The universe’s latest mystifying denizens continue to flummox and divide astronomers.
Almost as soon as NASA’s James Webb Space Telescope (JWST) turned on in 2022, gathering light from the first few billion years after the big bang, it saw an ancient sky festooned with tiny flecks of glowing red. Ever since, these “little red dots” (LRDs) have challenged practically everything scientists thought they knew about the early universe.
Most astronomers now agree that each of these minuscule crimson specks—which bear a striking resemblance to enormous, faraway stars—actually has a burgeoning black hole at its center. But the size of these black holes—and consequently their origin and role in the grand arc of cosmic history—remains a subject of intense debate.
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A paper published today in Nature stakes a claim on the “heavy” side of this cosmic guess-my-weight competition. Using JWST to gaze back to just 700 million years after the big bang, the paper’s authors report their measurement of an LRD’s mass by a novel, purportedly less equivocal method: they have found it to be some 50 million times the mass of our sun. The result has spurred skepticism ever since it appeared as a preprint last August, however, because its conclusions would overturn the beliefs of most astronomers. Finding such massive black holes so early in the universe’s life would suggest that they predate the galaxies that engulf them—and that they were perhaps born at the dawn of time itself.
“If everything in this paper is true at face value, then we are living in a stranger world,” says Jenny Greene, an astronomer at Princeton University, who was not involved in the study. “That’s why this is very important.”
The debate boils down to which came first: the vast agglomerations of stars and gas we call galaxies or the giant black holes we usually see at their core. If black holes came first, serving as gravitational seeds for the growth of galaxies, then they must have somehow gotten very large very early in the universe’s life.
Following the researchers’ discovery, early follow-ups suggested LRDs each weighed millions of solar masses—a potential lynchpin for this controversial “black holes first” chronology. But then astronomers challenged those initial estimates. These first attempts at gauging LRD masses used a common, indirect technique for weighing black holes in later, more contemporary cosmic epochs—the “supermassive” black holes at the center of every galaxy. But that technique assumed the LRDs had similar surroundings to their modern counterparts.
Unlike supermassive black holes, the critics argued, LRDs appeared occluded by much denser clouds of gas, potentially mandating a more direct method for accurately measuring their mass. Many of these critics believe LRDs are a totally new class of object called “black hole stars.” From the outside, a black hole star would look much like a red giant star: a glowing, swollen ball of ionized gas. But instead of cooking up thermonuclear fusion reactions at its unseen core as an ordinary star would, an LRD would harbor a growing—but not full-grown—black hole at its heart. Feasting on gas, this baby black hole would generate enough energy to prop up the surrounding cocoon and maintain its glow.
Either explanation would constitute an astrophysical revolution. If these black holes reached multiple millions of solar masses so early, their origin would become even more mysterious. “It points you to some exotic stuff,” Greene says.
Assigning LRDs a smaller size sidesteps the problem of nigh inexplicably overgrown black holes but only by branding them as an unprecedented, newfound celestial species—the black hole star. “There is a tendency to rebrand well-known phenomena as something new,” says Roberto Maiolino, an astrophysicist at the University of Cambridge and co-author of the Nature study. Cambridge Ph.D. student Ignas Juodžbalis, Maiolino’s collaborator and the study’s first author, agrees. “I think with LRDs, it’s more likely that we’re seeing a familiar object from an unfamiliar angle,” he says, adding that vanilla supermassive black holes are already “plenty weird.”
The new measurement attempts to settle this debate with a technique called “spectroastrometry,” which studies have recently used to unequivocally determine the mass of supermassive black holes in some of today’s galaxies. In this case, the study authors used JWST to gather light emitted by excited hydrogen atoms in a swirling maelstrom of gas in far-flung orbits around the black hole.
This light has a very specific wavelength, or color, but the JWST observations showed a minuscule shift in this color from one end of the maelstrom to the other because of the hydrogen’s motion. Much like an ambulance siren that rises in pitch as it approaches and then falls in pitch as it recedes, the light is slightly bluer where hydrogen atoms are moving toward us and slightly redder where they are moving away. From this shift, the researchers determined the gas’s velocity at different orbital distances from the LRD. “You have independent velocity and distance measurements,” Juodžbalis says, “which means that you know exactly the mass of the object inside.”
And to explain the velocity differences the researchers saw, this central black hole would need to weigh a hefty 50 million solar masses. If true, the result “would absolutely be a direct contradiction to the black hole star hypothesis,” says Raphael Hviding, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, who wasn’t involved in the work.
In fact, the measurement implies that the black hole outweighs its surroundings. “The black hole appears to be more massive than the host galaxy itself—if a host galaxy is present at all,” Maiolino says. Such an isolated but huge black hole could be the product of a direct collapse of gas clouds shortly after the big bang—or it might even be “primordial,” a hypothetical type born in the first second of cosmic time. “This result may help shed light on the nature of the seeds of today’s supermassive black holes and how they formed in the very early universe,” says astronomer Dale Kocevski, who was not involved in the work.
But the LRD is so distant that some have questioned whether such a subtle technique can be trusted. “It’s a really brave and hard measurement,” Greene says. “If someone is able to reproduce it independently, then I will pay more attention.” Juodžbalis is also looking for further ways to bolster this work, which he describes as “pushing the data to its limits.”
Beyond JWST, other cutting-edge observatories such as Europe’s ground-based Extremely Large Telescope in Chile will likely resolve the debate one way or the other when they come online near or in the 2030s. The coming decades, it seems, will see astronomers at last filling in the picture of the biggest objects in the universe, which most everything else literally revolves around.
“We’ll have the data to do it,” Juodžbalis says. “Definitely within my lifetime, we’ll figure out not only LRDs but where the supermassive black holes in general come from.”
