Null geodesics and observational cosmology

Understand

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.