Gravitational waves are ripples in spacetime created when a massive object undergoes accelerated motion. The genius physicist Albert Einstein predicted their existence in his general theory of relativity in 1915, but they were first observed a century later in 2015. Since then, astronomers have opened a new era of gravitational wave astronomy, observing phenomena that were previously difficult to witness, including the merger of two massive celestial bodies such as black holes and neutron stars. An international research team studying gravitational waves has captured the largest recorded black hole merger among those observed to date. Scientists believe the day may come in the next few years when modern astronomical theories regarding black hole formation will need to be revised.
The LIGO-VIRGO-KAGRA collaboration, an international group of scientists detecting gravitational waves, announced that during the 24th General Relativity and Gravitation International Conference (GR24) and the 16th Edoardo Amaldi Gravitational Waves Conference held in Glasgow, Scotland from 14th to 18th (local time), they detected the heaviest black hole merger since observing gravitational waves at the two LIGO observatories in the U.S. in November 2023.
The LIGO observatory that detected the heaviest black hole collision this time is based in Livingston, Louisiana, and Hanford, Washington. Operated under the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT), this facility first directly detected gravitational waves, waves in spacetime, on September 14, 2015, rewriting the history of astronomy. The gravitational waves were produced during the merger of two black holes, each weighing 30 times the mass of the Sun, resulting in a colossal black hole weighing 62 times the mass of the Sun. After this first gravitational wave observation, LIGO researchers have carried out joint observations with Italy's Virgo detector and Japan's KAGRA detector, which have been set up to measure the minute distortions in spacetime caused by violent cosmic events such as black hole collisions.
This signal, named 'GW231123', was captured during the fourth observation (O4) period of the LIGO-VIRGO-KAGRA network, conducted on November 23, 2023. The research team detected the phenomenon of two massive black holes colliding in a spiral manner beyond the edge of the Milky Way, generating waves in spacetime.
Analysis of the signal revealed that the newly created black hole after the collision weighs over 225 times the mass of the Sun. The two black holes were found to orbit each other before colliding approximately 2.3 billion to 1.34 billion light-years away from Earth, forming this colossal black hole. The research team commented that "this collision represents the largest scale of black hole mergers in history since the detection of gravitational waves began."
The largest black hole merger to date was the GW190521 event that occurred in 2021. The black hole formed by that collision reached 140 times the mass of the Sun. The black holes involved in this current merger are significantly larger than those previously observed. The pre-collision black holes also exceed 100 times the mass of the Sun. The actual analysis showed that they reached a mass of 103 times and 137 times the mass of the Sun, respectively.
Black holes are usually formed when massive stars die and collapse at the end of their life cycles. These extremely dense celestial bodies warp spacetime so much that not even light can escape. The merged black hole is considered excessively large to have formed directly from collapsing aging stars. Before the collision, the two black holes were found to be rotating at speeds 400,000 times faster than Earth. This contrasts sharply with the mostly slowly rotating black holes that have accompanied previous gravitational wave detections. The research team has indicated that these rapidly rotating, massive black holes are difficult to explain using existing standard stellar evolution models.
Mark Haynam, a professor at Cardiff University and a member of the collaboration, noted, "The masses of these black holes are too large to be considered directly formed from aging stars, leading us to suspect that they were likely formed from smaller black holes merging multiple times."
There is relatively abundant evidence supporting the existence of stellar mass black holes and supermassive black holes that are over 1 million times the mass of the Sun in the universe. However, how supermassive black holes found in the centers of galaxies like our own evolved in the early universe remains an unanswered question. Intermediate-mass black holes, ranging from 100 times to 100,000 times the mass of the Sun, are also rarely found. Scientists believe the characteristics of the speed and mass of the black hole recently captured will provide crucial clues. They have suggested that the intermediate-mass black holes being repeatedly discovered may play an important role in galaxy evolution.
Davide Gerosa, a professor at Bicocca University in Milan, said in an interview with the British science magazine New Scientist, "Ten years ago, I was very surprised that black holes with 30 times the mass of the Sun existed. Now, I’m amazed to see that there are black holes exceeding 100 times the mass of the Sun."
Gravitational waves produced by large, rapidly rotating black holes are more difficult to detect than those produced by small black holes. The merger of the black holes captured this time is exceptionally notable for its large mass and extremely high rotation speed, revealing the limitations of current gravitational wave detection technology and theoretical models.
