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Dark Energy in the Accelerating Universe

Posted on: Wednesday, 1 January 2003, 06:00 CST

The Door to Discovery

Berkeley Lab -- In 1929, when Edwin Hubble announced that the universe is expanding, he opened a door to unexpected discovery. The knowledge that expansion is accelerating opens the way to new advances, many of them unpredictable.

The Supernova Cosmology Project collaborators and their colleagues in the High-Z Supernova Search team, whose results agree on the acceleration, use instruments more powerful and sensitive than anything Hubble dreamed of, including giant telescopes on the ground, the Space Telescope named for Hubble himself, charge-coupled devices instead of photographic plates, and supercomputers.

Yet the basic strategy is much the same — to measure cosmic expansion by comparing the distances of far-off objects with their redshifts. A star's distance can be estimated from its brightness as seen on Earth, if its total emitted light is known — the farther away it is, the dimmer it appears.

Accurate estimates of total emitted light are possible for only a few kinds of astronomical objects; these "standard candles," like an ordinary candle seen across a dark room, reveal their distance by their apparent brightness.

The Supernova Cosmology Project uses type Ia supernovae as standard candles — exploding stars as bright as entire galaxies that can be seen across billions of light years.

These thermonuclear cataclysms emit most of their energy in a few weeks, and during that time each gives off nearly the same amount of light. The challenge is to catch them before they reach their brightest emission, then follow them until they fade.

In a typical galaxy, type Ia supernovae occur only two or three times in a thousand years; a decade ago, astronomers thought they were too rare and unpredictable to waste valuable telescope time searching for them.

Then the Supernova Cosmology Project demonstrated that if a moonless patch of sky filled with tens of thousands of galaxies is photographed digitally and then photographed again three weeks later, over a dozen bright spots will appear on the second set of images that were not on the first — a batch of supernova candidates whose identity can be quickly confirmed with follow-up observations.

Using these methods, the Supernova Cosmology Project showed that a few nights on the world's best telescopes can guarantee a bevy of "supernovae on demand."

Redshift in an Accelerating Universe

Light coming from far beyond Earth must cross space that is expanding. The effect is to stretch the light wave as surely as if it had been drawn on the skin of an expanding balloon; as it travels, its color shifts toward the red end of the spectrum.

The redshift of astronomical objects is measured by comparing characteristic spectral lines of elements in them with spectral lines of the same elements measured in the laboratory. The higher the redshift, the more distant the object that emitted the light.

The farthest redshifted galaxies discussed by Edwin Hubble in 1929 were about 6,000,000 light-years away; the light of such "close" galaxies was emitted recently, and the expansion of the universe since then has been relatively small.

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 now slowing under the influence of gravity, as astronomers expected before 1998, supernovae in distant galaxies should appear brighter and closer than their high redshifts might otherwise suggest.

The distant supernovae found so far tell a different story. At high redshifts, the most distant supernovae are dimmer than they would be if the universe were slowing under the influence of gravity; they must be located farther away than would be expected for a given redshift — larger-than-expected distances that can only be explained if the expansion rate of the universe is accelerating.

What do these observations imply about the geometry of the universe? What if that geometry is not Euclidean, or "flat," but "curved" instead?

If the universe were open, with negative curvature — and if observations of supernovae were subject to some systematic distortion, such as a novel form of intergalactic dust that absorbs their light — distant supernovae might appear deceptively fainter, mimicking acceleration. To determine the curvature of the universe and to detect possible distortions are among the goals of the Supernova Cosmology Project.

While it may be too soon to rule out a negatively curved universe, there is independent evidence against it. For example, measurements of the cosmic microwave background radiation hint that the universe is probably flat — its energy density equal to the critical energy density.

By far the most successful explanation for the flatness of the universe, which is otherwise extremely unlikely, is the theory known as inflation.

The Cosmological Constant

If the universe is flat and expanding ever faster, some invisible, unidentified energy must be offsetting gravity. In the beginning, when matter was close together and the universe was dense, gravitational attraction was much stronger. Now that matter is far apart and the density of the universe is low, this mysterious energy is pushing space itself outward at an accelerating rate. Its nature is unknown.

One proposal goes by the name of "the cosmological constant." In 1917 Albert Einstein, who assumed the universe was static, added an arbitrary term to the general theory of relativity to make sure his equations described it that way. When it became clear that the universe really is expanding, Einstein abandoned the cosmological term, later calling it his biggest blunder.

