On Wednesday, 7 October, the Geometry Comparison Report (GCR), which analyses the advantages and disadvantages of the possible configurations of the Einstein Telescope (ET) detector, providing an assessment independent of the candidate sites, was submitted to the Einstein Telescope Board of Governmental Representatives (BGR). The BGR is the body responsible for managing relations between the governments of the countries involved in the ET collaboration and the funding agencies and, above all, for making the final decision on the allocation of the experiment.
The report examines two possible geometries for ET: a single triangular-shaped detector, with sides approximately 10 kilometres long, to be built at a single site, and two twin L-shaped interferometers, with 15-kilometre-long arms, to be built at two different sites at least 1,200 kilometres apart.
The document submitted to the BGR, whose executive summary is available at this link, examines five key areas for comparing the two geometries: scientific added value, costs, risk analysis, governance, and socio-economic impact. Taking all the aspects examined in the report into account, the 2L configuration emerges as the best option.
«The Geometry Comparison Report is an extraordinarily important document for the Einstein Telescope community and a fundamental milestone on the path towards its realisation», says Antonio Zoccoli, President of the Italian National Institute for Nuclear Physics (INFN). «For the first time», Zoccoli continues, «we have a comprehensive, robust and shared comparative framework that provides a solid basis for moving beyond a phase of competition between the two configurations and entering a new phase of collaboration, focused on studying the double-L configuration. I would like to thank everyone who contributed to the report, bringing their expertise, rigour and collaborative spirit to the work, as well as the Einstein Telescope Organisation for effectively coordinating this complex effort».
The report was prepared by around 200 experts from more than 20 research institutions, with specific expertise in the different areas covered by the analysis, coordinated by the Einstein Telescope Organisation (ETO), the organisational structure responsible for coordinating the preparatory work for the construction of ET.
Regarding the scientific aspects, the working group, led by Alessandra Buonanno, Director of the Max Planck Institute for Gravitational Physics in Potsdam, Germany, defined a set of science golden channels to assess each of the proposed geometries. The findings show that the two-L configuration meets the requirements of the largest number of key scientific cases.
As for costs of the scientific instrumentation, the report provides a method for estimating them and indicate that the triangular configuration would be more expensive.
The risk analysis, on the other hand, does not identify a preferred geometry. On the one hand, the triangular configuration is more exposed to risks associated with technological challenges and scientific performance, while the two-L geometry is more susceptible to risks relating to governance and organisational issues.
With regard to governance, the report emphasises that the two-site option, associated with the double-L configuration, emerges as the one offering the best balance between opportunities and risks. At the same time, the document highlights that strong coordination and robust central management are essential conditions for effective ET governance, regardless of which option is chosen.
Finally, the analysis of socio-economic impact and sustainability did not identify an option that is universally preferable, as the two geometries have very different operational and risk profiles. In this case, the potential advantages and disadvantages are strongly linked to the geographical area in which the experiment will be built. For this reason, the report recommends that the decision on the geometry should be based primarily on scientific and technological criteria.
Therefore, taking all these aspects into account, the 2L geometry emerges as the most advantageous overall solution. The BGR will now assess the report’s findings and take them into account ahead of the final decision on the allocation of ET, which is expected in the second half of 2027.
There are currently three candidate sites to host ET: the Italian site, in the area surrounding the former Sos Enattos mine in the province of Nuoro; the Meuse-Rhine Euroregion, on the border between the Netherlands, Belgium and Germany; and the Lusatia site, in the German state of Saxony. Italy is a candidate to host ET under both the triangular and the double-L configurations.
«I am pleased that the document has been published, thanks to the dedication and hard work of the entire GCR team. The timeframe and circumstances have been extremely challenging, but the quality of the final result is entirely satisfactory», adds Alessandro Variola, ETO Project Manager. «With a further phase of coordinated work, we could have achieved even better results. Nevertheless, bringing the community together, having the opportunity to gather input from so many internal and external experts, and successfully organising all the information received undoubtedly represent an important step forward for Einstein Telescope».
«We congratulate everyone who contributed to the Geometry Comparison Report through intensive work and strong commitment», concludes Michele Maggiore, spokesperson for the Einstein Telescope Collaboration. «The GCR is an authoritative source of information for the decision on the geometry. Members of the ET Collaboration made significant contributions to all the work packages and, in particular, coordinated the work on Scientific Added Value. The findings confirm and extend the collaboration’s previous studies. We also appreciate the fruitful interaction between the ET Collaboration and the ETO».
Further information is available in the international news published by ETO, accessible at this link.