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Scientists first detected ripples in space known as gravitational waves from the merger of two black holes in September 2015. This discovery marked the culmination of a 100-year quest to prove one of Einstein’s predictions.
Two years after this watershed moment in physics came a second late-summer breakthrough in August 2017: the first detection of gravitational waves accompanied by electromagnetic waves from the merger of two neutron stars.
Gravitational waves are exciting to scientists because they provide a completely new view of the universe. Conventional astronomy relies on electromagnetic waves – like light – but gravitational waves are an independent messenger that can emanate from objects that don’t emit light. Gravitational wave detection has unlocked the universe’s dark side, giving scientists access to phenomena never observed before.
As a gravitational wave physicist with over 20 years of research experience in the LIGO Scientific Collaboration, I have seen firsthand how these discoveries have transformed scientists’ knowledge of the universe.
This summer, in 2025, scientists with the LIGO, Virgo and KAGRA collaboration also marked a new milestone. After a long hiatus to upgrade its equipment, this collaboration just released an updated list of gravitational wave discoveries. The discoveries on this list provide researchers with an unprecedented view of the universe featuring, among other things, the clearest gravitational wave detection yet.
What are gravitational waves?
Albert Einstein first predicted the existence of gravitational waves in 1916. According to Einstein’s theory of gravity, known as general relativity, massive, dense celestial objects bend space and time.
When these massive objects, like black holes and neutron stars – the end product of a supernova – orbit around each other, they form a binary system. The motion from this system dynamically stretches and squeezes the space around these objects, sending gravitational waves across the universe. These waves ever so slightly change the distance between other objects in the universe as they pass.
Detecting gravitational waves requires measuring distances very carefully. The LIGO, Virgo and KAGRA collaboration operates four gravitational wave observatories: two LIGO observatories in the U.S., the Virgo observatory in Italy and the KAGRA observatory in Japan.
Each detector has L-shaped arms that span over two miles. Each arm contains a cavity full of reflected laser light that precisely measures the distance between two mirrors.
As a gravitational wave passes, it changes the distance between the mirrors by 10-18 meters — just 0.1% of the diameter of a proton. Astronomers can measure how the mirrors oscillate to track the orbit of black holes.
These tiny changes in distance encode a tremendous amount of information about their source. They can tell us the masses of each black hole or neutron star, their location and whether they are spinning on their own axis.
A neutron star-black hole merger
As mentioned previously, the LIGO, Virgo and KAGRA collaboration recently reported 128 new binary mergers from data taken between May 24, 2023, and Jan. 16, 2024 – which more than doubles the previous count.
Among these new discoveries is a neutron star–black hole merger. This merger consists of a relatively light black hole with mass between 2.5 and 4.5 times the mass of our Sun paired with a neutron star that is 1.4 times the mass of our Sun.
In this kind of system, scientists theorize that the black hole tears the neutron star apart before swallowing it, which releases electromagnetic waves. Sadly, the collaboration didn’t manage to detect any such electromagnetic waves for this particular system.
Detecting an electromagnetic counterpart to a black hole tearing apart a neutron star is among the holy grails of astronomy and astrophysics. These electromagnetic waves will provide the rich datasets required for understanding both the extreme conditions present in matter, and extreme gravity. Scientists hope for better fortune the next time the detectors spot such a system.
A massive binary and clear gravitational waves
In July 2025, the LIGO, Virgo and KAGRA collaboration also announced they’d found the most massive binary black hole merger ever detected. The combined mass of this system is more than 200 times the mass of our Sun. And, one of the two black holes in this system likely has a mass that scientists previously assumed could not be produced from the collapse of a single star.
The most recent discovery announced by the LIGO, Virgo and KAGRA collaboration, in September 2025, is the clearest gravitational wave observation to date. This event is a near clone of the first gravitational wave observation from 10 years ago, but because LIGO’s detectors have improved over the last decade, it stands out above the noise three times as much as the first discovery.
Because the observed gravitational wave signal is so clear, scientists could confirm that the final black hole that formed from the merger emitted gravitational waves exactly as it should according to general relativity.
They also showed that the surface area of the final black hole was greater than the surface area of the initial black holes combined, which implies that the merger increased the entropy, according to foundational work from Stephen Hawking and Jacob Bekenstein. Entropy measures how disordered a system is. All physical interactions are expected to increase the disorder of the universe, according to thermodynamics. This recent discovery showed that black holes obey their own laws similar to thermodynamics.
The beginning of a longer legacy
The LIGO, Virgo and KAGRA collaboration’s fourth observing run is ongoing and will last through November. My colleagues and I anticipate more than 100 additional discoveries within the coming year.
New observations starting in 2028 may bring the tally of binary mergers to as many as 1,000 by around 2030, if the collaboration keeps its funding.
Gravitational wave observation is still in its infancy. A proposed upgrade to LIGO called A# may increase the gravitational wave detection rate by another factor of 10. Proposed new observatories called Cosmic Explorer and the Einstein Telescope that may be built in 10 to 20 years would increase the rate of gravitational wave detection by 1,000, relative to the current rate, by further reducing noise in the detector.
Chad Hanna, Professor of Physics, Penn State
This article is republished from The Conversation under a Creative Commons license. Read the original article.