Analytic description of dark matter clustering : beyond the perfect fluid approximation

Giulia Cusin

According to the standard paradigm for structure formation, baryonic matter has clustered in the dark matter (DM) potential wells. Depending on the prevalence of cold or warm dark matter, we have different scenarios for the formation of structures. Observations seem to indicate that structure formed with a bottom-up scenario and that the dominant component of DM is made of cold particle species. Cold dark matter is usually described as a perfect fluid with zero temperature. In this scenario a rotational component of the fluid velocity field (i.e. vorticity) can not be generated, as a consequence of the Helmotz’s theorem. On the other hand, we see that baryonic matter has non vanishing vorticity (i.e. galaxies rotate ect). We present a formalism that allows to go beyond the standard description of dark matter as a perfect fluid. In our model cold dark matter is described as a viscous fluid with finite temperature. We study the resulting dynamics of the fluid and we compute the vorticity field induced by the presence of viscosity. We comment on the comparison with N-body simulations.

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