Einstein Telescope (ET) will be Europe’s 3rd generation gravitational-wave (GW) observatory surpassing the ultimately achievable sensitivity of the present –2nd generation– GW observatories such as LIGO and Virgo by an order of magnitude. Whereas the LIGO-Virgo Collaboration opened a new window upon our Universe with the first and Nobel Prize winning direct detection of GWs following the merger of two black holes, ET’s superior sensitivity is essential to fully exploit the potential of GWs. E.g., LIGO/Virgo will at best be sensitive to black-hole mergers up to redshifts of z=1 (8 billion years after the Big Bang). ET will be sensitive to mergers up to z=20, allowing to probe the so-called ‘dark ages’ the few 100 million years period after the Big Bang prior to the formation of the first stars. Each year, ET will detect close to a million black-hole (and neutron-star) mergers with black-hole masses extending up to several thousand solar masses and often with huge signal-to-noise ratios. The wealth of data inherent to this combination of large redshift coverage, wide black-hole mass range, high statistics and excellent accuracy sets the scene for an enormous discovery potential in the fields of astrophysics, fundamental physics and cosmology. In addition to GWs from coalescing binaries, ET will also be able to detect GWs from several other sources such as supernovae and isolated pulsars. Possibly more speculative, ET might detect the stochastic GW background resulting from quantum fluctuations in the very early Universe and its subsequent brief period of rapid expansion –coined inflation. Furthermore, ET will play a pivotal role in the new field of multi-messenger astronomy which combines the detection of GWs with signals from radio, optical, IR, UV, X-ray, gamma-ray and neutrino telescopes as well as cosmic-ray observatories. ET’s superb sensitivity down to a few Hz allows to track GW signals for hours, permitting (profiting from the Earth’s rotation) accurate sky localisation to thereby alert other observatories ahead of time to point their instruments to observe e.g. the coalescence itself. And ET’s most exciting observations will be the surprises awaiting in our Universe to be unlocked by the combined power of GWs as a unique unstoppable messenger and ET’s unprecedented accuracy.
ET’s instrumentation is ‘simple’: three pairs of two laser interferometers –one optimised for low- and one optimised for high frequencies– installed in an underground triangular 6,5 meter diameter tunnel with large caverns at each corner. Nevertheless, the required technology for ET is mindboggling and is best illustrated by the projected precision with which the mirror jitter due to the passage of a GW will be recorded: 0,000 000 000 000 000 000 01 m. ET’s conceptual design was published in 2011 as the result of an EU-funded Design Study. It relies on driving the techniques pioneered and successfully implemented for GEO600, LIGO and Virgo: fabulous seismic isolation, large mirrors with low-noise dielectric coatings and silica fibre suspensions, high-power lasers and the use of squeezed light to reduce quantum noise to their physical limits. ET’s design relies on two distinct innovative features. Firstly, ET will be sited at a low-seismic-noise underground-location for which two candidates have been identified: Sardinia in Italy and the Euregion Meuse-Rhine on the Belgian-Dutch-German border. Secondly, ET will rely on the use of crystalline silicon mirrors operated at cryogenic temperatures to suppress thermal noise and mirror distortions to a minimum. Some of these new concepts are pursued in collaboration with the KAGRA project in Japan whereas others benefit from two recently approved EU Interreg projects. ‘ETpathfinder’ –a laser interferometer R&D laboratory being realised in Maastricht– dedicated to cryogenic silicon mirror technology and ‘E-TEST’ including detailed geology studies to identify the optimal ET site in the Euregio Meuse-Rhine.