
This result is of interest for numerical simulations of nonlinear structure
formation involving nonstandard dark-matter fluids. We obtain upper limits on
the magnitude of the viscous pressure by requiring that relevant perturbation
amplitudes should grow sufficiently to enter the nonlinear stage.
