No abstract

There is growing interest in using 3-dimensional neutral hydrogen mapping with the redshifted 21 cm line as a cosmological probe. However, its utility depends on many assumptions. To aid experimental planning and design, we quantify how the precision with which cosmological parameters can be measured depends on a broad range of assumptions, focusing on the 21 cm signal from 6 < z < 20. We cover assumptions related to modeling of the ionization power spectrum, to the experimental specifications like array layout and detector noise, to uncertainties in the reionization history, and to the level of contamination from astrophysical foregrounds. We derive simple analytic estimates for how various assumptions affect an experiment's sensitivity, and we find that the modeling of reionization is the most important, followed by the array layout. We present an accurate yet robust method for measuring cosmological parameters that exploits the fact that the ionization power spectra are rather smooth functions that can be accurately fit by 7 phenomenological parameters. We find that for future experiments, marginalizing over these nuisance parameters may provide almost as tight constraints on the cosmology as if 21 cm tomography measured the matter power spectrum directly. A future square kilometer array optimized for 21 cm tomography could improve the sensitivity to spatial curvature and neutrino masses by up to two orders of magnitude, to ∆Ω k ≈ 0.0002 and ∆mν ≈ 0.007 eV, and give a 4σ detection of the spectral index running predicted by the simplest inflation models.

We present the largest-volume (425 h −1 Mpc = 607 Mpc on a side) full radiative transfer simulation of cosmic reionization to date. We show that there is significant additional power in density fluctuations at very large scales. Although the halo power spectra are unaffected by this, there is strong modulation of the halo abundance reaching very large scales. We systematically investigate the effects this additional power has on the progress, duration and features of reionization, as well as on selected reionization observables. We find that comoving simulation volume of ∼ 100 h −1 Mpc per side is sufficient for deriving a convergent mean reionization history, but that the reionization patchiness is significantly underestimated. In the large-scale volume the isolated ionized regions reach volumes up to 4-10 times larger (depending on the measure being used) than they do in a smaller, 114 h −1 = 163 Mpc volume with the same source properties and reionization history, though the abundance of smaller ionized patches agrees well in the two cases. We use jackknife splitting of the large simulation volume to quantify the convergence of reionization properties with simulation volume for both mean-density and variable-density sub-regions. We find that sub-volumes of ∼ 100 h −1 Mpc per side or larger yield convergent reionization histories, except for the earliest times (corresponding to z ∼ > 20 − 25 for our parameters), but smaller volumes of ∼ 50 h −1 Mpc or less are not well converged at any redshift. Reionization history milestones, defined here as the redshifts at which the ionized fraction by mass reaches 10%, 50%, 90% and 99%, show significant scatter between the sub-volumes, of ∆z = 0.6 − 1 for ∼ 50 h −1 Mpc volumes, decreasing to ∆z = 0.3 − 0.5 for ∼ 100h −1 Mpc volumes, and ∆z ∼ 0.1 for ∼ 200 h −1 Mpc volumes. If we only consider mean-density sub-regions the scatter decreases, but remains at ∆z ∼ 0.1 − 0.2 for the different size sub-volumes. Consequently, many potential reionization observables like 21-cm rms, 21-cm PDF skewness and kurtosis all show good convergence for volumes of ∼ 200 h −1 Mpc, but retain considerable scatter for smaller volumes. In contrast, the three-dimensional 21-cm power spectra at large scales (k ∼ < 0.25 h Mpc −1 ) do not fully converge for any sub-volume size. These additional large-scale fluctuations significantly enhance the 21-cm fluctuations. At the rough beam-and bandwidth resolution expected for the LOFAR EoR experiment (3' and 440 kHz) and for our simulation parameters, the peak value of the rms 21-cm brightness temperature fluctuations as a function of frequency, derived from the large volume is ∼ 10% higher than for a ∼ 100 h −1 Mpc volume. At late times (high frequency), close to the overlap epoch, the signal derived from the large volume is up to 2.5 times larger, which should improve the prospects of detection considerably, given the lower foregrounds and greater interferometer sensitivity at higher frequencies.