Chapter 26: Ideas for 2.5G and 3G interferometric detectors

The direct detection of gravitational waves (GWs) and the beginning of GW astronomy were made possible by the success of the second generation of GW detectors: Advanced LIGO and Advanced Virgo. Further developments of these interferometers are required to achieve design sensitivity and extend their detection range. However, in order to contribute to future high-precision multi-messenger astronomy and to reach cosmological distances in the observation of GW sources, the construction of a third generation of GW observatories is indispensable. The Einstein Telescope (ET) in Europe and Cosmic Explorer (CE) in the US are planned to be the first nodes of a future network of 3G observatories. Some of the key design and technology elements of the ET project and the initial ideas of the CE concept are presented in this chapter.