The Einstein Telescope will not be the only third-generation terrestrial gravitational wave observatory. In addition to Europe, the United States also has a new experiment in the pipeline, which will serve as the ideal successor to the interferometers of the U.S. LIGO project, currently in progress with two instruments in Hanford (Washington) and Livingston (Louisiana).
The future observatory will be called Cosmic Explorer and, like LIGO, will consist of a pair of twin L-shaped interferometers built on the surface. However, compared to the current experiment, the size of the new observatory will be much more imposing: the two interferometers will have arms measuring 40 and 20 kilometers, respectively (compared to the four kilometers of the two LIGO instruments). This will result in a significant increase in sensitivity.
This is a very different approach from the Einstein Telescope (ET): the European project, in fact, aims to increase sensitivity not so much by building much longer interferometers, but by placing the infrastructure underground, at a depth of between 100 and 300 meters (although ET’s arms will be 10 or 15 kilometers long, depending on the chosen configuration, which is still longer than current instruments).
However, the two experiments have one thing in common: the time needed for their construction and operation will be long, no less than ten years. This period will coincide with a phase of enhancement for both LIGO and Virgo (the Italian-French-Dutch observatory based in Italy), whose role will be very important in testing technologies to be used in the next-generation observatories.
We discussed this with Peter Fritschel, director of the LIGO research unit at the Massachusetts Institute of Technology (MIT) in Cambridge, and Kevin Kuns, a researcher in the same unit, directly involved in the Cosmic Explorer project.
«At the moment, the LIGO detectors are operational with O4, the fourth observation run, which began last year and to which Virgo joined in recent months», Fritschel points out. «O4 will last until mid-2025, after which a phase dedicated to upgrading the interferometers will begin, lasting about two years, with the goal of reducing noise and further increasing sensitivity. At that point, the fifth observation cycle will begin, which will take us to the end of the decade».
The subsequent period, which will last at least until the mid-2030s (but more likely until the early 2040s), will be the real “middle ground” marking the transition between second- and third-generation experiments. «Our plan for the next decade is to push LIGO technology even further, particularly with the enhancement project called “LIGO-A#”. If funded, as we hope, it will achieve an even more significant increase in the experiment’s sensitivity, while also testing new technologies developed in preparation for the launch of Cosmic Explorer», Fritschel continues.
A similar approach applies to Virgo, which has the “Virgo_nEXT” enhancement project underway, also intended in part to serve as a “laboratory” for the Einstein Telescope.
About Cosmic Explorer, the path for the future U.S. experiment is still in its preliminary phase. «Right now, we are focusing on finding candidate sites that can host a 40-kilometer-long observatory. Some potentially suitable areas have already been identified: in the near future, we will seek to further explore the characteristics of these sites, involving stakeholders and the local community at every step», Kuns explains. «At the same time, we are also working on developing the conceptual design of the detector».
Compared to Cosmic Explorer, ET’s preparatory phase began several years earlier (candidate sites have already been identified, and the conceptual design has been developed, although the experiment’s configuration is still under discussion within the scientific community). However, the construction of the European experiment will likely take more time than the U.S. one, mainly due to the need to dig an underground cavern. «The two observatories will be complementary: it would be ideal if they started operating at around the same time», Kuns continues. «Of course, both Cosmic Explorer and Einstein Telescope will have specific scientific goals that can be pursued separately. However, at the same time, it will be crucial for the new third-generation observatories to operate as a network, sharing data and analyzing events together. This would lead to more accurate scientific results, such as better localization of gravitational wave sources».
The importance of cooperation between the European and U.S. scientific communities has long been recognized: the scientific collaborations working on the LIGO and Virgo gravitational interferometers, initially independent, signed an agreement in 2007 to share all aspects of research, forming a global network of large infrastructures and scientific expertise (recently, the new Japanese detector KAGRA also joined the network of interferometers). However, the future goal is to go even further. «Although they share scientific data, the LIGO, Virgo and KAGRA collaborations are effectively separate communities, with different management and internal rules», Fritschel says. «We are currently working to build a unified entity, called the International Gravitational-Wave Observatory Network (IGWN), which will more uniformly oversee the operations of the detectors and the analysis of the data. If this effort is successful, we expect that ET and Cosmic Explorer would naturally be part of this network, indeed forming its backbone in the future».
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Featured image: Artist’s concept of Cosmic Explorer, produced by Cal State Fullerton students Edward Anaya, Virginia Kitchen, and Angela Nguyen.