Euclid. I. Overview of the Euclid mission (2405.13491v2)

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.

• The paper presents the Euclid mission as a space-based initiative that maps the cosmic web using weak lensing and galaxy clustering.
• It employs a dual-instrument approach with VIS and NISP to capture high-resolution visible and near-infrared data over 14,000 square degrees.
• The mission’s rigorous survey design and calibration strategies are expected to refine cosmological parameters and test theories beyond ΛCDM.

An Overview of the Euclid Mission

The Euclid mission, a medium-class initiative under the European Space Agency (ESA) Cosmic Vision 2015-2025 programme, aims to explore some of the most compelling questions in cosmology—specifically the nature of dark matter and dark energy. Its observational strategy and scientific objectives are highly ambitious, capitalizing on the advantages of space-based observations to produce a high-resolution survey of the extragalactic sky. Key features of the Euclid mission include substantial observations using visible and near-infrared (NIR) imaging and spectroscopy over approximately 14,000 square degrees, intended to significantly advance our understanding of structure formation in the universe.

Science Objectives and Strategy

Euclid's primary scientific objectives center around two cosmological phenomena: the expansion history of the universe and the growth of cosmic structure. These objectives aim to provide insights into dark energy characteristics, test modifications to general relativity, and improve constraints on the sum of the neutrino masses. This necessitates high-precision measurements of large-scale structure through galaxy clustering and weak gravitational lensing.

To achieve these objectives, Euclid utilizes two fundamental techniques:

1. Weak Gravitational Lensing: This leverages the induced distortions in the shapes of distant galaxies due to intervening mass distributions. Euclid aims to map the cosmic web, examining the statistical properties of these distortions to infer the underlying matter distribution. It plans to achieve this by measuring shapes of approximately 1.5 billion galaxies with stringent control over potential systematic errors.
2. Galaxy Clustering: Utilizing spectroscopy, Euclid aims to chart a comprehensive 3D map of galaxies, using redshift space distortions and the Baryon Acoustic Oscillation (BAO) scales as cosmic rulers. This endeavor involves the spectroscopic measurement of about 25 million galaxies, which is crucial for understanding the dynamics of cosmic expansion and structure growth.

Both probes are expected to deliver complementary data that, when combined, enhance the precision of cosmological parameter estimation and provide a fertile ground for testing cosmological models beyond ΛCDM.

Spacecraft and Instruments

Euclid features a Korsch Telescope with a 1.2-meter primary mirror, split into two key instruments:

• VIS (Visible Imaging Channel): With a large field of view, this channel is optimized for weak lensing observations, delivering high-resolution optical data in the i-band (550-920 nm).
• NISP (Near-Infrared Spectrometer and Photometer): Responsible for both imaging and slitless spectroscopy, NISP operates across three NIR bands and performs spectroscopic assessments via grism dispersions in the 1.1-2.0 μm range.

The spacecraft's design ensures stability and minimizes systematic errors attributed to thermal and mechanical changes, taking advantage of ESA’s expertise in building precise and stable platforms—essential for achieving its ambitious scientific objectives.

Survey Design and Calibration

The Euclid survey strategy is meticulously planned to maximize scientific return. It consists of:

• The Wide Survey: This aims to map about 14,000 square degrees of sky in a step-and-stare approach, ensuring comprehensive trajectory coverage over its six-year operation window.
• The Deep Fields: These cover smaller areas but reach significantly deeper into the universe, serving as calibration fields and supporting detailed studies of cosmic structures.

Calibration is an integral part of the mission’s operations, featuring stringent requirements on photometric and astrometric precision necessary for achieving the mission goals. Regular calibration activities include monitoring instrument performance, cross-checking with ground-based observations, and adjusting operational parameters to counteract degradation over time.

Simulation and Data Processing

Comprehensive efforts have been made to simulate the Euclid mission's data, leveraging advanced N-body cosmological simulations to model the expected observations. These simulations are critical for testing the data processing pipeline and ensuring the robustness of scientific analyses against systematic errors. Data processing involves complex steps from initial calibration to the derivation of meaningful cosmological parameters, focusing on accuracy and precision.

Conclusion

In summary, the Euclid mission represents a step forward in observational cosmology, with its sophisticated instruments and carefully designed survey strategy set to deliver significant progress in our understanding of the universe's most fundamental components. By targeting the large-scale distribution of dark matter and the effects of dark energy with unprecedented precision, Euclid is expected to test existing theoretical models and potentially uncover new physics beyond our current understanding. It is a mission that embodies the synergy between technological innovation and scientific exploration, promising insights into the cosmos that will influence the field for decades to come.