Hubble diagram

The Hubble diagram is a graph that compares the brightness (or distance) of objects observed in the universe to their redshift (or apparent recessional velocity). Edwin Hubble, for whom the diagram is named, plotted his observations of galaxies outside the Milky Way in this format. In doing so, Hubble showed that the universe is expanding because the recessional velocities of the galaxies are proportional to their distances. Modern Hubble diagrams are based on observations of Type Ia supernovae, and suggest that the expansion rate of the universe is increasing.