Webb Space Telescope’s little red dots could hold key to first black holes

Johns Hopkins researchers searching the expanse of the universe for the existence of the first black holes are zeroing in on little red dots that may hold the key to the theoretical phenomenon. 

Using data from the James Webb Space Telescope (JWST), researchers from Johns Hopkins have tested whether one specific little red dot, called QSO1, could contain a primordial black hole—theoretical objects that predate the existence of stars, galactic dust particles and heavier elements. The research could offer insight into how primordial black holes, which astrophysicists have never found direct evidence for, may be formed.  

“A black hole is an object born from the gravitational death of a star. Primordial black holes are a very different story,” said co-author Valerio De Luca, a postdoctoral researcher in the Department of Physics and Astronomy at Johns Hopkins University. “They come from the physics of the early universe that predate the first stars, when the universe was expanding exponentially in time and space. So, the mechanisms for creating primordial black holes are different.”    

The findings were recently published in Physical Review Letters.  

Since Stephen Hawking first proposed the idea of primordial black holes in the 1970s, scientists have been working to determine what kind of evidence would be needed to prove their existence and differentiate them from astrophysical black holes. Astrophysicists have recently turned to swirling, super-hot discs of galactic dust particles, energy and gas, the little red dots on JWST images, to find answers. 

“These little red dots could surround astrophysical black holes, and most probably many of them do, but they could also be primordial black holes, if primordial black holes exist,” De Luca said. “So, the question becomes, ‘Which ones are primordial and how can we tell?’”  

As a proof of concept, the researchers turned to QSO1, which scientists suspected could contain a primordial black hole based on its age, size, and composition. QSO1 existed just 700 million years after the Big Bang and, unlike other little red dots, is almost entirely made of hydrogen and helium—the first elements that formed during the beginning of the universe—and lacks the heavier elements that formed later in the universe’s timeline. 

A primordial black hole would only be able to grow to QSO1’s size and obtain a swirling disc under special circumstances. 

“There are constraints on primordial black holes from cosmological observations. There can only be so many primordial black holes in the universe, for example, and there cannot be too many of them in a given cluster,” said co-author Loris Del Grosso, a postdoctoral researcher in the Department of Physics and Astronomy. 

Researchers at Johns Hopkins modeled three scenarios to see if a primordial black hole could possibly develop into a little red dot that is QSO1’s size and is surrounded by that concentration of other discs. 

The most likely scenario: a smaller primordial black hole pulled in and consumed surrounding gas and energy until it grew to be the size of QSO1.   

“The door was open on this theory, that little red dots could be primordial black holes. We’ve now cracked it open a little farther by showing some scenarios are more likely than others,” Del Grosso said. 

Next, the researchers plan to explore how many of the 100 observed and catalogued little red dots could possibly be primordial black holes.  

To definitively prove the existence of primordial black holes, the researchers said they would need to find a black hole that is either too light to be an astrophysical black hole or so old that it pre-dates the existence of stars.  

“We don’t have a clear understanding of what happens in the early universe right after the Big Bang,” said De Luca. “If primordial black holes do exist, then something had to happen to make them in the first place, which would be a groundbreaking discovery in physics.” 

Co-authors include Konstantinos Kritos, Emanuele Berti, and Joseph Silk from Johns Hopkins University; Gabriele Franciolini from Università degli Studi di Padova; and Daniel D’Orazio from the Space Telescope Science Institute.