ith a given recession velocity will be farther away than expected—and hence fainter—if the universe is accelerating.
To take advantage of this simple fact requires finding astronomical objects that have a known intrinsic luminosity—the amount of radiation per second produced by the object—and that can be seen across the universe. A particular class of supernovae known as type Ia are well suited to the task. Over the past decade, researchers have carefully calibrated the intrinsic luminosity of type Ia supernovae, so the distance to one of these explosions can be determined from its apparent brightness.
Finding such ancient and far-off supernovae is difficult, however. A type Ia supernova that exploded when the universe was half its present size is about one ten-billionth as bright as Sirius, the brightest star in the sky. Ground-based telescopes cannot reliably detect the objects, but the Hubble Space Telescope can. In 2001 one of us (Riess) announced that the space telescope had serendipitously imaged an extremely distant type Ia supernova (dubbed SN 1997ff) in repeated observations. Given the redshift of the light from this stellar explosion—which occurred about 10 billion years ago, when the universe was one third its current size—the object appeared much brighter than it would have been if [dust filling intergalactic space simply made the supernovae appear dim, as some researchers had proposed]. This result was the first direct evidence of the decelerating epoch. The two of us proposed that observations of more high-redshift supernovae could provide definitive proof and pin down the transition from slowdown to speedup.
The Advanced Camera for Surveys, a new imaging instrument installed on the space telescope in 2002, enabled scientists to turn Hubble into a supernova-hunting machine. Riess led an effort to discover the needed sample of very distant type Ia supernovae by piggybacking on the Great Observatories Origins Deep Survey. The team found six supernovae that exploded when the universe was less than half its present size (more than seven billion years ago); together with SN 1997ff, these are the most distant type Ia supernovae ever discovered. The observations confirmed the existence of an early slowdown period and placed the transitional “coasting point” between slowdown and speedup at about five billion years ago.