The first three-dimensional map of the distant universe, constructed using quasars to backlight never-previously imaged clouds of hydrogen gas, was released yesterday at the American Physical Society meeting in Anaheim, California. The technique used could help to shed light on the nature of the mysterious dark energy that is driving the universe’s expansion.
Unlike earlier, similar maps of the universe, which rely on direct detection of light from galaxies, the map put together by the Baryonic Oscillation Spectroscopic Survey (BOSS) experiment of the Sloan Digital Sky Survey shows the blobs of hydrogen gas between the galaxies. Because the gas doesn’t shine brightly enough to be directly detected, researchers looked for absorption lines, features caused by hydrogen molecules absorbing energy, in the light from distant quasars, the luminous nuclei of early galaxies. The technique had previously been tried using samples of 10 or 20 quasars at a time, but the new map (a single slice is pictured above) combines measurements from as many as 14,000 quasars, about 20 per square degree on the sky. “This is a proof of concept that this technique works,” says Anže Slosar of Brookhaven National Laboratory in Upton, New York, first author of a paper reporting the results.
The next step will be to scale the sample up to 150,000 quasars, enough to get a measure of the size of ripples in the pattern of the gas that can be compared to corresponding ripples in maps of the cosmic microwave background, the echo of the Big Bang. That comparison should reveal how fast the universe expanded, and so the strength of the mysterious dark energy at that time. Dark energy is thought to make up about 73% of today’s universe and is a key component of the standard model of cosmology, but astronomers would like to have a better handle on its strength throughout the universe’s history.
David Schlegel of Lawrence Berkeley National Lab, the principal investigator on BOSS, says he originally doubted the technique would be possible to deploy on such a grand scale. Detecting so many quasars involved spending a relatively short time – about 45 minutes – imaging each one, which meant the level of noise was around the same as the level of the signal the team was trying to detect. “That made me nervous and a lot of people sceptical and this paper puts that to rest,” he says.
The measurements were made using the 2.4 metre telescope at the Apache Point observatory in New Mexico. “It’s a technical tour-de-force” says astronomer David Kirkby at the University of California, Irvine. Kirkby says the most distant galaxies able to be imaged with good enough statistics to tell us the properties of the universe are about 6 billion light years from Earth, but the new map extends our knowledge for the first time into the range of 11 to 12 billion light years distant. (The universe is an estimated 13.7 billion years old.)
A slice from the map shows how the Sloan Digital Sky Survey has been able to image recent galaxies and distant gas, but has no coverage of the universe between about 6 billion and 12 billion light years away. This is of particular interest because, in standard model of cosmology, dark energy became the dominant driver of the universe’s expansion around 7 billion years ago . Schlegel says a proposed follow-on experiment called BigBOSS, to be based at Kitt Peak in Arizona, would be able to map that region.
Image Credit: Anže Slosar and BOSS Lyman-alpha cosmology working group
Correction: A sentence saying the most distant galaxy from Earth to be imaged is 10 billion light years away has been removed; the most distant galaxy to be imaged is actually about 13.2 billion light years away.