Astrophysicists from the LIGO-Virgo-KAGRA Collaboration have detected a further 35 gravitational waves since the last catalog release in October 2020, bringing to 90 the total number of observed events since gravitational-wave observations began. Of the 35 events detected, thirty two were most likely to be black hole mergers — two black holes spiraling around each other and finally joining together — and three were collisions between neutron stars and black holes. The black holes have a range of sizes, with the most massive around 90 times the mass of our Sun. Several of the resulting black holes that formed from these mergers exceed 100 solar masses and are classed as intermediate-mass black holes.
Gravitational waves were first predicted by Albert Einstein from his theory of general relativity in 1916.
Because the gravitational waves reaching Earth are so minuscule, it took many decades of work to build instruments precise enough to measure them.
Since the first gravitational wave detection in 2015, the number of detections has risen at a thundering rate.
In a matter of years, gravitational wave scientists have gone from observing these vibrations in the fabric of the Universe for the first time, to now observing many events every month, and even multiple events on the same day.
Gravitational wave detectors operate by using high power lasers to carefully measure the time taken for light to travel between mirrors along two perpendicular arms.
In the third observing run, which lasted from November 1, 2019 to March 27, 2020, the LIGO and Virgo detectors reached their best-ever performance.
To achieve this monumental progress, the pioneering instruments have been getting more sensitive thanks to 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 squeezed light,” said Dr. Madeline Wade, a physicist at Kenyon College.
“The excellent sensitivities of the detectors have allowed for the observation of so many more exciting gravitational wave events, including the first ever confident neutron star-black hole binary detection.”
“The latest discoveries represented a ‘tsunami’ and were a major leap forward in our quest to unlock the secrets of the Universes evolution,” said Professor Susan Scott, a researcher at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Australian National University.
“These discoveries represent a tenfold increase in the number of gravitational waves detected by LIGO and Virgo since they started observing.”
“We’ve detected 35 events. That’s massive! In contrast, we made three detections in our first observing run, which lasted four months in 2015-16.”
“This really is 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 throughout the Universe.”
Of 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 in GW191219_163120 is one of the least massive ever observed;
(ii) GW200210_092254: a merger between a black hole and an object which could either be 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 pair is spinning upright;
(v) GW191109_010717: 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;
(vi) GW191129_134029: a ‘light’ pair of black holes that together weigh only 18 times the mass of the Sun.
“Each new observing run brings new discoveries and surprises,” said Dr. Hannah Middleton, a postdoctoral researcher at OzGrav and the University of Melbourne.
“The third observing run saw gravitational wave detection becoming an everyday thing, but I still think each detection is exciting!”
“It’s fascinating that there is such a wide range of properties within this growing collection of black hole and neutron star pairs,” said Isobel Romero-Shaw, a Ph.D. student at OzGrav and 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.”
“Looking at the masses and spins of the black holes in these binary systems indicates how these systems got 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 cycles together and eventually became black holes?”
“Or were the two black holes thrust together in a very dense dynamical environment such as at the centre of a galaxy?”
“The continual improvement of gravitational wave detector sensitivity was helping drive an increase in detections.”
“This new technology is allowing us to observe more gravitational waves than ever before.”
“We are also probing the two black hole mass gap regions and providing more tests of Einstein’s theory of general relativity.”
“The other really exciting thing about the constant improvement of the sensitivity of the gravitational wave detectors is that this will then bring into play a whole new range of sources of gravitational waves, some of which will be unexpected.”
The team’s paper was published online on the arXiv.org preprint server.
R. Abbott et al. (LIGO Collaboration, Virgo Collaboration & KAGRA Collaboration). 2021. GWTC-3: Compact Binary Coalescences Observed by LIGO and Virgo During the Second Part of the Third Observing Run. arXiv: 2111.03606
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