A decade after the historic first detection of gravitational waves, UK scientists are reflecting on their vital contributions to one of the most significant discoveries in modern physics.
They are also looking ahead to the next chapter of cosmic exploration.
The LIGO-Virgo-KAGRA (LVK) collaboration announced the detection of GW250114, a ripple in spacetime that offers unprecedented insights into the nature of black holes and the fundamental laws of physics.
The study confirms Professor Stephen Hawking’s 1971 prediction that when black holes collide, the total event horizon area of the resulting black hole is bigger than the sum of individual black holes. It cannot shrink.
Confirmed Kerr nature of black holes
Research also confirmed the Kerr nature of black holes, a set of equations developed in 1963 by New Zealand mathematician Roy Kerr elegantly explaining what space and time look like near a spinning black hole.
Ten years ago on 14 September 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected ripples in space-time caused by two colliding black holes over a billion light-years away.
The discovery, which earned the Nobel Prize for Physics in 2017, opened an entirely new window on the Universe.
British researchers have been at the forefront of the science ever since leading to more than 300 detections in the past decade.
UK technology and expertise at the heart of gravitational wave detection
The first detection was made possible in part by crucial technology and analysis tools developed by UK scientists and engineers.
The Science and Technology Facilities Council Technology Department collaborated with the University of Glasgow, the University of Strathclyde, the University of Birmingham and Cardiff University during the advanced LIGO upgrade from 2002 to 2016.
This helped develop the passive suspension systems that provide isolation for the main and beamsplitter optics of the LIGO interferometer that gave the sensitivity necessary to detect a gravitational wave.
More recently, the UK has continued its technical contributions through the LIGO A+ upgrade project (2019 to 2024), developing upgraded Bigger Beam Splitter Suspension and new Ham Triple Relay Suspensions.
The UK also maintains ongoing research and development projects to design improved suspensions for future facility upgrades.
A census of the population of black holes in the Universe
The LIGO Scientific Collaboration 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.
Captured around 300 black hole mergers
Today an international network of detectors observes the gravitational Universe together.
The two LIGO detectors in the US, alongside the Virgo gravitational-wave detector in Italy and KAGRA in Japan.
Together known as the LVK, the network 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.
The clearest signal yet
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.
The University of Glasgow’s Dr John Veitch, who contributed to the design of one of the analyses used on the signal said:
This event was 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.
Best observational evidence captured to date
The research team was able to provide the best observational evidence captured to date for what is known as the black hole area theorem.
It is an idea put forth by Stephen Hawking in 1971 that says the total surface areas of black holes cannot decrease.
In essence, this new LIGO detection allowed the team to ‘hear’ two black holes growing as they merged into one, verifying Hawking’s theorem.
The initial black holes had a total surface area of 240,000 square km (roughly the size of the UK). The final area was about 400,000 square km (roughly twice the size of the UK), 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% as compared to 99.999% for the new data.
Confirm black holes behave as Einstein and Kerr worked out
Professor Alberto Vecchio from the University of Birmingham said of the new evidence:
Roy Kerr’s initially rather obscure paper from 1963 was a seminal piece of work and nowadays astrophysicists talk all the time about Kerr black holes.
I find it extraordinary that we can sit a billion light years away from a tiny 200 km region of the universe where two black holes collided and, by capturing this violent event’s imprint on gravitational waves, confirm that these cosmic monsters behave as Einstein and Kerr worked out on a piece of paper using the fundamental laws of physics.
Measuring the Universe’s expansion
British scientists have also used the growing catalogue of gravitational wave detections to tackle some of cosmology’s biggest questions.
Several hundred new gravitational wave events have now been brought into the public domain, enabling researchers to refine measurements of how fast the Universe is expanding.
Professor Tessa Baker from the University of Portsmouth, who manages the cosmology research within the latest catalogue release said:
These new events have allowed us to refine our measurements of how fast the universe is expanding – the Hubble constant – arguably the most crucial and hotly-debated number in current cosmology.
Multi-messenger astronomy revolution
The field took another leap forward in 2017 with the first ‘multi-messenger’ event, a gravitational wave detection that was simultaneously observed by traditional telescopes across the electromagnetic spectrum.
This breakthrough has had lasting impact on how scientists study the cosmos.
Professor Baker said:
The first multi-messenger event had a significant impact on the community, not just in astronomy but across fundamental physics.
It really kick-started the use of gravitational wave events to answer major questions in cosmology, something the LVK collaboration is still pushing the frontier of today. Whilst we haven’t detected any more multi-messenger events yet, this has actually made us develop some new and ingenious methods for doing gravitational wave science.
Looking to the future
As the field enters its second decade, UK scientists are already contributing to the next generation of gravitational wave detectors.
UK teams are active partners in proposals for involvement in future ‘next generation’ gravitational wave facilities, including the Einstein Telescope and Cosmic Explorer projects.
Cosmic Explorer would be an even larger detector, which would have arms 40 km long (the twin LIGO observatories have 4 km arms).
The continued UK involvement builds on a track record of scientific leadership within the LVK collaboration, with Professor Stephen Fairhurst from Cardiff University currently serving as LVK spokesperson.
UK research shapes how we explore the cosmos
From the initial detection to the latest cosmological discoveries, British researchers have helped transform our understanding of the Universe through gravitational wave science, and their contributions continue to shape how we explore the cosmos.
In the coming years, the scientists and engineers of LVK hope to further fine-tune the current machines, expanding their reach deeper into space.
They also plan to build another gravitational wave detector, LIGO India.
Professor Sheila Rowan, Director of the Institute for Gravitational Research at the University of Glasgow, said:
We’ve achieved a remarkable amount in the first 10 years of gravitational wave astronomy, but the universe is ringing with the faintest echoes of past cosmic events, and we’re only just getting started on listening to what they have to tell us.