Large scale structure from viscous dark matter

Understand

Cosmological perturbations of sufficiently long wavelength admit a fluid dynamic description.

• We consider modes with wavevectors below a scale $k_m$ for which the dynamics is only mildly non-linear.
• The leading effect of modes above that scale can be accounted for by effective non-equilibrium viscosity and pressure terms.
• For mildly non-linear scales, these mainly arise from momentum transport within the ideal and cold but inhomogeneous fluid, while momentum transport due to more microscopic degrees of freedom is suppressed.