Fluids often display dissipative properties. We explore dissipation in the form of bulk viscosity in the cold dark matter fluid. We constrain this model using current data from supernovae, baryon acoustic oscillations and the cosmic microwave background. Considering the isotropic and homogeneous background only, viscous dark matter is allowed to have a bulk viscosity $\lesssim {10}^7\mathrm{Pa}·\mathrm{s}$, also consistent with the expected integrated Sachs-Wolfe effect (which plagues some models with bulk viscosity). We further investigate the small-scale formation of viscous dark matter halos, which turns out to place significantly stronger constraints on dark matter viscosity. The existence of dwarf galaxies is guaranteed only for much smaller values of dark matter viscosity, $\lesssim {10}^{-3}\mathrm{Pa}·\mathrm{s}$.

• Received 5 June 2012

DOI:https://doi.org/10.1103/PhysRevD.86.083501