PUB - Publikationen an der Universität Bielefeld

Cosmic structures determine how light propagates through the Universe andconsequently must be taken into account in the interpretation of observations.In the standard cosmological model at the largest scales, such structures areeither ignored or treated as small perturbations to an isotropic andhomogeneous Universe. This isotropic and homogeneous model is commonly assumedto emerge from some averaging process at the largest scales. We study theeffects of averaging the geometry of space-time and derive an averaged versionof the null geodesic equation of motion. For the averaged geometry we thenassume a flat Friedmann-Lema\^{i}tre (FL) model and find that light propagationin this averaged FL model is not given by null geodesics of that model, butrather by a modified light propagation equation that contains an effectiveHubble expansion rate, which differs from the Hubble rate of the averagedspace-time.