Origin of the Cosmic Hum - Phase Transition or Primordial Black Holes?

4 November 2024

Photo: Carlo Tasillo & David Weir

A graphical representation of how gravitational waves from a cosmological phase transition or from merging black holes could have led to the signals recently observed by pulsar timing arrays. Understanding the sources of these gravitational waves helps us to explore fundamental aspects of the Universe.

New ideas for the origin of the ‘cosmic hum’ have been further developed in a couple of recent studies by Excellence Cluster Quantum Universe researchers from DESY, along with collaborators from Heidelberg, Mainz and Oslo. The cosmic hum of gravitational waves was recently observed by pulsar timing arrays yet its origin is not clear. Their scenarios involve the merger of primordial black holes or a cosmic phase transition. Either case could help to elucidate fundamental physics mysteries of our Universe.

Pulsar Timing Arrays Unlock Secrets of the Universe 

In 2023, the world of astrophysics was abuzz with exciting news—pulsar timing arrays (PTAs) had detected signals that might be evidence of gravitational waves at very low frequencies. These signals, found in the decade-long observational data of the regular radio pulses emitted by distant neutron stars (pulsars), revealed something new: a common background noise, or a hum, pervading space-time itself. This 'cosmic hum' is found to be caused by gravitational waves, the ripples in the fabric of the Universe predicted by Einstein’s theory of general relativity. But a big question remains: What is producing these waves? The conventional explanation for this hum is the merger of astrophysical supermassive black holes over cosmic history. However, two of the most exciting alternative possibilities are: A cosmic phase transition in the dark sector of the Universe or merging primordial black holes—both of which take us deep into the unknown territory of fundamental physics. 

A Phase Transition in the Dark Sector? 

The researchers’ first study delved into the idea that this gravitational wave background might come from a phase transition in the dark sector—a part of the matter content of the Universe that we cannot see or directly interact with, but which makes up about 85% of its mass. Imagine water boiling in a pot. At a certain temperature, liquid water undergoes a phase transition, turning it into steam. Something similar could have happened in the early universe—except here, the phase transition would have occurred in this hidden 'dark sector', forming bubbles in the primordial plasma, which then accelerate up to close to the speed of light and eventually collide, generating gravitational waves throughout the cosmos.

They explored this possibility in detail and found that a fully isolated dark sector —one that doesn’t interact with the visible Universe at all—would be an unlikely explanation for the gravitational wave signals, given the constraints we have from our precise understanding of the formation of the early elements and the cosmic microwave background. However, if the dark sector is connected to the visible Universe through very small interactions, this explanation remains a viable and exciting possibility. 

Could Primordial Black Holes Be Merging? 

The authors' second study explored a completely different idea for the origin of the cosmic hum: In this work, they hypothesized that supermassive primordial black holes, which formed right after the Big Bang, eventually formed binary systems. Due to their high masses, these black holes would orbit each other only very slowly, giving rise to a gravitational wave signal matching the observed very low frequency nano-Hertz scale. However, for this explanation to fit the data, the distribution of primordial black holes needs to be more clustered in some regions of the Universe than in others, rather than being evenly spread out. 

Their research shows that this clustering indeed enhances the merger rate of such black holes, making this hypothesis a strong candidate for explaining the PTA results. This setup is still speculative, and more work is needed to understand the influence of these clustered primordial black holes on other aspects of precision cosmology. However, the idea of gravitational waves revealing the merger events of ancient black holes that formed at the dawn of the Universe remains incredibly exciting. 

Significance

Gravitational waves offer a new way to explore the cosmos and may provide evidence for physics beyond the Standard Model, such as dark matter or hitherto unknown early universe phenomena. Whether the source of the waves is a dark sector phase transition, the inspiral of supermassive primordial black holes or rather the mergers of conventional astrophysical black holes as a result of structure formation, these results are a step towards solving some of the greatest cosmic mysteries.