As astronomers track pairs of pulsars in the sky, the timing differences in the light from them should become broadly less similar as the angle between them grows.
It is only now that pulsar timing array teams feel confident enough in their data to be able to spot the distinctive pattern within the signal predicted by general relativity. “It takes a lot of years of observations, it takes a lot of understanding the noise properties of spin irregularities, the interstellar medium, things like that.”Ī supermassive black hole is hurtling away from its home galaxy
“We have to be able to account for all of them and that takes a long time,” says McKee. If a constant background of gravitational waves is distorting all space-time, then it should also affect all the pulsars’ light pulses in the same way, but measuring this isn’t easy, due to the many other factors that might affect the timing of the signals from each pulsar in the array. That is why pulsars, which are suitably spaced and are some of the most sensitive clocks in the universe, are key to this search. The gravitational wave background is minuscule - the strength of the signal that astronomers need to extract compared with the noise that is also picked up at the same time equates to one part in a quadrillion, while the gravitational waves themselves stretch around a light year – more than 9 trillion kilometres – over one wavelength. “It’s on an enormous scale, with incredibly difficult data to work with.” “We’re kind of all seeing it, which is very exciting because that suggests that it is probably real.” Enormous scaleīut confirming these signals and gaining more confidence in them isn’t straightforward, says Aris Karastergiou at the University of Oxford. “They’re also starting to see this very characteristic correlation signal in their data,” says NANOGrav team member Megan DeCesar at George Mason University in Virginia.
The CPTA claims to have found the gravitational wave background at an even higher confidence level than NANOGrav, but for only one frequency, while both EPTA and PPTA are seeing hints of it at a slightly weaker statistical level. Three other pulsar timing array (PTA) collaborations, consisting of Europe and India (EPTA), China (CPTA) and Australia (PPTA), have also released their results today. This hasn’t passed the statistical threshold that scientists need to call it a definite detection of the gravitational wave background, but astronomers are comfortable calling it very strong evidence, at a 3-sigma level of statistical significance, meaning the odds of such a signal cropping up in the absence of the gravitational wave background are around 1 in 1000. “It’s gone from a tantalising hint to something that is very strong evidence for the gravitational wave background,” says team member James McKee at the University of Hull, UK. Now, after a total 15 years of observations, the NANOGrav team has seen this signature in the signal for the first time, across a range of different gravitational wave frequencies. NANOGrav used the Green Bank Telescope in West Virginia 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. 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. 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. To find this mysterious hum, astronomers have been tracking rapidly rotating neutron stars called pulsars that blast out light with extreme regularity. 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. 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. The fabric of the universe is constantly rippling, according to astronomers who have discovered a background buzz of gravitational waves. Pulsars have helped reveal ripples in space-time throughout the universe
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