  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 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. 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. 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 distribution of
hydrogen during the cosmic dark ages.
