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First hints of ‘background’ gravitational waves from supermassive black holes

(ABC News) -- You may not be able to feel it, but scientists believe the fabric of space-time around you is constantly rippling with waves created by monster black holes circling each other.
This rumble of low-frequency gravitational waves is impossible to detect using any telescope on Earth.

Supermassive black holes at the centre of galaxies may create low frequency gravitational waves.

Instead astronomers from around the globe, including Australia, have been peering at spinning stars called pulsars to try to measure these ripples as they slowly stretch and squeeze the universe.
After years of searching, they believe they may have found the first hints of the celestial fingerprint created by these waves.
“This is the first-ever evidence for the gravitational wave background. We’ve opened a new window of observation on the universe,” said Chiara Mingarelli, of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) in a statement.
In 2020, NANOGrav, which is observing 67 pulsars, detected a low frequency rumble, but it was not clear whether it was from gravitational waves or some sort of anomaly.
Then the same signal was seen by five separate groups using telescopes all around the world, including Murriyang - CSIRO’s radio telescope at Parkes in NSW.
The data appears today in several papers, published simultaneously in The Astrophysical Journal Letters and Research in Astronomy and Astrophysics.
Daniel Reardon, of Swinburne University and OZGrav, is the lead author of two research papers by the Australian collaboration known as the Parkes Pulsar Timing Array (PPTA).
The PPTA has been observing 30 pulsars over the past 20 years, the longest of all the telescopes.
“If you’re only observing northern hemisphere pulsars or only southern hemisphere pulsars, you’re really more restricted than if we combine northern and southern hemisphere collaborations and use all of the pulsars on the sky,” Dr Reardon said.
Gravitational waves were first discovered in 2015, with the Nobel Prize-winning detection of two merging black holes by the Laser Interferometer Gravitational-wave Observatory (LIGO).
LIGO used lasers to detect infinitesimally small changes in the length of 4-kilometre-long pipes, in detectors located 3,000 km apart.
Since then LIGO and another detector in Italy called VIRGO have detected dozens of instances of gravitational waves, created by merging black holes, black holes swallowing the dead cores of stars, and neutron stars crashing into each other.
The black holes at the centre of many of these discoveries are up to about 85 times the mass of our Sun, and the collisions produce sudden bursts of high frequency gravitational waves.
The black holes involved in today’s discovery are much larger – up to billions of times the mass of our Sun – and produce ultra-low frequency “background” gravitational waves that can take up to decades for their oscillations to snake through the cosmos.
Scientists believe these ultra-low frequency waves rumbles through the Universe, overlapping each other in a constant hum.
“It’s like an ocean, not just one single wave, but a whole spectrum of waves, which is the sum of all of the supermassive black hole binary systems in the whole Universe.”
“To pick up those we need a detector not four kilometres across like LIGO, but rather the size of a galaxy,” Dr Reardon said.
Pulsars are the lighthouses of the cosmos.
As these dead cores of once massive stars spin hundreds of times per second, they send out beams of radio waves from their poles. From Earth, those beams can be seen flashing with clockwork-like regularity.
According to Einstein’s theory of general relativity, ripples in the fabric of space-time created by gravitational waves affect the timing of when these pulses reach us.
“As the gravitational waves pass over Earth, they change the distances to pulsars by stretching and squeezing space,” Dr Reardon explained.
“We measure these changing distances as pulses that arrive at our telescopes earlier or later than we expect.”
Astronomers then compare signals from pairs of pulsars over time, to see if a pattern emerges.
The more pulsars, the easier it is to see if a signal is more than just an anomaly in the data.
But because the wavelength of ultra-low frequency gravitational waves is so long it can take many years to detect them.
“We’ve been looking for random fluctuations … for two decades [at Parkes],” Dr Reardon said.
Murriyang detected little evidence of a signal in the first decade of searching; it’s only been in the last few years using more modern technology that a pattern has started to emerge.
“It’s not as if we had a sudden burst and then came detection. It’s expected to build slowly. And that appears to be what’s happening now.”
Supermassive black holes, like the one at the centre of our galaxy, are very important to our understanding of how the Universe formed.

(Latest Update June 30, 2023)

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