What does it take to quantify a change in distance smaller than one-ten-thousandth the size of a proton? For the LIGO, Virgo, and KAGRA (LVK) scientists, it is the secret to opening some of the universe’s most extreme phenomena and this year, that accuracy has given rise to findings that contradict the very simulations of how black holes are created and act.
From May 24, 2023, to January 16, 2024, the LVK network observed 128 new binary mergers, at least doubling the known catalog. Among them was an unusual neutron star–black hole combination: a comparitively light black hole, between 2.5 and 4.5 solar masses, consuming a neutron star of 1.4 solar masses. In these kinds of systems, theory suggests that the neutron star can be ripped apart before swallowing, releasing a flash of electromagnetic radiation. No such light was observed this time, but the event highlights the promise of “multi-messenger” astronomy to study matter in extreme gravity.
The same run of observations also detected the most massive binary black hole merger ever observed. The first was the GW231123. It created a resulting black hole of approximately 225 solar masses the highest so far, beating the previous high of 140. The progenitors themselves, both around 100 and 140 solar masses, had spin rates near the theoretical maximum established by Einstein’s general relativity. “This is the most massive black hole binary we’ve observed through gravitational waves, and it presents a real challenge to our understanding of black hole formation,” Mark Hannam of Cardiff University. Our standard models of stellar evolution are unable to create black holes this large; an alternative is that they are themselves the result of previous mergers, grown in environments like nuclear star clusters or active galactic nuclei.
The technical achievement of picking up GW231123 was daunting. The signal lasted for a tenth of a second and needed sophisticated search algorithms to filter out background noise. “Without both detectors, we would have missed it,” said Surabhi Sachdev of the School of Physics, about LIGO’s Hanford, Washington, and Livingston, Louisiana, twin observatories. Its shortness and atypical properties strained current waveform models to the extreme, presenting a unique chance for improving theoretical instruments for analyzing very spinning, massive systems.
The LVK reported in September 2025 what scientists call the most discernible gravitational wave signal on record, GW250114. A near twin of the initial detection in 2015, it concerned two black holes some 1.3 billion light-years apart, each 30 to 40 solar masses. But ten years of engineering breakthroughs from quantum squeezing to better mirror coatings have diminished detector noise so much that the new signal was three times more distinct than the old one. “We can hear it loud and clear, and that lets us test the fundamental laws of physics,” said Caltech physicist Katerina Chatziioannou.
That accuracy enabled the team to pin down the “ringdown” phase the last quivering of the unified black hole to unprecedented precision. They also measured two different gravitational-wave modes, as predicted by the Teukolsky formalism introduced in 1972. The data also gave the most stringent observational test to date of Stephen Hawking’s area theorem for black holes. The first black holes together comprised a surface area of 240,000 square kilometers; the surface area of the last black hole was approximately 400,000 square kilometers. The rise validated that, as Hawking and Jacob Bekenstein theorized, black hole mergers bring about higher entropy, following thermodynamic-like principles.
The LVK’s instruments, which use laser interferometry to detect distortions in spacetime, remain the most precise rulers ever built. L-shaped arms up to 4 kilometers long bounce laser beams between mirrors, measuring changes in length as small as 10⁻¹⁸ meters. These minuscule shifts encode the masses, spins, and orbital dynamics of the colliding objects. Extracting such detail requires not only extreme mechanical stability but also innovations in quantum optics and data analysis.
As the fourth observing run runs through November, researchers anticipate more than 100 further detections within the year. By the year 2030, the number of detected binary mergers could be close to 1,000, particularly with anticipated facilities such as LIGO India, the proposed 40-kilometer Cosmic Explorer, and Europe’s Einstein Telescope. Every advance takes gravitational-wave astronomy further into the universe, with the potential to unveil events dating back to the earliest epochs of black hole creation.
