Understanding the universe we live in is one of the challenges that pushed mankind to look at the sky since the dawn of civilization. In modern times, accurate astrophysical observations have allowed us to turn questions like "how did the universe begin?" or "what the universe if made of?" into scientific problems.
The standard cosmological model predicts that the universe started in a hot and dense phase commonly known as Big Bang and then underwent an accelerated expansion called inflation where the seeds of the matter density perturbation that later evolved in galaxies and stars were generated. After inflation the universe expanded slowly since the beginning of a new accelerated phase a few billion years ago driven by an unknown energy component called dark energy. Understanding the nature of inflation and dark energy are two of the main questions that cosmologists hope to answer in the next decade using observations of the large scale structures (LSS) of the universe (galaxies, galaxy clusters etc.) and of the Cosmic Microwave Background (CMB), the relic light of the Big Bang. These observations will also help solving one of the key open question on the standard model of particle physics: the nature of the neutrino mass.
During its journey towards us, the CMB interacts with the LSS as they form. These leave several distinct imprints in the CMB photons: their gravitational force deflects the CMB photon's trajectories (CMB lensing) and CMB photons exchange energy with the energetic particles trapped in the LSS (SZ effect). Combining observations of the CMB and data of galaxy surveys mapping the LSS, we can thus study the properties of the matter distribution of the universe and its evolution with two different and complementary techniques. In particular, comparing the mass maps acquired with CMB and galaxy surveys (cross-correlation) will sharpen our understanding of the universe beyond what can be normally achieved and will allow us to go beyond the limiting factor affecting the analyses of both kinds of data sets when performed independently of each other.
With the new generation of CMB experiments (e.g. Simons Observatory -- SO) and galaxy surveys (e.g. the ESA mission Euclid) that have started observing the sky at the same time in 2023, this field has the potential to deliver multiple "firsts" in cosmology in the coming years. To fully exploit this potential, we need to better understand the statistical methods involved in the cross-correlation of CMB and galaxy surveys, test their robustness, design new techniques and observables capable of extracting the maximum amount of information from the data.
All these questions are at the core of the PiCOGAMBAS project which delivered robust data analysis methods, new data sets and cosmological constraints, and identified innovative approaches that will produce exciting results in cosmology in the coming years.
