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 deg² 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.

Long range scalar fields with a coupling to matter appear to violate known bounds on gravitation in the solar system and the laboratory. This is evaded thanks to screening mechanisms. In this short review, we shall present the various screening mechanisms from an effective field theory point of view. We then investigate how they can and will be tested in the laboratory and on astrophysical and cosmological scales.

The Near-Infrared Spectrometer and Photometer (NISP) on board the Euclid satellite provides multiband photometry and R ≳ 450 slitless grism spectroscopy in the 950–2020 nm wavelength range. In this reference article, we illuminate the background of NISP’s functional and calibration requirements, describe the instrument’s integral components, and provide all its key properties. We also sketch the processes needed to understand how NISP operates and is calibrated as well as its technical potentials and limitations. Links to articles providing more details and the technical background are included. The NISP’s 16 HAWAII-2RG (H2RG) detectors with a plate scale of 0″.3 pixel⁻¹ deliver a field of view of 0.57 deg². In photometric mode, NISP reaches a limiting magnitude of ~24.5 AB mag in three photometric exposures of about 100 s in exposure time for point sources and with a S/N of five. For spectroscopy, NISP’s pointsource sensitivity is a signal-to-noise ratio = 3.5 detection of an emission line with flux ~2 × 10⁻¹⁶ erg s⁻¹ cm⁻² integrated over two resolution elements of 13.4 Å in 3 × 560 s grism exposures at 1.6 µm (redshifted Hα). Our calibration includes on-ground and in-flight characterisation and monitoring of the pixel-based detector baseline, dark current, non-linearity, and sensitivity to guarantee a relative photometric accuracy better than 1.5% and a relative spectrophotometry better than 0.7%. The wavelength calibration must be accurate to 5 Å or better. The NISP is the state-of-the-art instrument in the near-infrared for all science beyond small areas available from HST and JWST – and it represents an enormous advance from any existing instrumentation due to its combination of field size and high throughput of telescope and instrument. During Euclid’s six-year survey covering 14 000 deg² of extragalactic sky, NISP will be the backbone in determining distances of more than a billion galaxies. Its near-infrared data will become a rich reference imaging and spectroscopy data set for the coming decades.

Accurate predictions for the nonlinear matter power spectrum are needed to confront theory with observations in current and near future weak-lensing and galaxy clustering surveys. We propose a computationally cheap method to create an emulator for modified gravity models by utilizing existing emulators for $\mathrm{\Lambda }\mathrm{CDM}$. Using a suite of $N$-body simulations, we construct a fitting function for the enhancement of both the linear and nonlinear matter power spectrum in the commonly studied Hu-Sawicki $f(R)$ gravity model valid for wave numbers $k\lesssim 5–10h{\mathrm{Mpc}}^{-1}$ and redshifts $z\lesssim 3$. We show that the cosmology dependence of this enhancement is relatively weak so that our fit, using simulations coming from only one cosmology, can be used to get accurate predictions for other cosmological parameters. We also show that the cosmology dependence can, if needed, be included by using linear theory, approximate $N$-body simulations (such as comoving lagrangian acceleration) and semianalytical tools like the halo model. Our final fit can easily be combined with any emulator or semianalytical models for the nonlinear $\mathrm{\Lambda }\mathrm{CDM}$ power spectrum to accurately, and quickly, produce a nonlinear power spectrum for this particular modified gravity model. The method we use can be applied to fairly cheaply construct an emulator for other modified gravity models. As an application of our fitting formula, we use it to compute Fisher forecasts for how well galaxy clustering and weak lensing in a Euclid-like survey will be at constraining modifications of gravity.

This paper presents the specification, design, and development of the Visible Camera (VIS) on the European Space Agency’s Euclid mission. VIS is a large optical-band imager with a field of view of 0.54 deg² sampled at 0″.1 with an array of 609 Megapixels and a spatial resolution of 0″.18. It will be used to survey approximately 14 000 deg² of extragalactic sky to measure the distortion of galaxies in the redshift range z = 0.1–1.5 resulting from weak gravitational lensing, one of the two principal cosmology probes leveraged by Euclid. With photometric redshifts, the distribution of dark matter can be mapped in three dimensions, and the extent to which this has changed with look-back time can be used to constrain the nature of dark energy and theories of gravity. The entire VIS focal plane will be transmitted to provide the largest images of the Universe from space to date, specified to reach m_AB ≥ 24.5 with a signal-to-noise ratio S/N ≥ 10 in a single broad I_E ≃ (r + i + z) band over a six-year survey. The particularly challenging aspects of the instrument are the control and calibration of observational biases, which lead to stringent performance requirements and calibration regimes. With its combination of spatial resolution, calibration knowledge, depth, and area covering most of the extra-Galactic sky, VIS will also provide a legacy data set for many other fields. This paper discusses the rationale behind the conception of VIS and describes the instrument design and development, before reporting the prelaunch performance derived from ground calibrations and brief results from the inorbit commissioning. VIS should reach fainter than m_AB = 25 with S/N ≥ 10 for galaxies with a full width at half maximum of 0″. 3 in a 1″.3 diameter aperture over the Wide Survey, and m_AB ≥ 26.4 for a Deep Survey that will cover more than 50 deg². The paper also describes how the instrument works with the Euclid telescope and survey, and with the science data processing, to extract the cosmological information.