Shedding light on dark energy

Hubble data support Einstein theory

November 17, 2006|By Frank D. Roylance | Frank D. Roylance,Sun reporter

Astronomers wielding the Hubble Space Telescope say they've pushed back the curtain a bit on a mystery so bizarre it caused even Albert Einstein to doubt himself.

Using Hubble to examine the light from 24 exploding stars as far as 9 billion light-years away, astrophysicists in Baltimore concluded that "dark energy," the unexplained force that accelerates the expansion of the universe, has been around - apparently unchanged - for at least 9 billion years.

Nobody knows what dark energy is or how it works. Some have dubbed it "a black cat in a coal bin." But it's a huge part of the universe around us, representing perhaps 70 percent of all the matter and energy in existence.

Scientists say the new data from Hubble should help theorists discard several competing theories, some of which had suggested a more rapidly changing nature for dark energy or a flawed understanding of gravity.

"This is a significant clue in the quest to understand what is probably one of the most pressing questions in physics," said Adam Riess, an astrophysicist at the Space Telescope Science Institute and the Johns Hopkins University, who was principal investigator in the study.

His work also helps to confirm a troublesome bit of Einstein's 1915 general theory of relativity and its conclusions about the nature of gravity.

"What we've learned is that Einstein was very smart, and his theory of gravity was really good," said Sean Carroll, a senior research associate at the California Institute of Technology who was not connected to the Hubble research.

Riess' findings are scheduled for publication in February in the Astrophysical Journal.

In 1998, Riess was the first to report the existence of "dark energy," so called because it appears to work even in the emptiest vacuum of space. He used Hubble to examine the light from a class of distant exploding stars called Type 1a supernovas - chosen because they're bright enough to see across the vast distances that astronomers needed to penetrate. Only Hubble can see them so far away.

Scientists also like Type 1a supernovas because their chemistry and physics are well understood - and they're known to have a consistent brightness. That allows astronomers to compute and compare their distance in space (and in time, because light from more distant objects takes longer to reach us). It's like judging the distance of fireflies across a field by how bright they appear.

Riess and his team also used the stars' light to measure how fast they were receding from Earth, carried away with the outward expansion of the universe.

"The supernovae are like smoke, riding in the wind - dark energy," Riess said. "We use the supernovae to tell us what dark energy is doing in the expansion of the universe at different times."

To their astonishment, they found that the farther away the stars were in space and time, the more slowly they were receding.

That meant the universe's rate of expansion has been increasing over time. It is responding to some kind of "repulsive force" that acted against the attractive force of gravity, which common sense suggested should be slowing things down, as gravity affects a ball rolled up a slope.

This repulsive force, or dark energy, was nevertheless just what Einstein had proposed. He called its acceleration rate the "cosmological constant."

Einstein later rejected the idea as his "biggest blunder." But Riess' 1998 discovery revived the notion, and scientists have been swarming over dark energy ever since trying to puzzle out its nature.

Is it accelerating the expansion of the universe at a constant rate everywhere? How fast can it take us? Will it eventually tear apart the galaxies, stars and even atoms?

In 2004, using Hubble observations of the most distant Type 1a supernovas they could find, Riess' team discovered that the early universe was dominated by the gravity of regular matter - stars and galaxies, dust and gas. This attractive force was still slowing down the expansion of the universe.

By 5 billion or 6 billion years ago, however, dark energy had become dominant, and the expansion has accelerated ever since.

What the latest findings show, the Hubble team said, is that the repulsive force of dark energy was already around 9 billion years ago, working against the still-dominant force of gravity.

"We not only found it was there, we found that the properties of dark energy back then are consistent with the properties now as well," said Mario Livio, senior astrophysicist at the Space Telescope Institute and the theoretician on Riess' team.

It doesn't prove that Einstein was right, he said: "All we can say is the results are consistent with Einstein's cosmological constant." Proving it will require more work by scientists in fields from astronomy to particle physics.

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