© High-Z Supernova Search Team.
Hubble's original diagram comparing the distances of other galaxies to their velocities showed that the universe is expanding. It was generally assumed that the expansion rate would slow over time as gravity pulled the universe back together. Scientists seeking to measure this slowing using Type Ia supernovae to probe further and deeper into the past were astonished when they added their new data points to the Hubble diagram. The top plot above is a modern version of Hubble's original, with brightness (in astronomer's units of relative magnitude) plotted as a function of redshift (denoted by z). The lower plot shows the difference between the data points and Hubble's law. If the universe were expanding at a constant rate, all the points in the lower plot would lie in a straight horizontal line at zero. Instead, the supernovae with higher velocities lie above the line. This was the first evidence that the universe is expanding at an accelerated rate, driven by something we choose to call "dark energy." (Unit: 11)