The expansion of the Universe is accelerating. Within the homogeneous and isotropic FLRW cosmology, the simplest solution points to a cosmological constant that, even though providing an excellent fit to the data, requires an extremely fine tuned non-vanishing vacuum energy, which remains unexplained. A different idea arises from the question whether the inhomogeneous structures present in the Universe might affect the global expansion and/or measurements in such a way that we infer accelerated expansion once we interpret these measurements within the framework of an FLRW model. A particularly attractive feature of this concept is that it would solve the coincidence problem, explaining why acceleration begins roughly at the same time that non-linear structures form. Furthermore, even if backreaction does not produce a fully-fledged dark energy model, perhaps it can alter quantities such as the spatial curvature by a significant amount. It is a matter of some importance to quantify these effects in more detail. I will introduce the cosmological back-reaction proposal and review the state of the art.
The cosmological back reaction