In September 2015, scientists made a groundbreaking discovery by detecting gravitational waves resulting from the merger of two black holes. This momentous event marked the culmination of a 100-year quest to validate one of Albert Einstein’s predictions. Two years later, in August 2017, a second milestone was achieved with the first detection of gravitational waves accompanied by electromagnetic waves from the merger of two neutron stars, further enriching our understanding of the universe.
Gravitational waves have revolutionized the field of astronomy by offering a new perspective of the cosmos. Traditional astronomy primarily relies on electromagnetic waves, such as light. In contrast, gravitational waves serve as an independent messenger, allowing scientists to observe celestial phenomena that do not emit light. This groundbreaking capability has enabled researchers to access the "dark side" of the universe, unveiling phenomena that have eluded observation until now.
As a gravitational wave physicist with over 20 years of experience within the LIGO Scientific Collaboration, I have witnessed firsthand how these discoveries have transformed our understanding of the universe. Recently, in the summer of 2025, the collaboration, which includes LIGO, Virgo, and KAGRA, marked another significant achievement by releasing an updated list of gravitational wave discoveries after an extensive equipment upgrade. This list features an unprecedented view of the universe, including the clearest gravitational wave detection to date.
The existence of gravitational waves was first predicted by Einstein in 1916, as part of his theory of gravity known as general relativity. This theory posits that massive and dense celestial objects bend space and time. When these objects, such as black holes and neutron stars, orbit each other, they form a binary system. The dynamic motion of these systems stretches and compresses the space around them, sending gravitational waves through the universe. As these waves pass, they induce slight changes in the distance between other objects.
Detecting gravitational waves requires exceptional precision in measuring distances. The LIGO, Virgo, and KAGRA collaboration operates four gravitational wave observatories: two LIGO observatories located in the United States, the Virgo observatory in Italy, and the KAGRA observatory in Japan. Each detector features L-shaped arms spanning over two miles, with cavities filled with reflected laser light that precisely measure the distance between mirrors. When a gravitational wave passes, it alters the distance between these mirrors by an astonishing 10-18 meters—equivalent to just 0.1% of a proton's diameter. By measuring the oscillations of these mirrors, astronomers can track the orbits of black holes and gather vital information about their masses, locations, and rotational dynamics.
Recently, the LIGO, Virgo, and KAGRA collaboration reported 128 new binary mergers from data collected between May 24, 2023, and January 16, 2024, effectively more than doubling previous counts. Among these findings is a neutron star-black hole merger involving a relatively light black hole, with a mass between 2.5 and 4.5 times that of our Sun, paired with a neutron star of 1.4 solar masses. In this scenario, it is theorized that the black hole tears the neutron star apart before consuming it, potentially releasing electromagnetic waves. Unfortunately, no electromagnetic counterpart was detected for this specific merger, but the search for such events remains a top priority in astronomy and astrophysics.
Another remarkable discovery announced in July 2025 was the identification of the most massive binary black hole merger ever recorded, with a combined mass exceeding 200 times that of our Sun. Notably, one of the black holes in this system challenges previous assumptions about stellar collapse. In September 2025, the collaboration unveiled the clearest gravitational wave observation to date, which closely resembled the first detection made a decade earlier. Thanks to advancements in LIGO’s detectors, the signal was three times clearer than the initial discovery, confirming that the newly formed black hole emitted gravitational waves consistent with Einstein's general relativity.
This recent discovery also demonstrated that the surface area of the resulting black hole was greater than the combined surface areas of the initial black holes, suggesting an increase in entropy—an essential concept in thermodynamics introduced by Stephen Hawking and Jacob Bekenstein. This finding indicates that black holes adhere to laws similar to those governing thermodynamics, where all physical interactions are expected to heighten the disorder of the universe.
The ongoing fourth observing run of the LIGO, Virgo, and KAGRA collaboration is set to continue until November 2025. My colleagues and I anticipate over 100 additional discoveries in the coming year. Looking ahead, new observations starting in 2028 could potentially increase the tally of binary mergers to nearly 1,000 by 2030, contingent on continued funding for this groundbreaking research.
