File:The Universe across space and time ESA24866637.png

English: One of the scientific goals of ESA’s Euclid mission is to chart the expansion history of the Universe. Recent cosmological observations showed that the Universe does not expand at a constant rate; rather, the Universe's expansion is accelerating. Before these observations, scientists thought that all forms of matter and energy in the Universe would only cause the expansion to slow down over time. To explain an accelerating expansion scientists had to introduce a new form of energy. One working hypothesis for this entity is the ‘cosmological constant’ suggested by Albert Einstein in 1917: a constant energy field present across the entire Universe. It is an intrinsic property of the vacuum of space, so the larger the volume of space, the more ‘vacuum energy’ (dark energy) is present and the greater its effects.
There are alternative suggestions. For example, the acceleration could be produced by a fifth fundamental force of nature that evolves with the expansion of the Universe. Contrary to the cosmological constant, this ‘quintessence’ is dynamic, time-dependent and not evenly distributed across space.
Each explanation for what dark energy is subtly alters the way the acceleration changes across cosmic time.
ESA’s Euclid observations will allow scientists to measure how the rate of expansion of the Universe has changed over time and map the last 10 billion years of cosmic history: from cosmic ‘noon’, the time when most stars were forming, until today. This 'looking back in time' will show us the variations in the cosmic acceleration with extreme precision, helping scientists pin down the nature of dark energy.
Euclid is a European mission, built and operated by ESA, with contributions from NASA. The Euclid Consortium is responsible for providing the scientific instruments and scientific data analysis. ESA selected Thales Alenia Space as prime contractor for the construction of the satellite and its service module, with Airbus Defence and Space chosen to develop the payload module, including the telescope. NASA provided the detectors of the Near-Infrared Spectrometer and Photometer, NISP. Euclid is a medium-class mission in ESA’s Cosmic Vision Programme.