Light propagation effects

The last-scattering surface is the most remote region which is observable using electromagnetic radiation. Since photons on their way pass through large-scale inhomogeneities such as voids, clusters and superclusters, it is important to know how the light propagation phenomena affect the CMB radiation. In the standard approach the CMB temperature fluctuations are analysed by solving the Boltzmann equation within linear perturbations around the homogeneous and isotropic FLRW model (Seljak and Zaldarriaga, 1996; Seljak et al., 2003). The use of the FLRW metric for the background model results in a remarkably good fit to the CMB data (Hinshaw et al., 2009). However, the assumption of homogeneity, which is also consistent with other types of cosmological observations, is not a direct consequence of them (Ellis, 2008). It is often said that such theorems as the Ehlers–Geren–Sachs (1968) theorem and the ‘almost EGS theorem’ (Stoeger et al., 1995), justify the application of the FLRW models. These theorems state that if anisotropies in the cosmic microwave background radiation are small for all fundamental observers, then locally the Universe is almost spatially homogeneous and isotropic. The founding assumption of these theorems, namely the local Copernican principle applied to the ‘U region’, i.e. ‘the region within and near our past light cone from decoupling to the present day’, is not mandatory and we have already stressed it needs still to be tested. Moreover, as shown by Nilsson et al. (1999), it is possible that the CMB temperature fluctuations are small but the Weyl curvature is large.