Einstein Telescope

Einstein Telescope (ET) or Einstein Observatory, is a proposed third-generation ground-based gravitational wave detector, currently under study by some institutions in the European Union. It will be able to test Einstein's general theory of relativity in strong field conditions and realize precision gravitational wave astronomy. Einstein Telescope (ET) or Einstein Observatory, is a proposed third-generation ground-based gravitational wave detector, currently under study by some institutions in the European Union. It will be able to test Einstein's general theory of relativity in strong field conditions and realize precision gravitational wave astronomy. The ET is a design study project supported by the European Commission under the Framework Programme 7 (FP7). It concerns the study and the conceptual design for a new research infrastructure in the emergent field of gravitational-wave astronomy. The evolution of the current gravitational wave detectors Advanced Virgo and Advanced LIGO, as second generation detectors, is well defined. Currently they have been upgraded to their so-called enhanced level and they are expected to reach their design sensitivity in the next few years. LIGO detected gravitational waves in 2015 and Virgo joined this experimental success with the first gravitational wave observed by three detectors GW170814 and shortly after with the first detection of a binary neutron star merger GW170817. Nevertheless, the sensitivity needed to test Einstein's theory of gravity in strong field conditions or to realize a precision gravitational wave astronomy, mainly of massive stellar bodies or of highly asymmetric (in mass) binary stellar systems, goes beyond the expected performances of the advanced detectors and of their subsequent upgrades. For example, the fundamental limitations at low frequency of the sensitivity of the second generation detectors are given by the seismic noise, the related gravitational gradient noise (so-called Newtonian noise) and the thermal noise of the suspension last stage and of the test masses. To circumvent these limitations new infrastructures are necessary: an underground site for the detector, to limit the effect of the seismic noise, and cryogenic facilities to cool down the mirrors to directly reduce the thermal vibration of the test masses.