and finished life, never to begin anew. Others suggest that the same energy driving the growth may eventually form fresh galaxies and stars.But some astronomers doubt the camera project will work, saying it can't find data to clearly support one explanation over the others.
'Smeared' data?
The proposal involves a satellite, called the Supernova Acceleration Probe (SNAP), costing probably more than $200 million. Scientists at UC Berkeley and a dozen other institutions are asking the Department of Energy and National Science Foundation for funding.
One group of skeptics wrote in a paper in the journal Astrophysics last July that this type of project will reveal "no useful information" about the universe's fate.
The researchers, from Princeton University and Ben-Gurion University in Israel, argued that the results depend on equations that "smear" the data. These multiply measurement errors as small as 1 percent into uncertainties of as much as 40 percent, they said.
Proponents, in contrast, claim the device can measure several key cosmic numbers, such as the total density of energy in the universe, to an accuracy of a few percent.
"This is one of the few instruments we know of where we can spot-check all the major sources of error," said Perlmutter.
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Perlmutter led one of two competing teams that rocked the scientific community in 1998 when they announced that the cosmic expansion seems to be speeding up. Scientists on both teams say they may drop their rivalry to collaborate on SNAP.
Supernova signposts
The probe would orbit Earth for three years, imaging 6,000 supernovae -- many at distances far beyond those visible today -- to glean data on the cosmic expansion.
The device would dovetail with several other planned telescope and satellite projects aimed at probing the farthest reaches of the universe. But SNAP is the only one devoted completely to studying the expansion.
A supernova of the type used to measure cosmic distances, photographed in 1994. These "Type 1a" supernovae are believed to erupt when a compact, dead star (known as a white dwarf) sucks in matter from a neighboring star. When the total mass of the white dwarf hits a certain limit, it blows to pieces.
SNAP would study a particular type of
supernova thought to serve as a milepost for cosmic distances. The belief that the universe is expanding ever-faster rests on measurements of several dozen of these explosions in the 1990s.Astronomers had known for decades that the universe is expanding, since virtually every galaxy appeared to be flying apart.
But the new discovery came when astronomers noticed that
galaxies birthed billions of years ago were moving away more slowly -- judging from the supernovae they contained. This acceleration differential between older and newer galaxies indicated the universe is expanding at an ever-increasing speed. All supernovae of this type have the same intrinsic brightness, so astronomers can calculate their distance by measuring how much the intervening space has dimmed their light before it reaches Earth.
Knowing that distance and the
speed of light, astronomers can also tell how long ago the flash appeared. And clocking the speed is a simple matter of measuring how much the light waves have been stretched by space as it balloons outward. The end, or a quintessential beginning
Once the apparent cosmic acceleration was announced, scientists proposed several explanations. One was that the measurements were simply wrong. Others involve a mysterious "dark energy" pushing the universe apart. SNAP is billed as the best available test of which theory is right.
The dark energy theory comes in various breeds.
One version, the "cosmological constant," says empty space has an unchanging energy that makes it swell. As it grows, more of the energy appears, propelling the growth more rapidly. Space expands forever; meanwhile the stars finish out their life cycles, biological life dies out and nothing ever replaces it.
Fortunately, there are some mathematical and physical problems with this theory. It also requires an odd coincidence to have happened -- that is, the acceleration started right about the time humans came along to view and measure it. Before then, the universe would have been compact enough so that matter's gravity would hold back the expansion.
Another group of explanations claim that the dark energy is something called "quintessence," with its strength changing over time. According to this account, the dark energy may end up decaying into regular matter, producing new stars and galaxies.
Support for string theory
This type of view fits into "
string theories," which postulate a universe peppered with extra dimensions and treat elementary particles as one-dimensional, thread-like objects.The data so far suggest "we're sort of perched on the edge of this much more crazy world" of mysterious extra dimensions and forces, said Andreas Albrecht, a physics professor at UC Davis.
"There are probably ways we could dig deeper. Maybe the accelerating universe is the first big example of that."
SNAP proponents say the device is custom-designed to test the different theories of the acceleration because each predicts a different ratio between the dark energy's pressure and density. The proposed satellite can accurately measure this number, its advocates say.
'I couldn't choose a better time'
Even some physicists who question the more extravagant claims for SNAP say it can produce interesting data.
The satellite will probably only be able to tell whether the cosmological constant exists, without discriminating much among the many competing theories, Albrecht said.
But this would add to a growing trickle of information from other projects that will unlock some of the deepest secrets of the
cosmos, he added. These include planned satellites to record the background radiation bathing the universe, radiation that can be affected by different types of cosmic expansion."You don't get to choose what period in history you're doing science," Albrecht said. "But if I had to choose, I couldn't choose a better time."
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