Structure formation in a $\Lambda$ $viscous$ CDM universe
  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 to 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 $\Lambda$CDM-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. 
Hermano Velten, Thiago R. P. Caram\^es, J\'ulio C. Fabris, Luciano   Casarini and Ronaldo C. Batista
astro-ph.CO