2.2. Problems with Cold Dark Matter?

A recurring question is whether the cold dark matter paradigm for structure formation is compatible with all the observational data (Navarro 2003; Abadi et al. 2002). One of the issues is the mass profile at the core of a galactic halo. As we heard at this meeting, density profiles clearly differ from naive power laws, and are much shallower than simple isothermal models. However, the haloes of < 2.5 seem to be in reasonable agreement with CDM sumulations, though there are still problems with > 2.5 galaxies. The theoretical predictions are not yet conclusive, though, with questions such as triaxiality, departures from equilibrium and time dependence remaining to be resolved. Another issue is halo substructure and the abundance of Milky Way satellites. However, the latest news is that there is apparently better agreement between the number of Milky Way satellites and the number of massive halo substructures predicted in CDM simulations. It has been argued that a Milky Way-like stellar disk would be thickened if there were substructures in the halo, but recent simulations do not display any significant such effect (Navarro 2003; Abadi et al. 2002).

Thus, the latest advice from the simulators seems to be that there is no showstopper for the CDM paradigm, so let us examine some of the candidates for the CDM.