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With its novel subterranean, triangular design, Europe’s Einstein Telescope would be a gravitational wave observatory unlike any other.

ET CONCEPTUAL DESIGN TEAM

European plan for gigantic new gravitational wave detector passes milestone

It’s far from a done deal, but plans by European physicists to build a huge new gravitational wave observatory with a radical design received a boost this week. The European Strategy Forum on Research Infrastructures (ESFRI), which advises European governments on research priorities, added the €1.9 billion observatory, called the Einstein Telescope, to a road map of large science projects ripe for progress. Developers hope the move will give them the political validation needed to transform the Einstein Telescope idea into a project.

“This isn’t a promise of any funding, but it shows the clear intention to pursue this,” says Harald Lück, a gravitational wave physicist at Leibniz University Hannover and the Max Planck Institute for Gravitational Physics and co-chair of the Einstein Telescope steering committee. “It is more of a political commitment.”

U.S. gravitational wave physicists welcomed the announcement, too, as they think it may bolster their plans to build a pair of detectors even bigger than the Einstein Telescope in a project called Cosmic Explorer. “In the U.S., I think the momentum is going to start to build,” says David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (LIGO) and a physicist at the California Institute of Technology.

Gravitational wave detectors sense tiny fleeting ripples in space itself when massive astrophysical objects, such as black holes, whirl together and collide. In the past 5 years, scientists have spotted dozens of merging pairs of black holes, the ghostly superintense gravitational fields left behind when massive stars collapse to infinitesimal points, spiraling together. They have also spotted the gravitational waves—and spectacular explosion—set off by the merger of a pair of smaller neutron stars, the ultradense corpses of middle-weight stars that burn out and blow up. This week, researchers announced they had twice sensed gravitational waves from a black hole swallowing a neutron star.

To sense gravitational waves, physicists employ gigantic L-shaped optical devices called interferometers. They use laser light to compare the lengths of an interferometer’s arm to exquisite precision and look for evidence that space is stretching more in one direction than the other. In the United States, LIGO consists of twin interferometers in Louisiana and Washington state, each with arms 4 kilometers long. In Italy, Europe’s Virgo detector has arms 3 kilometers long.

But scientists want even bigger, more sensitive interferometers. LIGO and Virgo can sense black hole mergers more than 10 billion light-years away. But if scientists had detectors 10 times more sensitive, they could spot black hole mergers all the way out of the edge of the observable universe, 45 billion light-years away. To achieve such sensitivity, Cosmic Explorer would consist of one or more L-shaped interferometers with 40-kilometer arms. In contrast, the Einstein Telescope would be a subterranean equilateral triangle housing a total of six V-shaped interferometers (two in each corner) with 10-kilometer arms.

Physicists in the United States and Europe both hope to build the detectors by the mid-2030s. Inclusion in ESFRI’s road map is a key first step toward realizing the Einstein Telescope, says Michele Punturo, a physicist and director of research at Italy’s National Institute of Nuclear Physics and co-chair of the Einstein Telescope steering committee. Over the next 3 or 4 years, Einstein Telescope developers will flesh out their existing conceptual design for the observatory in a more detailed technical design report, Punturo says. More important, he says, they will start the process of expanding the international collaboration to support the project. Currently, the Einstein Telescope team receives support from Belgium, Italy, the Netherlands, Poland, and Spain.

In fact, Punturo says, ESFRI exists because the European central government, the European Commission, has no default mechanism by which to organize and fund such large international projects. (The European Space Agency, the European Southern Observatory, and CERN, the European particle physics laboratory, have their own unique organizational structures, but none of them has the scientific expertise needed to build a gravitational wave observatory.) So ESFRI, which is run by the European Council and consists of representatives from the national scientific funding agencies, aims to help set priorities for big international facilities in Europe.

However, it remains up to the Einstein Telescope team to develop the organization that will support the project, Punturo says. For example, he says, the organization might be modeled after that of CERN. The ESFRI imprimatur will be vital in gaining support and funding from individual nations, he says. “The ESFRI road map is opening the preparatory phase that should perform all the technical, legal, and financial steps in order to reach the point where we can say, ‘OK, we’re ready to proceed.’”

“Building community is certainly a big part of facility design,” says Jocelyn Read, a gravitational wave physicist and LIGO member at California State University, Fullerton. She notes that the push to build the next generation of gravitational wave detectors is less a competition than a collaboration: “Cosmic Explorer and Einstein Telescope would do the best science working together.”