The puzzle: what is the nature of the dark energy that fills the universe? To solve it, Berkeley Lab physicists, astronomers, and engineers, working with colleagues from the University of California at Berkeley and other institutions, propose to launch a satellite named SNAP - the SuperNova/Acceleration Probe.

Through Berkeley Lab's Physics Division the Department of Energy has funded a year of development for the project, which is meant to explain what one prominent theorist, Frank Wilczek of MIT, calls "maybe the most fundamentally mysterious thing in basic science" - a conundrum that, more colorfully, cosmologist Michael Turner of the University of Chicago refers to as "the biggest embarrassment in theoretical physics."

SNAP would orbit a three-mirror, 2-meter reflecting telescope in a high orbit over Earth's poles, circling the globe every week or 14 days. By repeatedly imaging 20 square degrees of the sky, SNAP will discover and accurately measure 2,000 type Ia supernovae a year, 20 times the number of these unique exploding stars - the key evidence in the dark-energy mystery - that were found in a decade of ground-based search.

Studies of this type of supernova by the international Supernova Cosmology Project headquartered at Berkeley Lab and the High-Z Supernova Search Team centered in Australia led to the announcement, in 1998, of what Science magazine named its "breakthrough of the year" - that the universe is expanding at an accelerating rate.

Combined with other cosmological studies, such as the BOOMERANG and MAXIMA cosmic microwave background radiation results reported earlier this year, the supernova data strongly suggest that the universe is geometrically "flat" and that as much as two-thirds of its density is due to an unidentified form of energy - the cause of the accelerating expansion.

To confirm these results, SNAP would discover many type Ia supernovae at redshifts greater than any yet found. Because of SNAP's ability to measure their light curves and spectra to high precision, any uncertainties concerning the brightness and redshift of very distant supernovae can be minimized.

SNAP's optics will serve a set of precision instruments:

Unlike most astronomical CCDs currently in use, which have relatively poor response to red and infrared light and are difficult to combine in large arrays, SNAP will use radiation-tolerant, high-resistivity CCDs based on Berkeley Lab's experience with detectors developed for high-energy physics. These can be combined in large-format mosaics and will extend sensitivity into the infrared, creating an ideal tool for finding distant, high-redshift supernovae.

Prelude to discovery

For over ten years the Supernova Cosmology Project, supported by the Department of Energy, the National Science Foundation, and NASA, has been studying the expansion of the universe by measuring the redshift and brightness of distant type Ia supernovae.

Type Ia supernovae - stars that explode in thermonuclear cataclysms brighter than entire galaxies - make ideal "standard candles" with which to survey the universe. Their light curves and spectra are all nearly alike and they are bright enough to be seen across billions of light years.

By 1998 a few score type Ia supernovae had been analyzed in detail, enough to lead the Supernova Cosmology Project and their colleagues in the High-Z Supernova Search Team to the startling discovery that the expansion of the universe is not slowing, as had been expected, but accelerating.

Redshift in an accelerating universe

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.