KIPAC Seminar: Instrument Design Theory for 21 cm Cosmology

Abstract: Much of the cosmic volume between the nearby universe and the CMB remains only sparsely charted, and redshifted 21 cm emission is our most promising route to mapping it. The difficulty is that the cosmological signal must be extracted in the presence of astrophysical foregrounds that are many orders of magnitude brighter, making the central challenge not only one of sensitivity, but of avoiding mode-mixing between foregrounds and the 21 cm line. In this talk, I will discuss two ways in which that requirement can be translated into instrument-design principles. The first is a recent result on array layout: by carefully choosing where the antennas are placed so as to sample interferometric separations exhaustively over a specified range, the so-called “foreground wedge” can be fully suppressed at the level of array geometry alone. The second is ongoing work on mutual coupling: the Fourier-conjugate relationship between the antenna aperture and its beam enables an intuitive assessment of mitigation strategies based on the inversion of a coupling matrix, making clear how structural-mode scattering complicates those approaches, and highlighting potential pitfalls in conventional hardware design choices. The broader goal is to treat foreground control not as a purely downstream analysis problem, but as a design principle that helps determine which cosmological modes can be cleanly accessed in the first place.