Using a signal from dozens of fast-spinning dead stars, astrophysicists have come closer to realizing their goal of detecting a background rumble of gravitational waves in the universe.
When the existence of gravitational waves was confirmed in 2016, opened a new field of astrophysical research. Two black holes collided, causing a ripple in the space-time fabric detected on Earth when it bleeped the sensitive instruments of the Laser Interferometer Gravitational-Wave Observatory. Since then, scientists have picked up more gravitational waves produced by massive smash-ups, but they are also looking for ways to see the so-called gravitational wave background. To use a metaphor, we’ve discovered big waves rocking our planetary boat, and now we want to see the whole chaos of waves swirling in the cosmic ocean.
Last month, the North American Nanohertz Observatory for Gravitational Waves published the latest dataset in The Astrophysical Journal Letters. The data – 12 and a half years of it – is compiled from observations from the Green Bank Telescope in West Virginia and the recently collapsed Arecibo Observatory in Puerto Rico. The article describes what could be a telltale pattern in the light of 45 pulsars. It is a step towards identifying the gravitational wave background.
“What we find specifically is a low-frequency signal, and it’s a common signal among all the pulsars in the array,” Joseph Simon, an astrophysicist at the University of Colorado at Boulder and lead author of the recent paper, said in a press. conference today. Simon said the signal “is what we expect the first hints of the gravitational wave background to look like.”
Pulsars are the dense, spinning remnants of some dead stars. Millisecond pulsars spin extremely fast – hundreds of times per second – and a select few do so reliably enough to allow researchers to catalog the minute changes in our planet’s relative position to those pulsars. By using the radio wave pulses from the Milky Way’s pulsars in an array, the team essentially created a galaxy-sized network of detectors for low-frequency gravitational waves generated by the orbits of supermassive black holes rather than their collisions. The gravitational background the team is looking for seems more like a constant, jumbled space-time murmur than an isolated blip as detected by LIGO in 2016.
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Gravitational waves were predicted by general relativity. Decades of astrophysical analysis have concluded that such waves would cause changes in the timing of the light from pulsars reaching Earth. A gravitational wave background would affect the light we pass from the pulsars based on everyone’s location and relative position, and some correlated pattern in changes in that light would indicate a gravitational wave background. The team has not officially found the pattern, but they think they have noticed its beginning.
Although the astrophysicists have examined more than 12 years of data from their pulsar array, they still need more time and more pulsars to be sure of the pattern. The waves the team documents have much longer wavelengths than the gravitational waves detected by LIGO in 2016, so the study’s progress is gradual.
One challenge is that the pulsars’ pulses are timed using atomic clocks, which can lose their precision. But atomic clock errors were ruled out in the recent data, according to Scott Ransom, a staff astronomer at the National Radio Astronomy Observatory and a co-author of the recent paper.
Ransom compares the gravitational waves to waves in the ocean of space-time, from various sources near and far. The gravitational waves interfere with each other and collide with an Earth dicing in that ocean and slightly stretching and compressing the planet.
“What we can deduce from that is like whether you can tell that the ocean is calm or rough,” Ransom said in a telephone conversation. “We can get a lot of information about the entire history of the universe and how galaxies merge and interact with each other just by seeing this background signal.”
Both Simon and Ransom mourned the loss of the Arecibo Observatory radio dish, which collapsed in December after two cable failures. The research team pulled data from the observatory until the first cable broke, and the recent paper only included data up to and including 2017. Their current dataset will provide some sort of the afterlife of Arecibo as it will contribute to the search for a gravitational wave background for years to come.