The cosmological constant has repeatedly been dismissed by physicists, only to return. If to Einstein it was only a mathematical term, today it is identified with the energy of the vacuum itself, a consequence of quantum theory.

Yet if it is confirmed to operate in our universe, the cosmological constant will present theorists with a formidable problem, which can be phrased as a simple question: why is it so small? Any attempt to calculate the cosmological constant from quantum theory gives an answer more than 50 orders of magnitude larger than what is observed. Other candidates for the mysterious energy component of the universe, called "dark" energy, have been proposed as well, some rather exotic.

Closed, open, and flat once described universes destined to recollapse, expand forever, or reach a tenuous balance. But invisible energy could propel even a closed universe to eternal expansion. Many different observations suggest that the universe is flat, not curved, and that some mechanism is forcing expansion to accelerate. Credit: Berkeley Lab

Of the dozens of type Ia supernovae discovered in over a decade of ground-based observation, most have redshifts (or Z) much less than 1.0. A single year of satellite observation could find thousands of supernovae at redshifts up to 1.5 and beyond, yielding critical data on the mass density, vacuum energy density, curvature of the universe, and the mysterious "dark energy." Credit: LBL

Hundreds of rugged chips will be assembled in a circular array over a third of a meter wide, the largest and most sensitive astronomical CCD imager ever constructed. Credit: Berkeley Lab

Different kinds of mechanisms driving accelerating expansion would produce different observable consequences, but only if much more data of much higher quality can be gathered, some from farther back in time. Better observations of more supernovae over a wider range of redshifts must be plotted before the question of what is causing expansion to accelerate can be answered with confidence.

A CAMERA TO PHOTOGRAPH THE PAST

Charge-coupled devices (CCDs), now commonly found in still cameras and video recorders, convert light images to electronic data that can be immediately processed by computer.

Because CCDs are far more sensitive than photographic emulsions, they revolutionized astronomy in the 1970s. But CCDs are individually smaller than photographic plates and difficult to combine in large arrays.

To detect short-wavelength blue light, astronomical CCDs must be carefully thinned and back-illuminated, so that electrons generated on the back of the chip can reach wiring on the front. Their sensitivity to longer wavelengths is poor, a major drawback to good measurement of high redshifts.

The Supernova Cosmology Project, drawing on experience with silicon detectors developed at Berkeley Lab for high-energy physics — detectors which can sort a few events of interest from a particle accelerator's storm of radiation, such as those used to identify top quark decays at Fermilab's Tevatron — devised a rugged CCD that mimics the electrical properties of a thin, blue-sensitive chip while extending sensitivity into the infrared.

Moreover, the new CCD is designed so that many chips can be placed side-by-side in large-format mosaics for astronomical imaging; the chip is already in preliminary use at Lick Observatory.

A Satellite to Gather Supernovae

The ideal place for a supernova telescope would be far from atmospheric distortion and cloudy nights; the telescope should always point away from the sun's glare and the moon's glow. The ideal place for a supernova telescope is aboard a satellite.

These considerations have led to a proposal for a satellite called SNAP, a SuperNova/Acceleration Probe, to orbit a 1.8-meter reflecting telescope fitted with a billion-pixel CCD camera, the largest astronomical CCD imager ever constructed.

By repeatedly imaging just one or two large patches of sky, SNAP could gather 2,000 type Ia supernovae in a single year, 20 times the number from a decade of ground-based search. Because of enhanced sensitivity to infrared light above the atmosphere, many of these new supernovae would be at distances and redshifts far greater than any yet found.

SNAP's optics would serve a simple set of instruments: a CCD with 10 times the area of the Sloan Digital Sky Survey camera and more efficient at all wavelengths than any current astronomical camera, plus a spectrometer system to record accurate and consistent spectra, from the near ultraviolet to the near infrared, for every supernova it captures.

With this proposed satellite, effects of dust and elemental composition on the brightness and redshift of very distant supernovae will be resolved. SNAP would also shed new light on galactic clusters, gamma-ray bursters, forms of cold dark matter, comets in our own solar system, and many other astronomical phenomena. The essential purpose of the SNAP proposal, however, is to

address the most fundamental cosmological questions.

Will the universe last forever? Is the universe infinite in extent? What is the universe made of? What dark energy accelerates the expansion of the universe?

The ancient light from thousands of exploding stars holds the answers to these questions and more. In this ancient light, the mysterious energy that fills the universe will be unveiled.

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