Euclid. I. Overview of the Euclid mission

TL;DR

Euclid addresses the fundamental mysteries of dark matter, dark energy, and gravity by delivering a space-based, wide-area optical/NIR survey optimized for weak lensing and galaxy clustering. By combining high-precision shape measurements with near-infrared spectroscopy and imaging across ~14,000 deg^2, the mission aims for a robust 3×2pt analysis that achieves FoM>400 and tight constraints on w0, wa, and γg, while exploring beyond-LCDM scenarios. The paper outlines the mission architecture, survey design, data-processing pipelines, simulations, and data products, emphasizing end-to-end calibration, systematic control, and the dual cosmological and legacy science cases. It also highlights planned data releases and the broad non-cosmological science return, from Milky Way archaeology to high-redshift galaxies and transient phenomena, establishing Euclid as a foundational dataset for decades of astronomy.

Abstract

The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015-2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14,000 deg^2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

Figures (49)

• Figure 1: Comparison of the number of redshifts and comoving volume covered by various previous and ongoing spectroscopic surveys against the predictions for (see text for details). The grey lines show lines of constant number density as labelled.
• Figure 2: Measured galaxy power spectrum multipoles calculated from dedicated mocks (Pezzotta et al. in prep.) based on the Flagship simulation (see \ref{['sec:flagship']}) of the emission-line sample for a redshift bin $1.3 < z < 1.5$, compared to a best-fit model based on EFT (EFT, also called EFTofLSS, see \ref{['sec:nonlinear']} for further discussion) assuming $k_{\rm max} = 0.25\,h$ Mpc$^{-1}$. Error bars here correspond to the full mission volume for this redshift bin prior to observational effects.
• Figure 3: Shear correlation function $\xi_+(\theta)$ for -1000 (left, from Asgari2021), Y3 (middle, from Amon2022), and Y3 (right, from Li2023), and expected for . Each panel uses sources distributed according to the tomographic bin with the highest of the respective survey. The for is an order of magnitude larger than that of the most recent surveys. The other shear correlation function $\xi_-(\theta)$ shows a similar improvement in S/N (not shown).
• Figure 4: Source-redshift distributions $n(z)$ of the , , and , and as expected for . Distributions are normalised to the mean number density of sources used in the lensing analyses.
• Figure 5: Left: Overview of the spacecraft with the principal axes highlighted. Right: The fully assembled spacecraft on February 2023 in the anechoic chamber of Thales Alenia Space in France, after completing final electromagnetic compatibility tests. The side shown here will always face away from the Sun. The large white structure below the cylindrical telescope baffle is the NISP radiator. The hydrazine thrusters still have their protective red covers on. The plaque with the miniaturised fingerprint galaxy created thanks to a https://www.esa.int/ESA_Multimedia/Videos/2022/07/The_Fingertip_Galaxy_Reflecting_Euclid_in_art and Euclid Consortium members can be seen at the lower left. Figure credit: -- M. Pédoussaut.