The 1.42 GHz (21 cm) hyperfine transition of neutral hydrogen is used to study many scales in astronomy. This probe is so important to our understanding of the Universe that it has been used to define fundamental units in messages to ET. In a cosmological context, the observed wavelength of the redshifted radiation tells us about the size of the Universe at the time it was emitted, which in turn can map to a time or distance from the observer. By observing the 21 cm signal over many frequencies we can in principle reconstruct a 3D map of the neutral hydrogen over the history of the Universe.
I am working with the Hydrogen Epoch of Reionization Array (HERA) and Murchison Widefield Array (MWA) collaborations to use 21cm tomography to study the neutral hydrogen between galaxies when stars and galaxies were forming. As bright objects formed, they emitted UV photons which ionized the intergalactic medium (IGM). This time is called the Epoch of Reionization (EoR), and can teach us about how galaxies formed, how they affected their environment, and how early stars influenced nearby star formation.
This is an extremely difficult measurement to perform because the signal is very faint (orders of magnitude fainter than the noise of the sky at these frequencies), and the galactic and extragalactic foregrounds are about 4-5 orders of magnitude brighter than the EoR. Luckily we can take advantage of some symmetries to filter out the foregrounds to recover a signal. However, because of the dynamic range we wish to achieve, we must understand the foregrounds and the instrumental systematics to extremely precise levels.