heory, the notion of gravity as an attractive force still holds for all known forms of matter and energy, even on the cosmic scale. Therefore, general relativity predicts that the expansion of the universe should slow down at a rate determined by the density of matter and energy within it. But general relativity also allows for the possibility of forms of energy with strange properties that produce repulsive gravity. The discovery of accelerating rather than decelerating expansion has apparently revealed the presence of such an energy form, referred to as dark energy.
Whether or not the expansion is slowing down or speeding up depends on a battle between two titans: the attractive gravitational pull of matter and the repulsive gravitational push of dark energy. What counts in this contest is the density of each. The density of matter decreases as the universe expands because the volume of space increases. Although little is known about dark energy, its density is expected to change slowly or not at all as the universe expands. Currently the density of dark energy is higher than that of matter, but in the distant past the density of matter should have been greater, so the expansion should have been slowing down then.
It is important to look for direct evidence of an earlier, slowing phase of expansion. Such evidence would help confirm the standard cosmological model and give scientists a clue to the underlying cause of the present period of cosmic acceleration. Because telescopes look back in time as they gather light from far-off stars and galaxies, astronomers can explore the expansion history of the universe by focusing on distant objects. That history is encoded in the relation between the distances and recession velocities of galaxies. If the expansion is slowing down, the velocity of a distant galaxy would be relatively greater than the velocity predicted by Hubble’s law. If the expansion is speeding up, the distant galaxy’s velocity would fall below the predicted value. Or, to put it another way, a galaxy with 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