Image: Artistic representation of Black Hole Merger GW250114 Credit: Aurore Simonnet (SSU/EdEon)
On September 14, 2015, a signal arrived on Earth, carrying information about a pair of remote black holes that had spiralled together and merged.
On that day 10 years ago, the twin detectors of the US National Science Foundation Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever direct detection of gravitational waves - whispers in the cosmos that had previously been unheard.
The historic discovery meant that researchers could now sense the universe through three different means. While 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, this was the first time anyone had witnessed a cosmic event through its gravitational warping of space-time.
Today, LIGO, which consists of detectors in both Hanford, Washington and Livingston, Louisiana, now routinely observes roughly one black hole merger every three days. LIGO now operates in coordination with two international partners, the Virgo gravitational-wave detector in Italy and KAGRA in Japan.
Together, the gravitational-wave-hunting network, known as the LVK (LIGO, Virgo, KAGRA), has captured a total of about 300 black hole mergers, some of which are confirmed while others await 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 dramatic rise in the number of discoveries by LVK over the past decade is owed to several improvements to their detectors - some of which involve cutting-edge quantum precision engineering. The LVK detectors remain by far the most precise rulers for making measurements ever created by humans. The space-time distortions induced by gravitational waves are incredibly miniscule. For instance, LIGO detects changes in spacetime smaller than 1/10,000 the width of a proton. That's 700 trillion times smaller than the width of a human hair.
Scientists from the University of Portsmouth are heavily involved, taking a leading role in identifying and filtering out environmental interference in the detectors, such as vibrations from seismic noise, nearby traffic, and the detector electronics themselves. They are also searching for, and finding, signals in noisy data and using gravitational waves to probe and better understand the origins and development of the Universe.
The Clearest Signal Yet
LIGO's improved sensitivity has been demonstrated 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: January 14, 2025).
"It's amazing that 10 years after the first discovery of gravitational waves we are still making unique and interesting finds. This signal is much louder in our detectors than anything we've seen to date, allowing us to put Einstein's theory of gravity to the test like never before," said Ian Harry , Professor of Gravitational Wave Astronomy at the University of Portsmouth, who was one of the published paper writing team.
By analysing the frequencies of gravitational waves emitted by the merger, the LVK team was able to provide the best observational evidence captured to date for what is known as the black hole area theorem, by Stephen Hawking in 1971 that says the total surface areas of black holes cannot decrease.
When black holes merge, their masses combine, increasing the surface area. But they also lose energy in the form of gravitational waves. Additionally, the merger can cause the combined black hole to increase its spin, which leads to it having a smaller area. The black hole area theorem states that despite these competing factors, the total surface area must grow in size.
Another study from the LVK 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.
Professor Laura Nuttall from the University of Portsmouth, said: "The LIGO-Virgo-KAGRA network of gravitational wave detectors has been incredibly successful over the last decade. It really is a global effort to bring discoveries together - from the instruments, to calibrating and understanding the quality of the data, to searching for and characterising the signals as well as alerting electromagnetic telescopes. At Portsmouth we're proud to be part of this global team."
Pushing the Limits
LIGO and Virgo have 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 "multimessenger" 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 kilonovae.
Professor Tessa Baker from the University of Portsmouth's Institute of Cosmology and Gravitation said: "The first multi-messenger event had a monumental 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 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 really ingenious methods for doing gravitational wave science."
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.
In the coming years, the scientists and engineers of LVK hope to further fine tune the machines, expanding the reach deeper and deeper into space. They also plan to build another gravitational- detector, LIGO India.
Looking to the future, the team is also working on a concept for an even larger detector, called Cosmic Explorer, which would have arms 40 kilometres long (the twin - LIGO observatories have 4-kilometre arms).
