In this letter, 21 cm intensity maps acquired at the Green Bank Telescope are cross-correlated with large-scale structure traced by galaxies in the WiggleZ Dark Energy Survey. The data span the redshift range 0.6 < z < 1 over two fields totaling ∼ 41 deg. sq. and 190 hr of radio integration time. The cross-correlation constrains Ω HI b HI r = [0.43 ± 0.07(stat.) ± 0.04(sys.)] × 10 −3 , where Ω HI is the neutral hydrogen (H I) fraction, r is the galaxy-hydrogen correlation coefficient, and b HI is the H I bias parameter. This is the most precise constraint on neutral hydrogen density fluctuations in a challenging redshift range. Our measurement improves the previous 21 cm cross-correlation at z ∼ 0.8 both in its precision and in the range of scales probed.

Fast radio bursts are bright, unresolved, non-repeating, broadband, millisecond flashes, found primarily at high Galactic latitudes, with dispersion measures much larger than expected for a Galactic source. The inferred all-sky burst rate is comparable to the core-collapse supernova rate out to redshift 0.5. If the observed dispersion measures are assumed to be dominated by the intergalactic medium, the sources are at cosmological distances with redshifts of 0.2 to 1 (refs 10 and 11). These parameters are consistent with a wide range of source models. One fast burst revealed circular polarization of the radio emission, but no linear polarization was detected, and hence no Faraday rotation measure could be determined. Here we report the examination of archival data revealing Faraday rotation in the fast radio burst FRB 110523. Its radio flux and dispersion measure are consistent with values from previously reported bursts and, accounting for a Galactic contribution to the dispersion and using a model of intergalactic electron density, we place the source at a maximum redshift of 0.5. The burst has a much higher rotation measure than expected for this line of sight through the Milky Way and the intergalactic medium, indicating magnetization in the vicinity of the source itself or within a host galaxy. The pulse was scattered by two distinct plasma screens during propagation, which requires either a dense nebula associated with the source or a location within the central region of its host galaxy. The detection in this instance of magnetization and scattering that are both local to the source favours models involving young stellar populations such as magnetars over models involving the mergers of older neutron stars, which are more likely to be located in low-density regions of the host galaxy.

The large-scale distribution of neutral hydrogen in the Universe will be luminous through its 21 cm emission. Here, for the first time, we use the auto-power spectrum of 21 cm intensity fluctuations to constrain neutral hydrogen fluctuations at z ∼ 0.8. Our data were acquired with the Green Bank Telescope and span the redshift range 0.6 < z < 1 over two fields totalling ≈ 41 deg 2 and 190 h of radio integration time. The dominant synchrotron foregrounds exceed the signal by ∼ 10 3 , but have fewer degrees of freedom and can be removed efficiently. Even in the presence of residual foregrounds, the auto-power can still be interpreted as an upper bound on the 21 cm signal. Our previous measurements of the cross-correlation of 21 cm intensity and the WiggleZ galaxy survey provide a lower bound. Through a Bayesian treatment of signal and foregrounds, we can combine both fields in auto-and cross-power into a measurement of Ω HI b HI = [0.62 +0.23 −0.15 ] × 10 −3 at 68% confidence with 9% systematic calibration uncertainty, where Ω HI is the neutral hydrogen (H I) fraction and b HI is the H I bias parameter. We describe observational challenges with the present data set and plans to overcome them.

The expansion of the universe appears to be accelerating, and the mysterious anti-gravity agent of this acceleration has been called "dark energy". To measure the dynamics of dark energy, Baryon Acoustic Oscillations (BAO) can be used. Previous discussions of the BAO dark energy test have focused on direct measurements of redshifts of as many as 10 9 individual galaxies, by observing the 21cm line or by detecting optical emission. Here we show how the study of acoustic oscillation in the 21 cm brightness can be accomplished by economical three dimensional intensity mapping. If our estimates gain acceptance they may be the starting point for a new class of dark energy experiments dedicated to large angular scale mapping of the radio sky, shedding light on dark energy.Introduction.-To understand dark energy and sharply test theories of its character, it is necessary to precisely measure the last half of the expansion history. This period corresponds to the redshift range 0 < z 2. Many techniques have been proposed to study the latestage expansion history, and some of the most promising make use of Baryon Acoustic Oscillations (BAO) [1]. In addition to producing CMB structure, the acoustic oscillations also produced density structure in the atomic gas and dark matter, which is still detectable today. Many groups have reported 2 to 3 σ detections, in the low redshift universe, of periodic structures in the density of galaxies at the predicted wavelengths across the sky [2]. Because the acoustic waves are frozen in after recombination, the BAO peak wavelengths can be used as a cosmological standard ruler: observation of the angular size of the peak wavelengths across a range of redshifts allows accurate measurement of the expansion history.In this letter we present calculations of structure in the three dimensional brightness due to the hyperfine transition of neutral hydrogen (HI) at 21 cm wavelength. We show that, via 21 cm emission, baryon oscillations could be precisely measured, using a telescope just 200 wavelengths in diameter, since each cosmic cell of the appropriate scale contains more than 10 12 M ⊙ of emitting neutral hydrogen gas. We present forecasts on the dark energy constraints from this new type of observation, which are competitive with the best proposed dark energy experiments [3]. Where needed in the analysis we adopt WMAP3 values for the cosmological parameters [4], Ω m = 0.24, Ω b = 0.04, Ω Λ = 0.76, and h = 0.73, where Ω m , Ω b , and Ω Λ are the matter, baryon, and dark energy fractional density, respectively, and h is the dimensionless Hubble parameter. We also use the WMAP3 error limits for these values.A recent paper proposed a test of dark energy models by measurement of baryon oscillations in the 21 cm brightness field at redshifts z > 3 [5]. Using only such high redshift data two cosmological parametersdepartures of spatial flatness Ω − 1 and a slow change of dark energy equation of state-are nearly indistinguish-