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 the mergers of supermassive black holes, but can we prove it?

Most of the gravitational waves we’ve observed so far have been from stellar-mass black hole mergers. These mergers create short-wavelength gravitational chirps that observatories such as 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 far too long and their frequencies far too low for conventional observatories.

So NANOGrav took a different approach using pulsars. Pulsars are rapidly spinning 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 timings. They found a shift in the timings consistent with low-frequency gravitational waves, which wobble the pulsars ever so slightly.

It’s an amazing discovery. But the authors of the NANOGrav papers are careful not to presume too much. While they note that supermassive black holes (SMBHs) are the likely source, the team doesn’t claim it to be proven. This is where a new paper comes in.

The authors agree with the idea that these background gravitational waves are likely caused by supermassive black holes but look at two different types. The first is the usual kind we’re familiar with. The supermassive black holes found at the center of most galaxies. When two galaxies collide, their black holes can enter a close mutual orbit, eventually ending with their merger. The second type, of most interest to the authors, is primordial black holes.

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 that of an asteroid. But some models argue for supermassive primordial black holes. These would have formed the seeds for early galaxies, allowing for them to form quickly, as some JWST observations suggest.

As the paper notes, the statistical signal from classic SMBHs and primordial SMBHs are different as are the predicted strength of the gravitational waves. The authors find that if we assume 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, the primordial black holes were clustered, then they might be the source of the observed waves. At the same time, standard supermassive black holes would need to be about 10 times more common than we’ve thought to account for the strength of these gravitational waves. So the results are inconclusive.

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

Reference: Agazie, Gabriella, et al. “The NANOGrav 15 yr data set: Evidence for a gravitational-wave background.” The Astrophysical Journal Letters 951.1 (2023): L8.

Reference: Depta, Paul Frederik, Kai Schmidt-Hoberg, and Carlo Tasillo. “Do pulsar timing arrays observe merging primordial black holes?” arXiv preprint arXiv:2306.17836 (2023).