Interview conducted for the INFN newsletter Particle Chronicle with Marica Branchesi, member of the technical and scientific committee appointed by the Ministry of Universities and Research to promote Italy’s candidacy to host the Einstein Telescope in Sardinia, and a key figure — as coordinator between the LIGO/Virgo gravitational interferometer collaboration and the network of electromagnetic telescopes — in one of the defining chapters in the history of gravitational waves: the first detection of gravitational waves produced by the merger of two neutron stars.
Just over a year after the historic announcement of 11 February 2016 of the first direct detection of gravitational waves, produced by the merger of two black holes, the LIGO and Virgo collaborations announced, on 16 October 2017, another first observation: that of gravitational waves produced by the merger of two neutron stars. Could you explain why that result was so extraordinary and why it marked the birth of multimessenger astronomy?
It was an exceptional event. We were able to observe the final phase of the inspiral of two neutron stars as they orbited one another until they merged. It was a very long signal, lasting about one minute, much longer than those produced by the merger of binary black hole systems. We therefore immediately realised that this was a different kind of event. What was truly extraordinary was that, at the same time as that signal, the FERMI space telescope also recorded a gamma-ray burst, and the INTEGRAL space telescope did the same. At that point, the most extensive observational campaign in human history began. We observed signals across every band of the electromagnetic spectrum: twelve hours after the gamma-ray signal we observed the optical signal, a few days later the X-ray signal arrived, then the radio signal; and, in essence, observations of the region where the merger took place are still continuing. For the first time, we succeeded in combining signals coming from two “cosmic messengers”, gravitational waves and photons, and this had an enormous scientific impact across many fields of research. On relativistic astrophysics, for example, because the observation of the neutron star merger through gravitational waves, followed by the detection of the gamma-ray burst and subsequent observations across almost the entire electromagnetic spectrum, particularly in the radio band, confirmed that a relativistic jet had been produced there, whose structure we were able to understand for the first time. And thanks to ESO (European Southern Observatory) telescopes, we also observed kilonova spectra for ten days, namely the chemical and thermal signature of the material ejected during the merger. These observations explained how and where the heavy elements in the universe are formed. From the intensity and colour evolution of these spectra, it is in fact possible to estimate which elements are formed and in what quantities; gold, to mention one example, is present in quantities ranging between 10 and 100 Earth masses, which are dispersed into the interstellar medium, from which stars and planets, such as the Earth, are formed. The event also had important implications from the point of view of fundamental physics. By combining the gravitational and electromagnetic signals, we began to gain a better understanding of the nature of neutron stars, although estimates of the source parameters are still affected by significant uncertainties. And by combining the distance derived from gravitational waves with the redshift of the host galaxy, we were able to estimate – again with large uncertainties – the Hubble constant, namely the expansion rate of the universe, thereby inaugurating a new way of doing cosmology. Finally, by measuring the time delay between the arrival of the gravitational wave and that of the gamma-ray burst – just 1.7 seconds – we were able to determine the speed at which gravitational waves travel, that enabled us to rule out many modified gravity models, namely models that modify or extend Einstein’s theory of general relativity. So far, the observations are entirely consistent with the theory of general relativity, and it is significant that a single measurement allowed us to rule out so many alternative models.
Which questions remain unanswered? Will the future European gravitational-wave detector, Einstein Telescope, enable us to find answers?
