# Cosmology: collisionless vs collisional fluids?

I try to understand the difference between collisionless and collisional fluids in cosmology. My first question is the following.

In the context of FLRW cosmology, we suppose that the Universe can be described in terms of a mix of fluids with:

$T_{\mu\nu}=\left(\rho c^2 +P\right)u_{\mu}u_{\nu}+Pg_{\mu\nu}$

• When we write that, do we suppose a collisionless or collisional nature of the fluids?
• If this description corresponds to collisional fluids, why cosmological simulations are N-body simulations (collisionless) and are not simply based on hydrodynamics?
• Can we solve equations of a collisionless system without using particles (and just cells with physical properties like in the collisional case)?
• At very large scale (scale of homogeneity), when we are not interested in the formation of local structures (like galaxies and superclusters), does the collisionless/collisional description is important?
• You can (and should!) assume both types of fluid models and calculate which one fits better. I am not aware that all cosmological calculations (numerical or not) are n-body simulations or that n-body simulations rule out modeling of collisions (they don't). Do collisions matter at very large scale? Depends on what you mean by "matter" and "very large scale", doesn't it? To first order the distribution of matter doesn't "matter" at all, as long as its homogeneous "enough". – CuriousOne May 10 '15 at 23:09
• The density and temperatures in the interstellar medium are low and high enough that the plasma can be considered collisionless. In fact, the collision time scale in even the solar wind is roughly one Coulomb collision per day. Interstellar dust, however, may be neutral and could be influenced by collisions. Yet, I am not sure whether there is enough dust to significantly impact galaxies or clusters (an expert might help me here). – honeste_vivere May 11 '15 at 12:40