The discovery of gravitational waves from merging black holes and neutron stars by Advanced LIGO and Advanced Virgo marked the beginning of new era in observing our universe and scientists are keen to fully open up this new window to the (dark side of the) cosmos.

ET will detect several hundred thousand gravitational wave signals per year and allow us listen to the sound of merging black holes across the entire universe. For the first time, researchers will be able to study the precise structure of neutron stars, the birth of black holes and the structure of the universe immediately after the Big Bang.

To reach the full potential of such a third generation gravitational wave detector, several new techniques need to be developed. A few key topics include:

- better seismic isolation with advanced suspensions systems for the mirrors

- new wavelengths for the laser system

- cryogenically cooled mirrors

To develop these techniques a new facility is set up to work with full-scale prototypes of the essential optical elements: ET Pathfinder. This facility is hosted at Maastricht University, and operated by a consortium of Dutch, Belgian and German institutes.