Now let’s find a pair of black holes

Last week, scientists studying black holes reported that they’d managed to turn the entire Earth into a giant virtual telescope that allowed them to make an image of a supermassive black hole 55 million light years away.

Two supermassive black hole engines are seen as red dots
In this radio image, two supermassive black hole engines are seen as red dots, their large-scale jet structures clearly visible. Credit: NANOGRAV

Now, another group of black hole researchers is reporting on a way to turn our entire galaxy into an even more gargantuan black hole detector – this time looking for pairs of such supermassive black holes, orbiting each other in distant galaxies. 

The project, called NANOGrav, was described at a meeting of the American Physical Society in Denver, Colorado. It is attempting to spot supermassive black hole pairs via the effect of gravitational waves created by them on a class of astronomical objects known as millisecond pulsars.

Gravitational waves are ripples in the fabric of space-time, created by movements of massive objects, including black holes. These waves cause space to expand, contract, or vibrate, thereby distorting the medium in which we all live. 

Pulsars are the collapsed remnants of dead stars, which emit radio beams that sweep across the heavens like the blink-blink-blink of cosmic lighthouses. Millisecond pulsars blink so fast that they emit numerous pulses per second. 

“They’re like really stable clocks, scattered all over the Milky Way,” says Joseph Simon, an astrophysicist at NASA’s Jet Propulsion Laboratory, in Pasadena, California.

“Pulsars are some of the most accurate clocks we have in the universe,” says Brad Tucker, an astrophysicist and cosmologist at Australian National University, who is not a member of the NANOGrav team. “Observations of them are even used to calibrate GPS satellites.”

Black holes have no direct effect on pulsars, but when galaxies merge, astrophysicists believe, the supermassive black holes at their centres go into orbit around each other for a long time before they too eventually merge.

As these pairs circle each other, they should emit gravitational waves that oscillate in tandem with their orbital cycle. The goal of NANOGrav is to detect these waves via their effect on the otherwise-precise timing of pulsar signals coming through them. 

“As a gravitational wave passes by the Earth, it will stretch and squeeze space-time,” Simon says. “So, the pulse from that pulsar will have to travel a slightly longer distance or a slightly shorter distance. It will get here a bit sooner or slightly after what we expect.” 

Not that it’s a huge effect. “The change we are searching for is less than a microsecond,” Simon says – a formidable challenge to detect, given that our planet also spins and orbits the Sun, both of which create far greater differences in the arrival time of any given pulsar signal at any given radio telescope than the tiny effect the NANOGrav project is looking for.

Nor is it a rapid effect. The “nano” in the project’s name doesn’t refer to nanoseconds. Rather, it refers to nanohertz: events that complete only 1 billionth of a cycle per second. 

In other words, a full cycle takes about 30 years.

To detect this, the NANOGrav team has been monitoring 48 pulsars since late 2006. That means they’ve accumulated 12½ years of data, but that’s not yet a large enough fraction of a nanohertz cycle to be able to spot it.

It is, however, getting close. 

“We are expecting that within the next three to four years, we will be able to detect this, depending on how strong it actually is,” Simon says.

The goal, he adds, is very different from that of the LIGO project (and its European counterpart, Virgo), which have successfully used multi-kilometer-long laser detectors to spot the much more rapid gravitational-wave oscillations created by the mergers of much smaller (stellar mass) black holes and neutron stars. 

It’s also a world apart from a project at Louisiana State University, Baton Rouge, which has built a “table-top” version of LIGO that incorporates extremely tiny mirrors, about the diameter of a human hair, in an effort to ratchet up the sensitivity of the next round of advanced detectors used in LIGO and Virgo themselves. 

But gravitational wave researchers of all types are impressed by NANOGrav’s vision. 

“The work that NANOGrav does is fantastic,” says Thomas Corbitt, leader of the Louisiana State University team. “It’s amazing to see that the same physics governs these vastly different black holes.”

“This is yet another clever way to probe extreme environments in space,” adds Tucker. “These are the sorts of ideas that get me excited – using a precise observation for something completely different – much like how the Kepler Space Telescope, which was designed to find planets, has told us a lot about exploding stars and black holes.”

Learning more about supermassive black holes, he continues, is important in and of itself. “We think nearly every large galaxy has them,” he says. “[They] are the ultimate laboratory for testing extreme physics—not only of gravity but time itself.”

“The great thing,” adds David McClelland, director of Australian National University’s Centre for Gravitational Physics, is that LIGO and Virgo have already proved that gravitational waves exist, “and can be detected directly.” It’s only a matter of time, he says, until other projects, such as NANOGrav also detect them.

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