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Researchers from the University of Glasgow are celebrating the 10th anniversary of one of the 21st century’s key scientific achievements – the first direct detection of gravitational waves.

https://youtu.be/SqhFtkQ4f2c
 
On14 September 2015, a signal arrived on Earth from a pair of remote black holes that had spiralled together and merged. The signal had travelled for about 1.3 billion years to reach us at the speed of light—but it was not made of light. It was a different kind of signal: a ripple in space-time called a gravitational wave, first predicted by Albert Einstein 100 years before. On that day 10 years ago, the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever direct detection of gravitational waves, whispers in the cosmos that had gone unheard until that moment.
 
Glasgow scientists have been involved in gravitational research since the middle of the 20th century. Researchers from the University's Institute for Gravitational Research (IGR) played key roles in the development of the field, including the design and development of the Advanced LIGO detectors, developing the delicate mirror suspensions which make the detections possible by isolating them from external vibrations.
 
Professor Sheila Rowan, director of the IGR, said: “The first signal LIGO picked up came before the detectors had even begun their first official observing run, which was a remarkable gift from nature. As it became clear that this signal was genuine, it caused intense relief and excitement around the world – we knew that our years of hard work and international collaboration had paid off, and that we had a whole new way of probing our universe.”
 
The historic discovery meant that researchers could now sense the universe through three different means. Light waves, such as X-rays, optical, radio, and other wavelengths of light as well as high-energy particles called cosmic rays and neutrinos had been captured before, but this was the first time researchers had witnessed a cosmic event through its gravitational warping of space-time.
 
Today an international network of detectors observe the gravitational Universe together. The two LIGO detectors in the USA, alongside the Virgo gravitational-wave detector in Italy, and KAGRA in Japan. Together known as the LVK (LIGO, Virgo, KAGRA), has captured a total of about 300 black hole mergers, some of which are confirmed while others are still undergoing further analysis. During the network's current science run, the fourth since the first run in 2015, the LVK has discovered about 220 candidate black hole mergers, more than double the number caught in the first three runs.

Image credit: LIGO/Caltech/MIT/R. Hurt (IPAC)

Today, researchers at the IGR and the University's School of Physics & Astronomy continue to work at the forefront of developments at the LVK Collaboration, in partnership with their counterparts around the world. They have helped guide upgrades to the detectors which have sharpened their ability to pick out gravitational wave signals from the background noise of the universe. They have also helped to develop software capable of analysing more efficiently the increased frequency of detections which the upgraded detectors have enabled.
 
“When I joined the group in Glasgow on 14 September 2015, we had just detected the first gravitational wave. By the end of 2015 we’d detected two more. Ten years later we’ve detected well over three hundred, and Glasgow’s become a leader in analysing and interpreting the signals,” said Dr Daniel Williams, who led the development of the collaboration’s recent analysis which doubled the count of known mergers.

The dramatic rise in the number of LVK discoveries over the past decade is owed to several improvements to their detectors—some of which involve cutting-edge quantum precision engineering. LIGO remains by far the most precise ruler for making measurements ever created by humans. The space-time distortions induced by gravitational waves are incredibly miniscule. To sense them, LIGO must detect changes in space-time smaller than 1/10,000 the width of a proton. That's 700 trillion times smaller than the width of a human hair.
 
Professor Sir James Hough has been working on gravitational research at Glasgow since 1972, early in his post-PhD research career. He founded the IGR in 2000 and was knighted in 2019 in recognition of his leading roles in international research collaborations including LIGO and the British-German GEO600 detector which pioneered novel detector technologies later adopted by and used in the Advanced LIGO observatories.
 
