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When black holes need a place to crash, they prefer a nice, bright quasar.

So says Chiara Mingarelli, an assistant professor of physics in Yale's Faculty of Arts and Sciences and a key member of an international research team exploring fundamental questions about the structural foundations of the universe. Mingarelli's latest research puts quasars—the brightest objects in space, fueled by gases falling into supermassive black holes—at the center of this work.

Two years ago, Mingarelli and the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) made a major splash in the scientific community with the first direct evidence of a "background" of gravitational waves in the universe. The discovery suggested that gravitational waves, which are caused by slowly merging pairs of supermassive black holes, could be detected from Earth within a background field of low-frequency energy.

NANOGrav centered its detection methods around pulsars, which are the collapsed cores of massive stars that have exploded. Pulsars, which rotate rapidly, emit precisely timed radio signals.

In in The Astrophysical Journal, Mingarelli and her colleagues used pulsars to show that black hole collisions are five times more likely to be found in galaxies with a quasar.

Mingarelli spoke with Yale News about the new finding, how it relates to gravitational waves, and what comes next. The conversation has been edited and condensed.

What is the significance of being able to identify and catalog a gravitational wave network? How would it benefit society?

Right now, we are combining traditional astronomy—which looks at the universe with radio waves, X-rays, optical waves, and more—with gravitational wave astronomy. It's like we've discovered the fact that we have ears and can now hear the universe instead of just looking at it. Gravitational waves come straight from the source—merging supermassive black hole binaries—and aren't affected by gas and dust on their way to the Earth. This makes them exceptionally clean probes of extreme physics, not accessible by any other means.

It's exciting to think about how this work will eventually benefit society. Einstein's General Relativity gave us GPS [global positioning satellites] about 100 years later, and lasers, MRI, and Wi-Fi all came from scientists asking "why" before anyone knew "what for." Plus, imagine 100 years ago telling people about GPS—could they even imagine what we mean? I can't wait to see what the eventual applications of this work will be.

How does your new study fit into NANOGrav's overall work?

We believe there is a gravitational wave background that is composed of millions of slowly merging pairs of supermassive black holes. In 2023 we found evidence of this, and the next big thing is the detection of individual black hole pairs.

In this paper, we predict that quasars are up to five times more likely than any other type of galaxy to host these pairs of black holes. We conclude that quasars should be the number one targets to search for pairs of merging supermassive black holes.

Furthermore, my team of Yale graduate and undergraduate students and I are currently using the results of this paper to identify target galaxies that host supermassive black hole binaries. A paper with those results will likely come out by the end of the summer.

What other aspects of the current study stand out?

This is the first study to statistically constrain the supermassive black hole binary population by combining the gravitational wave background measurement with quasar variability. It represents a novel approach to characterizing the binary population.

If confirmed, even a small population of binary quasars could anchor our model of gravitational wave sources at low frequencies and pave the way for direct detections.

Your approach hinges on the use of pulsar timing arrays. What makes pulsars an advantageous tool for locating black hole mergers?

Pulsar timing arrays monitor ultra-stable stars called pulsars, which emit signals that are, in effect, excellent clocks.

Gravitational waves stretch and squeeze the fabric of space and time itself. When space/time is squeezed, pulsar pulses arrive early. When space/time is stretched, the pulses arrive late. The overall stretch and squash is about 1 part in a million billion—or the size of a virus divided by the diameter of the Earth. Very small!

The fact that we can monitor these pulsars for years to decades, with amazing accuracy—within 100 nanoseconds—means that we can detect gravitational waves with intervals of years-to-decades. It makes pulsar arrays the perfect instruments to detect gravitational waves from supermassive black holes, since they create gravitational waves with such long periods.

But without a list of targets, we can't localize a pair of merging supermassive black holes to anything smaller than an error box big enough to contain thousands of galaxies.

Beyond establishing the gravitational wave background network, what can we learn from the new study?

Constraining the demographics of supermassive black hole binaries is central to our understanding of galaxy evolution, black hole growth, as well as the gravitational wave background. This work provides a data-driven framework for identifying the host galaxies of supermassive black hole pairs and lays the foundations for future searches.