Supermassive black holes usually require billions of years to be formed. However, the James Webb Space Telescope is discovering them appearing soon after the Big Bang—much earlier than expected.
Forming supermassive black holes, such as the one located at the center of our Milky Way, is a lengthy process. Typically, a black hole’s formation begins with the death of a massive star—one at least 50 times the mass of our sun—which undergoes a billion-year life cycle before its core collapses.
The result of this collapse is a black hole that might only have about 10 times the sun’s mass, a far cry from the approximately 4 million-solar-mass black hole, Sagittarius A*, found in our Milky Way, or the billion-solar-mass supermassive black holes in other galaxies. These enormous black holes gradually form from smaller ones through the accretion of gas and stars, alongside mergers with other black holes, taking billions of years.
The recent findings from the James Webb Space Telescope raise a perplexing question: How are supermassive black holes being discovered in the early universe, eons before they should have had time to come into existence? Astrophysicists from UCLA propose an intriguing answer: Dark matter may have played a crucial role in maintaining the temperature of hydrogen, preventing it from cooling too rapidly. This allowed gravity to clump it into sizable clouds that eventually transformed into black holes rather than stars. This research can be found in the journal Physical Review Letters.
“It’s astonishing to find a supermassive black hole weighing a billion solar masses when the universe is only half a billion years old,” commented Alexander Kusenko, the senior author and a physics and astronomy professor at UCLA. “It’s akin to discovering a modern vehicle among dinosaur fossils, pondering how that vehicle could exist in prehistoric times.”
Some astrophysicists propose that a massive gas cloud could directly collapse into a supermassive black hole, skipping the long processes of star burning, accretion, and mergers. However, there’s a challenge: While gravity indeed pulls gas clouds together, it doesn’t create a single big cloud. Instead, it forms smaller halos of gas, which remain near each other but fail to evolve into a black hole.
The reason for this is the rapid cooling of the gas cloud. If the gas stays hot, its pressure can counteract gravity. But once the gas cools down, the pressure drops, allowing gravity to dominate in various regions, leading to the formation of dense objects before an entire cloud can condense into a single black hole.
“The cooling rate of the gas is significantly influenced by the amount of molecular hydrogen,” stated Yifan Lu, the primary author and a Ph.D. student. “When hydrogen atoms bond into molecules, they lose energy upon interacting with free hydrogen atoms. These hydrogen molecules act as cooling agents, absorbing thermal energy and radiating it away. In the early universe, excessive molecular hydrogen led to rapid cooling, resulting in the formation of small halos instead of larger clouds.”
Lu and postdoctoral researcher Zachary Picker developed algorithms to evaluate all potential processes in this scenario. They discovered that additional radiation could heat the gas and help break apart hydrogen molecules, changing the cooling dynamics.
“Introducing radiation within a specific energy range breaks down molecular hydrogen, creating circumstances that prevent the fragmentation of large gas clouds,” Lu explained.
But what is the source of this radiation?
A small fraction of the universe’s matter comprises the familiar elements that make up our bodies, the Earth, stars, and other observable entities. In contrast, the majority is believed to consist of a different form of matter that scientists are still trying to identify, detectable mainly through its gravitational interactions with celestial objects and its influence on the paths of light from distant sources.
The characteristics of dark matter remain largely enigmatic. Despite this uncertainty, particle theorists have suggested that dark matter could comprise unstable particles that decay into photons, the fundamental particles of light. Including this type of dark matter in their simulations provided the necessary radiation for the gas to maintain the structure of a large cloud while collapsing into a black hole.
Dark matter might consist of slowly decaying particles or various particle types, with some being stable and others decaying earlier. Whichever the case, the decay products could be in the form of radiation, such as photons, which disrupt molecular hydrogen and inhibit the quick cooling of hydrogen clouds. Even a minimal decay of dark matter can produce sufficient radiation to maintain large clouds, ultimately leading to the formation of supermassive black holes.
“This could clarify why we observe supermassive black holes in the early universe,” Picker noted. “Optimistically, this finding could also serve as fruitful evidence for a specific type of dark matter. If these supermassive black holes formed from gas cloud collapse, the necessary additional radiation might originate from yet unknown aspects of dark matter physics.”
Key takeaways
- Supermassive black holes generally take billions of years to emerge. However, the James Webb Space Telescope is uncovering them shortly after the Big Bang—much sooner than anticipated.
- Research from UCLA astrophysicists found that the decay of dark matter emits photons that help keep hydrogen gas sufficiently hot, allowing gravity to pull it into massive clouds that can later form supermassive black holes.
- This discovery not only clarifies the early existence of supermassive black holes but also supports the theory of a type of dark matter that can decay into particles such as photons.
