Spherical collapse for unified dark matter models
  We study the non-linear spherical "top hat" collapse for Chaplygin and
viscous unified cosmologies. The term unified refers to models where dark
energy and dark matter are replaced by one single component. For the
generalized Chaplygin gas (GCG) we extend previous results of [R. A. A.
Fernandes {\it et al}. Physical Review D 85, 083501 (2012)]. We discuss the
differences at non-linear level between the GCG with $\alpha=0$ and the
$\Lambda$CDM model. We show that both are indeed different. The bulk viscous
model which differs from the GCG due to the existence of non-adiabatic
perturbations is also studied. In this case, the clustering process is in
general suppressed and the viable parameter space of the viscous model that
accelerates the background expansion does not lead to collapsed structures.
This result challenges the viability of unified viscous models.
