Quantifying the impact of lunar and planetary occultation on experimental global 21 cm cosmology

ABSTRACT The global 21 cm signal from cosmic atomic hydrogen can be used to track the state of the early Universe via its emission and its absorption of the radio background. Detecting this signal has been a challenge due to the presence of strong galactic foregrounds obfuscating the transition. Forward modelling techniques aiming to simulate and remove these foregrounds have been suggested as a workaround to this problem. This technique requires a precise and accurate understanding of the foregrounds in question. Moving into the next major lunar standstill, the moon will be able to occult high-power areas of the sky in ways that are unaccounted for by maps used to simulate these foregrounds. We show that occultation by the moon can change the foreground emission by up to 15 parts per million (ppm) and increase the error in signal recovery for an injected cosmic dawn signal by up to 20 per cent. The effect is greatest when the moon occults bright regions near the centre of the Galaxy, and when it is aligned with high sidelobes of the antenna beam. This allows us to identify scenarios that should be actively avoided to preserve signal fidelity. We also demonstrate that bodies with a smaller apparent size than the moon, like Venus, are unlikely to cause any signal disruption due to occultation, giving a base map error of <2 ppm.