Scientists detect a "tsunami" of gravitational waves

Monday 8 November 2021

A team of international scientists, including researchers from The Australian National University (ANU), have unveiled the largest number of gravitational waves ever detected.
 
The discoveries will help solve some of the most complex mysteries of the Universe, including the building blocks of matter and the workings of space and time.

The global team’s study, published today on ArXiv, made 35 new detections of gravitational waves caused by pairs of black holes merging, or neutron stars smashing together, using the LIGO and Virgo observatories between November 2019 and March 2020.

This brings the total number of detections to 90 after three observing runs between 2015 and 2020.
 
The new detections are from massive cosmic events, most of them billions of light years away, which hurl ripples through space-time. They include 32 black hole pairs merging, and likely three collisions between neutron stars and black holes.

ANU is one of the key players in the international team making the observations and developing the sophisticated technology to hunt down elusive gravitational waves across the vast expanse of the Universe. 
 
Distinguished Professor Susan Scott, from the ANU Centre for Gravitational Astrophysics, said the latest discoveries represented “a tsunami” and were a “major leap forward in our quest to unlock the secrets of the Universe’s evolution”.

“These discoveries represent a tenfold increase in the number of gravitational waves detected by LIGO and Virgo since they started observing,” Distinguished Professor Scott said.  
 
“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.

"Amongst the data were some remarkable findings:

  • Two mergers between neutron star - black hole pairs (GW191219 and GW200115). The neutron star in GW191219 is one of the least massive ever observed at 1.17 solar masses, and merged with a black hole of 31 solar masses, making this the most extreme mass ratio of the events we have observed so far.
  • A merger between a black hole of 24 solar masses and an object of 2.83 solar masses (GW200210) which could either be a light black hole or a heavy neutron star.
  • A massive pair of black holes orbiting each other, with masses of 87 and 61 solar masses (GW200220). It's likely the larger black hole has a mass more than 65 solar masses, which is approximately the maximum mass of black holes expected to be formed from stellar collapse before encountering pair-instability supernovae, where the progenitor stars would be disrupted leaving no remnant behind.  The resultant black hole has mass 141 solar masses which makes it an intermediate black hole, a type that is very difficult to detect.
  • A light pair of black holes that together weigh only 17 times the mass of the Sun (GW191129) with component black hole masses of 10.7 and 6.7 solar masses - the least massive pair in this observing run.
  • A pair of black holes orbiting each other, in which at least one of the pair is spinning upright (GW191204). It has χeff = 0.16.
  • 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 (GW191109). It has χeff = -0.29. 

“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.

“Looking at the masses and spins of the black holes in these binary systems indicates how these systems got together in the first place.

“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?”
 
Distinguished Professor Scott, who is also a Chief Investigator of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), said 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,” she said.
 
“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.”

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