You can't see it happening on Earth, but space itself is stretching. Ever since the Big Bang happened 13.8 billion years ago, the universe has been getting bigger.
"If you go into the distant future, everything that we see in the universe right now will expand away from us so much that we won't be able to see it anymore," said David Schlegel of the Lawrence Berkeley National Laboratory.
The universe grows every year, and by an increasingly larger amount. Now, scientists have measured for the first time how fast the universe was expanding 10.8 billion years ago with unprecedented precision. The data come from that time period because scientists were looking at very distant celestial objects, whose light has been traveling for that long.
Results were presented at this week's American Physical Society meeting in Savannah, Georgia.
"What is impressive, in my opinion, is that we measure the expansion with a better accuracy (than the expansion today), even though we are looking further in time," said Andreu Font-Ribera of the Lawrence Berkeley National Laboratory, lead author of a new study, in an e-mail.
Font-Ribera and colleagues used the Baryon Oscillation Spectroscopic Survey, part of the Sloan Digital Sky Survey, to look at quasars. Quasars are very bright galaxy cores, powered by supermassive black holes, emitting massive amounts of energy.
Hydrogen gas in space partly absorbs light from distant quasars; when this happens, the gas gets imprinted with a ring-like pattern from a phenomenon called baryon acoustic oscillations. This is a marker of how matter is distributed. Patches in the gas that are denser absorb more light.
From the principles of physics, scientists can determine how big these imprints should be and compare that with their maps incorporating data observed with BOSS.
Two groups of scientists have analyzed data from more than 100,000 quasars, looking for hints of how matter is distributed. One study was led by Timothee Delubac of Centre de Saclay in France (PDF), the other by Font-Ribera. Collectively, their results establish an expansion rate of the universe.
What's driving the accelerated expansion of the universe is a mysterious entity that exists only in theory: dark energy. We think that dark energy is a force that pushes matter apart and that it makes up about 71% of the universe.
"The aim is to understand something more about dark energy and what it's doing to the universe," Schlegel said.
A brief history of expansion
Less than a trillionth of a second after the Big Bang, scientists believe, the universe expanded faster than the speed of light in an event called inflation. This theory was recently supported by a landmark study.
But then, because of the force of gravity, the universe's expansion decelerated and was in a slowing-down period 10.8 billion years ago. For that time, the expansion rate scientists have measured using quasars is 68 kilometers (42 miles) per second per million light-years.
Then, about 7 billion years ago, the expansion rate of the universe sped up again. Scientists attribute this mysterious rebound to dark energy.