The Euclid project

Cristobal Padilla

Euclid is a mission for the European Space Agency (ESA) Cosmic Vision (CV) 2015-25 programme to explore how the Universe evolved over the past 10 billion years to address questions related to fundamental physics and cosmology on the nature and properties of dark energy, dark matter and gravity, as well as on the physics of the early universe and the initial conditions which seed the formation of cosmic structure.

INTRODUCTION

The Satellite was launched on July 1st, 2023 from Cape Canaveral, Florida (US) with a Falcon 9 rocket from Space X and travelled 1.5 million kilometers up to the Lagrangian 2 point. After a period of commissioning the survey proper started in February 2024 and will take data until at least 2030. Euclid uses a high quality panoramic visible imager (VIS), and a near infrared 3-filter photometer (NISP-P) and a slitless spectrograph (NISP-S) to map 15,000 sqr. Deg. of the sky free of contamination by light from the Milky Way and the Solar System and a 40 deg2 deep survey to measure the high-redshift universe. The NISP Filter Wheel Assembly (NISP-FWA) was IFAE’s responsibility.

The complete Euclid survey represents hundreds of thousands of images and several tens of Petabytes of data. Euclid will observe about 10 billion sources out of which more than one billion will be used for weak lensing. Several tens of million galaxy redshifts will be also measured and used for galaxy clustering. With these images Euclid will probe the expansion history of the Universe and the evolution of cosmic structures by measuring the modification of shapes of galaxies induced by gravitational lensing effects of dark matter and the 3-dimension distribution of structures from spectroscopic redshifts of galaxies and clusters of galaxies. Euclid data will provide improvement factors of ~30 in the measurement of the neutrino mass and up to ~400 in the uncertainty of the parameters of the cosmology state equation and will leave legacy catalogs in many areas of galaxy science with exquisite imaging quality and superb Near Infrared Spectroscopy.

2025 Activities

During 2025, the group has continued activities inside the Euclid consortium to exploit the data and contribute to the scientific output of the mission. We have contributed to crucial aspects of the scientific analysis exploitation. Using detailed simulations, one of our PhD students has shown that the clustering redshifts technique consisting of crosscorrelating the photometric and spectroscopic galaxy samples will be able to calibrate the photometric reshifts to the level that fulfills Euclid’s requirements. This is shown in the Figure below from W d’Assignies, et al., A&A, 702, A155 (2025). This method will serve to verify the n(z) distribution and check for possible biases in this crucial ingredient for the extraction of the cosmological parameters.

Another PhD student is applying unsupervised machine learning techniques to denoise the images and measure the galaxy shear, a crucial parameter in the weak-lensing signal of the Euclid project.

Our three postdocs are also contributing to several working groups in Euclid. One postdoc has developed a source-injection pipeline that, by injecting galaxies to Euclid images and processing them, enables the study of detection bias and photometric systematics. Another postdoc is contributing to the calibration of the redshift distribution of galaxies by implementing self-organized maps and modelling the relationship between the photometry of deep fields and wide fields in Euclid. Finally a third postdoc is contribution to the intrinsic alignment modelling and pipeline in Euclid, an important systematic for the weak lensing measurements. Thus, we are active in several of the key working groups in Euclid involving catalogue creation (OU-MER), redshift calibration (ZTF), Weak Lensing (WL SWG), Shear measurements (OU-SHE). Additionally, Marc Manera is the deputy coordinator of the Photometric Redshifts Organizational Unit (OU-PHZ) of Euclid’s Science Ground Segment (SGS) and Cristobal Padilla is the chair of the Euclid Speaker’s Bureau (ECEB).