• An international consortium proposes the creation of a terrestrial observatory for gravitational waves, to target the new scientific challenges in the gravitational wave astronomy.

• The Einstein Telescope would enable scientists to detect any merge of two intermediate-mass black holes in the entire universe and thus contribute to the understanding of its evolution.

The Era of Gravitational Waves Astronomy

The remarkable scientific achievements of Advanced Virgo (in Europe) and Advanced LIGO (in the USA) in the last 5 years initiated the era of Gravitational Waves astronomy. The adventure began with the first direct detection of gravitational waves in September 2015 and continued in August 2017 when the two observatories observed gravitational waves emitted by two merging neutron stars. Simultaneously, signals of this event were observed with a variety of electromagnetic telescopes —on the ground and in space— over the entire observable wavelength range —from radio waves to gamma rays—. This marked the beginning of the era of multi-messenger astronomy with gravitational waves. Recently, the collaboration unveiled the existence of an extraordinarily massive merging binary system, the most massive black hole ever detected with gravitational waves.

Now, scientists propose the creation of a new observatory capable of observing GWs with sensitivity at least one order of magnitude better than the current detectors (the so-called second generation). The Einstein Telescope will be located in new infrastructure and will apply technologies that are dramatically improved over the current ones. It will enable scientists to detect any merge of two intermediate-mass black holes in the entire universe and thus contribute to the understanding of its evolution. This will shed new light on the Dark Universe and will clarify the roles of dark energy and dark matter in the structure of the cosmos. ET will explore the physics of black holes and will detect thousands of coalescences of neutron stars, improving our understanding of the behaviour of matter under such extreme conditions of density and pressure. Also, we will have a chance to explore the nuclear physics underlying the supernova explosions of the stars.

The Einstein Telescope has aroused great interest in the Spanish scientific community involved in gravitational waves, which includes all the centres that currently participate in ground-based (LIGO, Virgo and KAGRA) and space programs (LISA). Spanish researchers have contributed significantly to the development of the ET physics program, as well as to the preparation of its technical design report.

Our Institute joined the Virgo collaboration in July 2018, becoming a full member of Virgo in 2019. The ICCUB's Virgo team, led by researcher Jordi Portell, contributes to the electronics and instrumentation upgrades of Virgo and to improvements in the data analysis techniques. They also contribute to the review of the overall computing model, the software management and the data handling approach, aiming at efficient use of the computing facilities. These contributions are possible thanks to their expertise in other international projects such as the LHCb detector at CERN and ESA's astrometric satellite Gaia.

Two sites for the development of the ET infrastructure are currently being evaluated: The Euregio Meuse-Rhine, at the borders of Belgium, Germany and the Netherlands; and Sardinia, in Italy. It is hoped that a companion project in the US, Cosmic Explorer, will follow. With a successful ESFRI proposal, the project will enter its preparatory phase, which foresees the beginning of construction in 2026 intending to start observations in 2035.

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