This computer simulation shows the collision of two black holes, which produces gravitational waves. Photo: SXS

Gravitational waves, the historic discovery announced early last year, were a breakthrough in humankind’s understanding of the universe. Albert Einstein had been proven right, and other cosmological pieces of the puzzle had fallen into place.

But the gravitational waves detected by LIGO and Virgo, ripples in space-time created by the collision of two black holes, are just small ripples by comparison with some of the largest phenomena in the universe: the merger of supermassive black holes. The wavelengths of the energy created by those incredible unions cannot be detected directly by our most advanced tools, because of their low frequency – like it’s the deep roar of a large species animal, as opposed to the birdsong we’ve now become accustomed to hearing. 

Now, a team of scientists say that an indirect way of measuring the ripples in the very fabric of the universe, by looking at the fluctuations from radio pulses from dead stars, will provide confirmed observations of the merging supermassive black holes, they write in a new study in the journal Nature Astronomy this week.

“Observing low-frequency gravitational waves would be akin to being able to hear bass singers, not just sopranos,” said Joseph Lazio, co-author and chief scientist for the Deep Space Network based at NASA’s Jet Propulsion Laboratory.

The Pulsar Timing Array is made up of three sites: the Parkes Pulsar Timing Array in Australia, the North American Nanohertz Observatory for Gravitational Waves, and the European Pulsar Timing Array.

These three facilities can look at the timing of the spinning dead pulsars – which act as indirect detectors astrophysicists can use.

These “celestial lighthouses” constantly emit radio waves. But when gravitational waves ripple in the space between those dead stars and the Earth, they cause a distortion that can be measured and detected, said the group of scientists, whom included other institutions like the Center for Computational Astrophysics at the Flatiron institute in New York City.

The scientists did a massive mapping of the galaxies and the pulsars within a distance of 225 megaparsecs (roughly 730 million light-years) of the Earth.

They found roughly 91 continuous sources of gravitational waves. That translated to almost assuredly discovering the fluctuations in the next 10 years, as the pulsar detections continue to ramp up internationally, said Chiara Mingarelli, the lead study author, now of the Flatiron Institute but who worked on the scientists as a postdoc at Caltech and NASA’s JPL.

“By expanding our pulsar timing array over the next 10 years or so, there is a high likelihood of detecting gravitational waves from at least one supermassive black hole binary,” said Mingarelli.

“Detecting gravitational waves from billion-solar mass black hole mergers will help unlock some of the most persistent puzzles in galaxy formation,” said Leonidas Moustakas, a NASA scientist at JPL, in a release.

The most likely target yet identified is the galaxy NGC 3115 in the constellation Sextans, which is about 32 million light years away, and which was first seen by the human eye in the 18th century.

“The gravitational waves from these supermassive black hole binary mergers are the most powerful in the universe,” added Mingarelli.