Did the Pulsar Timing Array actually detect colliding primordial black holes?

The universe is filled with gravitational waves. We know this thanks to the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which recently announced the first observations of long-wavelength gravitational waves rippling through the Milky Way. The waves are likely caused by supermassive black hole mergers, but can we prove it?

Most of the gravitational waves we’ve observed so far come from stellar-mass black hole mergers. These mergers create short-wave gravitational squeals that observatories like LIGO and Virgo can detect. Given the scale of supermassive black holes, the gravitational waves they generate have wavelengths on the order of light years. Their wavelengths are too long and their frequencies too low for conventional observatories.

So NANOGrav took a different approach using pulsars. Pulsars are rapidly rotating neutron stars with very regular radio pulses, like a cosmic clock. NANOGrav observed the pulses of 67 pulsars for 15 years, looking for small changes in their pulse times. They found a shift in time consistent with low-frequency gravitational waves, which cause pulsars to wobble slightly.

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It’s an amazing find. But the authors of the NANOGrav papers are careful not to assume too much. While they note that supermassive black holes (SMBHs) are the likely source, the team doesn’t say it’s proven. This is where a new document comes into play.

The authors agree with the idea that these background gravitational waves are likely caused by supermassive black holes, but consider two different types. The first is the usual type we know. Supermassive black holes found at the center of most galaxies. When two galaxies collide, their black holes can enter into a close orbit with each other, ultimately resulting in their merger. The second type, of greater interest to the authors, is that of primordial black holes.

Formation of the universe with and without primordial black holes. Credit: European Space Agency

Primordial black holes are hypothetical black holes that formed in the earliest moments of the universe. They are usually thought to be tiny, with masses roughly equal to that of an asteroid. But some models support supermassive primordial black holes. These would have formed the seeds for the first galaxies, allowing them to form rapidly, JWST observations suggest.

As the paper notes, the statistical signal of classical SMBHs and primordial SMBHs is different, as is the predicted strength of gravitational waves. The authors find that if we assume that primordial black holes were evenly distributed in the early universe, then the observed gravitational waves are too strong to be caused by primordial black hole mergers. If, however, primordial black holes were clustered, then they could be the source of the observed waves. At the same time, standard supermassive black holes should be about 10 times more common than we thought to explain the strength of these gravitational waves. So the results are inconclusive.

The NANOGrav result is only the first observation of cosmic gravitational waves. With more data, astronomers will be able to distinguish between the two sources. It is only a matter of time.

Reference: Agazie, Gabriella, et al. “The 15-year NANOGrav Dataset: Evidence for a Gravitational Wave Background”. The letters from the astrophysicist diary 951.1 (2023): L8.

Reference: Depta, Paul Frederik, Kai Schmidt-Hoberg and Carlo Tasillo. “Do Pulsar Timing Arrays Observe the Merging of Primordial Black Holes?” prepress arXiv arXiv:2306.17836 (2023).

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