The ET is a proposed underground infrastructure to host a third-generation, gravitational-wave observatory. It builds on the success of the current, second-generation laser-interferometric detectors Advanced Virgo and Advanced LIGO. The Einstein Telescope will make it possible, for the first time, to explore the Universe through gravitational waves along its cosmic history up to the cosmological dark ages, shedding light on open questions of fundamental physics and cosmology. With a successful European Strategy Forum on Research Infrastructures (ESFRI) proposal, the project will enter its preparatory phase, which foresees the beginning of construction in 2026 with the goal to start observations in 2035.

It will probe the physics near black-hole horizons (from tests of general relativity to quantum gravity), help understanding the nature of dark matter (such as primordial black holes, axion clouds, dark matter accreting on compact objects), and the nature of dark energy and possible modifications of general relativity at cosmological scales. Exploiting the ET sensitivity and frequency band, the entire population of stellar and intermediate mass black holes will be accessible over the entire history of the Universe, enabling to understand their origin (stellar versus primordial), evolution, and demography.