Several Pulsar Timing Array (PTA) Observatories, including NanoGRAV (North American Nanohertz Observatory for Gravitational Waves) and the EPTA (European Pulsar Timing Array) announced the discovery of gravitational waves at nanoHertz frequencies. Binaries of supermassive black holes are arguably the most vanilla explanation; the last stage of their merger is expected to generate a stochastic signal at these frequencies. And that might very well be the right explanation. However, there are still holes in this understanding; in reality, we do not understand that well how do they merge, as standard estimations say that it takes more than the age of the universe for the binaries to cross the last parsec and merge. The data also seems to suggest a spectrum slightly different than that expected from binaries, although not at a significant level yet.
The case for new physics
All in all, there is space to consider alternative explanations, in particular those that include new physics. That was what I did with my collaborators in this article where we argued that the gravitational waves signals might instead have originated in the early universe collapse of certain cosmological structures called domain walls. These cosmological beasts are often unavoidably formed in many theories that extend the Standard Model of Particle Physics to address some of the open problems.
Remarkably, the latest dedicated analysis of PTA data has shown that the domain wall hypothesis performs significantly better than the supermassive black holes thus making the case for domain walls quite interesting. Since then we have looked further into the details of the domain wall collapse and in a recent work we derived a few more insights. In particular, we found that from the network’s collapse, there will be some rare highly energetic pockets that convert into stellar-mass black holes which we can also look for through gravitational wave observations. In a different region of parameters, these black holes might even constitute the totality of the dark matter.