Scientists around the globe are preparing for the upcoming LISA mission, which will launch a detector into space capable of discovering gravitational waves emanating from merging galaxies and colliding supermassive black holes. Illinois Physics is contributing expertise in theoretical astrophysics, relativity, and advanced computational modeling and data analysis.
Next-generation gravitational-wave detector will orbit the Sun
Sydnee O’Donnell
for Illinois Physics
To call the international Laser Interferometer Space Antenna (LISA) project ambitious is almost an understatement. Funded by the European Space Agency (ESA) in collaboration with NASA, the estimated $1.5 billion mission will extend our gravitational-wave observational capabilities into space, providing an unprecedented view into the universe and shedding light on some of the most profound as-yet-unanswered questions in astrophysics. Scientists around the globe are working to prepare for LISA’s planned launch in the mid-2030s. At Illinois Physics, scientists are preparing for the enormous data LISA will generate by developing the theoretical framework and technology needed to identify the signatures of gravitational waves and their sources.
Gravitational waves are ripples in the fabric of spacetime originating from the universe’s most violent events, such as collisions of black holes or neutron stars. The first direct detection of gravitational waves was achieved in 2015 when Advanced LIGO (the Advanced Laser Interferometer Gravitational-Wave Observatory) detected waves from a pair of merging black holes. This discovery confirmed a major prediction of Einstein’s theory of general relativity and opened a new era in physics and astronomy. Since then, ground-based detectors in the LIGO-Virgo-KAGRA collaboration have detected numerous gravitational-wave events, providing insights into the properties and behaviors of black holes and neutron stars.
The LISA mission will deploy three spacecraft positioned precisely 2.5 million kilometers apart, forming an equilateral triangle that will trail Earth in its orbit around the Sun. Each spacecraft will emit and receive laser beams from the others, detecting minute changes in distance caused by passing gravitational waves. The instrumentation build is set to begin in January 2025.
Why put a detector in space? Illinois Physics Professor Helvi Witek, co-chair of the LISA Waveform Working Group from 2018–2024 and in this role co-lead author of a white paper, explains, “Ground-based instruments have detected gravitational waves in the few hundred hertz range, produced by neutron stars or light black holes. LISA, being in space, will be sensitive to millihertz frequencies and will allow us to detect waves from supermassive black holes, the kind that live in the center of galaxies and are a hundred thousand to a million times as massive as our Sun.”
The complexity of the signals being measured and the sheer volume of data being collected mean that data processing and analysis will be enormous undertakings requiring sophisticated algorithms and computing techniques. Witek and her team at Illinois are already applying their specialized understanding of relativity and their expertise in generating numerical relativistic models toward LISA’s scientific goals.
Witek’s group is generating complex computer simulations that will serve as templates for interpreting signals in the LISA data. To achieve this, they are simulating the dynamics of the inspiral and merger of binary black holes of varied masses and the gravitational waves their collision emits, as described by general relativity or its extensions. By creating highly accurate source-specific models, the Witek group aims to provide a comprehensive template bank for interpreting LISA data. Once LISA is operational, these templates will enable scientists to identify the sources of gravitational waves—supermassive black holes or other violent cosmic phenomena—and to determine their properties, including mass, spin, and distance from Earth. The comparison of computational models to LISA’s physical observations will provide a test of Einstein’s theory of general relativity and will give LISA researchers the opportunity to explore dense dark-matter environments.
THE LISA MISSION
The LISA mission consists of three spacecraft orbiting the Sun in a triangular configuration. The three satellites are separated by a distance of 2.5 Mio km. Ground-based detectors are limited by their sensitivity to high-frequency gravitational waves, typically in the range of tens to thousands of hertz. LISA’s position in space will allow it to detect much lower-frequency waves, in the millihertz range, which are produced by different types of astrophysical events, including the inspiral and merger of supermassive black holes. LISA’s observations will complement those made by ground-based detectors, providing a more complete picture of the gravitational- wave background (GWB)—the constant rippling of spacetime.
Image courtesy of the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) / Milde Marketing Science Communication / Exozet Effects
Illinois Physics Professor Nicolás Yunes, director of the Illinois Center for Advanced Studies of the Universe (ICASU), recently served as co-chair of LISA’s Fundamental Physics Working Group. In this role, he co-led the writing of a white paper, published in the journal Living Reviews in Relativity, on our ability to test general relativity and extract fundamental physics from future LISA data.
