The BOSS proves it can do the job with quasars
BERKELEY, Calif., May 4, 2011 — The biggest 3-D map of the distant universe ever made, using light from 14,000 quasars, 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.
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. BOSS’s principal investigator is David Schlegel of the US Department of Energy's Lawrence Berkeley National Laboratory.
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). (Image: Sloan Digital Sky Survey)
"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," said co-investigator Anže Slosar of Brookhaven National Laboratory in Upton, N.Y. "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," 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."
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 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, said 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.
Measuring the expansion history of the universe
Baryon acoustic oscillation is the periodic clustering (oscillation) of matter (baryons), which originated as pressure waves moving through the hot, opaque, liquidlike early universe. The pressure differences resulted in differences in density and left their signature as small variations in the temperature of the cosmic microwave background. Because the denser regions formed by the pressure waves seeded galaxy formation and the accumulation of other matter, the original acoustic waves were echoed in the netlike filaments and voids of the clustering of galaxies and in variations in the density of intergalactic hydrogen gas.
Zooming in on a slice of the BOSS map shows areas with more (red) and less (blue) intergalactic gas, as revealed by correlations of the Lyman-alpha forest data from the spectra of thousands of quasars. A distance of 1 billion light-years is indicated by the scale bar. (Image: Anže Slosar and BOSS Lyman-alpha cosmology working group)
The oscillations repeat at intervals of about 500 million light-years, and because this scale is firmly anchored in the cosmic microwave background, it provides a ruler — a very big one — to measure the history of the expanding universe. With this cosmic yardstick, it will be possible to determine just how fast the universe was expanding at the redshift of the objects in the BOSS survey — in other words, how the expansion rate has changed over time. Knowing whether expansion has accelerated at a constant rate or has varied over time will help decide among the major theories of dark energy.
BOSS is using two distinct methods to calibrate the markings on the cosmic yardstick. The first method, well tested, will precisely measure 1.5 million luminous red galaxies at "low" redshifts around Z = 0.7. The second method will eventually measure the Lyman-alpha forest of 160,000 quasars with high redshifts around Z = 2.5. These redshifts correspond to galaxies at distances of 2 to 6 billion light-years and quasars at 10 to 11 billion light-years.
Lyman-alpha is the name given to a line in the spectrum of hydrogen, marking the wavelength of light emitted when an excited hydrogen electron falls back to its ground state; it's a strong signal in the light from quasars. As the quasar's light passes through intervening clouds of hydrogen gas, additional lines accumulate where the gas clouds absorb the signal, echoing it but shifting by different degrees according to factors including the redshift of the gas cloud and its density. The spectrum of a distant quasar may have hundreds of lines, clumped and blended into a messy, wiggly structure in the spectrum, which came to be called the Lyman-alpha forest.
"In theory, you can turn any of these absorption lines directly into redshifts and locate the gas cloud precisely," said Bill Carithers of Berkeley Lab, who concentrates on extracting relevant information from the noisy data that comes straight from the telescope. "But in practice, only the spectra of the very brightest quasars are clean enough to make things that simple."
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. (Image: Anže Slosar and BOSS Lyman-alpha cosmology working group)
Carithers said that “while a very long exposure could improve the signal-to-noise ratio, that comes at a price. We need lots and lots of quasars to make a map. We can only afford to spend so much telescope time on each.”
Since the heart of BAO is the correlation distance among density oscillations, the trick turns out to be not overconcentrating on individual spectra but instead measuring the correlations among them. "For any correlation distance, many quasars will contribute," Carithers said, "so the noise will average and the signal will get stronger. We can say, ‘I'll use my data, noise and all.’ ”
Targeting the search
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, the scientists placed “plug plates” — drilled with tiny holes at positions of known galaxies and quasars. These holes are plugged with optical fibers that channel the light from each target galaxy or quasar to a spectrograph that isolates the spectrum of each object.
“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,” Sloas 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.” 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.
For more information, visit: www.lbl.gov
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