He said: “It’s wonderful to be celebrating the 10th anniversary of the first detection, and to still be involved in the expanding field of gravitational wave astronomy. In the 1970s, a prototype aluminium bar detector we built here in Glasgow picked up a signal we thought might well have been evidence of a gravitational wave, but we weren’t able to verify it at the time. Having that experience helped keep me going through decades of work with colleagues around the world to build much more sophisticated detectors. It’s fantastic to be able to look back on nearly 300 confirmed detections and realise that our perseverance has paid off so well.”

LIGO's improved sensitivity is exemplified in a recent discovery of a black hole merger referred to as GW250114 (the numbers denote the date the gravitational-wave signal arrived at Earth: 14 January 2025).
 
“This event was really reminiscent of our first detection in September 2015; both involved a pair of black holes with masses somewhere between 30 and 40 times as heavy as the Sun colliding with each other around 1.3 billion light-years from the Earth. The difference, however, is that almost a decade of technological improvements mean that our detectors could make a much clearer detection of this year’s signal”, said the IGR’s Dr John Veitch, who contributed to the design of one of the analyses used on the signal.
 
“The new signal was detected so clearly by the detectors in January that it gives us a way to make very fine measurements and test our fundamental understanding of physics,” said Dr Rachel Gray at the IGR.
 
The LVK team was able to provide the best observational evidence captured to date for what is known as the black hole area theorem, an idea put forth by Stephen Hawking in 1971 that says the total surface areas of black holes cannot decrease.
 
“When the two black holes collide we can measure the mass of the merged black hole, which tells us directly about its surface area. The new signal confirms Hawking’s area theorem which says the surface area must always increase”, said Professor Martin Hendry, who was head of the School of Physics & Astronomy at the time of the first detection and now serves as the University of Glasgow’s Clerk of Senate.
 
Later, Hawking and physicist Jacob Bekenstein concluded that a black hole's area is proportional to its entropy, or degree of disorder. The findings paved the way for later groundbreaking work in the field of quantum gravity, which attempts to unite two pillars of modern physics: general relativity and quantum physics.

Image credit: Aurore Simonnet (SSU/EdEon)/LVK/URI
 
In essence, the LIGO detection allowed the team to ‘hear’ two black holes growing as they merged into one, verifying Hawking's theorem. (Virgo and KAGRA were offline during this particular observation.) The initial black holes had a total surface area of 240,000 square kilometers (roughly three times the size of Scotland), while the final area was about 400,000 square kilometers (roughly twice the size of Great Britain)—a clear increase. This is the second test of the black hole area theorem; an initial test was performed in 2021 using data from the first GW150914 signal, but because that data was not as clean, the results had a confidence level of 95 percent as compared to 99.999 percent for the new data.
 
Josh Sharkey, a PhD student at the IGR who works on interpreting signals like these, said: “The collision and merger of the two black holes built up  a lot of energy in the new, heavier black hole this created, and it continued to ‘ring’ like a bell, producing gravitational-waves which carried that energy away. We can then learn about the left over black hole by measuring the properties of these gravitational waves.”
 
In the new study, the researchers were able to precisely measure the details of the ringdown phase, which allowed them to calculate the mass and spin of the black hole, and subsequently determine its surface area. More precisely, they were able, for the first time, to confidently pick out two distinct gravitational-wave modes in the ringdown phase. The modes are like characteristic sounds a bell would make when struck; they have somewhat similar frequencies but die out at different rates, which makes them hard to identify. The improved data for GW250114 meant that the team could extract the modes, demonstrating that the black hole's ringdown occurred exactly as predicted by mathematical models developed in the 1970s by Saul Teukolsky, now a professor at Caltech and Cornell University, and others.
 
Another study from the LVK, submitted to Physical Review Letters today, places limits on a predicted third, higher-pitch tone in the GW250114 signal, and performs some of the most stringent tests yet of general relativity's accuracy in describing merging black holes.
 
The IGR’s Deputy Director, Professor Giles Hammond, said: “We’ve gone from making our very first detection ten years ago, to having hundreds today. While it’s hard to be sure what we’ll detect in the next decade, the leaps which we’ve made since 2015 make me confident that the future is very bright.”
 