So far, we have witnessed the extraordinary power of gravitational-wave astronomy and multimessenger astronomy in understanding the mergers of binary systems of black holes and neutron stars. However, what we observe still represents only a limited portion of the universe. And, as I mentioned, we still have large uncertainties in estimating the source parameters, which means that many questions remain open concerning the nature and evolution of compact objects throughout cosmic history. With the Einstein Telescope (ET) we will make an enormous leap, comparable to the leap from Galileo’s telescope to the James Webb Space Telescope. ET will allow us to broaden our perspective to the entire observable universe, and to do so with extremely high precision. Thanks to its sensitivity at low frequencies, we will be able to reach higher redshifts and much more distant sources. In other words, we will gain access to the early universe; we will be able to observe not only black holes originating from stars, but also primordial black holes, produced by density fluctuations in the universe. We will be able to understand how populations of stellar-mass black holes are connected to the giant black holes at the centres of galaxies (with masses millions or billions of times greater than that of the Sun), and determine whether the latter are the result of the growth and merger of intermediate-mass black holes. The extremely intense signals produced by these black hole mergers will allow us to subject general relativity to precision tests without precedent, and perhaps discover phenomena that will require a new theory of gravity. But we will not observe only 10⁵ binary black hole signals per year; we will observe 10⁵ neutron star signals, and we also expect to observe entirely new sources, such as core-collapse supernovae and exotic compact objects that we have never seen before. Einstein Telescope will be a revolutionary instrument for fundamental physics, nuclear physics, astrophysics, multimessenger astronomy, cosmology (thanks to much smaller uncertainties and much larger samples), and potentially even for the study of dark matter. Precisely because of its scientific potential, it is already attracting the interest of many different communities: gravitational-wave physicists, astrophysicists, cosmologists and nuclear physicists, for a total of more than 2,000 researchers currently active within the collaboration.
How will Einstein Telescope differ from current interferometers?
What certainly distinguishes Einstein Telescope is the fact that it will be built underground. It will have a so-called “xylophone” configuration, with two separate interferometers, one optimised for high frequencies and the other for low frequencies. The crucial decision to build it underground is mainly linked to the low-frequency interferometer: being underground, at extremely low temperatures thanks to the use of a cryogenic system, is necessary to access frequencies below 10 hertz, down to 2–3 hertz, and therefore the early universe. Clearly, we will also work on the power of the laser beams and on the mirrors. Indeed, an important study has recently used artificial intelligence to improve mirror control, which could have applications for precision instruments other than Einstein Telescope and that are used in other fields. However, the real challenge, both from the technological and scientific point of view, remains precisely the low-frequency interferometer: it is the one that requires the most ambitious developments and has the potential to become a major driver for industry.
Looking further ahead, how will it differ from, and interact with, its contemporaries Cosmic Explorer and LISA?
Cosmic Explorer will be an instrument similar to current detectors, but with greater sensitivity at high frequencies. Unlike Einstein Telescope, it is not designed to reach the lowest frequencies, and precisely for this reason the combination of observations from the two detectors will be extremely interesting: it will make it possible to exploit the strengths of both and increase the overall scientific potential. LISA, by contrast, will operate at extremely low frequencies, inaccessible to ground-based detectors, and will enable observations of sources that Einstein Telescope will not be able to detect, but also of sources accessible to both, such as binary black hole systems with masses of around 30 solar masses. If the two instruments operate simultaneously, LISA could provide advance warning of a future merger that would later be observed by Einstein Telescope, enabling multi-band observations and the determination of the source parameters with unprecedented precision. In this sense, the possibility of combining different observatories opens up an entirely new perspective: not only seeing more sources, but studying them from complementary viewpoints.
Speaking of complementarity, over the coming decades new major observatories will come into operation across almost the entire electromagnetic spectrum. How will Einstein Telescope fit into this ecosystem, and what will be the most promising synergies for multimessenger astronomy?
From the very beginning, we decided to work in close collaboration with the other observatories. Within the Einstein Telescope Observational Science Board, we established a working group dedicated to collaboration with other observatories, and I currently coordinate the activities devoted to multimessenger and multi-band astronomy. This is because, by establishing now which instruments should work together and how they should coordinate, we will be able to exploit the full potential of the entire observational network, achieving an unprecedented scientific return. In the optical, we already have instruments such as the Vera Rubin Observatory in Chile, with its very wide field of view, and the ELT for spectroscopy. Initiatives are also under way such as ESO Expanding Horizons, the programme through which ESO is defining its next major observational infrastructure. This process will, in all likelihood, identify an ideal candidate to operate in synergy with Einstein Telescope. At the highest energies, the distributed telescopes of CTAO (Cherenkov Telescope Array Observatory) will be able to observe together with ET. And in radio astronomy we are also working with SKAO. Our greatest concern at present is the lack of approved plans for a high-energy satellite for 2040 – such as Fermi, Swift or SVOM – which we need in order to carry out high-redshift multimessenger astronomy (thereby observing and localising, for example, gamma-ray bursts).
