Abstract

On the largest scales, the Universe is remarkably simple. The growth of structure is governed primarily by gravity, allowing cosmology to make spectacularly precise predictions that have been confirmed by observations. Yet on smaller scales, this simplicity breaks down. The ordinary matter, consisting of gas, stars, black holes and galaxies, introduces complex physical processes that reshape the distribution of matter in ways that remain poorly understood. In this talk, I will argue that this “messy” regime is not merely a complication, but a new frontier. Baryonic physics sits at the interface between cosmology and astrophysics: it governs how galaxies form and evolve, while simultaneously limiting our ability to extract cosmological information from small-scale structure. Understanding this interplay requires connecting the galaxy-halo relation, the thermodynamic state of cosmic gas, and the impact of feedback from stars and supermassive black holes. I will highlight how a new generation of observations is beginning to directly map the distribution and properties of baryons across cosmic time. By combining large-scale galaxy surveys such as DESI and Rubin Observatory with measurements of cosmic microwave background secondary anisotropies and X-ray emission, we can probe gas density, pressure, temperature, and total matter in a unified framework. These measurements open the door to tracking baryons as a function of halo mass, redshift, and galaxy type, transforming them from a source of uncertainty into a powerful cosmological probe. Ultimately, bridging the gap between small-scale astrophysics and large-scale cosmology will be essential for the next era of precision cosmology, and may offer new insights into the nature of dark matter, dark energy, and structure formation itself.