Baryonic acoustic oscillations in 21-cm emission : a probe of dark energy out to high redshifts

Abstract

Low-frequency observatories are currently being constructed with the goal of detecting red-shifted 21-cm emission from the epoch of reionization. These observatories will also be able to detect intensity fluctuations in the cumulative 21-cm emission after reionization, from hydrogen in unresolved damped Ly alpha absorbers (such as gas-rich galaxies) down to a redshift z similar to 3.5. The inferred power spectrum of 21-cm fluctuations at all redshifts will show acoustic oscillations, whose comoving scale can be used as a standard ruler to infer the evolution of the equation of state for the dark energy. We find that the first generation of low-frequency experiments (such as MWA or LOFAR) will be able to constrain the acoustic scale to within a few per cent in a redshift window just prior to the end of the reionization era, provided that foregrounds can be removed over frequency bandpasses of >= 8 MHz. This sensitivity to the acoustic scale is comparable to the best current measurements from galaxy redshift surveys, but at much higher redshifts. Future extensions of the first-generation experiments (involving an order of magnitude increase in the antennae number of the MWA) could reach sensitivities below 1 per cent in several redshift windows and could be used to,study the dark energy in the unexplored redshift regime of 3.5 less than or similar to z less than or similar to 12. Moreover, new experiments with antennae designed to operate at higher frequencies would allow precision measurements (less than or similar to 1 per cent) of the acoustic peak to be made at more moderate redshifts (1.5 less than or similar to z less than or similar to 3.5), where they would be competitive with ambitious spectroscopic galaxy surveys covering more than 1000 deg(2). Together with other data sets, observations of 21-cm fluctuations will allow full coverage of the acoustic scale from the present time out to Z similar to 12.