LIGO has also unveiled neutron stars over the past decade. Like black holes, neutron stars form from the explosive deaths of massive stars, but they weigh less and glow with light. Of note, in August of 2017, LIGO and Virgo witnessed an epic collision between a pair of neutron stars—a kilonova—that sent gold and other heavy elements flying into space and drew the gaze of dozens of telescopes around the world, which captured light ranging from high-energy gamma rays to low-energy radio waves. The ‘multi-messenger' astronomy event marked the first time that both light and gravitational waves had been captured in a single cosmic event. Today, the LVK continues to alert the astronomical community to potential neutron star collisions, who then use telescopes to search the skies for signs of kilonova.
 
LIGO spokesperson Professor Stephen Fairhurst of Cardiff University said: “A decade ago we couldn’t be certain that black holes ever collide in our universe. Now we observe several black-hole mergers per week. With the three hundred gravitational-wave candidates observed to date, we are beginning to provide a census of the population of black holes in the universe.  We have already found several surprises, including black holes which are less massive (about four times the mass of the sun) and more massive (over 100 times the mass of the sun) than expected.”
 
"The global LVK network is essential to gravitational-wave astronomy," says Gianluca Gemme, Virgo spokesperson and director of research at INFN (Istituto Nazionale di Fisica Nucleare). "With three or more detectors operating in unison, we can pinpoint cosmic events with greater accuracy, extract richer astrophysical information, and enable rapid alerts for multi-messenger follow-up. Virgo is proud to contribute to this worldwide scientific endeavor."

Other LVK scientific discoveries include the first detection of collisions between one neutron star and one black hole; asymmetrical mergers, in which one black hole is significantly more massive than its partner black hole; the discovery of the lightest black holes known, challenging the idea that there is a "mass gap" between neutron stars and black holes; and the most massive black hole merger seen yet with a merged mass of 225 solar masses. For reference, the previous record-holder for the most massive merger had a combined mass of 140 solar masses.
 
Even in the decades before LIGO began taking data, scientists were building foundations that made the field of gravitational-wave science possible. For example, breakthroughs in computer simulations of black hole mergers allow the team to extract and analyse the feeble gravitational-wave signals generated across the universe.
 
LIGO's technological achievements, beginning as far back as the 1980s, include several far-reaching innovations, such as a new way to stabilize lasers using the so-called rf reflection locking technique. Invented in 1983 based on the earlier work of physicist RV Pound at Harvard and developed at JILA, Boulder by John Hall and Ronald Drever and the University of Glasgow by James Hough and Henry Ward, this technique is widely used today in other fields, such as the development of atomic clocks and quantum computers. Other innovations include cutting-edge mirror coatings that almost perfectly reflect laser light; ‘quantum squeezing’ tools that enable LIGO to surpass sensitivity limits imposed by quantum physics; and new AI methods that could further hush certain types of unwanted noise.
 
In the coming years, the scientists and engineers of LVK hope to further fine tune their machines, expanding their reach deeper and deeper into space. They also plan to use the knowledge they have gained to build another gravitational-wave detector, LIGO India. Having a third LIGO observatory would greatly improve the precision with which the LVK network can localize gravitational-wave sources. Looking farther into the future, the team is working on a concept for an even larger detector, called Cosmic Explorer, which would have arms 40 kilometers long (the twin LIGO observatories have 4-kilometer arms). An observatory on this scale would allow scientists to hear the earliest black hole mergers in the universe.
 
The University of Glasgow’s research is supported by funding from UKRI’s Science and Technology Facilities Council (STFC), as are other gravitational research groups across the UK including the Universities of Birmingham, Cambridge, Cardiff, Kings College London, Nottingham, Portsmouth, Sheffield, Strathclyde, University College London, Queen Mary University, and the University of the West of Scotland.

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First published: 10 September 2025