Here’s How Scientists Plan to Watch Supermassive Black Hole Collisions

“When these objects merge, there is more energy produced than the rest of the universe put together.”

May 24 2019, 1:00pm

Concept art of a supermassive black hole merger. Image: ESA

It’s difficult to fathom the immensity of a supermassive black hole, a special kind of object that sits in the center of large galaxies and can grow to billions of times the mass of the Sun.

But what’s even more mind-boggling is imagining the cosmic fireworks when these giants crash into each other and unite as one.

“When these objects merge, there is more energy produced than the rest of the universe put together,” Paul McNamara, a scientist at the European Space Agency (ESA), told Motherboard over the phone.

Scientists have never directly witnessed supermassive black hole mergers, but McNamara and his colleagues plan to change that in the coming decades with two next-generation space observatories, according to an ESA release published on Thursday.

The missions—the Advanced Telescope for High-ENergy Astrophysics (Athena) and the Laser Interferometer Space Antenna (LISA)— are both unprecedented.

Overview of the Athena and LISA missions. Image: ESA/S. Poletti

Athena, currently on track for a 2031 launch, will be the largest X-ray observatory ever built, capable of spotting high-energy sources with up to 100 times the precision of past missions.

LISA, slated for liftoff in 2034, is a constellation of three satellites that will fly 1.5 million miles apart in an orbit that trails Earth’s path around the Sun. It will be the first gravitational wave detector in space, attuned to hear ripples in the fabric of spacetime produced by disruptive cosmic events such as supermassive black hole collisions.

Though it will take more than a decade to develop the advanced missions, it will be worth the wait if they are able to give humanity its first glimpse of these colossal mergers.

“If any of these objects ever merge in the universe, LISA will detect them,” said McNamara, who is on the LISA team at ESA. “It’s quite a phenomenal statement to say, but we can measure these objects out beyond what we call cosmic dawn, before the first stars in the universe.”

LISA is like a super-sized outer space version of LIGO, the ground observatory that earned the 2017 Nobel Prize for Physics for the first detection of gravitational waves. But where LIGO can only hear mergers of small black holes, involving objects about 10 times the mass of the Sun, LISA is designed to pick up low-frequency gravitational waves produced by holes containing millions or billions of solar masses.

Once LISA detects the waves released by an impending crash between two supermassive black holes, ESA scientists can point Athena at that part of the sky and capture the burst of high-energy radiation from the merger.

This combination of multiple lines of observational evidence is known as multi-messenger astronomy. Applying it to supermassive black hole mergers could help resolve major mysteries about the universe, such as why some galactic cores are far more active and luminous than others.

“If we can pinpoint the source, we may actually see one of these active galaxies switch on,” McNamara said. “At the moment, we don’t have any idea how or why it happens.”

“That’s what I love about science,” he added. “Gravitational wave astronomy is such a new field and we don’t know the answers.”