14,000 quasars shine a light on the distant universe
Scientists from the Sloan Digital Sky Survey (SDSS-III) have created the largest ever three-dimensional map of the distant universe by using the light of the brightest objects in the cosmos to illuminate ghostly clouds of intergalactic hydrogen. The map provides an unprecedented view of what the universe looked like 11 billion years ago.
Anze Slosar, a physicist at the U.S. Department of Energy’s Brookhaven National Laboratory, presented the new findings May 1, 2011 at a meeting of the American Physical Society. The findings appear in an article posted online on the arXiv astrophysics preprint server.
The new technique used by Slosar and his colleagues turns the standard approach of astronomy on its head. Slosar explained:
Usually we make our maps of the universe by looking at galaxies, which emit light. But here, we are looking at intergalactic hydrogen gas, which blocks light. It’s like looking at the moon through clouds — you can see the shapes of the clouds by the moonlight that they block.
Instead of the moon, the SDSS team observed quasars, brilliantly luminous beacons powered by giant black holes. Quasars are bright enough to be seen billions of light-years from Earth, but at these distances they look like tiny, faint points of light. As light from a quasar travels on its long journey to Earth, it passes through clouds of intergalactic hydrogen gas that absorb light at specific wavelengths, which depend on the distances to the clouds. This patchy absorption imprints an irregular pattern on the quasar light known as the Lyman-alpha forest.
An observation of a single quasar gives a map of the hydrogen in the direction of the quasar, Slosar explained. The key to making a full, three-dimensional map is numbers. He said:
When we use moonlight to look at clouds in the atmosphere, we only have one moon. But if we had 14,000 moons all over the sky, we could look at the light blocked by clouds in front of all of them, much like what we can see during the day. You don’t just get many small pictures — you get the big picture.
The big picture shown in Slosar’s map contains important clues to the history of the universe. The map shows a time 11 billion years ago, when the first galaxies were just starting to come together under the force of gravity to form the first large clusters. As the galaxies moved, the intergalactic hydrogen moved with them. Andreu Font-Ribera, a graduate student at the Institute of Space Sciences in Barcelona, created computer models of how the gas likely moved as those clusters formed. The results of his computer models matched well with the map.
That tells us that we really do understand what we’re measuring. With that information, we can compare the universe then to the universe now, and learn how things have changed.
The quasar observations come from the Baryon Oscillation Spectroscopic Survey (BOSS), the largest of the four surveys that make up SDSS-III. Eric Aubourg, from the University of Paris, led a team of French astronomers who visually inspected every one of the 14,000 quasars individually. Aubourg explained:
The final analysis is done by computers. But when it comes to spotting problems and finding surprises, there are still things a human can do that a computer can’t.
David Schlegel, a physicist at Lawrence Berkeley National Laboratory in California and the principal investigator of BOSS, said:
BOSS is the first time anyone has used the Lyman-alpha forest to measure the three-dimensional structure of the universe. With any new technique, people are nervous about whether you can really pull it off, but now we’ve shown that we can.
In addition to BOSS, Schlegel noted, the new mapping technique can be applied to future, still more ambitious surveys, like its proposed successor BigBOSS.
When BOSS observations are completed in 2014, astronomers can make a map ten times larger than the one being released today, according to Patrick McDonald of Lawrence Berkeley National Laboratory and Brookhaven National Laboratory, who pioneered techniques for measuring the universe with the Lyman-alpha forest and helped design the BOSS quasar survey. The ultimate goal of BOSS is to use subtle features in maps like Slosar’s to study how the expansion of the universe has changed during its history. McDonald said:
By the time BOSS ends, we will be able to measure how fast the universe was expanding 11 billion years ago with an accuracy of a couple of percent. Considering that no one has ever measured the cosmic expansion rate so far back in time, that’s a pretty astonishing prospect.
Quasar expert Patrick Petitjean of the Institut d’Astrophysique de Paris, a key member of Aubourg’s quasar-inspecting team, is looking forward to the continuing flood of BOSS data:
Fourteen thousand quasars down, one hundred and forty thousand to go. If BOSS finds them, we’ll be happy to look at them all, one by one. With that much data, we’re bound to find things that we never expected.