On the dark energy clustering properties

We highlight a viable mechanism leading to the formation of dark energy structures on subhorizon cosmological scales, starting from linear perturbations in scalar-tensor cosmologies. We show that the coupling of the dark energy scalar field, or quintessence, to the Ricci scalar induces a “dragging” of its density perturbations through the general relativistic gravitational potentials. We discuss, in particular, how this process forces dark energy to behave as a pressureless component if the cosmic evolution is dominated by nonrelativistic matter. This property is also analyzed in terms of the effective sound speed of the dark energy, which correspondingly approaches the behavior of the dominant cosmological component, being effectively vanishing after matter-radiation equality. To illustrate this effect, we consider extended quintessence scenarios involving a quadratic coupling between the field and the Ricci scalar. We show that quintessence density perturbations reach nonlinearity at scales and redshifts relevant for the structure formation process, respecting all the existing constraints on scalar-tensor theories of gravity. This study opens new perspectives on the standard picture of structure formation in dark energy cosmologies, since the quintessence field itself, if nonminimally coupled to gravity, may undergo clustering processes, eventually forming density perturbations exiting from the linear regime. A non-linear approach is then required to further investigate the evolution of these structures, and in particular their role in the dark halos surrounding galaxies and clusters.