Scientists create largest-ever three-dimensional map of distant universe

Scientists have created the largest-ever three-dimensional map of the distant universe using the light of the brightest objects in the cosmos.

Scientists used light from 14,000 quasars, supermassive black holes at the centres of galaxies billions of light years away, with the third Sloan Digital Sky Survey (SDSS-III) to construct the 3-D map.

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 US 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 Anze Slosar of Brookhaven National Laboratory.

"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 said.

"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," Slosar stated.

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 centres of active galaxies.

"BOSS is the first attempt to use the Lyman-alpha forest to measure dark energy," principal investigator Schlegel said.

"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," he stated.

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," Slosar said.

The wide-field Sloan Telescope covers a wide expanse of sky at moderate magnification. To measure both galaxies and quasars, a thousand targets for each BOSS exposure are selected in advance from existing surveys.

At the telescope's focal plane, "plug plates" are precision-machine-drilled with tiny holes at positions of known galaxies and quasars.

These holes are plugged with optical fibres that channel the light from each chosen galaxy or quasar to a spectrograph, which isolates the spectrum of each individual object.

Schlegel credits Berkeley Lab's Nicholas Ross for doing much of the "incredibly hard work" involved in this targeting.

"Our exploratory paper includes less than a tenth of the 160,000 quasars that BOSS will study, but already that's enough to establish a proof of the concept," Slosar said.

"This is a potentially revolutionary technique for mapping the very distant universe. We're paving the way for future BAO experiments like BigBOSS to follow suit," Slosar stated.

BigBOSS is a proposed survey that will find precise locations for 20 million galaxies and quasars and go beyond BOSS to encompass 10 times the volume of the finished BOSS map.

"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," Patrick McDonald of Berkeley Lab and Brookhaven, who pioneered techniques for measuring the universe with the Lyman-alpha forest, said.

"Considering that no one has ever measured the cosmic expansion rate so far back in time, that's a pretty astonishing prospect," he stated.