Noncosmological redshifts

Six decades ago Edwin Hubble found his velocity-distance relation that soon became the observational foundation of modern cosmology. According to the Cosmological Hypothesis (CH), all extragalactic objects — galaxies and the quasi-stellar objects — derive their redshifts from the expansion of the Universe. This article reviews the evidence for and against the CH. To what extent is it universal? Does it provide the entire redshift of an extragalactic object? If an extra, noncosmological component of redshift is present, what is it due to? On the observational side the evidence presented here is of three kinds: (i) evidence that is prima facie consistent with the CH, (ii) neutral evidence that can be reconciled with the CH with a few epicycles, and (iii) discordant evidence which, if accepted, suggests that some objects at least possess substantial noncosmological redshifts. The final part of this review discusses the various theories proposed to account for noncosmological redshifts and outlines further tests to establish the validity or otherwise of the CH.