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Australian astrophysicists have led an international effort to create the most detailed maps of gravitational waves across the Universe to date.
The maps reveal insights into the most massive black holes known, how they shaped the Universe and the cosmic architecture they left behind.
‘What we’re seeing hints at a much more dynamic and active universe than we anticipated,’ says lead author Matt Miles, a researcher at OzGrav and Swinburne University.
Gravitational waves were first predicted by Albert Einstein in 1916 as consequence of his theory of relativity, which proposes that objects with mass distort the fabric of spacetime like a ball on top of a sheet.
According to Einstein, when massive objects move, spacetime changes with them. If an object accelerates then it sends out ripples – gravitational waves – in all directions.
But even the waves from supermassive black hole collisions fade over cosmic distances. Indeed, these faint ripples have expected wavelengths smaller than an atom’s nucleus by the time they reach Earth. The first such ripples were detected in 2016.
The new gravitational wave maps – with an unexpected hotspot in the signal – are published this week, in a series of three papers.
‘Looking at the layout and patterns of gravitational waves shows us how our Universe exists today and contains signals from as far back as the Big Bang,’ says Rowina Nathan, first author on one of the studies and researcher at OzGrav and Monash University.
‘The presence of a hotspot could suggest a distinct gravitational wave source, such as a pair of black holes billions of times the mass of our Sun,’ says Nathan, who notes that many expected the background signal to be uniform.
‘There’s more work to do to determine the significance of the hotspot we found, but this an exciting step forward for our field,’ adds Nathan.
To create the maps, the research team built the largest ever galactic-scale gravitational wave detector, the MeerKAT Pulsar Timing Array. The detector uses the MeerKAT Radio Telescope in South Africa to observe pulsars.
Pulsars, which are rapidly spinning neutron stars, can serve as natural clocks that allow researchers to detect the miniscule changes caused by rippling gravitational waves with nanosecond-level precision.
The researchers used the MeerKAT Pulsar Riming Array to capture a stronger signal of background gravitational waves than similar global experiments and in one-third of the time.
‘Studying the background lets us tune into the echoes of cosmic events across billions of years,’ says Miles. ‘We know supermassive black holes are out there merging, but now we’re starting to ask: where are they, and how many are out there?’
Continued monitoring with the MeerKAT array will allow researchers to further refine the gravitational wave maps. This in turn will help astrophysics uncover new cosmic phenomena, explore the origins of supermassive black holes, understand the formation of galaxies, and even infer major events in our Universe’s early history.
‘By looking for variations in the gravitational wave signal across the sky, we’re hunting for the fingerprints of the astrophysical processes shaping our Universe,’ says Kathrin Grunthal, coauthor and researcher from the Max Planck Institute for Radio Astronomy in Germany.
All three papers are published in the journal Monthly Notices of the Royal Astronomical Society.
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