Celebrating 10 Years of Gravitational-Wave Discoveries

By: Colin Stuart September 12, 2025 0

The LIGO gravitational-wave detector celebrates its 10th birthday with the clearest signal yet from a pair of merging black holes.

This plot shows two similar gravitational-wave signals recorded by the LIGO Hanford detector almost 10 years apart, one in 2015 and one in 2025. Both events involve colliding black holes about 1.3 billion light-years away with masses between 30 to 40 times that of our Sun. The data (purple line) includes both the signal and noise, while the green line overlaid shows the best match predictions from general relativity. The noise comes from a variety of sources, including seismic motions that jiggle giant mirrors inside LIGO; the signal is much clearer noawadays, thanks to cutting-edge improvements made to the LIGO detectors.
*LIGO / J. Tissino (GSSI) / R. Hurt (Caltech-IPAC)*

Astronomers have used the Laser Interferometer Gravitational-wave Observatory (LIGO) to detect the clearest-ever gravitational-wave signal from colliding black holes. Released to coincide with the 10th anniversary of the first LIGO signal, these new measurements also confirm key predictions made by physicists Roy Kerr and Stephen Hawking.

Gravitational waves are ripples in the very fabric of the universe itself. Predicted by Einstein in 1915, it took a full century to find them. Since the first discovery in 2015, the floodgates have opened — LIGO now observes a black hole merger every three days.

Much has changed over the last decade, with LIGO now four times as sensitive as it was then. Improvements have allowed astronomers to look at new mergers with sharper eyes. As an example of this enhanced sensitivity, the LIGO team investigated an event seen as part of LIGO’s fourth observing run, in January 2025. The signal is known as GW250114.

“This specific collision involved two black holes that looked pretty much identical to the first two we saw,” says team member Maximiliano Isi (Columbia University). “Intrinsically, the signal is equally loud, but our detectors are just so much more high-fidelity now.”

This video compares the same two gravitational-wave signals. The visuals illustrate the increase in gravitational wave frequency as the two black holes spiral together. The data have also been converted to audio frequencies, so we can hear these cosmic collisions. The video plays each detection twice, with the first round at the original frequencies, and the second round at a pitch 30% higher, making it easier for us to hear the low “whoosh” of rippling spacetime that rises out of the background static. The background behind the more recently detected event, GW250114, is hushed compared to the 2015 signal.
LIGO / Derek Davis (URI)

LIGO's increased sensitivity enabled researchers to measure a key property of the single black hole created by the merger: ringdown. “The ringdown is what happens when a black hole is perturbed, just as a bell rings when you strike it,” says team member Katerina Chatziioannou (Caltech). Except these oscillations are in space and time. By analyzing the ringdown, the team found that the mass and spin of the new black hole was consistent with the solutions to Einstein's equations that Roy Kerr found 60 years ago. Those results appear in Physical Review Letters.

“The observations suggest that Einstein was right, yet again,” says Nils Andersson (University of Southampton, UK), who was not involved in the research. “It is in many ways remarkable that his 100-year-old theory keeps passing ever more precise experimental tests.”

The team used the data to confirm that the total surface area of the black hole’s event horizon increased, in line with a prediction made by Stephen Hawking. The black hole itself is a singularity in space and time, but its mass determines the size of its event horizon — the sphere around the singularity from which nothing, not even light, can escape. Hawking’s rule, which states that the event horizon area can only increase, is often called the Second Law of Black Hole Mechanics, mirroring the Second Law of Thermodynamics that says entropy can only increase.

In the case of GW250114, the initial black holes had event horizons with a total surface area of 240,000 square kilometers (90,000 square miles, or roughly the size of Oregon). The area of the final black hole’s horizon was about 400,000 square kilometers (roughly the size of California).

Gravitational-wave explainer — *Lucy Reading-Ikkanda / Simons Foundation*

Hawking once telephoned Kip Thorne to ask him whether LIGO could help prove his second law. "If Hawking were alive, he would have reveled in seeing the area of the merged black holes increase," Thorne says.

Astronomers performed a similar test in 2021, using the initial signal from 2014, but they were only 95% sure that Hawking’s theorem held. Now, thanks to LIGO's increased sensitivity, that certainty has risen to 99.999%.

There should be more landmark observations to come, particularly when LIGO observations are combined with those of the Virgo and KAGRA detectors (which were both offline for GW250114).

Plot showing discoveries versus mass for four observing runs — This chart plots the distance versus detection date for gravitational-wave events discovered in LIGO's four observing runs so far. During the current, fourth science run, the LVK detectors have spotted about 220 mergers, which more than doubles the number found in the first three runs combined. The closest event observed to date, shown in Run 2 and indicated by the down arrow, is a binary neutron star merger known as GW170817, located only 130 million light-years away. The total masses of the initial objects are represented by size, while the signal strength is indicated by color. Over time the gravitational-wave observatories are both finding *more* black holes and detecting them with higher signal-to-noise. Note that the black hole detections in the latter half of the fourth run are grey and appear to be the same size because these data have not been released in full, with the exception of the event called GW250114 (bright orange dot).
*LIGO / Caltech / MIT / R. Hurt (IPAC)*

However, one area where the project has struggled is with mergers involving neutron stars.

There was one landmark event in 2017, when LIGO and Virgo spotted two neutron stars merging. That signal was among the first multi-messenger astronomical objects, with the detection of both light and gravitational waves. “It established that gamma-ray bursts originate from neutron star mergers, that they also create a large fraction of the heavy elements like gold and platinum in the universe, placed some constraints on matter at extreme densities and even tested the rate of expansion of the universe,” says Andersson. “But since then, we have only seen one much weaker event of this kind.”

If future neutron star observations provide anywhere near the level of detail as these recent black hole measurements, more cosmic mysteries are likely to be solved.