Supernovas shed light on dark energy in report

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Suns that exploded 9 billion years ago have helped prove that a mysterious force called dark energy was pushing the universe apart even back then, scientists reported on Thursday.

They still do not understand dark energy -- which appears to be everywhere, pushing everything apart with its repulsive power. But now they may be on the right track.

"We think this is a significant clue in the quest to understand what is probably one of the most, if not the most, pressing questions in physics," said astrophysicist Adam Riess of Johns Hopkins University and NASA's Space Telescope Institute.

And their findings, made with the help of the orbiting Hubble Space Telescope, suggest that Albert Einstein was right yet again when he predicted that an energy force, which he called the Cosmological Constant, permeated the universe.

Gravity would cause a universe that was initially at equilibrium to contract. To counteract this possibility, Einstein added the cosmological constant, a repulsive energy.

He later abandoned the cosmological constant and called it the "biggest blunder" of his life. The discovery of dark energy in 1998 revived the theory.

"Its exotic repulsive gravity, this strange feature that Einstein first predicted, appears to be making the expansion rate of the universe speed up today," Riess told reporters in a telephone briefing.

"It appears that this dark energy was already boosting the expansion of the universe 9 billion years ago."


This strengthens evidence suggesting the expansion of the early universe after the Big Bang first slowed but then began to accelerate around 4 to 5 billion years ago.

The scientists came to this conclusion by studying 24 distant stars that exploded in blasts known as supernovae 9 billion years ago, when the universe was half the size it is now.

"Supernovae are cosmic mile markers," Riess said.

These particular supernovae are helpful because they all exploded at the same size -- 1.4 times the mass of our own sun. So by measuring their apparent brightness, astronomers can measure how far away they were and thus how fast the universe is expanding.

Although there are competing theories about what the universe is doing, the most accepted idea currently is that the universe is expanding at a rate that will allow it to continue expanding forever, as opposed to a "steady-state" universe or one that will eventually contract.


The expansion of the universe, and the rate of this expansion, is affected by how much matter there is, and thus how much gravity there is, and how much it is counteracted by this dark energy.

The researchers will publish their findings in the February issue of the Astrophysical Journal.

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