SCIENCE

Berkeley Lab-led BOSS proves it can do the job with quasars

BOSS is extending the existing Sloan Digital Sky Survey map of the universe based on galaxies, center, into the realm of intergalactic gas in the distant universe, using the light from bright quasars (blue dots). (Sloan Digital Sky Survey)

The biggest 3-D map of the distant universe ever made, using light from 14,000 quasars — supermassive black holes at the centers of galaxies many billions of light years away — has been constructed by scientists with the third Sloan Digital Sky Survey (SDSS-III).

The map is the first major result from the Baryon Oscillation Spectroscopic Survey (BOSS), SDSS-III’s largest survey, whose principal investigator is David Schlegel of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). The huge new map was presented at the April meeting of the American Physical Society in Anaheim, CA, by Anže Slosar of Brookhaven National Laboratory.

BOSS is the first attempt to use baryon acoustic oscillation (BAO) as a precision tool to measure dark energy. Baryon oscillation refers to how matter clumps in a regular way throughout the universe, a physical manifestation of the expansion of the universe. Until now, 3-D maps showing this oscillation have been based on the distribution of visible galaxies. BOSS is the first survey to map intergalactic hydrogen gas as well, using distant quasars whose light is produced by supermassive black holes at the centers of active galaxies.

“Quasars are the brightest objects in the universe, which we use as convenient backlights to illuminate the intervening hydrogen gas that fills the universe between us and them,” Slosar says. “We can see their shadows, and the details in their shadows” – specifically, the absorption features in their spectra known as the Lyman-alpha forest – “allowing us to see how the gas is clumped along our line of sight. The amazing thing is that this allows us to see the universe so very far away, where measuring positions of individual galaxies in large numbers is impractical.”

“BOSS is the first attempt to use the Lyman-alpha forest to measure dark energy,” says principal investigator Schlegel. “Because the Sloan Telescope has such a wide field of view, and because these quasars are so faint, there was no one who wasn’t nervous about whether we could really bring it off.”

A 2-D slice through BOSS’s full 3-D map of the universe to date. The black dots going out to about 7 billion light years are relatively nearby galaxies. The colored region beginning at about 10 billion light years is intergalactic hydrogen gas; red areas have more gas and blue areas have less. The blank region between is inaccessible to the Sloan Telescope, but the proposed BigBOSS survey would be able to observe it. (Anže Slosar and BOSS Lyman-alpha cosmology working group. Click on image for best resolution.)

By using 14,000 of the quasars collected by the Sloan Telescope at Apache Point Observatory in New Mexico during the first year of BOSS’s planned five-year run, the new map demonstrates that indeed it is possible to determine variations in the density of intergalactic hydrogen gas at cosmological distances and thus to measure the effects of dark energy at those distances.

Slosar, who leads BOSS’s Lyman-alpha cosmology working group, says that while similar measurements have been made with individual quasars or small groups of quasars in the past, “These have given only one-dimensional information about fluctuations in density along the line of sight. Before now there has never been enough density of quasars for a 3-D view.”

The distance scale of the new map corresponds to an early time in the history of the universe, when the distribution of matter was nearly uniform. Any effects of dark energy detected so early would settle basic questions about its nature.