Charlie Hoy, a professor at the University of Portsmouth, stated, "This black hole is rotating at a speed close to the limits allowed by general relativity," and he added that this indicates the need for new theoretical developments due to the difficulty in modeling and interpreting the signals. Scientists explained that in the future, dramatic observations similar to this one will be needed to explain the complex dynamics of high-speed rotating black holes, which could provide insights into how black holes grow and why they spin so fast.
The LIGO observatory is a device that detects minute changes in spacetime when gravitational waves pass through. The observatory consists of two massive vacuum tubes each 4 km long. There are mirrors at both ends of the vacuum tubes, and a laser beam travels back and forth 300 times, covering about 1,000 km. When a gravitational wave, a ripple in spacetime, passes by, it warps the vacuum tubes, slightly altering the distance over which the laser beam travels, resulting in an interference phenomenon where the waves of the two beams cancel each other out. This fluctuation in the laser detector captures the presence of the gravitational wave.
Before the development of gravitational wave detectors, scientists could only observe the universe using electromagnetic waves such as visible light, infrared, and radio waves. Visible light enables the determination of temperature, mass, brightness, chemical composition, and the shapes and structures of galaxies. Infrared is useful for observing celestial bodies hidden by dust and gas clouds, star formation regions, and celestial bodies within the galactic center that light cannot penetrate. Gravitational waves open a new avenue for exploring celestial bodies like black holes that do not emit light or electromagnetic waves. Observing both gravitational waves and electromagnetic waves together enhances the accuracy of determining the location of the gravitational wave source and allows for more detailed studies of the physical properties of celestial phenomena. Lee Chang-hwan, a physics professor at Pusan National University, remarked in the Horizons journal of the Korea Advanced Institute of Science and Technology in 2018 that "the detection of gravitational waves has ushered in a full-fledged era of multi-signal astronomy."
In fact, with every upgrade of the LIGO detector, more black hole mergers have been discovered. Since the first observations began in 2015, gravitational wave detectors have observed approximately 300 black hole mergers to date. The fourth joint observation started in May 2023, during which more than 200 black hole mergers were recorded.
The Nobel Prize in Physics in 2017 was awarded to Rainer Weiss, a professor emeritus at MIT, Barry Barish, a professor emeritus at Caltech, and Kip Thorne, a professor emeritus at Caltech for their contributions to the first detection of gravitational waves. Scientists expect that gravitational wave detectors will set more records, including even larger black hole collisions than those currently detected. Professor Haynam expressed, "We expect to observe all black hole merger phenomena in the universe within 10 to 15 years," and added, "We may even discover unexpected cosmic phenomena."
Recent research on gravitational waves has become a testing ground for scientific diplomacy and advanced industrial technologies. Currently, more than 1,600 scientists from around the world are participating in observations and analyses in the LIGO collaboration. The Virgo collaboration, centered around the European Gravitational Observatory (EGO), has around 880 scientists from 152 institutions across 17 countries. The Virgo detector is located near Pisa, Italy. KAGRA is a 3 km long laser interferometer installed in Kamioka, Gifu Prefecture, Japan, with approximately 400 scientists from 128 institutions in 17 countries involved in the collaboration. While Korea does not have a standalone observatory, various research institutions and universities, including the Korea Astronomy and Space Science Institute (KASI), the Korea Advanced Institute of Science and Technology, and Ewha Womans University, participate in the LIGO, Virgo, and KAGRA collaborations.
Gravitational wave astronomy is becoming a testing ground for advanced optics and laser technology, precision measurement, and metrology techniques used in communications, manufacturing, and healthcare. Gravitational wave observatories implement advanced vibration damping and ultra-high vacuum technologies that are unaffected by earthquakes or environmental noise. Recently, the introduction of data analysis techniques, machine learning, and artificial intelligence (AI) technologies to process the vast amount of information generated from gravitational wave detection has drawn renewed attention to their economic value.
Concerns have been raised within the scientific community regarding the U.S. government's announcement in May that half of the LIGO facility would be shut down. The international journal Nature reported that closing half of the LIGO facility, which has yielded the most achievements in gravitational wave detection, could effectively make it nearly impossible to detect new signals.
References
arXiv(2025), DOI : https://doi.org/10.48550/arXiv.2507.08219
Nature(2025), DOI : https://doi.org/10.1038/d41586-025-02212-7