Cosmological and astrophysical parameter measurements with 21-cm anisotropies during the era of reionization

Abstract

We study the prospects for extracting cosmological and astrophysical parameters from the low radio frequency 21-cm background due to the spin-flip transition of neutral hydrogen during and prior to the reionization of the Universe. We make use of the angular power spectrum of 21-cm anisotropies, which exists due to inhomogeneities in the neutral hydrogen density field, the gas temperature field, the gas velocity field, and the spatial distribution of the Lyman-$\ensuremath{\alpha}$ intensity field associated with first luminous sources that emit UV photons. We extract parameters that describe both the underlying mass power spectrum and the global cosmology, as well as a set of simplified astrophysical parameters that connect fluctuations in the dark matter to those that govern 21-cm fluctuations. We also marginalize over a model for the foregrounds at low radio frequencies. In this general description, we find large degeneracies between cosmological parameters and the astrophysical parameters, though such degeneracies are reduced when strong assumptions are made with respect to the spin temperature relative to the cosmic microwave background (CMB) temperature or when complicated sources of anisotropy in the brightness temperature are ignored. Some of the degeneracies between cosmological and astrophysical parameters are broken when 21-cm anisotropy measurements are combined with information from the CMB, such as the temperature and the polarization measurements with Planck. While the overall improvement on the cosmological parameter estimates is not significant when measurements from first-generation interferometers are combined with Planck, such a combination can measure astrophysical parameters such as the ionization fraction in several redshift bins with reasonable accuracy.