Matter in the Universe appears today in the form of a structured cosmic web made of galaxies, clusters of galaxies, and superclusters. These nodes are connected by filaments and walls of matter, and are surrounded by large cosmic voids. Dark Matter mainly shapes the skeleton of this cosmic web into which ordinary, baryonic matter flows to finally collapse into dense and cold objects such as stars and galaxies. However, these collapsed objects represent less than 10% of the total budget of ordinary matter, as measured thanks to the cosmic microwave background. At high redshifts (z > 2), ordinary baryonic matter is observed in an isonised, gaseous and hot state, which accounts for the total budget. At lower redshift (z < 1) however, about half of the baryons remain unobserved! Those hidden baryons in the late Universe are thought to be in a warm-hot and isonised state, and they continue to represent one of the most intriguing enigmas in astrophysics and cosmology.

Understanding the complex, non-linear physical processes at play in the assembly of ordinary baryonic matter into the web skeleton will also open a new window on the relation (and reciprocal backreactions) between the environmental skeleton of Dark Matter and the formation of structure of ordinary matter !

ByoPiC follows a novel viewpoint on the cosmic web focusing on the hot ionised baryons embedded in the largest cosmic scales traced by the Sunyaev-Zel'dovich (SZ) effect. Unveiling these baryons is the sole way to answer a central question in cosmology and astrophysics: How, where and in what form does structure formation hide half of the ordinary matter at late times? This will be done thanks to joint analysis of heterogeneous data sets tracing the cosmic skeleton (i.e. the location sites of the hidden baryons) from galaxy surveys on the one hand, and the hot ionised gas (the hidden baryons themselves) thanks to their SZ signature on the other hand.