Astronomers have detected a record number of gravitational waves, in a discovery they say will shed light on the evolution of the universe, and the life and death of stars.
An international team of scientists have made 35 new observations of gravitational waves, which brings the total number of detections since 2015 to 90.
Gravitational waves are ripples in the fabric of spacetime, created by massive cosmic events – such as pairs of black holes smashing together – up to billions of light years away.
Waves from these cataclysmic collisions were detected by the Laser Interferometer Gravitational-Wave Observatory (Ligo) observatory in the US and the Virgo instrument in Italy between November 2019 and March 2020.
The first detection of gravitational waves, announced in 2016, confirmed a prediction Albert Einstein made a century earlier based on his general theory of relativity.
Monash University researcher Shanika Galaudage, a collaborator in the Australian branch of the project known as OzGrav, described gravitational waves as a game-changing “new window into the universe”.
“Gravitational waves are not [electromagnetic] light,” Galaudage said. “We can see things that are invisible, such as binary black hole mergers.”
Of the 35 new detections, 32 likely resulted from pairs of black holes merging.
Notable discoveries included two massive pairs of black holes orbiting each other – one pair that was 145 times as heavy as the mass of the sun, and the other 112 times. The scientists also discovered a “light” pair of black holes with a combined mass only 18 times that of the sun.
Two of the 35 detections are believed to have originated from a neutron star merging with a black hole.
Neutron stars are small, incredibly dense objects: although they weigh around 1.4 times the mass of the sun, they are city-sized, with a radius of approximately 15km, said research collaborator Prof Susan Scott, of the Australian National University.
Scott said the detections were helping scientists understand both the evolution of the universe and also the nature of stellar objects.
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The number of recent detections were up tenfold from when the Ligo and Virgo instruments first started observing for gravitational waves, Scott said. She attributed the increase to upgrades in the detecting instruments, such as an improvement in laser power.
“Eventually as we make the detectors even more sensitive, we’ll be able to see all the binary black hole pairs coming together throughout the whole universe,” Scott said.
“Neutron stars when they collide – they don’t create as strong gravitational waves as the black holes because they’re not as dense, and therefore we can’t see them out as far.”
In future, astronomers may also be able to detect gravitational waves from stars as they become supernovae. “This would help us to understand the process of stars when they finish their life cycle and run out of fuel and blow up and then collapse,” Scott said.
Analysing certain properties of the mergers allowed scientists to determine how they were formed, Galaudage said.
“By looking at how a black hole is spinning, for example – how fast it’s spinning, versus … which way it’s pointed – can tell us more about how it came to be: whether these black holes lived their lives apart and met at some point or whether they were stars to begin with, and then collapsed down [separately] to form black holes, and then went on to merge and produce these gravitational waves,” she said.
Some of the new detections are still mysterious. The researchers believe the 35th event could be either a pair of black holes, or a merger between a black hole and a neutron star.
The lighter object in this event had a mass greater than would be usual for a neutron star, but smaller than that of a black hole. “We’re seeing features … that we cannot explain yet,” Galaudage said.