The study, published in the journal Astronomy & Astrophysics, shows the universe went through a phase roughly three billion years after the Big Bang when expansion actually started to slow, before the force of so-called 'dark energy' kicked in and sent galaxies accelerating away from each other.

Much is known about the immediate aftermath of the Big Bang from studies of its afterglow in the cosmic background radiation, and its accelerating expansion over several billion years can be seen with a look at the way distant galaxies are moving.

"Only now are we finally seeing its adolescence... just before it underwent a growth spurt," said Mat Pieri at the University of Portsmouth in Britain, one of the authors of the study.

Little is known about dark energy, and its counterpart dark matter, but astronomers argue the force must exist to account for the speed at which the universe is expanding. Together, dark energy and dark matter are believed to make up about 96 percent of the universe.

The new study supports the theory that dark energy was somehow created as the universe expanded, by detailing a period when gravity was winning the tussle and slowing the expansion.

"If we think of the universe as a roller coaster, then today we are rushing downhill, gaining speed as we go," said Pieri. "Our new measurement tells us about the time when the universe was climbing the hill - still being slowed by gravity."

The map, the work of 63 scientists from nine countries, was compiled using a novel technique for studying the intense light from 50,000 distant quasars as it passes through clouds of hydrogen in space on its way to Earth.

They produce a picture of the ancient universe in same way thousands of flashlight beams would light up a bank of fog.

"The quasars are back-lights," Pieri told Reuters, and the way the gas in front of them absorbs some of the light allows astronomers to get a detailed picture of these distant clouds of gas known as the intergalactic medium.

The study is the first fruit from a five-year project started in 2009. The team, from the third Sloan Digital Sky Survey, expect to expand the survey with light from about 160,000 quasars by the end of the project.

"We're essentially measuring the shadows cast by gas along a series of lines, each billions of light-years long," said Will Percival, a cosmology professor the University of Portsmouth.

"The tricky part is combining all those one-dimensional maps. The problem is like trying to recognize an object from a picture that's been painted on the quills of a porcupine," he said.

(Editing by Michael Roddy)

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