The Universe is not completely homogeneous. Even if it is sufficiently so on large scales, it is very inhomogeneous at small scales, and this has an effect on light propagation, so that the distance as a function of redshift, which in many cases is defined via light propagation, can differ from the homogeneous case. The corresponding distance is traditionally known as the Dyer–Roeder distance. I sketch the history of this model and some applications, then suggest some reasons why it is still relatively obscure.
This text is an `executive summary' of and borrows heavily from my recent review of this topic
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