Dark Cosmology: Investigating Dark Matter and Exotic Physics using the Redshifted 21-cm Global Signal with the Dark Ages Polarimeter Pathfinder (DAPPER)

We report on the results of a NASA-funded Astrophysics SmallSat concept study for a proposed lunar-orbiting experiment, the Dark Ages Polarimeter PathfindER (DAPPER), that is designed to observe the unexplored Dark Ages epoch of the early Universe. The Dark Ages, probed by the highly redshifted 21-cm neutral hydrogen global signal, is the ideal epoch for a new rigorous test of the standard LCDM cosmological model. DAPPER will search for divergences from the standard model that will indicate new physics such as heating or cooling produced by dark matter. A broad absorption trough in the redshifted 21-cm spectrum is expected during the Dark Ages, prior to the formation of the first stars and thus determined entirely by cosmological phenomena. DAPPER will observe this pristine epoch (17-38 MHz; z 83-36), and will measure the amplitude of the 21-cm spectrum to the level required to distinguish (at >5-σ) the standard cosmological model from that of additional cooling derived from recent EDGES results. The main challenge of this measurement is the removal of bright foregrounds. DAPPER is designed to overcome this by utilizing two pioneering techniques: (1) a polarimeter to measure polarization induced by the anisotropic foregrounds and large antenna beam to aid in the separation of the foregrounds from the isotropic, unpolarized global signal, and (2) a pattern recognition analysis pipeline based on well-characterized training sets of foregrounds from sky observations, instrument systematics from simulations and laboratory measurements, and signals from theoretical predictions. End-to-end simulations of the DAPPER instrument including thermal noise, systematics from the spectrometer/polarimeter and the beam-averaged foreground, along with 21-cm models which include added cooling meet our sensitivity requirements to separate the standard cosmological models from ones that point toward new physics. DAPPER's science instrument consists of dual orthogonal dipole antennas and a tone-injection receiver based on high TRL components from the Parker Solar Probe/FIELDS, CURIE, and WIND/WAVES. DAPPER will be deployed into a frozen 50x125 km lunar orbit to provide 4615 hours of radio-quiet integration over a 26 month lifetime.