Strong dark matter constraints on GMSB models
  We reconsider the dark matter problem in supersymmetric models with gauge
mediated supersymmetry breaking, with and without R-parity breaking. In these
classes of models, a light gravitino forms the dark matter.Consistency with the
experimental data, in particular the dark matter abundance and the small-scale
power spectrum, requires additional entropy production after the decoupling of
the gravitino from the thermal bath. We demonstrate that the usual mechanism
via messenger number violating interactions does not work in models where the
messenger belongs to SU (5) representations. This is mainly a consequence of
two facts: (i) there are at least two different types of lightest messenger
particles and (ii) the lightest messenger particle with SU (2) quantum numbers
decays dominantly into vector bosons once messenger number is broken, a feature
which has been overlooked so far. In case of SO(10) messenger multiplets we
find scenarios which work if the SM gauge singlet component is rather light.
