The remnant of a supernova ... stars that exploded 9 billion years ago have led to new insights into dark energy.
Credit: Chandra X-ray Observatory
WASHINGTON: That mysterious force called dark energy is not a new constituent of space, but was pushing the early universe apart, evidence from nine billion-year-old supernovae has revealed.
"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 the U.S. space agency NASA's Space Telescope Institute in Maryland.
Scientists still do not understand dark energy - which appears to be everywhere, pushing the universe apart with its repulsive power. But these new findings, published in the Astrophysical Journal, might have set them on the right track.
The discovery was made with the help of the orbiting Hubble Space Telescope, and suggests 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 said. "It appears that this dark energy was already boosting the expansion of the universe nine 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 four to five billion years ago. The scientists came to this conclusion by studying 24 distant stars that exploded in blasts known as supernovae nine billion years ago, when the universe was half the size it is now.
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.
Sean Carroll of the California Institute of Technology in Pasadena, who did not work on the report, said many experts believe dark energy helped drive the first expansion of the universe after the Big Bang, during a time called inflation.
"It fits with the data we have. Something like dark energy was driving inflation. Something like dark energy is around now. It is natural to wonder if it is the same thing," Carroll said.
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.
