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 detectedgravitational 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 mergerGW170817. 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 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 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.
Technical groups
The ET-FP7 project, through its four technical working groups is addressing the basic questions in the realization of this proposed observatory: site location and characteristics, suspension design and technologies, detector topology and geometry, detection capabilities requirements and astrophysics potentialities.
Participants
ET is a design study project in the European Framework Programme. It has been proposed by 8 European leading gravitational wave experimental research institutes, coordinated by the European Gravitational Observatory:
Although still in the early design study phase, the basic parameters are established. Like KAGRA, it will be located underground to reduce seismic noise and "gravity gradient noise" caused by nearby moving objects. The arms will be 10 km long, and like LISA, there will be three arms in an equilateral triangle, with two detectors in each corner. In order to measure the polarization of incoming gravitational waves and avoid having an orientation to which the telescope is insensitive, a minimum of two detectors are required. While this could be done with two 90° interferometers at 45° to each other, the triangular form allows the arms to be shared. The 60° arm angle reduces each interferometer's sensitivity, but that is made up for by the third detector, and the additional redundancy provides a useful cross-check. Each of the three detectors would be composed of two interferometers, one optimized for operation below 30 Hz and one optimized for operation at higher frequencies. The low-frequency interferometers will use optics cooled to, with a beam power of about 18 kW in each arm cavity. The high-frequency ones will use room-temperature optics and a much higher recirculating beam power of 3 MW.
Prototype
A prototype, or testing facility, called the ET Pathfinder will be built at Maastricht University in the Netherlands.
