Pavel Kroupa, University of Bonn
The dual-dwarf-galaxy theorem, according to which two types of galaxies must exist and which must be true in the standard model of cosmology, appears to be ruled by astronomical data: both types of dwarf galaxy, those with putative exotic dark matter and those known to not contain dark matter even if it were to exist, cannot be distinguished by observation. Furthermore, the arrangement of satellite galaxies in rotating disk-like vast near-polar structures around the Milky Way and Andromeda galaxies and the frequent occurrence of anisotropic flattened satellite populations around major galaxies, seem to very strongly support the conclusion that only one type of satellite dwarf galaxy exists, namely the type without dark matter. Also, the orbital decay implied by dynamical friction on the putative dark matter halos is not evident in interacting galaxies. Dynamically relevant cold or warm dark matter therefore seems not to be present. Instead and as suggested by Milgrom, scale-invariant dynamics is showing a new direction for understanding the astrophysics of galaxies. Galaxies are observed to be simple systems following laws that result from scale-invariant dynamics which do not emanate from the haphazard merging history of halos of exotic dark matter. As a result, the present-day cosmological description of galaxy formation and evolution appears to need major revision.