A graph that plots the recession velocity of a galaxy, measured by its redshift, against its distance from us. The first such plot was made in 1929 by E. P. Hubble. He found a linear relation between velocity and distance, implying that a galaxy’s recession velocity is proportional to its distance from us; this is known as the Hubble law. Hubble correctly interpreted this observation as meaning that the Universe is homogeneous and is expanding uniformly in all directions. He also realized that for very distant galaxies the observed relation would no longer be linear on account of the curvature of spacetime and other effects caused by the expansion of the Universe. Observations of distant galaxies can therefore be used to constrain cosmological models.

However, light from very distant galaxies takes a significant fraction of the age of the Universe to reach us, so that high-redshift galaxies are seen when they were young while nearby galaxies are seen when much older. Hence allowance must be made for any evolution in the properties of galaxies with time (see Galaxy Evolution). For most galaxy properties, such as luminosity, evolutionary uncertainties are larger than the differences between the models. In the late 1990s, astronomers made Hubble diagrams using Type Ia supernovae in distant galaxies to infer that the Universe is currently undergoing an acceleration in its rate of expansion.

Hubble diagram:

Distance of galaxies is plotted against their speed of recession. For nearby galaxies, there is a linear relationship between distance and speed (the Hubble law). For great distances the departure of the relationship from a straight line can, in principle, be used to test cosmological theories. The four lines show the predictions of four different cosmological theories. If the galaxies follow curve A, then the Universe was once expanding much faster than today and will eventually collapse (i.e. the Universe is said to be closed). Curve C means that the Universe will continue to expand forever (i.e. the Universe is open). Curve B is the intermediate case, in which the expansion will stop infinitely far in the future (i.e. the Universe is spatially flat). Curve D is for a steady-state universe. Unfortunately, galaxies far enough out in space for deviations from the straight line to be apparent are also being observed so far back in time that the effects of galaxy evolution are much greater than the predicted differences between the different theories.