Einstein’s zoo: LIGO and Virgo confirm gravitational waves from 50 cosmic collisions

The LIGO and Virgo Collaborations, which includes researchers from the University of Birmingham, have announced a further 39 gravitational-wave events, bringing the total number of confident detections to 50.

These 50 events include the mergers of binary black hole, binary neutron stars and, possibly, neutron star-black holes. The 39 events announced [today], in the release of the Collaboration’s second Gravitational-Wave Transient Catalogue (GWTC-2) also span a wide range of masses and contain a wealth of information on the history and formation of black holes and neutron stars throughout the universe. The events were detected during the first half of the third observing run, between 1 April and 1 October 2019.

The University of Birmingham has been a key member of the Advanced LIGO project since its inception. Researchers from the Institute for Gravitational Wave Astronomy have contributed to the design and construction of the LIGO detectors, have developed accurate models for the gravitational radiation emitted by binary systems and have pioneered the techniques used to mine astrophysical information from the gravitational-wave data. This work allows us to study the physics of binary systems, their astrophysical evolution and perform precision tests of Einstein’s theory with gravitational-wave observations. Members of the Institute for Gravitational Wave Astronomy have played a leading role in the analysis and interpretation of the data collected throughout the three observing runs.

Dr Patricia Schmidt , lecturer at the Institute for Gravitational Wave Astronomy, says: “Only five years after the very first detection of gravitational waves, we already have 50 events. Gravitational-wave astronomy is rapidly becoming an indispensable tool for studying some of the most fascinating objects such as black holes and neutron stars as well as the universe as a whole.’

These new observations are a treasure chest to peek into the evolutionary paths of black holes and neutron stars in binary systems. GWTC-2 contains a population of binary black holes that span a much wider mass range than previously observed, from approximately 5 to 85 times the mass of the Sun. The lower end of the range is where theorists expect the lightest black holes to be formed in Nature, a prediction that can now be tested observationally. The higher end of the mass spectrum is too high for standard stellar evolution models to produce a black hole as the end state of massive stars. To find out how and where in the Universe the systems found by LIGO-Virgo were formed, will keep astrophysicists busy for quite some time.

For the first time, this sample also provides a clear indication that at least some black holes spin and tumble in their death-dance around each other. A small fraction of them, around 20 per cent, seem to like to dance upside-down. Measuring the masses and spins of the binary companions is of extreme importance towards connecting the compact objects to their stellar progenitors, allowing us to solve the puzzle of their formation and evolution which can take many different paths and crucially depends on them.

Riccardo Buscicchio , a PhD candidate at the University of Birmingham School of Physics and Astronomy and member of the LIGO-Virgo Collaboration says: “It’s like reconstructing the entire history of primates, their origin and migration across the continents only by looking at Neanderthals and Homo Sapiens. Except that black holes progenitors are much older, 200 times further in the past than primates’ history on Earth.’

These new gravitational-wave detections provide yet another stress-test of Einstein’s General Theory of Relativity. By comparing the observed gravitational-wave signals to our best theoretical predictions we can search for small deviations from General Relativity: once more Einstein’s theory passes every test unscathed.

Dr Geraint Pratten , a researcher at the University of Birmingham, who played a leading role in the confirmation of Einstein’s theory, says: “Gravitational-wave observations provide us with a unique arena in which we can perform novel tests of fundamental physics. The wealth of observed binary black holes allow us to scrutinise Einstein’s theory of relativity in unprecedented detail, gaining remarkable insights into the astrophysical and fundamental nature of black holes.’

The future of gravitational-wave astronomy seems to be as bright as ever. The analysis of the second half of O3 is currently in progress and will further expand the growing gravitational-wave transient catalog. Following O3, detectors will undergo additional engineering improvements to further increase their astrophysical reach in time for the fourth observing run, currently scheduled to start in 2022. While scientists await instrumental improvements and the construction of even more ambitious detectors, GWTC-2 offers unprecedented opportunities to explore the nature of black holes and neutron stars throughout the universe.

Professor Alberto Vecchio , director of the Institute for Gravitational Wave Astronomy says: “I remember when over 20 years ago I started to devote most of my time to gravitational wave science many of my former PhD colleagues thought I was mad: I would spend my scientific career just staring at terabytes of noise. I am so glad I was foolish enough not to pay much attention to their remarks. Now it’s surprise after surprise in an exhilarating journey across the Universe, and this is just the very beginning.’