A Mystery About the Universe’s First Black Hole Might Finally Be Solved

Astrophysicist Priyamvada Natarajan has predicted that black holes can form without the help of stars. New observations support her theory.

As astronomers have delved into the earliest chapters of the universe’s history, they have uncovered a plethora of massive black holes that appear to have matured far more rapidly than scientists previously thought.

Priyamvada Natarajan is akin to a cosmic biologist. She studies the lives of these early black holes—objects so dense that they trap all matter and light within their grasp. As an astronomy graduate student, Natarajan was among the first to consider black holes not just as individual objects but as populations, akin to bats in a rainforest, by studying their general classification and evolution. Now an astrophysicist at Yale University, Natarajan continues to study these cosmic entities' behavior, focusing on understanding how they are born.

Traditionally, black holes form as a result of massive stellar explosions, growing in mass as they consume nearby gas reserves. However, observations of supermassive black holes in the early universe suggest that there is more to the story. In 2006, Natarajan and her colleagues proposed a groundbreaking explanation for how gas disks could collapse directly into unusually large infant black holes without forming stars first. Last year, a joint observation by the James Webb Space Telescope (JWST) and the Chandra X-ray Observatory spotted a distant, luminous black hole that appears to validate Natarajan’s prediction.

“This certainly makes a strong case for these heavy black hole seeds,” says astrophysicist Raffaella Schneider of Sapienza University of Rome. Natarajan's proposition of this concept significantly broadened our outlook regarding the myriad of potentialities embraced by the scientific community.

Natarajan spoke with Scientific American about how recent observations support her proposal of “direct-collapse black holes” and what they reveal about the lineage of these cosmic entities.

What ignited your curiosity to delve into the exploration of black holes and their genesis?

I have always been fascinated by invisible entities in the universe. My work has primarily focused on trying to understand the basic nature of the dark components of the cosmos—dark matter, dark energy, and black holes. I find these objects incredibly captivating and mysterious. They remind us of the limits of our knowledge, where the known laws of physics break down.

Over the past few decades, black holes have transitioned from being a purely mathematical concept to real objects we can observe. They have become central to our understanding of how galaxies form. The universe is filled with black holes of all sizes. They are a crucial part of our cosmic inventory, so understanding their formation is a fundamental open question.

What do we not know about black hole formation?

Typically, black holes are born when stars die. When the most massive stars undergo gravitational collapse, the small remnant they leave behind is a black hole. This is the well-established origin story.

However, about two decades ago, when we started looking deeper into the universe with missions like the Sloan Digital Sky Survey, we found a handful of extremely large black holes—about a billion times the mass of the sun—when the universe was only one to two billion years old. Given the known rate at which black holes consume matter, there wasn’t enough time for small seeds formed from the first stellar explosions to grow into these gigantic black holes. In the following years, we realized these weren’t just a few oddities; a whole population of supermassive black holes existed in the early universe. And that’s when the puzzle began.

Some people started investigating whether there could be ways for black holes to consume matter far faster than the known limits. There are theoretical possibilities, but we haven’t yet seen concrete observational evidence. So I began to wonder, what if we start with larger seeds? My team and I realized that if a gas disk were irradiated by nearby stars in a galaxy, it could disrupt star formation and collapse directly into a black hole. This direct-collapse black hole would be very massive at birth—about 1,000 to 100,000 times the mass of the sun. That black hole could then merge with nearby galaxies and easily grow to the sizes we have observed.

This image reveals the farthest black hole ever observed in X-rays, providing insight into the formation of the universe's earliest supermassive black holes. This extremely distant black hole is located in the galaxy UHZ1 and was captured using NASA’s Chandra X-ray Observatory (shown in purple) and infrared imaging from NASA’s James Webb Space Telescope (illustrated in in red, green, and blue). X-ray: NASA/CXC/SAO/Ákos Bogdán; Infrared: NASA/ESA/CSA/STScI; Image Processing: NASA/CXC/SAO/L. Frattare & K. Arcand

[An edited transcript of the interview follows.]

What sparked your interest in studying black holes and their origins?

