Euclid in a nutshell: looking through the Universe

Euclid is an astronomical spacecraft project set up by a European consortium and selected by the European Space Agency (ESA) in 2011 as a medium class mission of the Cosmic Vision 2015-2025 program. Recent astronomical observations indicate that our Universe is dominated by dark matter and energy of a so-far unknown nature. The Euclid plan is to survey almost  all of the extragalactic sky, with a mosaic of sharp images and spectra for the relatively bright sources. The main goal is to infer the amount and the distribution of these obscure and mysterious  matter / energy from the detailed properties of the distant light-emitting galaxies. This will be a major step forwards for a better understanding of the structure and of the history of our Universe.

The Euclid spacecraft (credits ESA).

Euclid primary science goals

Our understanding of cosmology is that of the Universe evolving from a homogeneous state after the Big Bang to a hierarchical assembly of galaxies, clusters and superclusters at our epoch. However this view relies on two untested assumptions about the initial conditions of the Universe and the nature of gravity itself, and the existence of two dominant components whose nature is entirely unknown. Of these unknown components, 76% of the energy density is in the form of dark energy, which is causing the Universe expansion to accelerate. Dark energy is in conflict with our knowledge of fundamental physics. If it behaves as predicted from the cosmological constant (introduced by Einstein), then its value is 1060 times smaller than that predicted from theory; this is the largest discrepancy between theory and observation ever encountered in modern physics. Another 20% of the energy density of the Universe is in the form of dark matter, which exerts gravitational attraction as normal matter, but does not emit or absorb light. While several candidates for dark matter exist in particle physics, its nature remains unknown. Another possibility to explain one or both of these puzzles is that Einstein's General Relativity, and thus our understanding of gravity, may need to be revised. This diversity of theoretical ideas shows our current ignorance but also defines the need for future observations. Based on our present-day knowledge, the existing plausible models will only change observational signatures by tiny amounts that can only be decisively distinguished using high-precision astronomical surveys covering a major fraction of the sky.

Euclid is a survey mission designed to understand the origin of the Universe ’s accelerating expansion. It will use cosmological probes to investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the universe and on the cosmic history of structure formation. Euclid will map large-scale structures over a co smic time covering the last 10 billion years, more than 75% of the age of the Universe. The mission is optimized for two independent primary cosmological probes: Weak gravitational Lensing (WL) and Baryonic Acoustic Oscillations (BAO).
 
WL is a technique to map dark matter and mea sure dark energy by quantifying the apparent distortions of galaxy images, a change in a galaxy’s observed ellipticity, caused by mass inhomogeneities along the line-of-sight. The lensing signal is derived from the measurement of shape and distance of galaxies. BAO are wiggle patterns imprinted in the clustering of galaxies that provide a standard ruler to measure the expansion of the Universe. The properties of the wiggles are derived from accurate distance measurements of galaxies. Surveyed in the same cosmic volume, these two probes provide necessary cross-checks on systematic errors. They also provide a measurement of the large-scale structure via different physical fields (potential, density and velocity), which are required for testing dark energy and gravity at all scales. In addition, the Euclid surveys yield data for several important complementary cosmological probes, such as galaxy clusters, redshift space distortions and the integrated Sachs-Wolfe effect. WL requires a high image quality on sub-arcsec scales for the galaxy shape measurements, and photometry at visible and infrared wavelengths to measure the photometric distances of each lensed galaxy out to z≤2.
BAO requires near-infrared spectroscopic capabilities to measure accurate redshifts of galaxies out to z≤0.7. Both probes require a very high degree of system stability to minimize systematic effects, and the ability to survey a major fraction of the extragalactic sky. Such a combination of requirements cannot be met from the ground, and demands a wide-field-of-view space mission.
 
Euclid is designed for that purpose. To understand the nature of dark energy its equation of state needs to be determined. Euclid uses WL and BAO to measure the constant and time varying terms of the dark energy equation of state to a 1σ precision of 0.02 and 0.1 respectively, sufficient to make a decisive statement on the nature of dark energy. Euclid tests the validity of General Relativity by measuring the rate of cosmic structure growth to a 1σ precision of <0.02, sufficient to distinguish General Relativity from a wide range of modified-gravity theories. As Euclid maps the dark matter distribution with unprecedented accuracy, subtle features produced by neutrinos are measured, providing constraints on the sum of the neutrino masses with a 1σ precision better than 0.03 eV. Likewise, the initial conditions of the seeds of cosmic structure growth are unveiled by determining the power spectrum of density perturbations to one percent accuracy. Euclid and Planck together measure deviations to a Gaussian distribution of initial perturbations with a precision one order of magnitude better than current constraints, allowing Euclid to test a broad range of inflation models. Euclid is therefore poised to uncover new physics by challenging all sectors of the cosmological model.
 

A journey across cosmic times with Euclid (credits: University of Colorado).