Yunes’s team at Illinois is leveraging its expertise in gravitational physics—within and beyond Einstein’s theory—and its proficiency in analytical and semi-numerical methods to help achieve LISA’s goals. The group uses post-Newtonian theory and black-hole perturbation theory to create highly precise models of the inspiral and merger of coalescing binaries and the gravitational waves they emit. His group is also developing sophisticated data-analysis tools to study synthetic LISA data and predict the physics that future LISA data will demonstrate.
Recently, the Yunes group has focused on making gravitational-wave models more realistic by accounting for the astrophysical environments in which binaries exist. This work will minimize the introduction of systematic errors that would be introduced by incorrect models.
Yunes comments, “Our ability to extract robust science from the data hinges on being able to model realistic signals and control systematic errors. I first became interested in these problems about two decades ago, and we have made a lot of progress toward this objective.”
Illinois Physics and Astronomy Professor Stuart Shapiro and his group, as well as Illinois Physics Research Professor Antonios Tsokaros, were the first at Illinois to join the LISA collaboration. They are members of several LISA working groups, including LISA’s Theoretical Astrophysics Working Group. Additionally, a new faculty member, Illinois Physics Professor Hector Okada da Silva, an expert on black holes and gravitational-wave science, will soon form his research group and will contribute to the Illinois LISA effort.
Shapiro is a pioneer in the physics of compact objects (black holes, white dwarfs, and neutron stars). He helped to develop the tools of numerical relativity that enable scientists to solve Einstein’s equations of general relativity computationally and to simulate the exotic cosmic scenarios that LISA hopes to detect. Shapiro’s research group was among the first to simulate the merger of binary compact objects and the subsequent generation of gravitational waves.
Shapiro comments, “The gravitational waves LISA will measure may provide clues to the type of environment in which really big black holes reside and help us solve the mysteries of exactly how and when supermassive black holes form—what are their seeds and how do they grow? LISA may also help us unlock the nature of ever-present dark matter, which may affect the mergers and resulting waveforms. Most importantly, the waves we measure will test our fundamental theory of relativistic gravitation—that is, general relativity—as well as our ideas about cosmological structure formation. It is all very exciting.”
The LISA experiment is perhaps most exciting to early-career physicists. Witek’s postdoctoral research associate Deborah Ferguson chairs the LISA Early-Career Scientist Group. She emphasizes the mission’s significance for up-and-coming gravitational-wave scientists.
Ferguson says, “Given its timeline, the adoption of LISA is particularly important for early-career scientists in the field. Many current graduate students and postdocs will be forming their own research groups around the time LISA launches, so this experiment will really define our careers.”
LISA demands innovations
Building and deploying LISA presents numerous technological challenges. One of the most significant is the need for extreme precision in measuring the distances between the spacecraft. The laser interferometry technique used by LISA must detect changes in distance as small as a few picometers—less than the diameter of an atom—over a baseline of 2.5 million kilometers. This requires incredibly stable and precise instrumentation.
Additionally, the spacecraft must maintain formation with a high degree of accuracy, despite the influences of solar radiation, gravitational forces, and other environmental factors in space. Advanced propulsion and navigation systems will be employed to keep the three spacecraft in the correct positions relative to one another.
Another challenge is the transmission of data from the spacecraft to Earth. Given the vast amount of data that will be collected, sophisticated data processing and compression techniques will be necessary to ensure that the most critical information is transmitted efficiently. Ground stations on Earth will have to be equipped with advanced receivers to handle the data flow and to integrate it into the scientific analysis.
A rigorous threshold of evidence must be met before a gravitational wave can be considered discovered. The process includes cross-verifying signals with multiple templates, ensuring that the detected signals are not contaminated by instrumental noise, and controlling other systematics introduced by astrophysical environments or other artifacts.
Research in the Helvi Witek group related to the LISA Mission is supported by the National Science Foundation (NSF) under Grant Nos. PHY-2409726, OAC-2004879, and OAC-2411068. Research in the Shapiro group related to the LISA Mission is funded by the NSF under Grant Nos. PHY-2308242 and PHY-2006066. Research in the Yunes group related to the LISA Mission is supported by NASA under Grant No. 80NSSC22K0806 and by the Simons Foundaton through a grant shared with Brown University. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the funding agencies.