Astrophysicists detect 35 new gravitational wave events


Astrophysicists in the LIGO-Virgo-KAGRA collaboration have detected 35 more gravitational waves since the catalog was last published in October 2020, bringing the total number of events observed since the start of gravitational wave observations to 90. Of the 35 events detected, thirty-two were most likely to be black hole mergers – two black holes spiraling around each other and eventually joining – and three were collisions between neutron stars and black holes. Black holes have a range of sizes, the most massive being about 90 times the mass of our Sun. Several of the black holes resulting from these mergers exceed 100 solar masses and are classified as black holes of intermediate mass.

Graphical table of all gravitational wave events discovered from 2015 to the end of the third LIGO / Virgo observation cycle. The table includes the name of the gravitational event, the type of binary component (black hole, neutron star or uncertain), the masses of the primary and secondary, and the mass of the final merged object. Image credit: LIGO / Virgo / KAGRA / C. Knox / H. Middleton.

Gravitational waves were first predicted by Albert Einstein from his theory of general relativity in 1916.

Because the gravitational waves reaching Earth are so tiny, it took decades of work to build instruments precise enough to measure them.

Since the first detection of gravitational waves in 2015, the number of detections has increased at a dazzling rate.

Within a few years, gravitational wave scientists have gone from observing these vibrations in the fabric of the Universe for the first time, to observing many events each month, and even multiple events on the same day.

Gravitational wave detectors work by using high powered lasers to carefully measure the time it takes for light to travel between mirrors along two perpendicular arms.

During the third observation period, which lasted from November 1, 2019 to March 27, 2020, the LIGO and Virgo detectors achieved their best performance.

To achieve this monumental progress, pioneer instruments have become more responsive through a program of constant upgrades and maintenance.

“The LIGO and Virgo detectors continue to improve with, for example, increased laser power and the installation of compressed light,” said Dr. Madeline Wade, physicist at Kenyon College.

“The excellent detector sensitivities have enabled the observation of so many exciting gravitational wave events, including the very first reliable neutron star-black hole binary detection.”

“The latest findings represented a ‘tsunami’ and were a great step forward in our quest to unlock the secrets of the evolution of universes,” said Professor Susan Scott, researcher at the ARC Center of Excellence for the discovery of gravitational waves (OzGrav) at Australian National University.

“These findings represent a tenfold increase in the number of gravitational waves detected by LIGO and Virgo since they began to observe.”

“We detected 35 events. It’s huge ! In contrast, we made three detections during our first observation campaign, which lasted four months in 2015-16. “

“This is truly a new era for gravitational wave detections and the growing population of discoveries is revealing so much information about the life and death of stars across the Universe.”

Among the 35 new events, here are some notable discoveries:

(i) GW191219_163120 and GW200115_042309: two mergers between possible neutron star-black hole pairs; the neutron star of GW191219_163120 is one of the least massive ever observed;

(ii) GW200210_092254: a merger between a black hole and an object which could be either a light black hole or a heavy neutron star;

(iii) GW200220_061928: a massive pair of black holes orbiting each other, with a combined mass 145 times heavier than the Sun;

(iv) GW191204_171526: a pair of black holes orbiting each other, in which at least one of the two rotates vertically;

(v) GW191109_010717: a pair of black holes orbiting each other that have a combined mass 112 times heavier than the Sun, which appears to spin upside down;

(vi) GW191129_134029: a “light” pair of black holes which together weigh only 18 times the mass of the Sun.

“Each new round of observations brings new discoveries and surprises,” said Dr Hannah Middleton, postdoctoral researcher at OzGrav and the University of Melbourne.

“The third round of observations saw the detection of gravitational waves become a daily thing, but I still think every detection is exciting! “

“It is fascinating that there is such a wide range of properties within this growing collection of neutron star pairs and black holes,” said Isobel Romero-Shaw, PhD student. student at OzGrav and Monash University.

“Properties like the masses and rotations of these pairs can tell us how they form, so seeing such a diverse mix raises interesting questions about their origin.”

“Looking at the masses and spins of black holes in these binary systems indicates how these systems came together in the first place,” Professor Scott said.

“It also raises some really fascinating questions. For example, did the system originally form with two stars that went through their life cycle together and eventually became black holes? “

“Or were the two black holes pushed against each other in a very dense dynamic environment like in the center of a galaxy?”

“Continuous improvement in the sensitivity of gravitational wave detectors has helped increase detections. “

“This new technology allows us to observe more gravitational waves than ever before.”

“We are also probing the two mass black hole regions and providing more tests of Einstein’s general theory of relativity.”

“The other really exciting thing about constantly improving the sensitivity of gravitational wave detectors is that it will then bring into play a whole new range of gravitational wave sources, some of which will be unexpected.”

The team’s paper was posted online at the preprint server.


R. Abbott et al. (Collaboration LIGO, Collaboration Virgo & Collaboration KAGRA). 2021. GWTC-3: Compact binary coalescences observed by LIGO and Virgo during the second part of the third cycle of observation. arXiv: 2111.03606

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