"Galaxy-Sized" Space Observatory May Have Detected Subtle Gravitational Waves Background

Representative illustration of the Earth embedded in space-time which is deformed by the background gravitational waves and its effects on radio signals coming from observed pulsars. Tonia Klein / NANOGrav

The gravitational waves that our Earth-based detectors catch are not the only ones present in the universe. Scientists expect that a more subtle and persistent murmur permeates the universe, known as the gravitational wave background. Researchers have now reported the first possible hint of detection.

Detecting gravitational waves from black holes or neutron star collisions is not an easy task. Gravitational waves squeeze and distort space-time but only by a fraction of the size of an atom. To catch the gravitational wave background researchers had to be even more creative with how they looked for it.

Enter pulsars. Pulsars are neutron stars pulsating at extremely regular intervals, a characteristic that makes them an incredible cosmic clock. The joint US and Canadian project, North American Nanohertz Observatory for Gravitational Waves (NANOGrav), monitored pulsars across the Milky Way as a galactic gravitational wave observatory. Changes to timings of the pulsar pulsations could be a gravitational wave signal but a lot of data is required to confirm and understand.

In The Astrophysical Journal Letters, the team report finding the first exciting result from 13 years of data, a low-frequency signal. The team was able to rule out many sources of errors and effects unrelated to gravitational waves, such as matter in our own Solar System. What's left is a strong hint that could be the gravitational-wave background.

“It is incredibly exciting to see such a strong signal emerge from the data,” lead author Dr Joseph Simon, from the University of Colorado Boulder, said in a statement. “However, because the gravitational-wave signal we are searching for spans the entire duration of our observations, we need to carefully understand our noise. This leaves us in a very interesting place, where we can strongly rule out some known noise sources, but we cannot yet say whether the signal is indeed from gravitational waves. For that, we will need more data.”

If the gravitational wave background signal is confirmed, it will provide a new look into some of the mysteries of the universe, such as how the supermassive black holes at the center of galaxies merge. This has been a head-scratching issue for physicists for quite some time.

"These enticing first hints of a gravitational wave background suggest that supermassive black holes likely do merge and that we are bobbing in a sea of gravitational waves rippling from supermassive black hole mergers in galaxies across the universe," said Professor Julie Comerford, also of CU Boulder and a NANOGrav team member, in a statement.

This work, presented virtually at the 237th meeting of the American Astronomical Society, has a bittersweet note. A huge part of the data collection came from the now-defunct Arecibo telescope that collapsed last year and has now been decommissioned.

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