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We show that in clustering dark energy models the growth index of linear matter perturbations, γ, can be much lower than in ACDM or smooth quintessence models and presents a strong variation with redshift. We find that the impact of dark energy perturbations on γ is enhanced if the dark energy equation of state has a large and rapid decay at low redshift. We study four different models with these features and show that we may have 0.33 < γ(z) < 0.48 at 0 < z < 3. We also show that the constant γ parametrization for the growth rate, f = d ln δm/d ln a = Ωm#947;, is a few percent inaccurate for such models and that a redshiftdependent parametrization for y can provide about four times more accurate fits for f. We discuss the robustness of the growth index to distinguish between general relativity with clustering dark energy and modified gravity models, finding that some f(R) and clustering dark energy models can present similar values for γ. [ABSTRACT FROM AUTHOR]

The possibility of dark matter being a dissipative component represents an option for the standard view where cold dark matter (CDM) particles behave on large scales as an ideal fluid. By including a physical mechanism in the dark matter description like viscosity, we construct a more realistic model for the Universe. Also, the known small scale pathologies of the standard CDM model either disappear or become less severe. We study clustering properties of a ACDM-like model in which dark matter is described as a bulk viscous fluid. The linear power spectrum, the nonlinear spherical "top hat" collapse, and the mass functions are presented. We use the analysis with such structure formation tools in order to place an upper bound on the magnitude of the dark matter's viscosity. [ABSTRACT FROM AUTHOR]