Glossary term: Hubble Diagram
Description: The original Hubble diagram is a graph of velocity (y-axis) versus distance (x-axis) of galaxies. The graph shows a linear relationship between velocity and distance, providing evidence that distant galaxies are moving away faster than closer galaxies, and overall galaxies seem to be moving away from "us". This is used as one line of evidence for an expanding Universe. The slope (gradient) of the line is referred to as the Hubble parameter (H), and the equation of the line is called the Hubble–Lemaître Law. The value of the Hubble parameter in the current era (13.8 billion years after the Big Bang) is called the Hubble constant (H₀). Modern iterations of the Hubble diagram, based on observations of Type Ia supernovae, plot distance modulus (indirect measure of distance using brightness) versus redshift. In fact, the velocity of galaxies in the original Hubble diagram is measured indirectly from the redshift.
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