* Astronomy

Title: Measuring Baryon Acoustic Oscillations on 21 cm intensity fluctuations at moderate redshifts
Authors: Xiao-Chun Mao

After reionisation, emission in the 21 cm hyperfine transition provides a direct probe of neutral hydrogen distributed in galaxies. Different from galaxy redshift surveys, observation of baryon acoustic oscillations in the cumulative 21 cm emission may offer an attractive method for constraining dark energy properties at moderate redshifts. Keys to this program are techniques to extract the faint cosmological signal from various contaminants, such as detector noise and continuum foregrounds. In this paper, we investigate the possible systematic and statistical errors in the acoustic scale estimates using ground-based radio interferometers. Based on the simulated 21 cm interferometric measurements, we analyse the performance of a Fourier-space, light-of-sight algorithm in subtracting foregrounds, and further study the observing strategy as a function of instrumental configurations. Measurement uncertainties are presented from a suite of simulations with a variety of parameters, in order to have an estimate of what behaviours will be accessible in the future generation of hydrogen surveys. We find that 10 separate interferometers, each of which contains ~ 300 dishes, observes an independent patch of the sky and produces an instantaneous field-of-view of ~ 100 deg², can be used to make a significant detection of acoustic features over a period of a few years. Compared to optical surveys, the broad bandwidth, wide field-of-view and multi-beam observation are all unprecedented capabilities of low-frequency radio experiments.

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If dark energy is real and not a flaw in our understanding of gravity, then the best way to understand it is by studying the expansion history of the Universe, according to Martin White, an astrophysicist at Berkeley Lab and a professor of astronomy and physics at UC Berkeley.

"One way is with 'standard candles' - that is, supernovae. Another way is with a 'standard ruler'" - Martin White

Baryon acoustic oscillation, or BAO, may provide the ideal standard ruler. The scale is calibrated by the cosmic microwave background (CMB), which recorded the state of the Universe roughly 400,000 years after the Big Bang. At this early epoch, the standard ruler for BAO is detectable as periodic, minute variations in the temperature of the CMB. More recently, the ruler's scale is evident in the regular clustering of galaxies and intergalactic gas, and is also present in the clumping of invisible dark matter. These oscillations can be measured both across the sky and in the line of sight (back in time).

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BOSS, the Baryon Oscillation Spectroscopic Survey, is the most ambitious attempt yet to map the expansion history of the Universe using the technique known as baryon acoustic oscillation (BAO). A part of the Sloan Digital Sky Survey III (SDSS-III), BOSS achieved "first light" on the night of September 14-15, when it acquired data with an upgraded spectrographic system across the entire focal plane of the Sloan Foundation 2.5-meter telescope at Apache Point Observatory in New Mexico.
BOSS is the largest of four surveys in SDSS-III, with 160 participants from among SDSS-III's 350 scientists and 42 institutions. BOSS's principal investigator is David Schlegel, its survey scientist is Martin White, and its instrument scientist is Natalie Roe; all three are with the Physics Division of the U.S. Department of Energy's Lawrence Berkeley National Laboratory. Daniel Eisenstein of the University of Arizona is the director of SDSS-III.

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