Now let's find a pair of black holes

Last week, scientists studying black holes announced they managed to turn the entire Earth into a giant virtual telescope allowing them to create the image of a supermassive black hole at 55 million-year-olds. light.

Now another group of black hole researchers is investigating how to turn our entire galaxy into an even bigger gigantic black hole detector – this time looking for pairs of supermassive black holes, rotating around each other in distant galaxies.

The project, called NANOGrav, was described at a meeting of the American Physical Society in Denver, Colorado. It attempts to locate pairs of supermassive black holes through the effect of gravitational waves created by them on a class of astronomical objects called millisecond pulsars.

Gravitational waves are ripples in the structure of space-time, created by massive object movements, including black holes. These waves cause an expansion, a contraction or a vibration of the space, which distorts the environment in which we all live.

Pulsars are the collapsed remains of dead stars, which emit radio beams that sweep the sky like flashing blinking cosmic lighthouses. Millisecond pulsars flash so fast that they emit many pulses per second.

"It's 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 among the most accurate clocks in the universe," says Brad Tucker, astrophysicist and cosmologist at Australian National University, who is not part of the NANOGrav team. "Their observations even serve to calibrate GPS satellites."

Black holes have no direct effect on pulsars, but when galaxies fuse, say astrophysicists, supermassive black holes in their center are long in orbit before melting.

When these pairs surround themselves, they must emit gravitational waves that oscillate with their orbital cycle. The goal of NANOGrav is to detect these waves via their effect on the otherwise accurate synchronization of the pulsar signals that pass through them.

"When a gravitational wave passes over the Earth, it expands and compresses space-time," Simon explains. "Thus, the pulse of this pulsar will have to travel a slightly longer or slightly shorter distance. This will happen a little earlier or slightly after what we expect. "

It's not a huge effect. "The change we're looking for is less than a microsecond," says Simon – a big challenge to detect, as our planet revolves around the Sun, creating much greater differences in the time of day. Arrival of a given pulsar signal. whatever the minimal effect sought by the NANOGrav project.

This is not a quick effect either. The "nano" in the project name does not refer to nanoseconds. It's more like nanohertz: events that only end up in a billionth of a cycle per second.

In other words, a complete cycle lasts about 30 years.

To detect this, the NANOGrav team has been monitoring 48 pulsars since the end of 2006. This means that they have accumulated 12½ years of data, but they do not yet represent a fraction of the nanohertz cycle sufficient to detect it.

He is getting closer though.

"We expect that we can detect it in the next three to four years, depending on its real strength," says Simon.

The objective, he adds, is very different from that of the LIGO project (and its European counterpart, Virgo), which have successfully used laser detectors of several kilometers to identify the much faster oscillations of the gravitational waves created. by fusions of black holes and much smaller neutron stars (stellar mass).

It's also a world apart from a project at Louisiana State University in Baton Rouge, which has developed a "table" version of LIGO that incorporates extremely tiny mirrors, the diameter of a human hair, in order to increase the sensitivity. of the next series of advanced detectors used in LIGO and Virgo themselves.

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

"The work done by NANOGrav is fantastic," says Thomas Corbitt, team leader at Louisiana State University. "It's amazing to see that the same physics governs these very different black holes."

"It's another nifty way to probe extreme environments in space," says Tucker. "It's the kind of ideas that motivate me – using accurate observation for something completely different – much like the Kepler Space Telescope, designed to find planets, taught us a lot about stars and holes. blacks exploding. "

To know more about supermassive black holes, he continues, is important in itself. "We think almost all big galaxies have some," he says. "[They] are the ultimate laboratory for testing extreme physics – not just gravity, but time itself. "

"The big advantage," says David McClelland, director of the gravitational physics center at the Australian National University, is that LIGO and Virgo have already proven that gravitational waves exist and can be detected directly. "It's only a matter of time," he says, "until other projects such as NANOGrav detect them as well.