Using a signal from dozens of fast-spinning, dead stars, astrophysicists have come closer to realizing their goal of detecting a background noise of gravitational waves in the universe.
When the existence of gravitational waves was confirmed in 2016, a new field of astrophysical research opened up. Two black holes collided and emitted a ripple in the substance in space-time, which was discovered on Earth when it caused a flap in the sensitive instruments of the Laser Interferometer Gravitational-Wave Observatory. Since then, researchers have found several gravitational waves produced by massive smash-ups, but they have also been looking for ways to see the so-called gravitational wave background. To use a metaphor: We have discovered large waves that shook our planetary boat, and now we will see the whole mess of waves plunge into the cosmic sea.
Last month the North American Nanohertz Observatory for Gravitational Waves published its latest dataset in The Astrophysical Journal Letters. The data – 12 and a half years of this – were compiled from observations made by the Green Bank Telescope in West Virginia and recently collapsed the Arecibo Observatory in Puerto Rico. The paper describes what can be a revealing pattern in the light of 45 pulses. It is a step towards identifying the gravitational wave background.
“What we find specifically is a low-frequency signal, and it is a common signal among all pulsars in the matrix,” Joseph Simon, an astrophysicist at the University of Colorado Boulder and lead author of the latest paper, said in a news 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, rotating remnants of some dead stars. Millisecond pulsars rotate extremely fast – hundreds of times per second – and a select few do so reliably enough to allow scientists to catalog the small changes in our relative relative position to these pulsars. Using radio wave pulses from the Milky Way’s pulsars in a series, the team effectively evoked a galaxy-sized network of detectors for low-frequency gravity waves, generated by the orbits in supermassive black holes instead of their collisions. The gravitational background that the team is looking for looks more like a constant, mixed-up murmur in space-time than an isolated blip like the one discovered by LIGO in 2016.
Gravitational waves were predicted by general relativity. Decades of astrophysical analysis have concluded that such waves would cause changes in the timing of pulsar light reaching the earth. A gravitational wave background will affect the light we see from pulsars based on each individual’s location and relative position and a particular correlated pattern in changes to this light would indicate a gravitational wave background. The team has not officially found the pattern, but they think they have seen the start of it.
Although astrophysicists have studied over 12 years of data from their range of pulsars, they still need more time and more pulsars to be sure of the pattern. The waves that the team documents have much longer wavelengths than the gravitational waves discovered by LIGO in 2016, so the research has been gradual.
One challenge is that the pulses of the pulser are timed using atomic clocks, which can lose their precision. But atomic clock errors were ruled out in recent data, according to Scott Ransom, a staff astronomer at the National Radio Astronomy Observatory and co-author of the latest paper.
Ransom compares the gravitational waves to waves in the ocean of space coming from various sources near and far. The gravitational waves interfere with each other and bounce up against a land that bubbles in the ocean that stretches and compresses the planet ever so little.
“What we can deduce from this is like you can see the sea calm or rough,” Ransom said in a phone call. “We can get a lot of information about the full history of the universe and how galaxies merge and interact just by seeing this background signal.”
Both Simon and Ransom mourned the loss of the Arecibo Observatory radio bowl, 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 contained data through 2017. Their current data set will provide a sort of aftermath of Arecibo, as it will contribute to the search for a gravitational wave background in the coming years.