Dr. Valerio Marra
at the Federal University of Espírito Santo,
Within the standard paradigm, dark energy is taken as a homogeneous fluid that drives the accelerated expansion of the universe and does not contribute to the mass of collapsed objects such as galaxies and galaxy clusters. The abundance of galaxy clusters has been extensively used to constrain the normalization of the matter power spectrum: it is an important probe as it allows us to test if the standard ΛCDM model can indeed accurately describe the evolution of structures across billions of years. It is then quite significant that the Planck satellite has detected less clusters than expected. One of the simplest generalizations that could reconcile these observations is to consider models in which dark energy is allowed to cluster. In this case, however, the standard methods to compute the abundance of galaxy clusters need to be adapted to account for the contributions of dark energy. In particular, we examine the case of clustering dark energy – a dark energy fluid with negligible sound speed – with a redshift-dependent equation of state. We carefully study how the halo mass function is modified in this scenario. Our results show that cluster counts are modified by about 30\% at a redshift of unity.