Name: Ole Bjaelde
 Institution: RWTH Aachen
 Title: Dark Energy and the Spherical Collapse – a Scale-dependent Approach
 Key Words: Dark Energy, Structure Formation
 Abstract: The apparent acceleration of our universe is, according to the standard model of cosmology, best described by the presence of a dark energy component with a strong negative pressure, dominating the gravitational attraction on large scales. However, our knowledge about the actual properties of dark energy is very limited and there are several open questions such as: Is the dark energy dynamical or not?, Does the dark energy take part in clustering or not?, Does it act differently on different scales?, etc. In order to address some of these open questions, we study the effect of dark energy in the spherical collapse model for gravitational clustering. Our setup is a matter component with zero sound speed and zero equation of state and a generic dark energy component with an arbitrary sound speed and an (almost) arbitrary equation of state. We show that the presence of the (clustered) dark energy component has a significant effect on the formation of structure – so-called halos - and interestingly this effect depends on the mass of the formed halo. As the approach is essentially model independent starting from first principles, it can be used to study the effect of other cosmological and scale-dependent ingredients taking part in structure formation.