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Hubble evidence pointing to ever-expanding universe


Astronomers using the Hubble Space Telescope say they have glimpsed the ultimate fate of the universe.

In what some are calling a landmark study, a team led by Adam Riess and Louis-Gregory Strolger said yesterday they have found the most reliable measure yet of the mysterious "dark energy" that's pushing everything in the universe apart.

Captured from the light of exploding stars, the data suggest that dark energy is pushing at a nearly constant rate, just as Albert Einstein predicted in 1920.

If those findings hold up, it means the universe will expand forever. Whatever galaxies, stars, planets and people are still around billions of years from now won't be ripped apart by runaway acceleration of that expansion - or crushed by its reversal into a "big crunch."

"This dark energy and cosmological acceleration are the deepest mystery in all science," said University of Chicago astrophysicist Michael S. Turner, who was not involved in the study. "This is the first time the measurements have been good enough to see whether or not the dark energy is changing with time ... I think it is a landmark."

Appeals for Hubble

Riess and other astronomers at the Space Telescope Science Institute in Baltimore immediately gave credit to Hubble, which faces an early retirement after a NASA decision to cancel its final servicing mission.

"The Hubble Space Telescope is the only tool able to peer far enough out in time and distance to observe these supernovae and track the rate of expansion of the universe over a large range of its history, and gauge the strength of dark energy," Riess said.

Neither the James Webb Space Telescope, scheduled for launch in 2011, nor the Spitzer Space Telescope, launched in August, can monitor the correct wavelengths, or scan a broad enough expanse of sky to search for the distant supernovae critical to measuring dark energy.

"Realistically, this problem will be with us for the next decade or two," Riess said. With Hubble expected to fail as early as 2007 if it's not serviced, he added, "I hope it isn't too long until there is some kind of dedicated telescope to do this kind of work."

Cosmologists, who study the origins, nature and fate of the universe, were startled in 1998 by the discovery that the expansion of the universe, which began with the Big Bang 13.7 billion years ago, was speeding up.

Some sort of "anti-gravity" force had overcome the expected attraction of all the matter in the universe, and was now pushing - or pulling - it apart at an accelerating rate. It was dubbed "dark energy."

Enduring mystery

Although the dark energy constitutes 70 percent of the total energy of the universe, "we don't have a clue what it is," said Mario Livio, a theorist on Riess' team in Baltimore.

To begin to explain it, Livio said, scientists needed to find a way to measure how strong this energy is, and whether it was changing over time. Riess' team attacked the problem by measuring how fast objects were moving at various times throughout the history of the universe.

To do that, they needed to put a kind of radar gun on objects at increasing distances from Earth. Because light from more distant objects began its journey to Earth longer ago than light from nearer objects, it's a way of looking back through time.

Riess' team chose Type 1A supernovas because they are so bright they can be seen by Hubble across billions of light-years.

It worked. The Hubble team found 16 faraway Type 1A supernovas, then measured their distance and the speed at which they appeared to be moving away from us. That gave them the expansion rates for the universe at the times when the light left each star.

Their accuracy was eight times better than in 1998, Riess said - good enough to determine when the expansion of the universe stopped slowing, and began to speed up - about 5 billion years ago.

Blunder no more

The study's bottom line is that the universe appears to be accelerating at something close to a constant rate.

And the strength of the dark energy is at least close to the value that Einstein predicted - his famous "cosmological constant," which he once dismissed as "my greatest blunder."

The more supernovas astronomers can locate and measure, the more precise their estimates of the expansion will become. But they will have to quadruple the number they find just to double their accuracy.

It won't be easy. "We need to search tens of thousands of galaxies to find one," Riess said.

But "at the very least," Livio added, "it appears that for the next 25 billion years or so, we may be safe. The universe is neither ripping apart nor recollapsing."

The Hubble team's paper has been accepted for publication by the Astrophysical Journal. It will be formally presented next week at a scientific conference in California.

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