Dark Ages Radio Explorer mission: Probing the cosmic dawn

The period between the creation of the cosmic microwave background at a redshift of ∼1000 and the formation of the first stars and black holes that re-ionize the intergalactic medium at redshifts of 10–20 is currently unobservable. The baryonic component of the universe during this period is almost entirely neutral hydrogen, which falls into local regions of higher dark matter density. This seeds the formation of large-scale structures including the cosmic web that we see today in the filamentary distribution of galaxies and clusters of galaxies. The only detectable signal from these dark ages is the 21-cm spectral line of hydrogen, redshifted down to frequencies of approximately 10–100 MHz. Space-based observations of this signal will allow us to determine the formation epoch and physics of the first sources of ionizing radiation, and potentially detect evidence for the decay of dark matter particles. JPL is developing deployable low frequency antenna and receiver prototypes and calibration techniques to enable both all-sky spectral measurements of neutral hydrogen and ultimately to map the spatial distribution of the signal as a function of redshift. Such observations must be done from space because of Earth's ionosphere and ubiquitous radio interference. Both lunar orbiting and lunar surface based instruments are under development. A specific application of these technologies is the Dark Ages Radio Explorer (DARE) mission. This small Explorer class mission is designed to measure the sky-averaged hydrogen signal from the shielded region above the far side of the Moon. Observations of neutral hydrogen from the dark ages and the cosmic dawn (when light from the first generation of stars appeared) will provide unique and critical information on the early evolution of the universe. These data will complement ground-based radio observations of the final stages of intergalactic re-ionization at higher frequencies. DARE will also serve as a scientific percursor for space-based interferometry missions to image the distirbution of hydrogen during the cosmic dark ages.