The expansion of the universe has been speeding up for the past 5 billion years, but that hasn't always been the case.
Shortly after the Big Bangthe event scientists believe occurred 13.8 billion years ago that created the universe and everything in itthe universe was expanding at a much slower rate.
"If we think of the universe as a roller coaster, then today we are rushing downhill, gaining speed as we go," Nicolas Busca of the Laboratoire Astroparticule et Cosmologie of the French Centre National de la Recherche Scientifique (CNRS), one of the lead authors of the study, said in a statement. "Our new measurement tells us about the time when the universe was climbing the hillstill being slowed by gravity."
Directly following the Big Bang, all of the matter that makes up the universe was still relatively densely packed together, meaning the mutual gravitational pulls of all the galaxies in the universe were pulling each other toward one another. However, as those galaxies continued to move farther apart, another force took over: dark energy.
Dark energy is a constant, repulsive force that scientists believe is accelerating the expansion of the universe. The findings, published in the journal Astronomy and Astrophysics and providing a snapshot of the universe about 11 billion years ago, show that dark energy has been part of the universe from the beginning.
The large group of astronomers who contributed to the research are part of the third Sloan Digital Sky Survey (SDSS-III).
In order to measure the expansion and history of the universe, scientists look at matter and how it is distributed. The universe is roughly composed of 70 percent dark energy, 20 percent dark matter and 5 percent matter. However, scientists can't see dark matter, but they can see the prevalent gas hydrogen, and one follows the other, said Kyle Dawson an assistant professor in the department of physics and astronomy at the University of Utah who worked on the study.
"What we care about is dark matter, but what we can measure is the hydrogen," said Dawson. "It's a gas. If I have a flashlight behind these clouds of gas, I can see the shadows of these gas clouds on matter."
Previously, astronomers have used a technique that would have required them to look at galaxies that are extremely far away, and therefore extremely faint.
Instead, the astronomers looked at hydrogen gas and how it clusters across the universe, Dawson said. The group used super-bright quasars as that flashlight to measure the expansion of the universe.
As the light from a quasar passes through a hydrogen gas cloud, some of that light is absorbed, so astronomers know where the gas cloud is, said Dawson, whose job is to look at the data the astronomers collected from the 50,000 quasars and make sure it is sound.
But the technique has its challenges.
"It's a cool technique because we're essentially measuring the shadows cast by gas along a single line billions of light-years long," says Anze Slosar of Brookhaven National Laboratory. "The tricky part is combining all those one-dimensional maps into a three-dimensional map. It's like trying to see a picture that's been painted on the quills of a porcupine."
The findings paint one of the earliest pictures scientists have been able to create of the universe.
"No technique has ever been able to probe this ancient era before," said principal investigator David Schlegel of the Lawrence Berkeley National Laboratory. "Back then, the expansion of the universe was slowing down; today, it's speeding up. How dark energy caused the transition from deceleration to acceleration is one of the most challenging questions in cosmology."