Null geodesics and observational cosmology
  The Universe is not isotropic or spatially homogeneous on local scales. The averaging of local inhomogeneities in general relativity can lead to significant dynamical effects on the evolution of the Universe, and even if the effects are at the 1% level they must be taken into account in a proper interpretation of cosmological observations. We discuss the effects that averaging (and inhomogeneities in general) can have on the dynamical evolution of the Universe and the interpretation of cosmological data. All deductions about cosmology are based on the paths of photons. We discuss some qualitative aspects of the motion of photons in an averaged geometry, particularly within the context of the luminosity distance-redshift relation in the simple case of spherical symmetry. 
A. A. Coley