BioEd Online

Astronomers have determined that dark energy, a force that counters gravity, existed at least 9 billion years ago. And the mysterious force seems to have been much the same then, in the infancy of our 13.7-billion-year-old Universe, as it is today.

"This is a significant clue in the quest to understand one of the most interesting questions in physics," says Adam Riess, an astronomer at Johns Hopkins University in Baltimore, Maryland, and a member of the study.

Dark energy is a poorly understood force that seems to be pulling the Universe apart. It was first spotted in 1998 by astronomers, including Riess, who were studying the violent deaths of distant stars.

Further observations of the afterglow of the Big Bang, known as the cosmic microwave background, have strengthened the case for dark energy. Those measurements show that it could make up as much as 74% of the Universe, while another unknown substance called 'dark matter' makes up 22%. That leaves just 4% for 'ordinary' matter including stars, planets and people.

At the core of the new work are 23 observations of very old, distant star-explosions called supernovae, spotted with the Hubble Space Telescope. The supernovae came from a specific type of star that astronomers believe explodes with a uniform brightness, so the explosions' observed brightness can be used to determine how far away, and how old, the stars are.

The survey shows that the stars are further away than would be expected if the Universe were expanding without the added force of dark energy to push things apart, the researchers say. The power of the dark energy seems to be much the same back then as it is today, says Riess, although the error bars on this are still very large.

Constant question

This will constrain theories about how our Universe works. Some say that dark energy is a constant, unchanging quality. Others suggest that it, like the electromagnetic force, has changed over time. Then there are those who think that dark energy does not exist at all; instead, they say, gravity has behaved differently over time, accounting for the behaviour that dark energy was invented to describe.

"We still don't understand some very basic things," says Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena. "Every clue on dark energy is important."

But there's still a good deal of work to be done before astronomers will be able to determine whether dark energy really is a constant, unchanging force of nature, according to Saul Perlmutter, an astronomer at Lawrence Berkeley National Laboratory in California. Perlmutter says it will take thousands, rather than dozens, of supernovae measurements to determine whether dark energy has changed over time. That is likely to require a dedicated, space-based telescope to hunt for supernovae.

Nevertheless, he says, it's important to take at least some measurements now to get an early hint of the final conclusion. "These are very important steps to be taking," Perlmutter says. "I'll look forward to seeing the paper." The work is due to be published in the Astrophysical Journal next year.

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