Survey reveals how dark energy shaped universe

Posted by Kate Taylor

After two years' work, the Sloan Digital Sky Survey (SDSS-III) has  measureed the precise distances of over a quarter of a million galaxies.

The data gives scientists more information on the period when the universe first began to expand at an ever-increasing rate.

"There's been a lot of talk about using galaxy maps to find out what's causing accelerating expansion," says David Schlegel of the Lawrence Berkeley National Laboratory.

"We've been making a map, and now we're using it - starting to push our knowledge out to the distances when dark energy turned on.

The team's already been able to measure how fast the Universe was expanding six billion years ago, to an accuracy of two percent.

The reason for the universe's expansion is generally believed to be dark energy - although some suggest that at very large distances the force of gravity deviates from Einstein's General Theory of Relativity and becomes repulsive.

The new survey is designed to help solve the mystery by measuring accurate distances to as many galaxies as possible, helping astronomers to trace the history of the universe's expansion.

The BOSS project uses a new custom-designed spectrograph of the SDSS 2.5-meter telescope at Apache Point Observatory in New Mexico, allowing it to measure spectra of more than a million galaxies over six years.

The maps analyzed so far are based on data from the first year and a half of observations, and contain more than 250,000 galaxies, some more than six billion light years away.

As well as providing highly accurate distance measurements, the BOSS data also enable a stringent new test of General Relativity, says Beth Reid, a NASA Hubble Fellow at Lawrence Berkeley National Laboratory.

"Since gravity attracts, galaxies at the edges of galaxy clusters fall in toward the centres of the clusters," she says.

"General Relativity predicts just how fast they should be falling. If our understanding of General Relativity is incomplete, we should be able to tell from the shapes we see in BOSS's maps near known galaxy clusters."

And the team found that the rate at which galaxies fall into clusters is consistent with Einstein's predictions.

"We already knew that the predictions of General Relativity are extremely accurate for distances within the solar system," says Reid, "and now we can say that they are accurate for distances of 100 million light-years.

We're looking a billion times further away than Einstein looked when he tested his theory, but it still seems to work."