An underground observatory that could detect gravitational waves from the far reaches of the universe is one step closer to reality. Last week, the Dutch government said it was prepared to foot about €900 million of the project’s roughly €1.9 billion construction cost if it is built near the border of the Netherlands, Germany, and Belgium. The pledge puts the Dutch proposal for Europe’s so-called Einstein Telescope ahead of rival bids, says Stan Bentvelsen, director of the Netherlands’s National Institute for Subatomic Physics and a leader of the Dutch proposal. “I think the Dutch government is sticking its neck out the farthest,” he says.
Gravitational waves were discovered in 2015 when the ripples in spacetime generated by a pair of merging black holes were detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO), two U.S. detectors each made up of 4-kilometer-long tubes arranged in an L-shape. By sending a laser beam down each tube and bouncing it off mirrors suspended at each end, physicists can look for fluctuations in the beams’ travel time, a sign that a passing gravitational wave has subtly stretched or squeezed the arms. Researchers at LIGO and Virgo, a detector in Italy with arms 3 kilometers long, have since gone on to detect dozens more black hole mergers, as well as collisions between pairs of neutron stars.
Now, researchers in Europe and the United States are thinking about bigger detectors that could survey most of the universe. The most sophisticated, the Einstein Telescope, might be able to detect hundreds of thousands of mergers per year, out to the distant reaches of the observable universe, or soon after the big bang. Three overlapping L-shaped detectors, each with arms 10 kilometers long, will watch for spacetime distortions, and the observatory will be buried in bedrock several hundred meters down to insulate it from the surface noise of wind and traffic. Each detector arm will contain two laser systems, including one cooled almost to absolute zero, giving it sensitivity to longer wavelength radiation from mergers of very large black holes, hundreds of times the mass of the Sun.
Last year, the European Union added the Einstein Telescope to an official wish list of major scientific facilities known as the European Strategy Forum on Research Infrastructures, and official project bids are due within the next 2 to 3 years. Bentvelsen says the Dutch site—somewhere between the cities of Maastricht, Liège, and Aachen—has a layer of soft soil above the bedrock. The contrast between the layers ensures that most surface vibrations would bounce off the bedrock rather than shake the observatory.
In a recent assessment of new proposals for public infrastructure, the Netherlands National Growth Funds Commission cited the region’s geology as one of its advantages. Last week, the Dutch Cabinet agreed to pay €42 million for preparatory research and €870 million for construction, should the site be chosen. To win the funding, however, the observatory’s backers in academia and provincial government also need to show how the project would benefit local companies. Bentvelsen appears confident, saying he has “positive feedback” from a range of companies and technology institutes in the region.
The Dutch bid has an Italian rival that would put the Einstein Telescope close to a former zinc mine near the town of Lula in Sardinia. It is one of the 30 most seismically quiet sites in the world, says Michele Punturo of Italy’s National Institute of Nuclear Physics, who co-leads the overall Einstein Telescope collaboration and is the scientific coordinator of the Sardinian proposal. He says he and his colleagues submitted a request in February to the Italian government for €100 million to develop detector technology and better scrutinize the site’s geology. They expect to hear the outcome in June. “The noises are positive,” he says.
Punturo is heartened that two separate governments are showing an interest in the project, but acknowledges it is not yet guaranteed. After a board of scientists compares site characteristics, the winner should emerge from negotiations between national governments, he says. Construction should begin in 2026 or 2027, with scientific operations starting up some 9 years later.
Even after the big pledge, the Dutch will still have to drum up support from other European nations. Belgium and Germany have been involved in preliminary site investigations but have yet to formally support the Dutch bid.
And Germany may launch its own bid. Günther Hasinger, head of the European Space Astronomy Centre near Madrid, is spearheading plans for a new astrophysics center in the state of Saxony, using federal funds intended for development in eastern Germany, where the coal industry has declined. Hasinger wants to site the Einstein Telescope in a strip of granite found in the region—but only if the site’s geology makes it “significantly better than the others.” With a site decision expected in 2025, he and his colleagues have a few years to find out.