The cosmic 21cm signal

The 21-cm line corresponds to the spin-flip transition in the ground state of neutral hydrogen. It is widely used to study HI in our galaxy and nearby galaxies, but its potential in cosmology is unmatched. Before the completion of reionization, the entire Universe was filled with neutral hydrogen. The 21-cm line can be used to map out the temperature, ionization and density of the intergalactic medium (IGM) during this period that spans over half of the observable Universe.

Such a dataset will revolutionize our understanding not just of how reionization unfolded, but also of the sources that were responsible. Over 99% of galaxies are likely unseen by even the most powerful telescopes like JWST, and yet they are the drivers of cosmic reionization, as well as the preceding epoch of X-ray heating. The properties of these first galaxies (e.g. star formation rates, masses, stellar populations, X-ray luminosities, etc.) are encoded in the timing and spatial structure of the 21-cm signal.

The 21-cm signal also probes fundamental cosmology. It can measure the matter distribution, provide insight into the Hubble tension through acoustic oscillations, as well as exotic dark matter heating / cooling processes.

The signal is there and it is only a matter of time until we use it to map out our Universe. Current interferometers are pushing towards a statistical detection, but in the following decade the multi-billion euro Square Kilometer Array (SKA) observatory has the potential to make a 3D map of the majority of our Universe: a cosmological dataset of unparalleled potential.


My collaborators and I are helping these efforts on several fronts, including developing simulation tools, interpreting current upper limits using a multi-tracer approach, paving the way for the first, credible detection, and preparing for future transformative images from the SKA.


Here are some relevant images and movies: