After decades of searching, astronomers have found a distinctive pattern of light, from spinning stars called pulsars, that suggests huge gravitational waves are creating gentle ripples in space-time across the universe
By Alex Wilkins
29 June 2023
Pulsars have helped reveal ripples in space-time throughout the universe
NANOGrav
The fabric of the universe is constantly rippling, according to astronomers who have discovered a background buzz of gravitational waves. These waves may be produced by supermassive black holes merging across the universe, but they might also have more exotic origins, such as leftover ripples in space-time created shortly after the big bang. Pinning down their true nature could tell us about how supermassive black holes grow and affect their host galaxies, or even about how the universe evolved in its first moments.
To find this mysterious hum, astronomers have been tracking rapidly rotating neutron stars called pulsars that blast out light with extreme regularity. By looking at different pulsars across the Milky Way, astronomers can effectively use them as a galaxy-sized gravitational-wave detector called a pulsar timing array.
While individual gravitational waves, which are ripples in space-time created by massive objects colliding, have been seen regularly since the first detection in 2015, the object of this search is different. Those previous gravitational waves all have a localised origin and rise and fall hundreds of times a second, but the newly-discovered signal is more like a gravitational wave background that would permeate the entire universe at much lower frequencies, similar in concept to the cosmic microwave background, which is radiation left over by the big bang and seen all over the universe today.
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In 2021, there were the first hints that the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a US-based collaboration that began in 2007 and that uses a pulsar timing array, had detected this gravitational wave background using radio telescopes.
By measuring the light signals from pulsars as they arrive at Earth and checking for tiny time fluctuations that may have been caused by ripples in space-time, astronomers thought they had found signs of a common process affecting all the pulsars’ timing in the same way. However, at that time they lacked a telltale signature predicted by Albert Einstein’s general theory of relativity that would confirm this cosmic-scale hum.