OCULIS · Observational Cosmology Using Large Imaging Surveys

The current standard model of cosmology successfully describes a variety of measurements, but its main ingredients, dark matter and dark energy, are a great mystery. ESA’s Euclid satellite is the biggest step forward towards solving this fundamental problem in physics. It is designed to reveal the nature of the dark constituents and gravity by measuring the growth of structure over most of cosmic time with unprecedented precision, but the interpretation of the data is complex: incorrect modelling of the galaxy populations, astrophysical processes or instrumental effects can easily be mistaken as evidence for new physics. To fully exploit the statistical power of Euclid, the intricacies of the data need to be understood, but we also need an accurate model of structure formation that can describe the interplay between galaxy formation, astrophysics, dark matter and cosmology.OCULIS tackles both aspects by providing the best measurements of the lensing signal and using the Euclid data in novel ways to relate observed galaxy properties directly to the matter distribution. To do so, we will (i) use in-flight data to correct for instrumental biases; (ii) measure the intrinsic alignments of galaxies that contaminate the lensing signal; (iii) exploit Euclid’s unique ability to determine the lensing signal on small scales to directly determine the stellar masses of galaxies for the first time, and to study the tidal stripping of dark matter halos; (iv) use the results to inform models that describe the small-scale astrophysics in hydrodynamic simulations, yielding robust predictions of the cosmological signals. As cosmology coordinator of the Euclid mission, the PI is uniquely positioned to achieve these objectives. The synergetic approach of OCULIS exploits that Euclid is much more than a cosmology mission: it is also a prime dataset for the study of the formation and evolution of galaxies. By linking both science cases, OCULIS maximizes the scientific return of Euclid.