Theory HEP seminar

Average evolution of locally inhomogeneous universes

Alexander Wiegand

Universität Bielefeld

Despite its global homogeneity and isotropy, the local matter distribution in the late universe is manifestly inhomogeneous. Cosmological backreaction suggests a link between the structure formation, that lead to this inhomogeneity, and the expansion history of the Universe. In order to quantitatively examine this connection there have been many studies using cosmological perturbation theory, but also some attempts to create non perturbative models.

This talk will first give a brief overview over the results of these studies. To this end, the averaging framework is introduced and its basic features are discussed.

It will then be shown how this formalism may be used to study the local dynamics and how local structure influences our ability to measure the underlying cosmic parameters. As many cosmological observations use measurements in the local universe to determine the global cosmic parameters, local fluctuations may pose problems in the era of precision cosmology. The question of how big this cosmic variance still is, given the recent deep galaxy surveys, will be addressed by quantifying this fundamental uncertainty in surveys with different geometries. Furthermore, the influence of these fluctuations on the evolution of the local volume will be discussed and possible measurements to quantify its importance are proposed.

Finally a partitionning model, that describes the universe by a separation into its under- and overdense regions is presented. It will be described how this model may be used to study the influence of large underdense regions and how we can use it to estimate the amount of kinematical backreaction at work in the late universe.