Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization

Abstract

A number of radio interferometers are currently being planned or constructed to observe 21 cm emission from reionization. Not only will such measurements provide a detailed view of that epoch, but, since the 21 cm emission also traces the distribution of matter in the universe, this signal can be used to constrain cosmological parameters. The sensitivity of an interferometer to the cosmological information in the signal may depend on how precisely the angular dependence of the 21 cm three-dimensional power spectrum can be measured. Using an analytic model for reionization, we quantify all the effects that break the spherical symmetry of the three-dimensional 21 cm power spectrum. We find that upcoming observatories will be sensitive to the 21 cm signal over a wide range of scales, from larger than 100 to as small as 1 comoving Mpc. Next, we consider three methods to measure cosmological parameters from the signal: (1) direct fitting of the density power spectrum to the signal, (2) using only the velocity field fluctuations in the signal, and (3) looking at the signal at large enough scales that all fluctuations trace the density field. With the foremost method, the first generation of 21 cm observations should moderately improve existing constraints on cosmological parameters for certain low-redshift reionization scenarios, and a 2 yr observation with the second-generation interferometer MWA5000 in combination with the CMB telescope Planck could improve constraints on Ω_w, Ω_(m)h^2, Ω_(b)h^2, Ω_ν, n_s, and α_s. If the universe is substantially ionized by z ~ 12 or if spin temperature fluctuations are important, we show that it will be difficult to place competitive constraints on cosmological parameters with any of the considered methods.