Research indicates that primordial black holes, which formed in the universe’s earliest moments, could have rapidly grown to enormous supermassive sizes.
This finding might address a major challenge in modern cosmology: the question of how supermassive black holes managed to accumulate masses millions or billions of times that of the sun in less than a billion years after the Big Bang.
This conundrum has been exacerbated by the discoveries made by NASA’s James Webb Space Telescope (JWST). With its advanced capabilities, the telescope has been exploring the early universe and uncovering an increasing number of supermassive black holes that existed a mere 700 million years after the Big Bang, or even sooner.
“The issue is that as we observe the early universe with increasingly advanced telescopes, which allow us to look back at the cosmos from various angles due to the finite speed of light, we continuously encounter supermassive black holes,” explained John Regan, a Royal Society University research fellow at Maynooth University in Ireland, to Space.com. “This suggests that supermassive black holes were already present early on, within just a few hundred million years.”
Previously proposed mechanisms for the growth of supermassive black holes, such as rapid accretion of matter and the merging of larger black holes, would typically require over a billion years to yield a supermassive black hole.
The earliest and most distant supermassive black hole identified by JWST is CEERS 1019, located just 570 million years after the Big Bang and weighing in at 9 million solar masses. This size is too large to exist based on what we know about black hole formation 13.2 billion years ago.
“This presents a puzzling situation: these black holes must have either formed at this large mass or somehow experienced incredibly rapid growth from a smaller mass,” Regan noted. “We lack evidence for how a black hole could emerge with such immense mass, leaving us uncertain about how smaller black holes could expand so quickly.”
The latest research suggests that primordial black holes might have provided early supermassive black holes with an advantage in their development.
Non-astrophysical black holes get a head start
Black holes come in various sizes. Stellar-mass black holes, 10 to 100 times the mass of the sun, form when massive stars exhaust their nuclear fuel and undergo supernova explosions.
Supermassive black holes, with a mass at least a million times that of the sun, reside at the centers of all large galaxies. Their formation cannot occur through the death of massive stars. Instead, they arise when smaller black holes merge repeatedly or voraciously consume surrounding matter, or through a combination of both methods.
These categories of black holes, along with the rare intermediate-mass black holes that exist between stellar-mass and supermassive black holes, are termed “astrophysical” black holes.
Scientists have long speculated about “non-astrophysical” black holes, specifically primordial black holes. This term refers to their formation processes that do not depend on collapsing stars or existing black holes.
Primordial black holes are thought to form directly from dense regions within the matter that filled the universe in the first seconds following the Big Bang.
Currently, there is no observational evidence confirming the existence of primordial black holes. Nonetheless, scientists propose that these hypothetical entities could explain dark matter, the elusive substance that constitutes 85% of the universe’s mass but remains undetectable as it doesn’t interact with light.
The new findings indicate that primordial black holes, expected to have masses ranging from 1/100,000th of a paperclip to 100,000 times that of the sun, might have an edge in rapidly forming supermassive black holes. This is because their maximum mass is not limited by the size of a dying star, unlike stellar-mass black holes.
“Primordial black holes would form in the early moments after the Big Bang, and if they exist, they possess advantages over their astrophysical counterparts,” Regan explained. “They can potentially be more massive from the outset and may settle into the centers of galaxies more easily, allowing for rapid growth.”
Additionally, primordial black holes have an advantage over stellar-mass ones since they don’t need to wait for massive stars to die — a process taking millions of years.
Regan clarified that astrophysical black holes can only come into existence after the first generation of stars exhaust their fuel. Even then, these astrophysical black holes tend to have only a few hundred solar masses. Moreover, the energy released by stars during their lifetimes and explosive deaths often clears nearby material, limiting the available resources for a black hole’s growth.
“This can lead to a scarcity of material for the young black hole to consume,” Regan noted.
In contrast, primordial black holes would not emit energy or explode, avoiding this problem. However, they still need to locate a rich source of matter to grow.
Do primordial black holes become supermassive at the center of galaxies?
In the simulations executed by…
Regan and his team suggested that primordial black holes must grow by attracting matter, while mergers with other black holes are secondary in importance.
“In the early universe, most matter was hydrogen and helium,” Regan added. “These primordial black holes are likely to mainly increase in size by attracting these gases. Although mergers with other primordial black holes could contribute, accretion is anticipated to be the primary process.”
For primordial black holes to accumulate enough matter to eventually form supermassive black holes, they must be effective at consuming matter quickly. This requires them to move towards areas in the universe where matter is densely packed—specifically, the centers of galaxies where supermassive black holes are currently found.
“To achieve this, primordial black holes have to drift towards a galaxy’s center,” Regan explained. “This is feasible if there are a sufficient number of primordial black holes. Only a few need to be fortunate!”
The availability of primordial black holes is crucial in determining if astrophysical black holes will contribute to the formation of early supermassive black holes.
“If primordial black holes are plentiful, they could potentially constitute the entire population of supermassive black holes,” Regan stated. “Whether these black holes are solely responsible for the mass of early supermassive black holes depends on their abundance. It’s theoretically possible, but I suspect astrophysical black holes are also involved.”
(Image credit: Robert Lea (created with Canva))
However, these conclusions are based on simulations, and confirming this theory still requires further investigation. One potential observational clue would be finding a massive black hole existing in the early universe, particularly before 500 million years post-Big Bang.
Another clue could come from discovering a black hole today with a mass less than three times that of our sun. Such a small black hole could not have formed from the collapse of a massive star, suggesting it originated from a primordial black hole.
“I was surprised to see that primordial black holes could grow so swiftly and that our simulations aligned with the conditions for their existence,” Regan remarked. “All we need now is solid evidence for a primordial black hole—either a very low-mass black hole now or a very high-mass one from the early universe.”
“If primordial black holes exist, they will likely be found in extreme circumstances!”
In the absence of concrete evidence, the team plans to enhance their cosmological simulations to reinforce the idea that supermassive black holes originated from primordial black holes.
“The next steps involve improving the realism of our simulations. This was just an initial attempt focusing solely on primordial black holes,” Regan concluded. “Our goal is to model primordial and astrophysical black holes in tandem and identify any unique features.”
The team’s findings are available as a pre-peer-reviewed paper on the arXiv repository.
