Ultralight dark matter could explain early black hole formation

A black hole is a region of space where gravity is so strong that nothing, not even light, can escape. There are two main types of black hole; stellar mass and supermassive black holes, and they differ in size, formation, and impact on their host galaxy. Stellar mass black holes, a few to dozens of times the mass of the sun, form from collapsing massive stars. Supermassive black holes, on the other hand, are millions to billions of times more massive and tend to live in the center of galaxies and grow through accretion and mergers.

The origin of supermassive black holes (SMBHs), especially those over a billion solar masses and at high redshifts, remains unclear. These objects, as their high redshift shows, are at vast distances from Earth and therefore exist at some of the earliest epochs of the universe. One key formation process is the direct collapse of cooling atomic gas clouds without becoming stars. It is thought this process could create massive “seed” black hole around 100,000 solar masses with “heavy” seeds that can grow to a billion solar masses.

In a paper posted to the arXiv preprint server by a team led by Hao Jiao from Cornell University, the team explores a process that may explain the existence of the SMBHs at such extreme high redshifts. At the center of their hypothesis is the flow of ultraviolet radiation that can dissociate hydrogen molecules which prevent fragmentation of the cloud and enable its collapse. In other words, they have a theory to explain how the UV radiation can help create the right conditions for the formation of SMBHs in the early universe.

They assume that dark matter consists of ultra-light wave-like particles, referred to as “axions,” which interact with electromagnetism through particular topological interactions known mathematically as the Chern-Simons term. In dark matter halos, axion field oscillations can amplify infrared photons through energy transfer from the axion field to the photons (parametric resonance.) This can also lead to the emission of ultraviolet radiation either by the thermal energy reaching equilibrium or the optically thick halos or through a strong photon cascade from the resonance process.

The study reveals that ultralight dark matter (axions) can generate enough UV photons for the direct collapse of black holes through the process of parametric resonance. This allows for black hole formation during the earliest periods of the history of the universe, without the need for starlight or stellar evolution and could explain the presence of supermassive black holes at high redshifts.

Their mechanism works and is in accordance with several modern theories, such as the standard cosmological model (which articulates the structure and evolution of the universe) and with cosmic string seeds (which are theorized as one-dimensional topological defects in spacetime.) Even elements of string theory support the model where cosmic string loops could help by seeding halos earlier and making them more concentrated, increasing the likelihood of their formation.