Euclid will map the large-scale structure of the Universe over 15,000 square degrees, nearly half of the full sky excluding the regions dominated by the stars in our Milky Way galaxy. It will measure galaxies out to distances which corresponds to a look-back time of about 10 billion years, covering the period over which dark energy accelerated the expansion of the Universe. Euclid is optimized for two primary cosmological probes, Weak Lensing and Galaxy Clustering. 

Euclid Weak Lensing measures the tiny distortions of ~1.5 billion galaxy images due to gravitational lensing by cosmological mass inhomogeneities along the line-of-sight, to map matter distribution and probe dark energy through a combined measurement of the cosmic expansion history and growth history of large scale structure. Gravitational lensing, the bending of light by gravity, is a prediction of Einstein's theory of general relativity, and has been observed over wide ranges of scales and has broad applications in cosmology. 

Euclid Galaxy Clustering measures angular positions and redshifts of tens of millions of galaxies, to obtain the 3D distribution of ~30 million galaxies. This enables the measurement of the cosmic expansion history through baryonic acoustic oscillations (BAO), and the growth history of large scale structure through redshift-space distortions (RSD). BAO originated from primordial sound waves that were frozen when the Universe first became transparent; they provide a standard ruler to measure cosmic geometry. RSD are artifacts in redshift space that scale with the growth rate of the Universe.

Euclid will enable other constraints on dark energy, as well as precise constraints on initial conditions in the Universe. In addition, Euclid will enable unprecedented advances across the range of astrophysics topics, from objects in our own Solar System to the light of the first stars detected in background fluctuations. Euclid will deliver high quality morphologies, masses, and star-formation rates for billions of galaxies out to a lookback time of ~10 billion years, over about half of the entire extra-galactic sky.  It will also revolutionize our understanding of the Milky Way halo.

Click here for a list of science topics (other than dark energy) enabled by Euclid data.