I've perpetually harbored a fascination for the unseen entities that populate the cosmos. My work has primarily focused on trying to understand the nature of these dark components of the cosmos—dark matter and dark energy, as well as black holes, at a fundamental level. I find these objects incredibly captivating and mysterious. They remind us of the limits of our knowledge, the places where the known laws of physics break down.

Over the past few decades, black holes have transitioned from being purely mathematical concepts to real objects we can observe, now playing a central role in our understanding of how galaxies form. The universe is filled with black holes of every size. They are a significant part of our cosmic inventory, so understanding how they form is a fundamental open question.

What do we not know about black hole formation?

Typically, black holes are born when stars die. As colossal stars succumb to gravitational collapse, they yield a diminutive residue that manifests as a black hole. This is the well-established origin story.

However, about two decades ago, when we started looking deep into the universe with missions like the Sloan Digital Sky Survey, we found a handful of very large black holes—almost a billion times the mass of the Sun—when the universe was only one to two billion years old. Given the rate at which we know black holes like to consume matter, there simply wasn't enough time for the small seeds left by the first stellar explosions to grow into these enormous black holes. Over the next few years, we realized that these were not just a few odd objects; there was an entire population of supermassive black holes in the early universe. And that's when the puzzle began.

Some people started exploring whether there could be ways for black holes to feed much faster than known limits. Theoretically, there are, but we haven't yet seen solid observational evidence for it. So I started thinking, what if we start with bigger seeds? My team and I realized that if a gas disc were irradiated by stars from a nearby galaxy, it could disrupt the star formation process and collapse directly into a black hole. This direct-collapse black hole would be very massive at birth—1,000 to 100,000 times the mass of the Sun. That black hole could then merge with a nearby galaxy and easily grow to the sizes we have observed.

The farthest black hole ever observed in X-rays resides in the galaxy UHZ1, captured by NASA's Chandra X-ray Observatory (shown in purple) and infrared data from NASA's James Webb Space Telescope (displayed in red, green, and blue). How was this proposal received by the community? We had many people pushing us back. They remarked that while the concept is theoretically sound and logical, they questioned whether this process truly possesses the necessary skill to occur in the universe. At that time, these ages of the universe were not easily visible. To witness the formation of these early seeds, we needed to look back billions of years after the formation of the universe. That's why the promise of JWST was so enticing; it kept us motivated to work on it. We started thinking about what clues we could see as evidence of supermassive black holes, and we came up with an idea. In nearby galaxies, the mass of all stars is often 1,000 to 10,000 times greater than the mass of a central black hole. But in these observational scenes, for a brief period, the mass of a black hole could actually be comparable to the mass of the stars. This means you should see a very bright, actively feeding black hole that inevitably dominates all the stars in the galaxy. Observing one of these galaxies in both X-ray and infrared wavelengths should reveal the unmistakable signs of a massive black hole at its core. I was intimately familiar with every detail of that dark matter map. However, even with JWST and Chandra, we can't see far enough to directly witness the formation of these early black hole seeds. But I realized that if nature was kind to us, one of these galaxies could be hidden behind a gravitational lens: a rich galaxy group enriched with dark matter acting as a dramatic gravitational lens. I was working with the Hubble Space Telescope to map some of these gravitational lenses, suggesting that we focus our new telescopes on the extremely complex cluster known as Abel 2744. I knew every part of that dark matter map inside and out. I was hopeful, but this was a shot in the dark. And how did it pay off? Well, lo and behold, at the beginning of last year, I got a call from my colleague, the astrophysicist Akos Bogdan, who had been studying the lensing of galaxies behind Abel 2744. He said, "Are you sitting down? I think we've found something." Remarkably, the spectrum of a galaxy perfectly matched a speculative prediction made by us in 2017. It was surreal. It validates every speculative hypothesis. It's very solid evidence that supermassive black holes form in the early universe. These are no longer just speculations. Now, there could be other ways to create black hole seeds. I'm exploring that as well: trying to illuminate other pathways and what their unique observational signatures might be. It opens up a whole treasure trove of intriguing questions. I can imagine. How did it feel to have your theoretical ideas validated by nature? As an astrophysicist, I find this incredibly exciting - I want to confront theoretical ideas with observational data. We're in this incredible era of history where you can make a prediction and it can be validated or invalidated within your lifetime. That's why people say we're living in the golden age of cosmology. I'm extremely grateful.