Scientists from the Sloan Digital Sky Survey (SDSS-III) collaboration, including an astronomer at Penn State, 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 how the universe looked 10 billion years ago.
The new research achievement is being presented today at a meeting of the American Physical Society and is described in a scientific article posted on an astrophysics preprint server (arxiv.org/archive/astro-ph). \”The novel approach employed by this investigation has great promise for future studies of the conditions present in the early universe,\” said Donald Schneider, distinguished professor of astronomy and astrophysics at Penn State and a coauthor of the study. Schneider is the Survey Coordinator of the large, international SDSS-III collaboration.
The new technique turns the standard approach of astronomy on its head, explained Anze Slosar, a physicist at the U.S. Department of Energy\’s Brookhaven National Laboratory who led this research study. \”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 nearby moon, the SDSS team observed distant quasars, brilliantly bright beacons of light 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, depending on the distances to the clouds. An observation of a single quasar gives a map of the hydrogen in the direction of the quasar, Slosar explained.
But the key to making the new, fully three-dimensional map was observing many quasars — 14,000 of them.
\”When we use moonlight to look at clouds in the atmosphere, we have only 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 get just many small pictures — you get the big picture,\” Slosar said.
The big picture shown in the new 3-D map contains important clues to the history of the universe. The map shows a time, 10 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 intergalacitc 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. \”This tells us that we really do understand what we\’re measuring,\” Font-Ribera said. \”With this 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 the SDSS-III project. Eric Aubourg, from the University of Paris, led a team of French astronomers who visually inspected every one of the 14,000 quasars individually. \”The final analysis is done by computers,\” Aubourg said, \”but when it comes to spotting problems and finding surprises, there are still things a human can do that a computer can\’t.\”
The patchy absorption of the ghostly hydrogen clouds imprints an irregular pattern on the quasar light known as the \”Lyman-alpha forest.
\”BOSS is the first time anyone has used the Lyman-alpha forest to measure the three-dimensional structure of the universe,\” said David Schlegel, a physicist at Lawrence Berkeley National Laboratory in California and the principal investigator of BOSS. \”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. BOSS\’s ultimate goal is to use subtle features in 3-D maps to study how the expansion of the universe has changed during its history.
\”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 percents. Considering that no one has ever measured the cosmic expansion rate so far back in time, that\’s a pretty astonishing prospect,\” McDonald said.