Hubble diagram


Hubble diagram. The points are labelled with the names of clusters of galaxies. The relationship showing is for a Hubble constant of 50 kilometres per second per megaparsec.
In its original form, a graph on which the redshifts of a sample of galaxies are plotted against their apparent magnitudes (see illustration). The term is now applied more generally to any plot of redshift or recession velocity against a distance indicator for a carefully selected sample of galaxies.
This diagram gave the first strong evidence (1929) that the universe is expanding. It continues to be a crucial tool in observational cosmology because the shape of the diagram at high redshifts is dependent on the geometry of the universe.

See also: Hubble's law, Hubble constant, expanding universe.