Since January 2026, more than forty members of the Einstein Telescope (ET) Collaboration and Laser Interferometer Space Antenna (LISA) Consortium have joined forces in a Synergy project. The aim of this first official shared project is to create a code that both collaborations can use for joint analyses. Because the LISA and the ET will detect different parts of the gravitational-wave spectrum, combining their analyses can give scientists a fuller picture of the signals and their origins, and better test theoretical models.
Searching for new classes of gravitational waves
Since the first detection of gravitational waves (GWs) a decade ago, gravitational-wave searches have become a valuable addition to the cosmological probes we use to study and unveil the mysteries of the universe. The LIGO/Virgo/KAGRA collaborations have already detected hundreds of GWs produced by mergers of compact objects, giving us a better understanding of the population properties of black holes or neutron stars. But a whole different class of signals is still out there, waiting to be detected. Those signals are produced by processes in the early Universe. Due to the weakness of gravitational interactions they travel freely through the cosmos, carrying information about physics that we cannot get in any other way.
These cosmological GW signals are among the main targets that future GW interferometers like the Laser Interferometer Space Antenna (LISA) and the Einstein Telescope (ET) will try to observe. These two detectors probe different frequency bands. LISA in space will cover the millihertz, while ET down here on Earth will cover the range from unity to thousands of hertz.
Different detectors, different frequency bands
Unfortunately, we do not know in advance at which frequency any specific signal will appear. This depends on the process that generated it which can span many orders of magnitude in energy. Roughly speaking, the frequency of a signal today is tied to the energy scale at which it was produced, which is unknown. This means that different frequency bands correspond to different epochs or periods of time. For example, GW associated with asteroid-mass primordial black holes would peak at LISA’s millihertz band, while the ones related to heavier black holes would produce a signal at smaller frequencies. A signal can appear strong in one band and only a bit in another, or be at its strongest somewhere in between where both experiments cannot detect it. In fact, the signals are not expected to completely appear in one detector’s band or in another. They could show up only partially, with the main contribution being outside of the detector range. So, if two (or more) GW detectors look at similar frequencies, their combined efforts could be crucial.
This is the main reason for our Synergy project, the first official project between two GW communities (ET and LISA). Combining what different detectors see across the bands should make it easier to work out the shape of the background, separate the cosmological part (coming from the early-Universe) from the astrophysical one (produced more recently by astrophysical sources), and potentially tell apart models that would look basically the same through a single instrument.
A step towards shared code for gravitational wave detectors
There is also a practical side to this project. LISA and ET already work on overlapping questions, but the analysis codes and conventions have mostly grown up separately. This makes sharing work across the two communities harder than it should be. One of the goals of the project is to overcome this and build something that both sides can use.
The plan is to create a code that the collaborations can use together to perform a joint analysis between GW detectors, and developed to make it easy to add other detectors to the network, including future ones such as Cosmic Explorer in the US.
The ET-LISA Synergy Project started in January 2026, and has grown into an ambitious global undertaking. The project is coordinated by Antonio J. Iovino (NYU, Abu Dhabi) and Gabriele Perna (KBFI, Tallinn). More than forty people are involved so far from both the ET Collaboration and the LISA Consortium, spanning Europe, the United States, the United Kingdom and Asia and bringing a mix of different expertise to both the theory and coding elements s of the project. Besides the actual coding work, the other main deliverables that will be produced are two scientific papers: one focuses on different cosmological GW signals and how they look across the bands, and the other introduces the code so the scientific community can easily use it.
Preliminary tests already showed the extreme gain in characterising GW signals when more detectors are considered together and we’re excited to explore the wider cosmological implications!
Antonio J. Iovino and Gabriele Perna, Project Coordinators