Cosmic Ripples in Space and Time: Largest Number of Gravitational Wave Detections to Date

Key points:

• An international team of scientists, including Australian researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), have collaborated on a study released today, presenting the largest number of gravitational wave detections to date — 90 detections!

• Gravitational waves are cosmic ripples in space and time that are caused by some of the most violent and energetic processes in the Universe, like supernovas, merging black holes, and colliding neutron stars — city-size stellar objects with a mass about 1.4 times that of the Sun.

• The newest gravitational wave detections come from the second part of the third observing run which lasted from November 2019 to March 2020. There were 35 new gravitational wave detections in this period: 32 detections were from pairs of merging black holes; 3 were likely to come from the collision of a neutron starA neutron star is the collapsed core of a large (between 10 and 29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of just 10 – 20 kilometers (6 – 12 miles), they can have masses of about 1.3 – 2.5 that of the Sun.”>neutron star and a black holeA black hole is a place in space where the pull of gravity is so strong not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.”>black hole.

The gravitational-wave Universe is teeming with signals produced by merging black holes and neutron stars. In a new paper released today, an international team of scientists, including Australian OzGrav researchers, present 35 new gravitational wave observations, bringing the total number of detections to 90!

All of these new observations come from the second part of observing run three, called “O3b,” which was an observing period that lasted from November 2019 to March 2020. There were 35 new gravitational wave detections in this period. Of these, 32 are most likely to come from pairs of merging black holes, 2 are likely to come from a neutron star merging with a black hole, and the final event could be either a pair of merging black holes or a neutron star and a black hole. The mass of the lighter object in this final event crosses the divide between the expected masses of black holes and neutron stars and remains a mystery.

Dr. Hannah Middleton, postdoctoral researcher at OzGrav, University of Melbourne, and co-author on the study says “Each new observing run brings new discoveries and surprises. The third observing run saw gravitational wave detection becoming an everyday thing, but I still think each detection is exciting!”

Highlights

Of these 35 new events, here are some notable discoveries (the numbers in the names are the date and time of the observation):

• Two mergers between possible neutron star — black hole pairs. These are called GW191219_163120 and GW200115_042309, the latter of which was previously reported in its own publication. The neutron star in GW191219_163120 is one of the least massive ever observed.
• A merger between a black hole and an object which could either be a light black hole or a heavy neutron star called GW200210_092254
• A massive pair of black holes orbiting each other, with a combined mass 145 times heavier than the Sun (called GW200220_061928)
• A pair of black holes orbiting each other, in which at least one of the pair is spinning upright (called GW191204_171526)
• A pair of black holes orbiting each other which have a combined mass 112 times heavier than the Sun, which seems to be spinning upside-down (called GW191109_010717)
• A ‘light’ pair of black holes that together weigh only 18 times the mass of the Sun (called GW191129_134029)

The different properties of the detected black holes and neutron stars are important clues as to how massive stars live and then die in supernova explosions.
“It’s fascinating that there is such a wide range of properties within this growing collection of black hole and neutron star pairs,” says study co-author and OzGrav PhD student Isobel Romero-Shaw (Monash University). “Properties like the masses and spins of these pairs can tell us how they’re forming, so seeing such a diverse mix raises interesting questions about where they came from.”

Not only can scientists look at individual properties of these binary pairs, they can also study these cosmic events as a large collection — or population. “By studying these populations of black holes and neutron stars we can start to understand the overall trends and properties of these extreme objects and uncover how these pairs came to be,” says OzGrav PhD student Shanika Galaudage (Monash University) who was a co-author on a companion publication released today: ‘The population of merging compact binaries inferred using gravitational wavesGravitational waves are distortions or ripples in the fabric of space and time. They were first detected in 2015 by the Advanced LIGO detectors and are produced by catastrophic events such as colliding black holes, supernovae, or merging neutron stars.”>gravitational waves through GWTC-3 P2100239’. In this work, scientists analyzed the distributions of mass and spin and looked for features which relate to how and where these extreme object pairs form. Shanika adds, “There are features we are seeing in these distributions which we cannot explain yet, opening up exciting research questions to be explored in the future.”

Detecting gravitational waves: a complicated global effort

Detecting and analysing gravitational-wave signals is a complicated task requiring global efforts. Initial public alerts for possible detections are typically released within a few minutes of the observation. Rapid public alerts are an important way of sharing information with the wider astronomy community, so that telescopes and electromagnetic observatories can be used to search for light from merging events — for example, merging neutron stars can produce detectable light.

Says Dr. Aaron Jones, co-author and postdoctoral researcher from The University of Western Australia, “It’s exciting to see 18 of those initial public alerts upgraded to confident gravitational wave events, along with 17 new events.”

After thorough and careful data analysis, scientists then decipher the shortlist of gravitational-wave detections, delving into the properties of the systems that produced these signals. They use parameter estimation, a statistical technique to learn information about the black holes and neutron stars, such as their masses and spins, their location on the sky and their distance from the Earth.

Future detections

All of these detections were made possible by the global coordinated efforts from the LIGOThe Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory supported by the National Science Foundation and operated by Caltech and MIT. It’s designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. It’s multi-kilometer-scale gravitational wave detectors use laser interferometry to measure the minute ripples in space-time caused by passing gravitational waves. It consists of two widely separated interferometers within the United States—one in Hanford, Washington and the other in Livingston, Louisiana.”>LIGO (USA), Virgo (Italy), and KAGRA (Japan) gravitational-wave observatories.

Between the previous observing runs, the detectors have been continually enhanced in small bursts which improves their overall sensitivity. Says Disha Kapasi, OzGrav student (Australian National UniversityFounded in 1946, the Australian National University (ANU) is a national research university located in Canberra, the capital of Australia. Its main campus in Acton encompasses seven teaching and research colleges, in addition to several national academies and institutes.”>Australian National University), “Upgrades to the detectors, in particular squeezing and the laser power, have allowed us to detect more binary merger events per year, including the first-ever neutron star-black hole binary recorded in the GWTC-3 catalogue. This aids in understanding the dynamics and physics of the immediate universe, and in this exciting era of gravitational wave astronomy, we are constantly testing and prototyping technologies that will help us make the instruments more sensitive.”

The LIGO and Virgo observatories are currently offline for improvements before the upcoming fourth observing run (O4), due to begin in August 2022 or later. The KAGRA observatory will also join O4 for the full run. More detectors in the network help scientists to better localize the origin or potential sources of the gravitational waves.

“As we continue to observe more gravitational-wave signals, we will learn more and more about the objects that produce them, their properties as a population, and continue to put Einstein’s theory of General Relativity to the test,” says Dr. Middleton.

There is a lot to look forward to from gravitational-wave astronomy in O4 and beyond. But in the meantime, scientists will continue to analyze and learn from the data, searching for undiscovered types of gravitational waves, including continuous gravitational waves, and of course new surprises!

For more on this research, read Massive “Tsunami” of Gravitational Wave Detections Breaks Record.