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Mysterious Dark Energy Has Existed For Most of Time, Scientists Say

Richard A. Lovett
for National Geographic News
November 17, 2006
 
Peeling back both space and time, scientists using the Hubble Space Telescope have found that the strange force known as dark energy appears to have existed for at least the past two-thirds of the universe's history.

Dark energy is a mysterious repulsive force that opposes gravity, causing the universe to expand. Albert Einstein posited that a form of the force existed nearly a century ago by, but it was not discovered until 1998.

Scientists believe dark energy makes up about 70 percent of the substance of the universe.

Scientists can measure the strength of dark energy by looking at distant galaxies for a type of exploding star known as a Type 1A supernova. (Related: "Galaxy Hunters" in National Geographic Magazine.)

Because these supernovae release known amounts of energy, measuring the amount of light arriving on Earth serves as a convenient distance marker to any galaxy in which one is observed.

"[It's] much like judging the distance of a faraway lighthouse by its apparent brightness," said Louis-Gregory Strolger, an astronomer at Western Kentucky University in Bowling Green, in a press conference yesterday.

Other instruments allow astronomers to determine how quickly distant stars are moving, giving hints to the forces acting on the galaxies they belong to.

The farther away the supernovae are, the longer their light has been traveling and the further back in time the explosion occurred.

To date astronomers have detected 23 such explosions from about 9 billion years ago, announced a team led by Adam Reiss, an astrophysicist at Baltimore, Maryland's Space Telescope Science Institute, yesterday afternoon.

Information garnered from these supernovae indicates that dark energy not only existed back then, but that it was about as strong then as today.

Physics' Biggest Problem

Because little is known about dark energy, this is a "significant clue" to what the force might be, scientists say.

The find indicates, for example, that dark energy isn't being diluted by the expansion of the universe. This supports theories that the force might be "vacuum energy," an underlying background energy present throughout the universe even in the absence of matter.

"Understanding the nature of dark energy is arguably the biggest problem physics is facing today," Mario Livio, a colleague of Reiss, said. (Related: "Journal Ranks Top 25 Unanswered Science Questions" [June 30, 2005].)

Sean Carroll, a theoretician at California Institute of Technology in Pasadena, agrees. "Every clue helps," he said at the press conference.

Reiss looks forward to additional data when the Hubble telescope undergoes servicing in 2008. (Related: "Hubble Repair Mission Approved by NASA" [October 31, 2006].)

"With luck," he said, "the astronauts will plug another new camera into Hubble that would let us work even farther back in time and see what dark energy was doing not 9 billion years ago, but 11 or 12 billion years ago."

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