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What’s Lurking in the Ticks from Cosmic Clocks?

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A pulsar timing array
Artist's illustration of a pulsar timing array. [Tonia Klein/NANOGrav]

Deep within the universe, supermassive black holes are swirling around each other, giving off an immense amount of energy that travels at the speed of light through a fabric of ticking stellar clocks. Have we been looking long enough to catch their distant background whisper?  

In 2015, LIGO announced the first detection of ripples in the fabric of spacetime, and since then, scientists have detected ~90 gravitational-wave events (35 of which were announced this week!), consisting of black hole–black hole mergers, neutron star–neutron star mergers, and black hole–neutron star mergers. These different sources each produce their own frequency of gravitational waves depending on the mass of the object. For instance, binary neutron star mergers are very energetic and happen quickly, so they produce high-frequency waves, but mergers of supermassive black hole binaries happen much more slowly and generate much weaker, low-frequency waves. Much like how you can’t detect a gamma-ray burst with an infrared telescope, you can’t observe gravitational waves from binary supermassive black holes with something meant for binary neutron star mergers. This is where pulsar timing arrays come in.  

A Galaxy-Sized Gravitational Wave Detector

Different periods of gravitational waves and what instruments can detect them

Different frequencies of gravitational waves that can be detected by various instruments that can detect them (left to right: BICEP2, the Green Bank Telescope LISA, LIGO). [Shami Chatterjee/NANOGrav]

Pulsars are super dense, rapidly rotating neutron stars that emit radio radiation that crosses our sightline like a lighthouse as the star spins. Pulsars are incredibly periodic and rival atomic clocks in terms of precision, so we can predict exactly when a pulse will arrive. Pulsar timing arrays are collections of millisecond pulsars (which are especially stable and spin hundreds of times per second) around the sky that are monitored for changes in the time of arrival of their pulses. Because they’re scattered throughout the galaxy, a pulsar timing array is essentially a galaxy-sized gravitational wave detector. If a gravitational wave comes between us and a pulsar, it will show up in the pulse arrival times because the distance between the pulsars will have changed slightly as space is stretched and squeezed by the wave.  

When searching for gravitational waves, the goal is not only to detect them but also to characterize them. The passage of a gravitational wave changes the arrival times of pulsar signals in a distinctive way that can be described by something known as the Hellings–Downs curve. By obtaining many times of arrival from pulsars all around the sky, astronomers can look for signs of this curve and use it to characterize the source of the gravitational waves. 

Angle between pulsars vs. the correlation between their times of arrival

The Hellings–Downs Curve, which plots the separation between pulsars against the correlation between their arrival times. If a gravitational wave passes through, it will show up in the times of arrival between different pairs of pulsars on the sky and follow this curve exactly. [Mike Zevin/Astrobites]

Hunting for the Hellings–Downs

But this is easier said than done! The gravitational waves that can be detected by timing arrays are incredibly weak, so an immense amount of data is needed to identify these elusive signals — and pulsar timing efforts around the world have been searching for decades without any luck. Recently, however, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration found a signal in their data that could be a sign of lurking gravitational waves.

While NANOGrav’s detection may be the long-awaited signal from these background gravitational waves, we can’t rule out the possibility that this is noise in the data, such as dust in the interstellar medium or certain phenomena in pulsars that change the spin period slightly, such as glitching. To confirm NANOGrav’s results, we ideally need independent observations of the Hellings–Downs curve from another group.  

The angle between the pulsars against the correlation between times of arrival the team measured. The dotted line is the predicted correlation, the solid lines are the distributions, and the red is the error in those measurements. Their measurements could indicate a Hellings–Downs curve but their errors show that this may not be the case. [Goncharov et al. 2021]

Recently, a team from Parkes Pulsar Timing Array, led by Boris Goncharov (Swinburne University of Technology, Australia), set out to look for these signals in their own data to see if they could verify NANOGrav’s results. Using the Parkes Telescope in Australia, the team measured the arrival times of 26 pulsars for up to 15 years. They performed Bayesian analyses on the pulsars and modeled the signals in their data. What they find is data consistent with what NANOGrav found: a low-frequency signal that’s common to all their pulsars. They also find a gravitational wave amplitude consistent with NANOGrav’s. Goncharov and collaborators were able to rule out some alternative explanations that could mimic a gravitational wave signal, but they couldn’t definitively link the low-frequency rumblings they observed to gravitational waves from supermassive black hole binaries. Though there is a hint of something, they do not find evidence for or against the holy grail of pulsar timing: the Hellings–Downs curve.  

An International Pursuit

International efforts are currently underway to combine data from telescopes around the world to combine data from telescopes around the world to create one big data set to search through for gravitational waves. With hints of some kind of signal in NANOGrav data, Parkes Pulsar Timing Array data, and, most recently, European Pulsar Timing Array data, we may not be far from hearing the whisper of supermassive black hole binaries.  

Citation 

“On the Evidence for a Common-spectrum Process in the Search for the Nanohertz Gravitational-wave Background with the Parkes Pulsar Timing Array,” Goncharov et al 2021 ApJL. 917 2. doi:10.3847/2041-8213/ac17f4