Recommended as one of the four notional missions in the Visionary Era by the NASA Astrophysics Roadmap in 2013, the Cosmic Dawn Mapper aims to understand how the first galaxies and stars form out of cold clumps of hydrogen and helium gas in the universe as early as 400,000 years after the Big Bang. The hyperfine transition line of a neutral hydrogen (HI) atom at wavelength of 21 centimeter is considered the most promising observatrional probe to date for studying the universe in such an early time. This transition occurs at the 1S ground state when magnetic dipole moments of the proton and electron flip from parallel (triplet state) to antiparallel (singlet state). As the universe expands, the 21-cm transition line emitted in the early time is redshifted to wavelengths between one to few tens of meters when being observed on Earth.
Three of the main observational challenges for the 21-cm signal are caused by ionospheric distortions, terrestrial radio frequency interference (RFI), as well as foreground radiation from the Galaxy and extragalactic radio sources. By carrying out observations on the lunar farside, either on lunar surface or in lunar orbit, removes the first two of these challenges. The Dark Age Radio Explorer (DARE) is a mission concept to realize the latter approach. DARE is a spacecraft equipped with a broadband low radio frequency antenna with high precision receiver system to carry out radiometric measurements of all sky-averaged 21-cm signal when it is on the farside of the lunar orbit. Nonetheless, the last remaining ambiguity is to separate the foreground radiation from the weak cosmological 21-cm background. The foreground emission is estimated to be at least four orders of magnitude brighter than the 21-cm signal.
In this study, we propose a new polarimetry technique and a novel signal processing approach in attempt to constrain and isolate this foreground from the background 21-cm signal. From the perspective of an observer, the Galaxy appears to evolve about a fixed position on the sky, such as the North Celestial Pole (NCP) for the Northern Hemisphere. By pointing a dual-polarization antenna, such as a crossed dipole, at the NCP, projection of the highly asymmetric foreground onto the antenna plane produces a cyclic and non-zero net composite polarization, whereas the isotropic 21-cm background does not due to symmetry. As a result, Fourier harmonic analysis of the cyclic foreground polarization provides a direct measurement of the sky-averaged foreground without the background signal embedded in it. Development of our ground-based prototype, the Cosmic Twilight Polarimeter (CTP), as well as preliminary evaluations based on numerical simulations combined with realistic full sky survey are presented. The CTP can potentially provide a blueprint for future observations on and above the lunar farside, such as DARE.