The Euclid Mission

Mission Overview

Our view of the Universe is incomplete. The laws of nature we are used to living with on a day-to-day basis cannot be translated to the large scale Universe. Either our theory of gravity is incorrect on large scales, or there is a significant hitherto unobserved contribution to the mass-energy content of the Universe. Which of the two options, if either, is correct? This is one of the most important questions in modern physics, and one that the Euclid Mission aims to answer.

The Euclid Space Mission with investigate two of the most puzzling quantities in modern cosmology: Dark Matter and Dark Energy. The Euclid Mission will constrain the nature of these physical phenomena in unprecedented detail, allowing different dark energy and dark matter models to be tested and limits of the theory of gravity to be investigated.

The large areas of the sky observed by the mission's high precision, cutting-edge, scientific instruments will also provide a treasure trove of information for many different areas of astronomy. Not only will Euclid revolutionize our understanding of the Universe on the largest scales, it will also provide a huge amount of valuable data for the wide astronomy community.

Two science techniques

Technically Euclid will utilize two methods to reach its goals:

First, in order to map the 3-dimensional Dark Matter distribution in the Universe, the effect of Weak Gravitational Lensing will be used. The shapes of a billion galaxies down to very faint fluxes and at redshifts out to z=2.0 will be measured. With assumptions on the intrinsic ellipticity of the galaxy distribution the gravitative effect of mass to distort the light-paths of background light-sources can be calculated and, with a measurement of the distances of each foreground galaxy, a 3-dimensional mass map, including all luminous and dark matter.

This approach requires an extremely high precision measurement of galaxy shapes, in Euclid realised with the Visual Imager (VIS), and the inference of galaxy distances by means of photometric redshifts. For this purpose, Euclid measures galaxy photometry in three near-infrared wavelengths (920 to 2000nm) with the dedicated Near Infrared Spectrophotometer (NISP) -- a task which is impossible to be carried out from the ground down to the required limiting sensitivities and over 1/3 of the sky.

With the second technique, the expansion history of the Universe is to be measured, hence the size of the Universe at 10 points in time over the past 10 billion years. This will by done by characterizing the so scale length of the called Baryon Accousting Oscillations as a function of time. Sound waves scalelengths we frozen into the matter distribution soon after the Big Bang. At very early times, i.e. redshift 1000, these can be seen as a clustering peak of structure in the Cosmic Microwave Background. Later these structures are propagated into the developing large-scale web of galaxies, ever expanding along with the Universe itself. By measuring galaxy clustering properties and the identification of this characteristic yard-stick at different points in time, the expansion history of the Universe itself is being measured.

For this approach exact 3-dimensional positions of tracer galaxies have to be measured for which the photometric redshifts used for the weak lensing approach are not sufficient. Hence actual spectra of several tens of millions of galaxies with emission lines have to be measured. The spectral positions of their emission line peaks will provide very precise redshift and hence distance coordinate. This task will be carried out by the spectroscopic channel of the NISP instrument.

Legacy Science

The statistical approach of the two main science probes requires a very large survey. The Euclid Mission aims to survey over 15,000 deg2 of the extragalactic sky with visible imaging, near-infrared photometry and near-infrared spectroscopy. As a result, the Euclid Mission will generate a vast dataset for legacy science including broadband visible images and near-infrared photometry of roughly 1.5 billion galaxies and near-infrared spectroscopy of roughly 50 million galaxies. Such a large dataset will touch on many aspects of astrophysics, on many different scales, from the formation and evolution of galaxies down to the detection of brown dwarfs.

The surveys required to meet Euclid's primary science goals can only be performed in space. The high precision visible shape measurements of lensed galaxies require highly stable imaging to prevent systematic effects dwarfing the shear signal. The turbulent nature of the atmosphere and less stable temperature environment makes such measurements from the ground very difficult. The mission also requires a deep photometric and a spectroscopic near-infrared survey. Space observations, without the bright sky background at these wavelengths, allow these surveys to be performed much more efficiently than ground-based observations. Essentially, to cover the survey area with the precision and the depth required to meet the high level science goals, there is no alternative to a space-based experiment.

In addition to the mission's 15,000 deg2 survey, called the Wide Survey, Euclid will also perform a Deep Survey by regularly observing two 20 deg2 patches of the sky at the ecliptic poles. This deeper survey is not only required for calibration purposes, but will also provide additional information for some of the secondary science cases, such as Type Ia Supernova searches.