An artist's impression of a hierarchical scheme for merging black holes.
Credit: LIGO/Caltech/MIT/R. Hurt (IPAC)
Science and Technology Facilities Council (STFC) funded researchers from the University of Portsmouth and across the UK, together with an international team of scientists studying gravitational waves caused by black hole and neutron star collisions, have released the latest available analysis.
This new analysis of data, more than doubles the number of black holes and neutron stars colliding, reported to date, from 90 to 218, with 128 new mergers being detected.
Black holes are invisible, but when they collide, they create ripples in space-time called gravitational waves. These waves travel across the universe at the speed of light, and by the time they reach Earth, they are almost imperceptibly tiny - smaller than 1/10,000th the width of a proton - so to pick up and observe them, powerful and sophisticated detectors are needed.
The data analysis published in Gravitational Wave Transient Catalogue (GWTC-4.0) is the result of observations gathered from advanced detectors located in the United States of America, Italy and Japan, the names of which form the title of the research collaboration ' LIGO-Virgo-KAGRA '.
Scientists from the University of Portsmouth are heavily involved, taking a leading role in monitoring the properties of the highly specialised monitoring equipment, analysing the data, and interpreting the results. Their UK collaborators include cosmologists and astrophysicists from the University of Glasgow, Cardiff University, Royal Holloway University of London, and the University of Birmingham.
The findings now available from the LIGO-Virgo-KAGRA collaboration are from the fourth run of observations gathered over nine months, from May 2023 until January 2024.
Professor Tessa Baker from the University of Portsmouth's Institute of Cosmology and Gravitation , who was manager for the Cosmology paper in this release, said: "It's really exciting to bring over a hundred new gravitational wave events into the public domain, after several years of quiet. These new events have allowed us to refine our measurements of how fast the universe is expanding - a.k.a. the Hubble constant - arguably the most crucial and hotly-debated number in current cosmology."
"As chair of one of the key analysis groups, I am particularly mindful of the scale of GWTC-4," added Dr Michael Williams from the University of Portsmouth. "The sheer number of events in this catalogue presented both a challenge and an opportunity. Coordinating the review and validation of these results required sustained collaboration, and I am proud of the rigor and consistency we achieved."
Research Fellow in the Institute for Gravitational Research at the University of Glasgow, Dr Daniel Williams, said: "This new update really highlights the capabilities of both the international network of gravitational-wave detectors, and the analysis techniques which have been developed to dig very faint signals out of the data. What we've observed has broadened our understanding of the cosmic graveyard."
The catalogue also contains signals from two mergers between black holes and neutron stars; one of these was previously published and is recorded as GW230529, but a second merger, GW230518, is presented in this analysis release for the first time.
The large number of new detections are thanks to sensitivity improvements which have been made to the LIGO detectors since 2020. The UK has made key contributions to the development of these detectors over the last three decades.
The enhanced sensitivity of the detectors means that more sources can be detected, and clearer measurements of gravitational-wave signals can be made. The observation GW230814 is the strongest gravitational-wave observation to date.
"Gravitational-wave signals are the perfect way to test Einstein's theory of gravity. The louder the signal, the more precise our measurements of any potential deviations", explained the University of Portsmouth's Professor of Gravitational Wave Astronomy, Ian Harry . "So far Einstein has passed every test, but we will keep looking closer! For these types of analysis, it is very important to have observations from multiple gravitational-wave detectors, so you can cross-reference the signal in both. As we gather more detections we can also learn more about the fundamental nature of the Universe."
The new variety of different mergers also casts new light on how stars have evolved over the history of the Universe.
"In a similar way to how a palaeontologist can learn about long-extinct dinosaurs by looking at their fossilised bones, we can learn about stars by looking at their black hole or neutron star remains," said Dr Greg Ashton of Royal Holloway University of London.
"Here, we might expect that following the merger of two black holes, the remnant black hole could find a new partner and merge again, forming an even bigger black hole. In GWTC-4.0 we've seen tell-tale hints that some of the sources could come from black holes that were themselves the result of previous mergers. Teasing out the black holes formed from collapsing stars and those formed from previous mergers will tell us about how stars live their lives, and where they live their lives across the Universe."
As a recognised international centre of research excellence, the University of Portsmouth's Institute of Cosmology and Gravitation (ICG) brings together more than 70 researchers - faculty, postdoctoral fellows and PhD students - tackling some of the Universe's most profound mysteries, from the earliest moments after the Big Bang to the large-scale structure of galaxies, dark energy and gravitational waves.
Its world-class impact was confirmed in REF 2021, where 100 per cent of ICG research was rated world-leading or internationally excellent.
The institute's contributions include roles in major international projects such as Euclid , LISA (Laser Interferometer Space Antenna) , the LIGO gravitational wave detectors , and the Dark Energy Spectroscopic Instrument (DESI) .
