The Dark Ages Lunar Interferometer (DALI)
Kurt Weiler (NRL)
The Dark Ages Lunar Interferometer (DALI) is a Moon-based radio telescope concept aimed at imaging highly-redshifted neutral hydrogen signals from the first large scale structures forming during the Universe’s “Dark Ages” and “Epoch of Reionization.” The Universe’s Dark Ages consist of the interval after recombination until the formation of the first luminous objects, when the Universe was unlit by any stars. During the Dark Ages, baryons -- neutral hydrogen atoms -- were able to collapse into dark matter-dominated, overdense regions. As the H I gas accumulated in overdense regions, its excitation temperature decoupled from, and became lower than, the temperature of the cosmic microwave background (CMB). Observations of the highly-redshifted hyperfine (21-cm) transition should show a patchwork of absorption features from the first large-scale structures against the CMB. Observing these features would probe structure formation in the relatively simple linear regime, and the H I line may represent the only means of obtaining information about this cosmic epoch. Later, at redshifts z ~ 10, the first stars and black holes formed in these overdense regions, and their collective UV radiation led to the Universe becoming nearly fully ionized, a state in which it remains today. The Epoch of Reionization (EoR) marks this second transition, during which time the 21-cm line excitation temperature should have risen, eventually exceeding the CMB temperature, until essentially all of the hydrogen was ionized. Imaging the (redshifted) 21-cm line of H I at different wavelengths will construct a tomographic or 3-dimensional view of the Dark Ages and EoR. Operating at 1 - 30 meter wavelengths (10 - 300 MHz), probing redshifts 6 < z < 100, DALI would be located on the far side of the Moon, where it would be shielded from terrestrial emissions and, for half of the Moon’s orbit, from solar radio emissions. In order to have sufficient sensitivity, the array must have an effective collecting area of at least 10 km^2 (10^7 m^2). As a secondary science goal, DALI will target auroral radio bursts from extrasolar planets in order to study their magnetospheres, potentially helping to distinguish habitable and non-habitable terrestrial-mass planets in the solar neighborhood. We illustrate the notional DALI concept and identify areas of technology development that will be required over the next decade that would allow the deployment of DALI in following decades.