  An array of low-frequency dipole antennas on the lunar farside surface will
probe a unique, unexplored epoch in the early Universe called the Dark Ages. It
begins at Recombination when neutral hydrogen atoms formed, first revealed by
the cosmic microwave background. This epoch is free of stars and astrophysics,
so it is ideal to investigate high energy particle processes including dark
matter, early Dark Energy, neutrinos, and cosmic strings. A NASA-funded study
investigated the design of the instrument and the deployment strategy from a
lander of 128 pairs of antenna dipoles across a 10 kmx10 km area on the lunar
surface. The antenna nodes are tethered to the lander for central data
processing, power, and data transmission to a relay satellite. The array, named
FARSIDE, would provide the capability to image the entire sky in 1400 channels
spanning frequencies from 100 kHz to 40 MHz, extending down two orders of
magnitude below bands accessible to ground-based radio astronomy. The lunar
farside can simultaneously provide isolation from terrestrial radio frequency
interference, the Earth's auroral kilometric radiation, and plasma noise from
the solar wind. It is thus the only location within the inner solar system from
which sky noise limited observations can be carried out at sub-MHz frequencies.
Through precision calibration via an orbiting beacon and exquisite foreground
characterization, the farside array would measure the Dark Ages global 21-cm
signal at redshifts z~35-200. It will also be a pathfinder for a larger 21-cm
power spectrum instrument by carefully measuring the foreground with high
dynamic range.
