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Artist's impression of the Einstein Telescope. Credit: Marco Kraan (Nikhef).

When he visits Lula, a small mining village in the heart of Sardinia, physicist, Alessandro Cardini, tells the story of the Swiss and French villages around CERN, the European Organization for Nuclear Research. Until the 1950s they had fewer than 1,000 inhabitants each. Today their populations are more than ten times bigger, and the area hosts a vibrant international community. Cardini, a physicist at the National Institute for Nuclear Physics (INFN) in Cagliari, explains to the people of Lula that this is what may happen to their town if it ends up hosting the Einstein Telescope (ET), the next-generation gravitational waves detector. The underground area near the decommissioned Sos Enattos mine in Lula is one of the two candidate sites to host the interferometer. The other is in the Meuse–Rhine Euroregion (EMR), at the border of the Netherlands, Belgium, and Germany.

The local population is enthusiastic about the project. “Entering the café in Lula’s central square, you can find the mayor talking about gravitational waves with his fellow citizens,” says Cardini. But the outcome is far from certain. Each site will prepare a bid, and a decision is expected in 2025.

The design of ET envisages a triangular geometry. At each corner of the triangle lies a pair of interferometers with the arms extending for ten kilometres. Three main caverns at each vertex will host the main optical apparatus, where two laser beams travelling along the arms will interfere and allow the detection of ripples in space time caused by accelerating massive objects, such as black holes or neutron stars orbiting each other. ET needs an extremely low level of seismic and environmental noise. “In this respect, the Sardinian site is the strongest candidate at the moment” says Michele Punturo, spokesperson of the ET international scientific collaboration. There are several noise sources near the EMR site, like wind farms, industrial plants, and a railway line, but the area benefits from a large network of companies and research institutions, he adds.

The bidding countries must commit to cover the construction costs, which are estimated around 1 billion euros, and part of the operating costs, which should amount at nearly 40 million euros per year. “The Netherlands has already committed to invest 900 million euros if the EMR site is selected, whereas the funding from the Belgian and German governments is still to be announced”, says Domenico D’Urso, physicist at the University of Sassari, and coordinator of the Italian host team that will prepare the Sos Enattos application.

The minister of university and research, Anna Maria Bernini, recently appointed a scientific advisory board chaired by the Nobel Prize-winner Giorgio Parisi, which will advocate for the Italian site.

The previous government also allocated 50 million Euros from the National Recovery and Resilience Plan to the ETIC project, that will use 20 million to further characterize the Sos Enattos site, measuring more accurately the seismic and environmental noises, and the remaining 30 million to develop technologies for the ET.

The first measurements began in 2010, two years after the first conceptual design of a third-generation gravitational wave detector was drafted. “Going underground is crucial to limit the noise. The region, with its decommissioned mines, low urbanization and one of the lower seismic activities in Italy, looked extremely promising”, Cardini comments.

As a first testbed, in 2021 INFN installed in Sos Enattos the Archimedes experiment, that looks for tiny variations in gravity caused by quantum effects, and that also requires very low noise. D’Urso says the apparatus was first tested in Naples and at Virgo, the existing gravitational wave experiment near Pisa, but is performing much better in Sos Enattos. “This shows why this site is so suitable to host ET,” he says.

At the end of January, the ET collaboration met in Maastricht for the second workshop of the site preparation board. “We are establishing the standards to follow when presenting the data for each site so that they can be fairly compared,” D’Urso explains.

The noise data analyses will be collected and recorded, along with estimates about the building costs and times, which strongly depend on the type of rocks in the underground of each site and on the regulatory frameworks of the two hosting countries.

At the EMR site, a layer of soft rock lies above a harder one. When seismic waves travel through the ground, they are reflected by the interface between the two layers and only partially transmitted. To exploit the noise attenuation effect, the tunnels and caverns of the interferometers need to be built below the interface, at a depth of nearly 250 meters.

The Sos Enattos soil instead is homogenous and made of granite, a hard rock where seismic waves travel better than in soft soil. However, the noise, both seismic and environmental, is so low that scientists first estimates indicate that the tunnels and caverns of ET could be placed 120 meters deep, with a significant reduction of the excavation costs and times.

In Maastricht, ET scientists also discussed the measures needed to preserve the characteristics of the sites on the long term. “The Netherlands government has instituted a ten kilometers exclusion area, where no wind turbines or other infrastructures could be built until the end of ET operations,” D’Urso says. “We should negotiate with our national government a similar agreement, but we will need a larger exclusion area because of the geophysics features of the Sos Enattos site”.

Here is where problems may arise. Several companies have proposed to build new wind farms near Sos Enattos. In October 2022, just a week before leaving office, Draghi approved the construction of a large wind farm on the Gomoretta plateau, 4km from one of the three vertices of the proposed ET triangle. “It would severely damage the Italian bid”, D’Urso comments. The area’s mayors have protested against the Gomoretta wind farm. “INFN addressed the regional authorities and the prime minister, asking for a fifteen kilometers exclusion area around each of the three vertices”, Punturo explains.

The ETIC project will also consider a different configuration, beside the triangular one, that may pave the way for a compromise. “We also studied a configuration for two sites, each hosting one L-shaped interferometer”, says Punturo. This is the same configuration used by the current detectors VIRGO near Pisa, LIGO in the US and KAGRA in Japan, that can work collaboratively. “This way, the Dutch and Italian bids would become a coordinated one.”