Spherical collapse for unified dark matter models

Abstract

We study the nonlinear 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 et al., [Phys. Rev. D 85, 083501 (2012)]. We discuss the differences at nonlinear level between the GCG with $\ensuremath{\alpha}=0$ and the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model. We show that both are indeed different. The bulk viscous model which differs from the GCG due to the existence of nonadiabatic 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.