Einstein Telescope could be built in Sardinia, in the area of the former Sos Enattos mine, in the province of Nuoro. Alongside Italy, Germany, with the Lusatia region in Saxony, and the Meuse-Rhine Euroregion, on the border between the Netherlands, Belgium and Germany, have also put forward candidacies to host the infrastructure. How competitive is the Sardinian candidacy?
A very large number of measurements have been carried out in Sardinia by INFN, INGV (National Institute of Geophysics and Volcanology) and INAF (National Institute for Astrophysics), and the result is unequivocal: the Sardinian site is perfect for hosting Einstein Telescope. Anthropogenic noise is extremely low, magnetic fields are extremely weak, and there is granite, which makes excavation easier than in the other countries. Sos Enattos has truly extraordinary, unmatched qualities for reaching low frequencies, unlike the other two sites. We therefore sincerely hope that the quality of the site will be the decisive criterion in the selection process; quality that is fundamental for achieving the scientific goals we have set ourselves. The Italian community believes so strongly in its site that it is prepared to submit a double proposal – to host the infrastructure both in the double-L configuration and in the triangular configuration – even though it is firmly convinced that the double-L configuration is the better option.
Why are we convinced that the double-L configuration is the best choice?
Numerous studies have been published, and they all converge on the conclusion that two L-shaped interferometers with 15-kilometre arms provide better scientific performance than a triangle with 10-kilometre sides, both in terms of the number of observable events and the precision of the measured parameters. In both respects, estimates indicate an improvement by a factor of between two and three for the double-L configuration compared with the triangular one. One might argue that, given 10⁵ events, a factor of two or three makes little difference; in reality, the opposite is true. The truly revolutionary events, those capable of leading to Nobel Prize-winning discoveries, are by definition very rare; and the same applies to the events that enable the most stringent tests of the theory of gravitation, whose number is limited. We can already see this today with Virgo and LIGO: for the most distant events, in the region being explored by the James Webb Space Telescope, the number of observable sources is small. There, an increase in sensitivity by a factor of two or three makes the difference between being able to detect these sources or not. But it is not only a scientific issue, there is also a technological one. In the triangular configuration there would be six interferometers on the same site, which could be affected by correlated noise, and which are, in general, more difficult to operate. It has also emerged that, with the double-L configuration, it is possible to achieve certain scientific cases relying solely on high-frequency performance, whereas the triangular configuration, to obtain the same results, would also have to achieve the planned low-frequency performance, which represents a greater technological challenge. For all these reasons, we are convinced that the double-L solution is the best one and the one involving the least risk.
To conclude, focusing on the scientific cases: if you had to choose a single event that you dream of observing with Einstein Telescope, what would it be, and why?
Something we cannot even imagine today: an unexpected phenomenon capable of revolutionising our understanding of physics. But I also have other dreams. Multimessenger astronomy will move from being limited to extremely rare events to becoming mainstream. We will be able to associate gamma-ray bursts systematically with gravitational waves, opening a new window onto the physics of the most energetic phenomena in the universe, which still remain among the greatest mysteries in astrophysics. And then there is the early universe, the most distant events. The James Webb Space Telescope is already showing us extraordinary things – stars and galaxies much closer to the Big Bang than we had expected. So I cannot wait to observe the first black holes and understand whether they originated from stars or from the initial density fluctuations of the universe.
Marica Branchesi is a Full Professor of Astrophysics at the Gran Sasso Science Institute, a member of the Accademia dei Lincei and of the Board of Directors of the Italian Space Agency. She is an associate researcher at the National Institute for Nuclear Physics and holds coordinating roles within the Einstein Telescope collaboration. She played a leading role in the discovery of GW170817, which marked the birth of multimessenger astronomy. In 2017, Nature named her one of the ten scientific figures of the year; she has received the Occhialini Prize (2020) and the Into Change Award (2025).