Advancing 21cm Cosmology with the Canadian Hydrogen Intensity Mapping Experiment

Intensity mapping of the 21cm line of neutral hydrogen is a promising new approach to charting the large scale structure of the Universe. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) has played a crucial role in establishing the observational viability of this probe, recently achieving the first independent detection of the cosmological 21cm power spectrum at redshift ~ 1. However, this detection is insensitive to the baryon acoustic oscillation (BAO) signal, whose evolution over cosmic time CHIME was built to trace to constrain the expansion history of the Universe and the underlying physics of dark energy. The large scales sensitive to BAO are obscured by bright astrophysical foregrounds which are 4-5 orders of magnitude brighter than the 21cm signal; characterizing and removing these foregrounds requires exquisite control of the telescope beam response, which imprints spurious spectral structure on otherwise smooth foreground components. In light of these challenges, I will describe two pathways for advancing CHIME's cosmology program to larger scales. First, I present a project which adapts the classic radio holography technique to address the challenge of beam calibration for a large telescope using stationary cylindrical reflectors. I will present the first suite of results using holographic measurements of six bright radio sources across the full 400–800 MHz observing band of CHIME to provide measurements of the beam response in both amplitude and phase for all 1024 dual-polarized feeds in the array. Second, I present the development of a pipeline for cross-power spectrum estimation between CHIME maps and traditional spectroscopic large scale structure surveys, tested on the sample of QSOs from the SDSS-IV eBOSS program. The combination of relaxed foreground filtering and the statistical power of cutting edge LSS surveys in cross-correlation may offer a new pathway to unlocking the BAO signal.