Evidence of background hum of gravitational waves in pulsars

More than 15 years of observations of pulsars has revealed the low-frequency hum as spacetime is stretched and squeezed by gravitational waves.

Evidence is now pointing to the fact that the rhythms of millisecond pulsars in our region of the Milky Way galaxy are affected by the long-wavelength gravitational radiation. A number of studies pointing to this have been published recently. Among the most comprehensive papers was one published by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) in the Astrophysical Journal Letters.

Pulsars are rotating neutron stars which give off pulses of radiation at very regular intervals ranging from milliseconds to seconds.

Gravitational waves

Gravitational waves were first detected in 2015. This phenomenon was predicted by Einstein more than 100 years ago as a result of his General Theory of Relativity which describes a fabric of spacetime moulded by massive objects like stars and black holes.

Short-wavelength gravity waves have been observed as fluctuations in spacetime caused by the merger of small black holes or neutron stars several hundred times the mass of the Sun.

Are the long-wavelength gravitational waves – which oscillate on a scale of years or decades – also produced by black holes?

Supermassive black hole mergers

Computer simulations of supermassive black hole mergers were compared with the signatures observed by NANOGrav. Over the course of the universe’s 13.8-billion-year lifespan, gravitational waves created by such mergers would overlap like ripples from many stones tossed into a pond, producing the background rumble.

“I guess the elephant in the room is we’re still not 100% sure that it’s produced by supermassive black hole binaries,” says co-author, University of California, Berkeley, physicist Luke Zoltan Kelley. “That is definitely our best guess, and it’s fully consistent with the data, but we’re not positive.”

“If it is binaries, then that’s the first time that we’ve actually confirmed that supermassive black hole binaries exist, which has been a huge puzzle for more than 50 years now.”

“Other groups have suggested that this comes from cosmic inflation or cosmic strings or other kinds of new physical processes which themselves are very exciting, but we think binaries are much more likely,” Kelley comments.

Most large galaxies have a supermassive black hole at their centre. These objects can be millions of times heavier than our Sun.

But it is impossible to rule out that at least some of these central black holes are actually supermassive black hole binaries.

Galactic collisions

It has not been clear if the collision of large galaxies can bring supermassive black holes close enough together to merge.

“Galaxy mergers bring the two supermassive black holes together to about a kiloparsec or so – a distance of 3,200 light years, roughly the size of the nucleus of a galaxy. But they need to get down to five or six orders of magnitude smaller separations before they can actually produce gravitational waves,” Kelley explains.

The fact that the team has found low-frequency gravitational waves suggests that the supermassive black holes in large galaxy mergers do get close enough to produce ripples in spacetime.

“This is key evidence for gravitational waves at very low frequencies,” says Vanderbilt University’s Stephen Taylor, who co-authored the research. “After years of work, NANOGrav is opening an entirely new window on the gravitational-wave universe.”

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