Francesco Pace (University of Manchester, United Kingdom)
Dark matter and dark energy are usually considered to be two separate components in the energy budget of the Universe. A simple extension to this hypothesis is to consider that they could be different aspects of a single component. This class of models is hence dubbed Unified Dark Matter (UDM) models. UDM models could cluster and hence give origin to cosmic structures if the adiabatic sound speed of the fluid is small enough, but this is achieved only with a severe fine-tuning of the parameters. A particularly interesting sub-class of UDM models solving this issue is characterised by a fast transition between a standard matter-like era and a late epoch with accelerated expansion (cosmological constant or dark energy). In addition these models can avoid the problems affecting the Chaplygin gas models. An important point lacking in literature is a rigorous analysis of structure formation in adiabatic fast transition models, clearly defining the equation of motions of the clustering part of the fluid. I will show results of structure formation in UDM fast transition models within the framework of the spherical collapse model and a comparison with the standard approach for LCDM and dark energy models.