Gravitational waves from supermassive black hole binaries might be ‘right around the corner’

In 2016, an international team co-led by Northwestern professor Vicky Kalogera used the Laser Interferometer Gravitational-Wave Observatory (LIGO) to first detect gravitational waves from the merger of two stellar-mass black holes, which resulted in obvious, short-lived ripples in space-time. But supermassive black hole binaries are too big and much too far apart for Earth-based equipment like LIGO to detect. These monster pairs create waves so long that it could take years or even decades for their gravitational waves to fully wash over Earth. Even when NASA and the European Space Agency launch LISA (a space-based gravitational-wave detector for which Northwestern professor Shane Larson is a co-principal investigator) in the early 2030s, it still will not be able to detect such enormous waves.

“LIGO can only detect wavelengths that fit within its arms,” Witt said. “We have to look for much lower wave frequencies. We are sensitive to supermassive black hole pairs that can take a month or even up to 15 years to orbit each other. So, we’re looking for a steady signal that could blend into the background.”

Pulsars tick like a clock

To overcome this obstacle, an international collaboration of researchers established the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which hunts for gravitational waves using pulsars, a type of rapidly rotating neutron star born in the supernova explosion of a massive star at the end of its life. Just like a lighthouse, a pulsar emits a light beam that flashes by as it rotates.

“Because pulsars spin so stably, we see little flashes of light that tick like a clock,” Witt said. “We watch that light with ground-based radio telescopes. If the clock ticks arrive either a little bit early or a little bit late, this is a sign that it could have been affected by a gravitational wave.”

Map of pulsar locations (shown as white stars) with new pulsar locations added from the new dataset (white stars outlined in red).

NANOGrav tracks 75 pulsars — 45 of which were used in this study — located all across the night sky. Their beams of light take mere milliseconds to flicker past Earth. So, in this case, “a little bit early or a little bit late” could mean a fraction of a nanosecond. Therefore, NANOGrav’s techniques must be incredibly sensitive to capture these nearly imperceptible changes. 

By looking across the entire sky, Witt and the NANOGrav team search for specific patterns from all pulsars together. According to theory, supermassive black hole binaries should emit gravitational waves that literally stretch and squeeze (or strain) space-time on their way to Earth. Warped space-time will affect pulsars’ light beams in such a way that indicates an elusive pair of monster black holes.

‘Red noise can trick us’

But, of course, pulsars also generate their own noise, which can muddy the signals.

“Pulsars do have some intrinsic noise called ‘red noise,’” Witt said. “Their insides might slowly wobble a little bit, which you wouldn’t be able to see unless you were looking as closely as we are. That red noise looks similar to the broad gravitational wave noise that we’re looking for. We have to tease that apart.”

Last year, the NANOGrav team published a study finding a red noise process in all pulsars that shares the same common characteristics. Without more evidence, however, NANOGrav cannot attribute this to gravitational waves. In the new study, Witt and her team found that this red noise still must be carefully considered in order to definitively detect gravitational waves from individual supermassive black hole binaries.

“When a gravitational wave becomes detectable, it looks very similar to red noise at first glance,” Witt said. “The red noise can trick us. Our new study tells us that we must look closely to avoid getting confused. That will be important to watch for when we do finally detect gravitational waves.”

Although NANOGrav has yet to detect supermassive black hole binaries with gravitational waves, Witt’s new paper brings the field closer than ever. By leveraging the 12.5-year dataset, the researchers created new models to accurately account for uncertainties in the pulsar data and implement new techniques to account for the red noise.

Confirming candidates

These new models provide the tightest limits yet on the strength of gravitational waves emitted from supermassive black hole pairs. Previously, other researchers discovered potential supermassive black hole binaries with light-based telescopes. NANOGrav could eventually confirm that these potential candidates are, indeed, supermassive black hole binaries.

“With our new methods, we might be able to confirm this sooner,” Witt said. “Or, if we continue gathering and analyzing data, then we might be able to rule it out as a candidate. It might just be something else weird going on in the galaxy.”