An international team of scientists, including Northwestern University astrophysicists, has detected 128 new gravitational-wave candidates - more than doubling the size of the current catalog.
Within the new dataset, the more unusual signals include the heaviest black hole binary detected to date, a binary where both black holes have exceptionally high spins (rotating at nearly 40% the speed of light) and an unusually lopsided pair of black holes, with one object twice as massive as the other. The catalog also holds two black hole-neutron star binaries.
Scientists detected these ripples in space-time with a global network of gravitational-wave observatories: the U.S.-based National Science Foundation Laser Interferometer Gravitational-Wave Observatory (NSF LIGO), the Virgo interferometer in Italy and the Kamioka Gravitational Wave Detector (KAGRA) in Japan.
The latest compilation of gravitational-wave detections will appear in a forthcoming special issue of The Astrophysical Journal Letters.
"Each new gravitational-wave detection adds another data point to our map of the universe's most extreme objects," said Northwestern astrophysicist Vicky Kalogera, a senior member of the LIGO Scientific Collaboration (LSC). "As our catalog grows, we're beginning to move from individual discoveries to seeing patterns begin to emerge. Those patterns are helping us understand how black holes and neutron stars form, evolve and merge throughout the cosmos."
An expert in gravitational-wave analyses, Kalogera is the Daniel I. Linzer Distinguished University Professor of Physics and Astronomy at Northwestern's Weinberg College of Arts and Sciences, director of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the NSF-Simons National AI Institute for the Sky (SkAI Institute).
A long history at Northwestern
When the densest objects in the universe collide and merge, the violence sets off ripples in the form of gravitational waves, which reverberate across space and time over hundreds of millions or even billions of years. By the time they pass through Earth, such cosmic ripples are barely discernible. Together, the LIGO-Virgo-KAGRA (LVK) observatories "listen" for faint wobbles in the gravitational field that could have originated from far-off astrophysical smash-ups.
Even before LIGO detected the first gravitational-wave signals in 2015, Northwestern was already playing a major role in the development of gravitational-wave astronomy. Kalogera's group helped build the theoretical framework for understanding the kinds of cosmic collisions that produce gravitational waves. Since then, Northwestern researchers have continued to analyze gravitational-wave events, leveraging them to understand how black holes form, how binary systems evolve and how these extreme objects shape the universe.
Five Northwestern researchers contributed to LVK's Gravitational-Wave Transient Catalog-4.0 (GWTC-4), which comprises detections of gravitational waves from a portion of the observatories' fourth and most recent observing run between May 2023 and January 2024. Those researchers are:
- Darsh Bellie, an NSF Graduate Research Fellow in physics and astronomy at Weinberg, helped analyze the astrophysical implications of the most massive binary black hole merger in GWTC-4.
- Debatri Chattopadhyay, a postdoctoral associate at CIERA, was a member of the core writing team, which structures, interprets and writes the scientific paper. In this role, she was responsible for determining the astrophysical implications of the new findings. Chattopadhyay also co-led the writing of the paper's summary, which distilled the main scientific results into a clear narrative for a general, non-expert audience.
- Shanika Galaudage, a CIERA-Adler postdoctoral fellow, contributed to the population analysis for GWTC-4, specifically determining which model best describes the growing population of binary black hole mergers observed so far. She also helped review the analyses in the scientific paper.
- Ish Gupta, an N3AS postdoctoral fellow at CIERA, used the updated sample of events to test Albert Einstein's theory of general relativity, which describes gravity as a geometric property of space and time. With the new dataset, Gupta and his collaborators placed the most stringent constraints on the theory to date. He also helped estimate the parameters that characterize binary systems, including the masses and spins of black holes.
- Anarya Ray, a CIERA postdoctoral fellow, helped analyze candidate signals to determine the probability that each one came from a real astrophysical event. He also worked on population studies that reveal patterns among merging black holes and neutron stars, helping scientists understand how these extreme systems form.
A growing, diverse population
Colliding black holes are the source of many gravitational waves. Such "bread-and-butter" binaries typically consist of two black holes of similar size - usually several tens of times more massive than the sun - that merge into one larger black hole. Gravitational waves also can be produced by the collision of a black hole with a neutron star, which is an extremely dense remnant core of a massive star, or by the collision of two neutron stars.
In its first three observing runs, the LVK observatories detected signals from a handful of collisions involving a black hole and a neutron star as well as two collisions between neutron stars.
The newest detections reveal a greater diversity in the binary systems that produce gravitational waves. The updated catalog includes the heaviest black hole binary, in which each object is about 130 times the mass of the sun; a binary with black holes of asymmetrical, lopsided masses; and a binary where both black holes have exceptionally high spins.
The expanding number of samples has also allowed scientists to test Einstein's general theory of relativity. Scientists put Einstein's theory to the test using a newly detected merger, which provided one of the "loudest" gravitational-wave signals observed to date. The surprisingly clear signal gave scientists a chance to probe it in detail, looking for any aspects of the signal that might deviate from Einstein's predictions. This signal pushed the limits of their tests of general relativity, mostly passing with flying colors but illustrating how environmental noise can challenge others in such an extreme scenario.
Next, scientists will use these new detections to start making connections about the properties of black holes as a population. They will also use this new information to gauge how fast the universe is expanding, which remains a long-enduring mystery in cosmology.
The study is titled "GWTC-4.0: An introduction to version 4.0 of the gravitational-wave transient catalog."
