Our Universe is undergoing an accelerated expansion. This unexpected observational discovery was made in 1998 and 1999, and awarded the Noble Prize in Physics in 2011. There are two possible causes for the observed cosmic accelation: a new, unknown energy component (i.e., "dark energy"), or a modification of Einstein's theory of gravity (i.e., "modifed gravity"). For convenience, the unknown cause for the cosmic acceleration has been dubbed "dark energy", although its true nature remains shrouded in mystery.

Euclid is an ESA-led mission to map the geometry of the dark Universe and probe the mystery of dark energy. Using two primary cosmological probes—weak lensing and galaxy clustering—in a wide-field survey, Euclid will precisely measure the expansion history of the Universe and growth rate of large scale structure in the Universe using two independent methods. This will enable the differentiation of dark energy and modified gravity as possible causes for the observed cosmic acceleration.

Euclid's Cosmological Probes:

Euclid will map the large-scale structure of the Universe over ~15,000 square degrees of the extragalactic sky - or 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:

1. Weak gravitational lensing:  This method measures the tiny distortions of galaxy images by cosmological mass inhomogeneities along the line-of-sight, and uses these 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.
2. Galaxy Clustering: This method measures the angular positions and redshifts of tens of millions of galaxies, and enables the measurement of the cosmic expansion history through baryonic acoustic oscillations (BAO), and the measurement of 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 z = 2, over the entire extra-galactic sky.  It will also revolutionize our understanding of the Milky Way halo. For a list of non-dark-energy science topics enabled by Euclid data, click here.

Euclid Main Surveys:

• Wide Survey:
  • Covering ~15,000 square degrees of the extra-galactic sky with galactic latitude |b| > 30 deg
  • Obtaining galaxy shear measurements for ~1.5 billion galaxies, and spectroscopic measurements for ~30 million galaxies

• Deep Survey:
  • ~ 2 magnitudes deeper than the wide survey, total area of ~50 deg² in patches of approximately 10-20 deg²

Resources to Learn More:

• Learn more about dark energy and dark matter from the NASA website: http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
• Euclid definition study report (Red Book):  http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=48983#
• ESA's Euclid Science Goals webpage: http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42267