The current standard model of cosmology successfully describes a wide variety of measurements, but its main ingredients, dark matter and dark energy, are a great mystery. The observations demonstrate that our theories of particle physics and gravity are either incomplete or incorrect, but we lack compelling theoretical guidance to solve this crisis in cosmology. The only viable course of action is to improve observational constraints by at least an order of magnitude. This is the objective of Euclid, the recently launched ESA mission that will survey more than half of the extragalactic sky over a period of six year. Its homogeneous, high-quality space data are unaffected by atmospheric blurring and infrared emission, resulting in unrivaled data fidelity, but the cosmological interpretation of the Euclid data is complex as information from multiple probes needs to be combined, while the observational signatures of new physics are subtle. Incorrect modelling of the galaxy populations, astrophysical processes or instrumental effects can easily be mistaken as evidence for new physics. Although Euclid is designed to minimize observational biases, these cannot be fully eliminated, and a thorough understanding of the intricacies of the data is essential. Similarly, the main probes were chosen to be robust, but they are not immune to astrophysics. To fully exploit the unprecedented precision of Euclid we need to disentangle the physics of galaxy formation and cosmology.
OCULIS focuses on several key areas where progress is needed to realize the full potential of the Euclid data. First, we will improve the actual measurement of the lensing signal by using in-flight data to correct for instrumental effects that would otherwise bias the shape estimates. OCULIS will also exploit the high-quality data from Euclid to improve our understanding of astrophysical sources of bias, in particular the effect of intrinsic alignments of galaxies that contaminate the lensing signal. By developing dedicated measurement tools we will explore the alignment signal as a function of galaxy properties, and compare these with predictions from state-of-the-art cosmological hydrodynamic simulations.
Thanks to the superb image quality of Euclid, the lensing signal can be measured on angular scales that are inaccessible with ground-based data. In particular, the lensing signal at small radii around foreground lenses can be used to determine the stellar and halo masses with a minimal reliance on the cosmological model and to study the tidal stripping of dark matter halos as a function of environment. This requires modifications to the standard shape measurement pipelines, so that they can account for the contaminating light from the lens galaxies. The improved shape measurement algorithms will also used to measure the intrinsic alignments of galaxies that contaminate the lensing signal. By relating the resulting measurements to the surrounding matter distribution we can advance a physical understanding of these phenomena, and use the findings to improve the fidelity of the modelling of small-scale astrophysics in hydrodynamical simulations. To predict the various cosmological signals consistently, whilst accounting for the uncertainty in the astrophysics (and fundamental physics), we will incorporate our results into an emulator, which enable the best possible measurements of cosmological parameters using Euclid.
In short, the aim of OCULIS is to use the Euclid data themselves to better understand the observational sources of bias, and using these findings to improve the overall interpretation of the cosmological signal. This allows us to probe smaller scales and use fainter galaxies, thus improving the overall ability of the mission to determine cosmological parameters.
