  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.
