Supernovae as distance markers
The discovery of dark energy was made possible thanks to the remarkable fact that the
calibrated brightness of a Type Ia supernova — an exploding star — is the same no matter how
far away it is. Supernovae therefore act as precise distance indicators, thus allowing
researchers to study cosmic dynamics. These data show the "light curves" of 73 supernovae
— the rise in brightness from about 18 days before the peak brightness (defined as day 0)
and subsequent fading — as measured by the Supernova Legacy Survey. The same light
curve is exhibited by supernovae at high redshifts, z > 0.589 (red), and lower redshifts of
z < 0.589 (blue).
Source: A Conley et al. 2006 Astron. J. 132 1707