Is Dark Energy Simulated by Structure Formation in the Universe ?
  The standard model of cosmology assumes that the Universe can be described to
hover around a homogeneous-isotropic solution of Einstein's general theory of
relativity. This description needs (sometimes hidden) hypotheses that restrict
the generality, and relaxing these restrictions is the headline of a new
physical approach to cosmology that refurnishes the cosmological framework.
Considering a homogeneous geometry as a template geometry for the in reality
highly inhomogeneous Universe must be considered a strong idealization.
Unveiling the limitations of the standard model opens the door to rich
consequences of general relativity, giving rise to effective (i.e. spatially
averaged) cosmological models that may even explain the longstanding problems
of dark energy and dark matter. We explore in this talk the influence of
structure formation on average properties of the Universe by discussing: (i)
general thoughts on why considering average properties, on the key-issue of
non-conserved curvature, and on the global gravitational instability of the
standard model of cosmology; (ii) the general set of cosmological equations
arising from averaging the scalar parts of Einstein's equations, the generic
property of structure formation interacting with the average properties of the
Universe in a scale-dependent way, and the description of cosmological
backreaction in terms of an effective scalar field.
