In the vast, mysterious expanse of the cosmos, black holes remain one of the most enigmatic and captivating phenomena. They are regions in space where gravity is so intense that not even light can escape their grasp, leading to their name. These gravitational giants come in two primary types: stellar mass black holes and supermassive black holes (SMBHs). Despite being widely known and studied, the origins of supermassive black holes, especially those that reside in the distant reaches of the universe and boast masses millions to billions of times that of the Sun, continue to baffle astronomers and physicists alike.
While stellar mass black holes form from the remnants of massive stars that collapse under their own gravity after exhausting their nuclear fuel, supermassive black holes are of a different, much grander scale. They reside at the centers of galaxies, shaping the evolution and dynamics of their host galaxies. Understanding how these behemoths form, particularly in the early universe, is a key question in modern astrophysics. A groundbreaking new theory proposes that the answer might lie in the properties of ultralight dark matter, specifically a hypothetical particle known as the axion. But how can axions, a form of dark matter that interacts with electromagnetic forces in an unusual way, potentially explain the formation of SMBHs in the early universe?
The Puzzle of Supermassive Black Hole Formation
One of the most intriguing mysteries in astrophysics is how supermassive black holes came into existence, particularly those found at extremely high redshifts, implying their presence in the very early stages of the universe. These distant SMBHs seem to have formed much earlier than we would expect, given the amount of time available for their growth through accretion—slowly accumulating mass by drawing in gas, dust, and even stars.
The conventional model for black hole formation hinges on the collapse of massive stars, where stellar remnants, usually with a mass several times greater than that of the Sun, end their lives in supernova explosions. These explosions leave behind a black hole core. However, this process does not account for the rapid formation of supermassive black holes that appear in the early universe, some as early as a few hundred million years after the Big Bang. Such rapid formation suggests a more exotic process, one that could bypass the normal stellar evolutionary pathway.
Supermassive black holes with masses millions to billions of times the Sun’s, located in the centers of galaxies, do not just form from ordinary stellar collapse. Their origins may lie in the collapse of primordial gas clouds in the early universe. However, the exact conditions that allow for such massive structures to form remain unclear. Could the solution lie in the interaction of dark matter and ultraviolet radiation in the early universe?
A New Theory: The Role of Ultralight Dark Matter (Axions)
In a paper published by Hao Jiao from Cornell University and his team, a novel theory is proposed that might explain the formation of SMBHs during the universe’s infancy. Their hypothesis centers on a fascinating idea: dark matter, particularly ultralight dark matter in the form of axions, could play a crucial role in creating the necessary conditions for black hole formation.
Axions are hypothetical particles that are thought to make up dark matter. Unlike traditional dark matter particles, which are typically modeled as heavy, axions are light and behave like waves, rather than individual particles. These wave-like axions are theorized to interact with electromagnetism in specific ways, particularly through a mathematical interaction known as the Chern-Simons term. This interaction, though not yet directly observed, could have profound implications for our understanding of dark matter and its role in the universe.
The key to Jiao and his team’s theory lies in the behavior of axions in dark matter halos. According to their model, axion field oscillations can amplify infrared photons through a process known as parametric resonance. This process transfers energy from the axion field to photons, causing a significant increase in the intensity of these photons, potentially pushing them into the ultraviolet (UV) spectrum.
Ultraviolet radiation, in this case, has a crucial role to play. When UV photons are generated in large quantities, they can dissociate hydrogen molecules in interstellar gas clouds. This dissociation prevents the cloud from fragmenting into smaller clumps, allowing it to collapse more easily under gravity. In other words, the UV photons generated by the axions could help prevent the hydrogen gas from clumping into stars, allowing the gas to collapse directly into a massive black hole seed.
The idea is that this process could create the “seeds” of supermassive black holes, large primordial black holes with masses around 100,000 times that of the Sun. These seeds could then grow rapidly through accretion and mergers, eventually evolving into the supermassive black holes we observe today, residing at the centers of galaxies.
The Process of Direct Collapse and Its Implications
The theory proposed by Jiao and his colleagues provides a possible mechanism for the direct collapse of gas clouds into black holes without the intermediate step of star formation. In the traditional model, gas clouds in the early universe collapse to form stars, and only the most massive of these stars end their lives as black holes. However, in this new scenario, the ultraviolet radiation from axion-generated photons disrupts star formation altogether, allowing the gas cloud to collapse directly into a black hole.
This process of direct collapse is particularly important for explaining the formation of supermassive black holes at high redshifts—those that existed in the early universe. The theory also offers an intriguing alternative to traditional models, which rely on a long and gradual accumulation of mass through accretion. The direct collapse model suggests that supermassive black holes could have formed much more quickly, potentially explaining the existence of SMBHs just a few hundred million years after the Big Bang.
The model is also consistent with several key features of the modern cosmological model. For instance, it aligns with the standard cosmological model, which describes the structure and evolution of the universe. It also integrates with ideas from string theory, which posits the existence of one-dimensional topological defects in spacetime known as cosmic strings. These cosmic strings are theorized to exist in the early universe and could have played a role in seeding dark matter halos. These halos, in turn, could have provided the ideal environment for the direct collapse of gas clouds into black holes.
How This Theory Fits into Current Understanding
The theory presented by Jiao and his team aligns with multiple strands of current scientific thought. First, it integrates the concept of ultralight axions, a form of dark matter, into the framework of cosmic evolution. While axions have not yet been directly detected, their theoretical properties make them a compelling candidate for dark matter. Second, the idea that axions can generate UV radiation that induces the collapse of gas clouds is consistent with modern theories of astrophysical processes, including those that govern the formation of stars, galaxies, and black holes.
The concept of cosmic strings as a possible seed for dark matter halos is also an exciting extension of existing ideas from string theory. While cosmic strings have yet to be observed, they provide a fascinating theoretical explanation for the structure of the early universe, particularly in the formation of dense regions that could collapse into black holes.
This theory also presents an exciting possibility for future research. If the formation of supermassive black holes in the early universe can be tied to axion-induced UV radiation, this could provide new avenues for testing both the nature of dark matter and the formation of black holes in the early universe. Future observations of distant galaxies, particularly those at high redshifts, could provide clues to the presence of these primordial black holes and help confirm or refute the axion hypothesis.
Conclusion: A New Era of Black Hole Research
The hypothesis that ultralight dark matter, in the form of axions, could play a central role in the formation of supermassive black holes is a groundbreaking concept that opens up exciting new avenues for research. If proven correct, it would not only provide a more complete understanding of how these cosmic giants form but also shed light on the nature of dark matter, one of the greatest mysteries in modern physics.
As scientists continue to study the earliest epochs of the universe, the possibility that dark matter particles like axions could facilitate the creation of black holes in the early universe promises to be a key part of unraveling the cosmic puzzle. Whether this new theory will stand the test of time remains to be seen, but it marks a significant step forward in our quest to understand the formation of the universe’s most fascinating and mysterious objects.
Reference: Hao Jiao et al, Direct Collapse Supermassive Black Holes from Ultralight Dark Matter, arXiv (2025). DOI: 10.48550/arxiv.2503.19414