The space between the stars is not empty at all, in fact, there is on average one particle per cubic centimeter in the so-called interstellar medium. This is still significantly more tenuous than even the best vacuum chamber on Earth, but since the interstellar medium is so vast there is enough gas in it to be visible for some telescopes. One of the most reliable ways to probe the interstellar medium is the 21-cm radio line of atomic hydrogen, by far the most abundant gas in space.
In our research we compare observations of this 21-cm line with cosmological simulations – computer models which try to emulate a synthetic universe. The goal of this project is to assess how realistic this synthetic universe is. We found that while the atomic hydrogen content of galaxies is broadly realistic, there are not enough high-density regions of atomic gas. Moreover, many galaxies in the simulation feature large central gas holes, which is much more rare in observations. These results allow us to improve the computer models to generate even more realistic synthetic galaxies. We also plan to use this framework to investigate observational uncertainties in 21-cm surveys, and foresee an extension to other gas species and lines.