Quantifying the Impact of Lunar and Planetary Occultation on Experimental Global 21 cm Cosmology

Abstract

The global 21 cm signal from the hyperfine transition of cosmic atomic hydrogen is theorised to track the state of the early Universe via the analysis of its absorption and emission with respect to the radio background. Detecting this signal has been a challenge for astronomers since it was first postulated due to the presence of strong galactic foregrounds obfuscating the transition. Forward modelling techniques that aim to simulate and then remove these foregrounds have been suggested as a workaround to this problem. This technique, however, requires a precise and accurate understanding of the foregrounds in question. As we move 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, and thereby disrupt signal recovery. We show that in toy cases an occultation from the moon, or other proximate object, leading to a mismatch in our expected and actual foregrounds of 15 parts per million increases the error in signal recovery of up to 20\%. We show that this level of disruption can happen when the moon is found towards the centre of the galaxy and is amplified when this alignment happens at high directivity regions of the antenna beam, causing a disruption of up to 180 parts per million, leading to a signal recovery error of 115\%. This allows us to identify lunar alignment scenarios that should be actively avoided to preserve signal fidelity. We also demonstrate that a body with a smaller apparent size than the moon, such as Venus, is unlikely to cause any signal disruption due to occultation, giving a base map error of <2 parts per million.