As light travels through space, space itself is expanding. The effect is to stretch light waves and shift their color toward the red end of the spectrum.
Light from the most distant galaxies has traveled billions of years, giving a snapshot of the universe at a fraction of its present age. If expansion were slowing under the influence of gravity, supernovae in distant galaxies should appear brighter and closer than their high redshifts suggest.
The distant supernovae found so far tell a different story. At high redshifts, the most distant are dimmer than they would be if expansion were slowing; they must be located farther away than would be expected for a given redshift -- powerful evidence that the expansion rate of the universe is accelerating.
The cosmological constant
In the very dense early universe, when matter was close together, gravitational attraction was strong and expansion was slowing. Today, because of continued expansion, matter is farther apart and the density of the universe is low -- so low that it has apparently dropped below the density of some unidentified dark energy now causing it to expand ever faster.
The dark energy may be what Albert Einstein called the "cosmological constant," an arbitrary term he added to the general theory of relativity to make sure it described a static universe. Although Einstein later abandoned the idea, evidence for an accelerating universe has forced cosmologists to consider the existence of a cosmological constant once again.
In a typical galaxy, type Ia supernovae occur only a few times in a millennium, and so far only several dozen have been measured with enough precision to answer key cosmological questions. Before the nature of the dark energy can be determined with confidence, observations of many more supernovae over a wider range of redshifts are needed -- observations with much better control on systematic uncertainties.
Enter SNAP, the SuperNova/Acceleration Probe. Not only will SNAP be able to find thousands of type Ia supernovae every year, it will also shed new light on galaxy clusters, gamma-ray bursters, cold dark matter, weak lensing, asteroids, astronomical transients, and many other phenomena. But its primary mission is to discover the nature of the dark energy that accelerates the expansion of the universe.
In the ancient light from thousands of exploding stars, the mysterious energy that fills the universe will be unveiled.