Listening for Gravitational Waves Using Pulsars

There are merging objects whose gravitational wave signals have not yet been detected: supermassive black holes, more than 100 million times more massive than our Sun. Most large galaxies have a central supermassive black hole.

This computer simulation shows the collision of two black holes, which produces gravitational waves. Image credit: NASA/SXS

When galaxies collide, their central black holes tend to spiral toward each other, releasing gravitational waves in their cosmic dance. Merging supermassive black holes create lower-frequency gravitational waves than the relatively small black holes LIGO (Laser Interferometer Gravitational-Wave Observatory) and similar ground-based experiments can detect.

To explore this uncharted area of gravitational wave science, researchers look to a natural experiment in the sky called a pulsar timing array. Pulsars are dense remnants of dead stars that regularly emit beams of radio waves, which is why some call them "cosmic lighthouses."

Because their rapid pulse of radio emission is so predictable, a large array of well-understood pulsars can be used to measure extremely subtle abnormalities, such as gravitational waves.

The new study concerns supermassive black hole binaries -- systems of two of these cosmic monsters. For the first time, researchers surveyed the local universe for galaxies likely to host these binaries, then predicted which black hole pairs are the likeliest to merge and be detected while doing so. The study also estimates how long it will take to detect one of these mergers.

Pulsar timing arrays are sensitive to gravitational wave signals from supermassive black holes that are spiraling toward each other and will not combine for millions of years. That's because galaxies merge hundreds of millions of years before the central black holes they host combine to make one giant supermassive black hole.

While bigger galaxies have bigger black holes and produce stronger gravitational waves when they combine, these mergers also happen fast, shortening the time period for detection. For example, black holes merging in the large galaxy M87 would have a 4-million-year window of detection.

By contrast, in the smaller Sombrero Galaxy, black holes mergers typically take about 160 million years, offering more opportunities for pulsar timing arrays to detect gravitational waves from them.

Black hole mergers generate gravitational waves because, as they orbit each other, their gravity distorts the fabric of space-time, sending ripples outward in all directions at the speed of light. These distortions actually shift the position of Earth and the pulsars ever so slightly, resulting in a characteristic and detectable signal from the array of celestial lighthouses.

Because all supermassive black holes are so distant, gravitational waves, which travel at the speed of light, take a long time to arrive at Earth. This study looked at supermassive black holes within about 700 million light-years, meaning waves from a merger between any two of them would take up to that long to be detected here by scientists.

Many open questions remain about how galaxies merge and what will happen when the Milky Way approaches Andromeda, the nearby galaxy that will collide with ours in about 4 billion years. (NASA)

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