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''Euclid'' (named after the ancient Greek mathematician Euclid of Alexandria, the "Father of Geometry"), is a space mission currently under development by the European Space Agency (ESA). The objective of Euclid is to better understand dark energy and dark matter by accurately measuring the acceleration of the universe. To achieve this, the spacecraft will measure the redshift of galaxies at varying distances from Earth and investigate the relationship between distance and redshift. Dark energy is generally accepted as contributing to the increased acceleration of the expanding universe, so understanding this relationship will help to refine how physicists and astrophysicists understand it. Euclid's mission advances and complements ESA's Planck mission, and other contemporary space missions. Euclid is a medium-class ("M-class") mission and is part of ESA's "Cosmic Vision" (2015–2025) scientific program. This class of missions have an ESA budget cap at around €500 million. Euclid was chosen in October 2011 together with Solar Orbiter, out of several competing missions. The launch date is planned for 2020. ==Scientific objectives and methods== Euclid will probe the history of the expansion of the universe (thought to be governed by dark energy) and the formation of cosmic structures by measuring the redshift of galaxies out to a factor of 2, which is equivalent to seeing back 10 billion years in the past. The link between galactic shapes and their corresponding redshift will give a look into how dark energy contributes to the increased acceleration of the universe. The methods employed exploit the phenomenon of gravitational lensing, measurement of Baryon acoustic oscillations, and measurement of galactic distances by spectroscopy. Gravitational lensing (or gravitational shear) is a consequence of the deflection of light rays caused by the presence of matter that locally modifies the curvature of space-time: light emitted by galaxies, and therefore observed images, are distorted as they pass close to matter lying along the line of sight. This matter is composed partly of visible galaxies but it is mostly dark matter. By measuring this "shear", the amount of dark matter can be inferred, furthering the understanding of how it is distributed in the universe. Spectroscopic measurements will permit measuring the redshifts of galaxies and determining their distances using the Hubble's Law. In this way one can reconstruct the three-dimensional distribution of galaxies in the universe. From this data, it is possible to simultaneously measure the statistical properties concerning the distribution of dark matter and galaxies, and measure how these properties change as the spacecraft looks further back in time. Highly precise images are required to provide the most accurate measurements, as any distortion inherent in the sensors themselves must be accounted for and calibrated out, otherwise the resultant data will be of questionable use.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Euclid (spacecraft)」の詳細全文を読む スポンサード リンク
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